BACKGROUND
most parasites are intimately dependent on one or a few hosts.
because of this host fidelity, parasites are expected to track speciating hosts by speciating themselves.
this process, known as cospeciation, will lead to cocladogenesis, the topological matching of symbiont phylogenies.
parasite and host phylogenies are rarely identical, however; forces such as duplication , sorting events , and host-switching   <cit>  can generate discordance between the phylogenies of hosts and their symbionts.
despite these complications, congruent phylogenies are known in host-parasite associations  <cit>  and in host-mutualist associations as well  <cit> .
the fungus-growing ant-microbe symbiosis is a novel example of a system in which cocladogenesis occurs between a host and both its mutualistic and parasitic symbionts.
research over the last decade has demonstrated the congruence of the phylogenies of fungus-growing ants, the fungi that they cultivate  and the cultivar-attacking pathogen escovopsis at ancient phylogenetic levels  <cit> .
genetic analyses of more recently diverged, younger lineages demonstrate discrepancies between ant and cultivar associations, which are likely due to a combination of lateral transfer of cultivars between colonies and occasional domestication of free-living fungi by the ants  <cit> .
to date, the two published phylogenetic studies of the escovopsis-cultivar association indicate no discordance between the phylogenies of the cultivars and escovopsis  <cit> .
ancient codiversification of fungus-growing ants and their cultivars is driven by the intimate dependence of the ants on fungus as their primary food source and the intimate dependence of the fungus on ants for protection, nutrition and dispersal  <cit> .
in ants, the ability to cultivate fungi for food arose only once, about 50– <dig> million years ago, and gave rise to roughly  <dig> described, extant species of fungus-growing ants  <cit> .
the long coevolutionary history of these mutualists has led to the specialization of each ant species on the cultivation of a unique, narrow range of cultivated fungi, most of which are in the family lepiotaceae.
as depicted in figure 1a, these lepiotaceous cultivars form two morphologically and molecularly distinct groups .
there has been one switch to a distantly related cultivar; most ants in the genus apterostigma now cultivate fungi in the family pterulaceae  <cit> , which is distantly related to the family lepiotaceae.
the pterulaceous cultivars fall into two monophyletic, morphologically distinct cultivar groups .
one apterostigma species, a. auriculatum, has retained the ancestral state of growing lepiotaceaous cultivars  <cit> .
currie et al.
<cit>  demonstrated that, at ancient levels, the phylogeny of escovopsis  , a genus of specialized, highly pathogenic microfungi that attack the ants' fungal cultivars, matches that of the ants' diverse cultivars and consequently that of the ants themselves.
escovopsis has only been found associated with nests of attine ants.
upon establishing infection, escovopsis consumes the ants' cultivated fungi and can devastate attine colonies  <cit> .
though infection rates vary across host species, infections are prevalent in colonies of many attine genera throughout their geographic ranges  <cit> .
escovopsis is thought to track the cultivars because of the coevolutionary specialization of each escovopsis lineage on attacking and overcoming defenses of only a narrow range of cultivar hosts  <cit> .
ancient phylogenetic congruence between cultivars and escovopsis suggests that these pathogens may be tightly tracking their speciating hosts by speciating themselves, and that escovopsis lineages have not switched to novel cultivar hosts over evolutionary time.
to test for host-switching, however, it is necessary to include extensive sampling across the diversity of both hosts and symbionts.
previous studies of escovopsis host-fidelity have included few samples of apterostigma-associated escovopsis despite the fact that they are an extremely diverse group of fungus-growing ant pathogens.
currie et al.
<cit> , the most extensive phylogenetic analysis of escovopsis to date, included only two apterostigma-associated escovopsis, which were morphologically similar and were isolated from ant colonies that raised closely-related fungi.
not surprisingly, these isolates formed a single monophyletic "apterostigma escovopsis" clade .
however, unlike the other fungus-growing ant genera, which each raise cultivars in a single cultivar group, apterostigma ants raise cultivars in three groups , which are each attacked by morphologically distinct escovopsis types <cit> .
more extensive sampling of these diverse apterostigma pathogens, therefore, can reveal the extent to which escovopsis species are host-faithful, tracking their particular hosts without host-switching.
through extensive geographic sampling and phylogenetic analysis of apterostigma-associated escovopsis, we ask whether host and pathogen phylogenies are still congruent when genetic analyses are extended to include the diversity of the apterostigma-associated escovopsis.
first, do apterostigma-associated escovopsis form a monophyletic clade as the apterostigma ants do, or are apterostigma-associated escovopsis polyphyletic like their cultivars?
second, do the apterostigma escovopsis form three distinct clades that correspond to the three cultivar groups  raised by the different species of apterostigma ants?
based on earlier findings that escovopsis is highly cultivar-type specific  <cit> , we hypothesize that more extensive sampling will reveal that the apterostigma-associated escovopsis are not monophyletic like their associated ant-hosts, because the escovopsis that infects lepiotaceous apterostigma cultivars  will be more closely-related to escovopsis isolated from lepiotaceous gardens of non-apterostigma ant species than to escovopsis isolated from pterulaceous apterostigma gardens .
this would support findings of currie et al.
