BACKGROUND
arbuscular mycorrhizal fungi  are widespread in terrestrial ecosystems and form obligatory symbiotic relationships with most land plants  <cit> .
these symbioses are not host specific but are to some extent host-preferential  <cit> .
amf may have a biogeographical distribution partially because of their soil-borne life form, host preference, and limited dispersal resulting from geographic isolation  <cit> .
increasingly, studies have shown that amf are cosmopolitan at the genus or higher taxonomic level but have limited distributions at the species level.
global-scale studies showed that amf have distinct distribution patterns  <cit>  and that habitat filtering or dispersal limitation affects these patterns  <cit> .
at the continental scale, a single widespread host plant was found primarily to harbor geographically generalist amf  <cit> .
at the regional scale, a typical distance-decay distribution of amf suggested that geographic distance and environmental heterogeneity determined the patterns of spatial scaling  <cit> .
at the local scale, amf distribution is closely related to host identity and habitat  <cit> .
at the fine scale, amf diversity has a patchy distribution  <cit> .
studies have also shown that amf distribution could be influenced by climate  <cit> , habitat  <cit> , geographical isolation, soil conditions, and anthropogenic activities  <cit> .
several studies have reported high selectivity between host plants and amf, which may affect amf distribution  <cit> .
bever et al  <cit>  reported host-specific differences in the population growth rates of amf.
helgason et al  <cit>  found physical and functional selectivity in amf, and zhang et al.
<cit>  recently documented that the invasive plant solidago canadensis promoted the growth of the most beneficial amf and thereby increased its own competitiveness.
distinctive amf communities were associated with coexisting plant species, including grasses  <cit> , forbs  <cit> , and trees  <cit> .
even plants of the same species that differed in age harbored distinctive amf  <cit> .
thus, it seems likely that plants can actively select with amf to associate.
some studies have suggested that plants have evolved an ability to recognize beneficial amf.
for example, bever et al  <cit>  found that the host plant preferentially allocated carbon to beneficial symbionts.
kiers et al  <cit> , who used qpcr and stable isotope probing methods, documented the existence of a reciprocal rewards mechanism that stabilizes the cooperation between plants and amf.
because host plants have distinct geographical distributions  <cit> , selection by host plants may result in distinct geographical distributions for amf  <cit> .
however, whether and how host selection affects the pattern of amf distribution at large scales has not been well studied.
recently, two studies have revealed global patterns of amf by using molecular approach.
one of these studies  <cit>  has developed a database  and demonstrated ecosystemic and global patterns for amf distribution.
this study also documented that amf had high specificity of host plant superorders and wider distribution with larger breadth of hosts.
the other study  <cit>  also found a high endemism of amf at global scales that could be affected by geographic distance, soil condition and plant community.
although both of these studies indicated that host plants related to amf distribution at a global scale, whether and how host plants affect amf distribution remain unknown.
here, we hypothesized that selective pressure from host plants at different scales  may be the main factor affecting the distribution of amf at large scales, and that the effects of biogeographical, ecosystemic and climatic factors may be mediated by host plants.
its , ssu  and lsu  are the three widely used marker genes in molecular diversity of amf.
ssu was utilized widely while lsu was relatively infrequent compared with its .
as relative conserved variability of sequences among species, ssu is widely used in molecular diversity of amf.
however, ssu has a limitation to resolve species in genera of ambispora, diversispora and scutellospora  <cit> .
this limitation can affect the designation of phylotype and further influence the analysis on the distribution of the whole amf  <cit> .
for lsu, although it had relative higher intra-specific variability than ssu and can also well resolve amf species, the lsu based data in genbank are limited for the study of amf distribution at global scale.
to avoid the limitations of ssu and lsu, we selected its as marker gene for our analysis.
although its has a high intra- and inter-specific variability and is not suitable for operational approaches, it can provide sufficient phylogenetic signal that fit well in the phylogenetic methods.
moreover, the complete its region could separate almost all amf species except glomus intraradices and its close relatives  <cit> .
in addition, abundant its sequences are stored in public databases that support our present study.
also, the online genus search tool emerencia  <cit>  provides great convenience for extracting all the amf its sequences from genbank.
in this study, we used data mining to explore the diversity and distribution of amf based on its sequences in public databases.
our objectives were: 1) to explore the global distribution patterns of amf; 2) to test whether selectivity for host at scales of taxonomic order and functional group is the main factor affecting the current distribution patterns of amf.
RESULTS
amf its-vt determination and phylogeny
using entire its sequences of each genus, we delimited its-vt  based on the maximum likelihood neighbor-joining phylogeny and 90% sequence similarity.
a total of  <dig> its-vts were obtained.
some its-vts included named glomeromycotan species, while some named species were grouped into its-vts.
the maximum likelihood tree distinguished nine glomeromycota families .
the genus glomus clustered into three distinct clades: clade a , b , and c .
glgra included three distinct subgroups.
diversisporaceae was composed of glgrc and diversispora.
similarly, taxa from gigaspora and scutellospora were grouped together in the clade of gigasporaceae.
however, the acaulosporaceae was represented by two phylogenetically distant clades, one corresponding to acaulospora and one to entrophospora.
the entrophospora clade was uncertain.
this clade included all the entrophospora its-vts, three acaulospora its-vts, and three glomus its-vts.
the three glomus its-vts were singletons, were annotated with uncultured fungi, and were assigned to glomus genus according to emerencia, but the maximum similarity with glomus was only about 80% through blast.
amf its-vt richness
the rarefaction curve did not reach an asymptote at the global scale, suggesting that some or many taxa have not yet been sequenced .
meanwhile, the rarefied relationship between accumulated plant species and its-vts showed a positive near-linear curve, indicating that more its-vts would be found with increasingly sampled plants .
of the  <dig> its-vts,  <dig> were singletons and  <dig> were doubletons.
glomeraceae was dominant and represented by  <dig> its-vts .
the acaulosporaceae-entrophospora branch was the second largest clade, with  <dig> its-vts.
the third and fourth largest clades were glgrb and gigasporaceae, each with  <dig> its-vts.
the acaulosporaceae-acaulospora branch was the fifth largest clade, with  <dig> its-vts.
smaller glomeromycotan families included diversisporaceae with  <dig> its-vts, paraglomeracae with  <dig> its-vts, and pacisporaceae with  <dig> its-vt .
the numbers and percentage of its-vts for each genus are shown in additional file  <dig>  with respect to genera, glomus was dominant, i.e., glomus was represented by the largest proportion of its-vts for continent, biogeographical realm, climatic zone and supercontinent, totally accounting for 74%.
the numbers of genera were highest for europe and north america among continents, palearctic and nearctic regions among biogeographical realms, regions with warm temperature among climatic zones, and laurasia with super-continent, indicating biased sampling intensity.
when the uneven sampling intensity was removed, the its-vt richness was not significantly different among plant functional groups, among ecosystems or among super-continents.
however, for climatic zones, snow and polar zones had lower its-vt richness than other regions ; for continents, its-vt richness in asia and south america was lower than in europe, north america, and oceania ; for biogeographical realms, its-vt richness in neotropic regions was lower than other regions  .
amf its-vt distribution patterns across spatial scales
the distribution of its-vts was assessed at spatial scales of super-continents, biogeographical realms, continents and climatic zones .
non-metric multidimensional scaling analysis  showed distinctive its-vt composition between laurasia and gondwana .
across supercontinents, totally 76% its-vt specifically occurred in laurasia or gondwana, while a small proportion of 24% were shared .
across continents, nmds revealed distinct its-vt composition in europe, north america, asia, and south america, but not evidently different between africa and oceania .
77% its-vts were unique to one continent .
at the scale of biogeographical realm, palearctic, nearctic and neotropic had distinctive its-vt composition, but not for afrotropic and australasia .
78% its-vts occurred in one type of biogeographical realms .
at the scale of climatic zones, evidently different its-vts occurred in warm temperate regions compared with other zones .
high endemism was shown with specific occurrence of 78% at the scale of climatic zones .
host plant distribution patterns across spatial scales
non-metric multidimensional scaling analysis  showed that spatial factors could drive host plant distributions .
amf associated with plant taxonomic orders were totally different between laurasia and gondwana .
at the scale of biogeographical realms, palearctic, neotropic and australasia had distinct amf based on plant order composition, but not for nearctic and afrotropic .
similar results were shown across continents .
in addition, the amf associated with taxonomic orders of plants was distinctive among different climatic zones .
host selectivity at the scales of ecological groups and the distribution for its-vts
amf host selectivity at the scale of ecological group  affected the distribution of amf .
nmds revealed that amf its-vt composition was totally different among grass, forb and wood .
about 71% its-vts were unique to one type of functional group while only a small proportion of 4% its-vts were shared by all three functional groups .
at the scale of ecosystems, grassland, forest and anthropogenic system had distinctive am fungal communities; while amf community in successional system was not evidently different with grassland and forest .
in addition, high ecosystemic specificity was found and the specific its-vts occurring in a single ecosystem accounted for 81%.
no its-vt was shared by four types of ecosystems .
host selectivity at the scale of plant taxonomic order and the distribution for amf its-vts
host plant order had high specific selectivity for amf and each taxonomic order had 53% specific its-vts ; meanwhile, its-vts showed high host specificity .
most its-vts were associated with only one host taxonomic order, and only a small proportion of its-vts occurred simultaneously in  <dig> host orders .
linear regression analysis showed a positive relationship between the distribution of amf across plant taxonomic order and across geographical locations .
its-vt geographical distribution increased as host range increased .
meanwhile, according to linear regression analysis, compositional similarity of plant taxonomic order and its-vt was significantly correlated across ecosystems , biogeographical realms , continents  and climatic zones .
mantel test also showed that amf distribution was significantly related to the distribution of plant taxonomic orders across ecosystem , biogeographical realms , continents  and climatic zones  .
our data fit the sem analysis .
sem indicated that ecosystem, climate, biogeography and continent directly affected the distribution of plants, and indirectly affected the distribution of amf .
these factors accounted for 52% variance for plant distribution and 59% variance for amf distribution.
sem showed that biogeography and plant were the two most important factors  that affected amf distribution .
however, biogeography had lower direct effect  but higher indirect effect  on amf.
plant had strong direct effects  but no indirect effects on amf distribution .
in addition, the effects of climate, ecosystem and continent on plant distribution were direct, while these effects on amf distribution were indirect .
discussion
identification of its-vts
in this study, we defined  <dig> its-vts of amf based on maximum likelihood phylogenetics for each genus.
our phylogenetic reconstruction of all its-vts was different from earlier studies  <cit> .
the genus glomus was grouped into three distinct clades , and glgra contained three subgroups.
glgra, glgrb, and glgrc were distantly related to each other; meanwhile, glgrc was placed in the diversisporaceae clade.
this polyphyletic pattern was consistent with classical taxonomic identification and description  <cit> .
glgrb was distantly related to glgra in our analysis.
this is not consistent with a previous phylogenetic reconstruction based on the ssu gene  <cit> .
this difference may result from different evolutionary rates of amf clades for the its and ssu gene.
in addition, the phylogenetic placement of acaulosporaceae remains controversial.
according to schüßler et al  <cit> , acaulosporaceae consists of acaulospora and entrophospora, but in the current study, acaulospora and entrophospora were phylogenetically separated at the family level, not at the genus level.
this might result from the misidentification of the entrophospora morphospecies that were the sources of the original its reference sequences.
meanwhile, three glomus its-vts occurred in the acaulosporaceae-entrophospora clade.
these its-vts were singletons, annotated with uncultured fungus and had low sequence similarity with glomus species.
this might be errors of emerencia.
in addition, previous studies  <cit>  suggested acaulospora and entrophospora were two genera in the family of acaulosporaceae.
however, both clades were phylogenetically distant at the family level.
therefore, it also seems possible that a new family should be defined for entrophospora clade.
high local diversity of amf and selectivity of host plants
our results showed that amf have high local diversity and that most amf have narrow distribution ranges.
these patterns might be explained by high selectivity of amf by their hosts.
the selectivity from host plants could occur at different scales, such as plant taxonomic levels of species, family and order as well as functional groups and plant communities.
in our study, about 60% of its-vts were host specific.
71% its-vts occurred in a single functional group.
only a small proportion of its-vts had broad host ranges and functional group ranges.
host selectivity for amf could result in host plants harboring distinctive amf and explain high amf diversity at the local scale.
vandenkoornhuyse et al  <cit>  found that coexisting grass species had distinct amf communities.
martinez-garcia et al  <cit>  reported high host preference of amf in semiarid environments.
in oak woodland, douhan et al  <cit>  found contrasting amf communities associated with three commonly occurring plants.
similar results were also obtained from trees in tropical forests  <cit> .
findings in these studies are consistent with the idea that hosts might exert selective pressure on amf communities  <cit>  and influence amf local diversity.
alguacil et al  <cit>  documented that plant types promoted amf diversity.
johnson et al  <cit>  found that plant communities could significantly affect amf diversity and composition.
in addition, interactions among host plants might also drive development of specific amf communities  <cit> .
thus, a host plant diversified community could result in high local diversity of amf.
effects of host plants on the distribution of amf
this study showed that amf exhibited distinct ecosystemic, climatic, biogeographical and continental patterns , and these patterns were correlated with host plant distribution .
for ecosystems, kivlin et al  <cit>  reported that amf had an ecosystemic distribution pattern, and argued this distribution was a confounding host plant effect.
in this study, amf distribution was significantly correlated with host plant distribution across ecosystems  and our sem also showed that ecosystems could directly influence plant distribution but indirectly affect amf distribution .
meanwhile, sem revealed host plant had strong direct effects on amf.
therefore, amf ecosystemic pattern might be mediated by host plants.
ecosystems consisted of distinctive plants, e.g., crop plants were limited to anthropogenic systems and most trees only occurred in forests; this is leading to high specificity of amf for certain ecosystems and distinct ecosystemic composition because of high host specificity of amf .
in addition, specific interactions between plants in certain ecosystems might exert direct effects on the ecosystemic pattern of amf  <cit> .
our results showed that amf had high specificity and distinct composition for climatic zones .
we determined that climatic factors maybe affect amf distribution.
however, our sem suggested that climate had no direct effect but relatively stronger indirect effects on amf .
the indirect effects on amf might be through plants and ecosystems because of strong host effects .
studies had showed that climate was a main driving factor for plant distribution and different climatic zones had distinct ecosystems  <cit> .
the selectivity from host plants and ecosystems might form the current distribution pattern of amf in climatic zones.
in addition, our study revealed high endemism and distinct distribution pattern of amf across continents .
our results also showed that plant had a continent distribution pattern and closely correlated to amf distribution.
our sem further showed that continent had strong indirect effect but weak direct effect on amf .
this indicated that the continental limitation for amf distribution might be a function of plant distribution.
long- range dispersal of host plants across ocean is relative uncommon, but plants dispersal within continents occurs more frequently.
dispersal limitation of amf exists among continents but is not significant within continents for amf which was reported by kivlin et al  <cit> .
previous studies suggested historical factors could affect the distributions of amf associated with plants and amf at global scale  <cit> .
in this study, the paleocontinents, laurasia and gondwana showed significantly different amf composition .
the amf composition among biogeographical realms was also distinctive .
our sem showed that biogeography had a strong direct effect on plant distribution but a strong indirect effect on amf distribution .
it suggested the effects of biogeography might be mediated by plants.
from geological study  <cit> , laurasia separated from gondwana at triasic of mesozoic ; at early cretaceous , gondwana departed into afrotropic, oceania, indo-malay and neotropic; and laurasia finally divided into palearctic and nearctic at  <dig> mya.
evidently, the ordovician origin  of amf far predated the separation of laurasia and gondwana as well as the formation of current continents  <cit> .
thus, we could assume that amf was randomly distributed and formed a 'common pool' at pangaea; as pangaea separated into laurasia and gondwana, amf composition in these two supercontinents would be similar.
however, our results and previous study  <cit>  showed that only a small proportion of amf was shared by laurasia and gondwana.
two possibilities could attribute to this phenomenon.
one is that new speciation of amf occurred after the division of laurasia and gondwana and after the separation of the current tectonic plates.
this could have been mediated by host plants.
at late cretaceous, angiosperm plants explosively appeared  <cit> , resulting in strong selective pressure to promote am fungal speciation.
another possibility was that there were far more amf in the 'common pool' than present.
after large numbers of angiosperm plants speciated, climate and other natural selective pressures would promote the revolution of different plants on different tectonic plates.
because of asymmetric and obligatory nature of mycorrhiza, some amf associated with compatible host, and co-evolved to form present patterns, while others did not and went extinction.
although our results suggest that host plant directly affects amf, climatic, ecosystemic, continental and biogeographical factors also indirectly influence the distribution of amf, all of these factors can only explain 58% variance of amf distribution.
there is still 42% variance waiting for explanation.
this might be the effects of soil, because soil conditions have been found to significantly affect amf distribution  <cit> .
CONCLUSIONS
this study shows that host plants have high selectivity for amf and that amf have high host specificity and local diversity.
amf with low host specificity have a wide geographical distribution, but most amf have a restricted distribution, i.e., most have been only found in ecosystems, or in biogeographical realms, or in climatic zones, or in continents.
consequently, amf have distribution patterns across ecosystems, across biogeographical realms, across climatic zones and across continents.
plant composition is significantly correlated with amf composition.
ecosystems, biogeographical realms, climatic zones, and continents had relatively stronger direct effects on plants than indirect effects, but stronger indirect effects on amf than direct effects.
biogeography and plant are the two strongest factors on amf, but the effect of biogeography is indirect while direct for plant.
it suggested the effects of ecosystems, biogeographical realms, climatic zones and continents on amf distribution might be mediated by plants through high host selectivity.

 arbuscular mycorrhizal fungi are widespread in terrestrial ecosystems and form obligatory symbiotic relationships with most land plants. these symbioses are not host specific but are to some extent host-preferential. amf may have a biogeographical distribution partially because of their soil-borne life form, host preference, and limited dispersal resulting from geographic isolation. increasingly, studies have shown that amf are cosmopolitan at the genus or higher taxonomic level but have limited distributions at the species level. global-scale studies showed that amf have distinct distribution patterns and that habitat filtering or dispersal limitation affects these patterns. at the continental scale, a single widespread host plant was found primarily to harbor geographically generalist amf. at the regional scale, a typical distance-decay distribution of amf suggested that geographic distance and environmental heterogeneity determined the patterns of spatial scaling. at the local scale, amf distribution is closely related to host identity and habitat. at the fine scale, amf diversity has a patchy distribution. studies have also shown that amf distribution could be influenced by climate, habitat, geographical isolation, soil conditions, and anthropogenic activities. several studies have reported high selectivity between host plants and amf, which may affect amf distribution. bever et al reported host-specific differences in the population growth rates of amf. helgason et al found physical and functional selectivity in amf, and zhang et al. recently documented that the invasive plant solidago canadensis promoted the growth of the most beneficial amf and thereby increased its own competitiveness. distinctive amf communities were associated with coexisting plant species, including grasses, forbs, and trees. even plants of the same species that differed in age harbored distinctive amf. thus, it seems likely that plants can actively select with amf to associate. some studies have suggested that plants have evolved an ability to recognize beneficial amf. for example, bever et al found that the host plant preferentially allocated carbon to beneficial symbionts. kiers et al, who used qpcr and stable isotope probing methods, documented the existence of a reciprocal rewards mechanism that stabilizes the cooperation between plants and amf. because host plants have distinct geographical distributions, selection by host plants may result in distinct geographical distributions for amf. however, whether and how host selection affects the pattern of amf distribution at large scales has not been well studied. recently, two studies have revealed global patterns of amf by using molecular approach. one of these studies has developed a database and demonstrated ecosystemic and global patterns for amf distribution. this study also documented that amf had high specificity of host plant superorders and wider distribution with larger breadth of hosts. the other study also found a high endemism of amf at global scales that could be affected by geographic distance, soil condition and plant community. although both of these studies indicated that host plants related to amf distribution at a global scale, whether and how host plants affect amf distribution remain unknown. here, we hypothesized that selective pressure from host plants at different scales may be the main factor affecting the distribution of amf at large scales, and that the effects of biogeographical, ecosystemic and climatic factors may be mediated by host plants. its, ssu and lsu are the three widely used marker genes in molecular diversity of amf. ssu was utilized widely while lsu was relatively infrequent compared with its. as relative conserved variability of sequences among species, ssu is widely used in molecular diversity of amf. however, ssu has a limitation to resolve species in genera of ambispora, diversispora and scutellospora. this limitation can affect the designation of phylotype and further influence the analysis on the distribution of the whole amf. for lsu, although it had relative higher intra-specific variability than ssu and can also well resolve amf species, the lsu based data in genbank are limited for the study of amf distribution at global scale. to avoid the limitations of ssu and lsu, we selected its as marker gene for our analysis. although its has a high intra- and inter-specific variability and is not suitable for operational approaches, it can provide sufficient phylogenetic signal that fit well in the phylogenetic methods. moreover, the complete its region could separate almost all amf species except glomus intraradices and its close relatives. in addition, abundant its sequences are stored in public databases that support our present study. also, the online genus search tool emerencia provides great convenience for extracting all the amf its sequences from genbank. in this study, we used data mining to explore the diversity and distribution of amf based on its sequences in public databases. our objectives were: 1) to explore the global distribution patterns of amf; 2) to test whether selectivity for host at scales of taxonomic order and functional group is the main factor affecting the current distribution patterns of amf.  amf its-vt determination and phylogeny using entire its sequences of each genus, we delimited its-vt based on the maximum likelihood neighbor-joining phylogeny and 90% sequence similarity. a total of its-vts were obtained. some its-vts included named glomeromycotan species, while some named species were grouped into its-vts. the maximum likelihood tree distinguished nine glomeromycota families. the genus glomus clustered into three distinct clades: clade a, b, and c. glgra included three distinct subgroups. diversisporaceae was composed of glgrc and diversispora. similarly, taxa from gigaspora and scutellospora were grouped together in the clade of gigasporaceae. however, the acaulosporaceae was represented by two phylogenetically distant clades, one corresponding to acaulospora and one to entrophospora. the entrophospora clade was uncertain. this clade included all the entrophospora its-vts, three acaulospora its-vts, and three glomus its-vts. the three glomus its-vts were singletons, were annotated with uncultured fungi, and were assigned to glomus genus according to emerencia, but the maximum similarity with glomus was only about 80% through blast. amf its-vt richness the rarefaction curve did not reach an asymptote at the global scale, suggesting that some or many taxa have not yet been sequenced. meanwhile, the rarefied relationship between accumulated plant species and its-vts showed a positive near-linear curve, indicating that more its-vts would be found with increasingly sampled plants. of the its-vts, were singletons and were doubletons. glomeraceae was dominant and represented by its-vts. the acaulosporaceae-entrophospora branch was the second largest clade, with its-vts. the third and fourth largest clades were glgrb and gigasporaceae, each with its-vts. the acaulosporaceae-acaulospora branch was the fifth largest clade, with its-vts. smaller glomeromycotan families included diversisporaceae with its-vts, paraglomeracae with its-vts, and pacisporaceae with its-vt. the numbers and percentage of its-vts for each genus are shown in additional file with respect to genera, glomus was dominant, i.e., glomus was represented by the largest proportion of its-vts for continent, biogeographical realm, climatic zone and supercontinent, totally accounting for 74%. the numbers of genera were highest for europe and north america among continents, palearctic and nearctic regions among biogeographical realms, regions with warm temperature among climatic zones, and laurasia with super-continent, indicating biased sampling intensity. when the uneven sampling intensity was removed, the its-vt richness was not significantly different among plant functional groups, among ecosystems or among super-continents. however, for climatic zones, snow and polar zones had lower its-vt richness than other regions; for continents, its-vt richness in asia and south america was lower than in europe, north america, and oceania; for biogeographical realms, its-vt richness in neotropic regions was lower than other regions. amf its-vt distribution patterns across spatial scales the distribution of its-vts was assessed at spatial scales of super-continents, biogeographical realms, continents and climatic zones. non-metric multidimensional scaling analysis showed distinctive its-vt composition between laurasia and gondwana. across supercontinents, totally 76% its-vt specifically occurred in laurasia or gondwana, while a small proportion of 24% were shared. across continents, nmds revealed distinct its-vt composition in europe, north america, asia, and south america, but not evidently different between africa and oceania. 77% its-vts were unique to one continent. at the scale of biogeographical realm, palearctic, nearctic and neotropic had distinctive its-vt composition, but not for afrotropic and australasia. 78% its-vts occurred in one type of biogeographical realms. at the scale of climatic zones, evidently different its-vts occurred in warm temperate regions compared with other zones. high endemism was shown with specific occurrence of 78% at the scale of climatic zones. host plant distribution patterns across spatial scales non-metric multidimensional scaling analysis showed that spatial factors could drive host plant distributions. amf associated with plant taxonomic orders were totally different between laurasia and gondwana. at the scale of biogeographical realms, palearctic, neotropic and australasia had distinct amf based on plant order composition, but not for nearctic and afrotropic. similar results were shown across continents. in addition, the amf associated with taxonomic orders of plants was distinctive among different climatic zones. host selectivity at the scales of ecological groups and the distribution for its-vts amf host selectivity at the scale of ecological group affected the distribution of amf. nmds revealed that amf its-vt composition was totally different among grass, forb and wood. about 71% its-vts were unique to one type of functional group while only a small proportion of 4% its-vts were shared by all three functional groups. at the scale of ecosystems, grassland, forest and anthropogenic system had distinctive am fungal communities; while amf community in successional system was not evidently different with grassland and forest. in addition, high ecosystemic specificity was found and the specific its-vts occurring in a single ecosystem accounted for 81%. no its-vt was shared by four types of ecosystems. host selectivity at the scale of plant taxonomic order and the distribution for amf its-vts host plant order had high specific selectivity for amf and each taxonomic order had 53% specific its-vts; meanwhile, its-vts showed high host specificity. most its-vts were associated with only one host taxonomic order, and only a small proportion of its-vts occurred simultaneously in host orders. linear regression analysis showed a positive relationship between the distribution of amf across plant taxonomic order and across geographical locations. its-vt geographical distribution increased as host range increased. meanwhile, according to linear regression analysis, compositional similarity of plant taxonomic order and its-vt was significantly correlated across ecosystems, biogeographical realms, continents and climatic zones. mantel test also showed that amf distribution was significantly related to the distribution of plant taxonomic orders across ecosystem, biogeographical realms, continents and climatic zones. our data fit the sem analysis. sem indicated that ecosystem, climate, biogeography and continent directly affected the distribution of plants, and indirectly affected the distribution of amf. these factors accounted for 52% variance for plant distribution and 59% variance for amf distribution. sem showed that biogeography and plant were the two most important factors that affected amf distribution. however, biogeography had lower direct effect but higher indirect effect on amf. plant had strong direct effects but no indirect effects on amf distribution. in addition, the effects of climate, ecosystem and continent on plant distribution were direct, while these effects on amf distribution were indirect. discussion identification of its-vts in this study, we defined its-vts of amf based on maximum likelihood phylogenetics for each genus. our phylogenetic reconstruction of all its-vts was different from earlier studies. the genus glomus was grouped into three distinct clades, and glgra contained three subgroups. glgra, glgrb, and glgrc were distantly related to each other; meanwhile, glgrc was placed in the diversisporaceae clade. this polyphyletic pattern was consistent with classical taxonomic identification and description. glgrb was distantly related to glgra in our analysis. this is not consistent with a previous phylogenetic reconstruction based on the ssu gene. this difference may result from different evolutionary rates of amf clades for the its and ssu gene. in addition, the phylogenetic placement of acaulosporaceae remains controversial. according to schüßler et al, acaulosporaceae consists of acaulospora and entrophospora, but in the current study, acaulospora and entrophospora were phylogenetically separated at the family level, not at the genus level. this might result from the misidentification of the entrophospora morphospecies that were the sources of the original its reference sequences. meanwhile, three glomus its-vts occurred in the acaulosporaceae-entrophospora clade. these its-vts were singletons, annotated with uncultured fungus and had low sequence similarity with glomus species. this might be errors of emerencia. in addition, previous studies suggested acaulospora and entrophospora were two genera in the family of acaulosporaceae. however, both clades were phylogenetically distant at the family level. therefore, it also seems possible that a new family should be defined for entrophospora clade. high local diversity of amf and selectivity of host plants our results showed that amf have high local diversity and that most amf have narrow distribution ranges. these patterns might be explained by high selectivity of amf by their hosts. the selectivity from host plants could occur at different scales, such as plant taxonomic levels of species, family and order as well as functional groups and plant communities. in our study, about 60% of its-vts were host specific. 71% its-vts occurred in a single functional group. only a small proportion of its-vts had broad host ranges and functional group ranges. host selectivity for amf could result in host plants harboring distinctive amf and explain high amf diversity at the local scale. vandenkoornhuyse et al found that coexisting grass species had distinct amf communities. martinez-garcia et al reported high host preference of amf in semiarid environments. in oak woodland, douhan et al found contrasting amf communities associated with three commonly occurring plants. similar results were also obtained from trees in tropical forests. findings in these studies are consistent with the idea that hosts might exert selective pressure on amf communities and influence amf local diversity. alguacil et al documented that plant types promoted amf diversity. johnson et al found that plant communities could significantly affect amf diversity and composition. in addition, interactions among host plants might also drive development of specific amf communities. thus, a host plant diversified community could result in high local diversity of amf. effects of host plants on the distribution of amf this study showed that amf exhibited distinct ecosystemic, climatic, biogeographical and continental patterns, and these patterns were correlated with host plant distribution. for ecosystems, kivlin et al reported that amf had an ecosystemic distribution pattern, and argued this distribution was a confounding host plant effect. in this study, amf distribution was significantly correlated with host plant distribution across ecosystems and our sem also showed that ecosystems could directly influence plant distribution but indirectly affect amf distribution. meanwhile, sem revealed host plant had strong direct effects on amf. therefore, amf ecosystemic pattern might be mediated by host plants. ecosystems consisted of distinctive plants, e.g., crop plants were limited to anthropogenic systems and most trees only occurred in forests; this is leading to high specificity of amf for certain ecosystems and distinct ecosystemic composition because of high host specificity of amf. in addition, specific interactions between plants in certain ecosystems might exert direct effects on the ecosystemic pattern of amf. our results showed that amf had high specificity and distinct composition for climatic zones. we determined that climatic factors maybe affect amf distribution. however, our sem suggested that climate had no direct effect but relatively stronger indirect effects on amf. the indirect effects on amf might be through plants and ecosystems because of strong host effects. studies had showed that climate was a main driving factor for plant distribution and different climatic zones had distinct ecosystems. the selectivity from host plants and ecosystems might form the current distribution pattern of amf in climatic zones. in addition, our study revealed high endemism and distinct distribution pattern of amf across continents. our results also showed that plant had a continent distribution pattern and closely correlated to amf distribution. our sem further showed that continent had strong indirect effect but weak direct effect on amf. this indicated that the continental limitation for amf distribution might be a function of plant distribution. long- range dispersal of host plants across ocean is relative uncommon, but plants dispersal within continents occurs more frequently. dispersal limitation of amf exists among continents but is not significant within continents for amf which was reported by kivlin et al. previous studies suggested historical factors could affect the distributions of amf associated with plants and amf at global scale. in this study, the paleocontinents, laurasia and gondwana showed significantly different amf composition. the amf composition among biogeographical realms was also distinctive. our sem showed that biogeography had a strong direct effect on plant distribution but a strong indirect effect on amf distribution. it suggested the effects of biogeography might be mediated by plants. from geological study, laurasia separated from gondwana at triasic of mesozoic; at early cretaceous, gondwana departed into afrotropic, oceania, indo-malay and neotropic; and laurasia finally divided into palearctic and nearctic at mya. evidently, the ordovician origin of amf far predated the separation of laurasia and gondwana as well as the formation of current continents. thus, we could assume that amf was randomly distributed and formed a'common pool' at pangaea; as pangaea separated into laurasia and gondwana, amf composition in these two supercontinents would be similar. however, our results and previous study showed that only a small proportion of amf was shared by laurasia and gondwana. two possibilities could attribute to this phenomenon. one is that new speciation of amf occurred after the division of laurasia and gondwana and after the separation of the current tectonic plates. this could have been mediated by host plants. at late cretaceous, angiosperm plants explosively appeared, resulting in strong selective pressure to promote am fungal speciation. another possibility was that there were far more amf in the'common pool' than present. after large numbers of angiosperm plants speciated, climate and other natural selective pressures would promote the revolution of different plants on different tectonic plates. because of asymmetric and obligatory nature of mycorrhiza, some amf associated with compatible host, and co-evolved to form present patterns, while others did not and went extinction. although our results suggest that host plant directly affects amf, climatic, ecosystemic, continental and biogeographical factors also indirectly influence the distribution of amf, all of these factors can only explain 58% variance of amf distribution. there is still 42% variance waiting for explanation. this might be the effects of soil, because soil conditions have been found to significantly affect amf distribution.  this study shows that host plants have high selectivity for amf and that amf have high host specificity and local diversity. amf with low host specificity have a wide geographical distribution, but most amf have a restricted distribution, i.e., most have been only found in ecosystems, or in biogeographical realms, or in climatic zones, or in continents. consequently, amf have distribution patterns across ecosystems, across biogeographical realms, across climatic zones and across continents. plant composition is significantly correlated with amf composition. ecosystems, biogeographical realms, climatic zones, and continents had relatively stronger direct effects on plants than indirect effects, but stronger indirect effects on amf than direct effects. biogeography and plant are the two strongest factors on amf, but the effect of biogeography is indirect while direct for plant. it suggested the effects of ecosystems, biogeographical realms, climatic zones and continents on amf distribution might be mediated by plants through high host selectivity. 
this pattern indicates high specificity of amf for host at different scales and high selectivity from host plants for amf. studies at large scales, however, are limited because intensive sampling is difficult. we also used these sequence metadata to investigate whether host plant is the main factor that affects the distribution of amf at large scales. the distribution of amf as indicated by its rdna sequences showed a pattern of high endemism at large scales. each host taxonomic order averaged about 53% specific its-vts, and approximately 60% of the its-vts were host specific. the distributions of most its-vts were limited across ecosystem, across continent, across biogeographical realm, and across climatic zone. non-metric multidimensional scaling analysis showed that amf community composition differed among functional groups of hosts, and among ecosystem, continent, biogeographical realm, and climatic zone. the mantel test showed that amf community composition was significantly correlated with plant community composition among ecosystem, among continent, among biogeographical realm, and among climatic zone. the structural equation modeling showed that the effects of ecosystem, continent, biogeographical realm, and climatic zone were mainly indirect on amf distribution, but plant had strongly direct effects on amf. the effects of ecosystemic, biogeographical, continental and climatic factors on amf distribution might be mediated by host plants. here, we used its rdna sequence metadata obtained from public databases to study the distribution of amf at continental and global scales. arbuscular mycorrhizal fungi can form obligate symbioses with the vast majority of land plants, and amf distribution patterns have received increasing attention from researchers. at the local scale, the distribution of amf is well documented. most its-vts occurred in only one type of host functional group.
 arbuscular mycorrhizal fungi can form obligate symbioses with the vast majority of land plants, and amf distribution patterns have received increasing attention from researchers. at the local scale, the distribution of amf is well documented. studies at large scales, however, are limited because intensive sampling is difficult. here, we used its rdna sequence metadata obtained from public databases to study the distribution of amf at continental and global scales. we also used these sequence metadata to investigate whether host plant is the main factor that affects the distribution of amf at large scales.  we defined its virtual taxa among all sequences of the glomeromycota by using a comprehensive maximum likelihood phylogenetic analysis. each host taxonomic order averaged about 53% specific its-vts, and approximately 60% of the its-vts were host specific. those its-vts with wide host range showed wide geographic distribution. most its-vts occurred in only one type of host functional group. the distributions of most its-vts were limited across ecosystem, across continent, across biogeographical realm, and across climatic zone. non-metric multidimensional scaling analysis showed that amf community composition differed among functional groups of hosts, and among ecosystem, continent, biogeographical realm, and climatic zone. the mantel test showed that amf community composition was significantly correlated with plant community composition among ecosystem, among continent, among biogeographical realm, and among climatic zone. the structural equation modeling showed that the effects of ecosystem, continent, biogeographical realm, and climatic zone were mainly indirect on amf distribution, but plant had strongly direct effects on amf.  the distribution of amf as indicated by its rdna sequences showed a pattern of high endemism at large scales. this pattern indicates high specificity of amf for host at different scales and high selectivity from host plants for amf. the effects of ecosystemic, biogeographical, continental and climatic factors on amf distribution might be mediated by host plants. 