<cit> , depicted in figure 1b, that the pterulaceous-attacking escovopsis form a monophyletic clade distinct from the escovopsis that infects lepiotaceous cultivars, but would contradict their findings of complete congruence between ant, cultivar, and escovopsis phylogenies.
we further hypothesize that more extensive sampling will reveal that pterulaceous-attacking escovopsis will fall into two clades associated with the two pterulaceous cultivar groups .
overall, in looking at the fungus-growing ant-microbe symbiosis as a whole, we predict four host-specific escovopsis clades that are each specialized at attacking one of the four known fungus-growing ant cultivar groups  .
RESULTS
diversity of apterostigma-associated escovopsis
of  <dig> colonies from which microbes were sampled, at least one fungal symbiont  was isolated from each of  <dig> colonies.
for the purpose of this study, based on field identification of the ants, garden architecture and growth form of cultivar isolates, each colony was classified as either a g <dig>  g <dig> or g <dig> colony, which raise respectively g <dig>  g <dig> and g <dig> cultivars .
escovopsis infection of these  <dig> colonies was common and pathogen phenotypes were diverse.
g <dig> colonies and g <dig> colonies had much higher infection rates than g <dig> colonies .
more than 50% of g <dig> and g <dig> colonies were infected with at least one escovopsis type, whereas only 11% of g <dig> colonies were infected .
escovopsis samples isolated from infected colonies were classified into four morphotypes based on spore-color: brown, yellow, white and pink.
these types have different micromorphological conidiophore structures  and likely represent different escovopsis lineages.
in the absence of proper species descriptions, we will refer to the different escovopsis lineages by their characteristic spore-color .
while white and pink escovopsis were each specific to a single cultivar group, brown escovopsis infected both g <dig> cultivars and g <dig> cultivars, and yellow escovopsis infected both g <dig> cultivars and g <dig> cultivars.
the yellow escovopsis isolates associated with these two clades, however, are micromorphologically distinct from one another  and likely are two separate species.
a small percentage of colonies  were infected by multiple escovopsis morphotypes .
escovopsis types are identified here according to spore-color.
<dig> for the g2-cultivar colonies, the number of infections based on spore-color exceeds the number of total infections because many colonies were infected with two or three escovopsis types.
<dig> though sharing a similar spore-color, the g2-attacking and g3-attacking yellow escovopsis have distinct micromorphologies .
phylogenetic relationships of apterostigma-associated escovopsis
the results of parsimony, likelihood and bayesian analyses were highly concordant.
three well supported clades were identified that correspond to brown, white and pink escovopsis .
yellow escovopsis is not monophyletic; g2-attacking yellow escovopsis is genetically distinct from the single isolate of g3-attacking yellow escovopsis.
overall, as predicted, these diverse apterostigma-associated escovopsis do not form a monophyletic clade.
both the yellow and pink escovopsis isolated from apterostigma colonies with g <dig> cultivars are nested within other g3-attacking escovopsis and are distinct from other apterostigma-associated escovopsis.
contradictory to our predictions, g2-attacking and g4-attacking escovopsis do not form separate, monophyletic clades.
within the brown escovopsis, there are two clades of g4-associated escovopsis.
parametric-bootstrapping verified the polyphyly of isolates of g4-associated escovopsis.
the null hypothesis of a single origin of g4-associated escovopsis was rejected at p <  <dig> .
this implies that brown escovopsis has switched multiple times between g <dig> and g <dig> hosts.
discussion
phylogenetic patterns of the fungus-growing ant microbe symbiosis reveal a coevolutionary history of host-fidelity punctuated by occasional host-shifts.
all known escovopsis lineages have some limitation to their host-range.
for example, we here show that pink escovopsis attacks only lepiotaceous g <dig> cultivars , white escovopsis attacks only g <dig> cultivars, and though escovopsis with yellow spores attacks both g <dig> and g <dig> cultivars, the yellow escovopsis lineages associated with each of these host groups are morphologically and genetically distinct .
despite this specificity, however, there is not complete congruence of host and pathogen phylogenies as suggested by previous studies  <cit> , indicating that escovopsis host ranges have shifted and may continue to shift .
this complex history parallels that of other symbiotic associations in which extensive sampling reveals that codiversification is interrupted often by host-switches  <cit> .
in fact, it appears that the cases where cocladogenesis persists over evolutionary time are mostly vertically-transmitted endosymbionts  <cit> , whereas most ectosymbionts, such as escovopsis and the fungal cultivar, show patterns of switching and absence of strict cocladogenesis with their hosts.
adaptive processes may explain the host-fidelity of most escovopsis types, which leads to the host-specific escovopsis clades revealed here.
gerardo et al.
<cit>  demonstrated through microbial bioassays that escovopsis lineages are attracted to chemical signals released by their host cultivars.
for example, in microbial bioassays, isolates of yellow escovopsis from g2-apterostigma colonies grow more rapidly towards chemical signals produced by g <dig> than by g <dig> and g <dig> cultivars, which is consistent with the host-range of yellow escovopsis.
unlike yellow escovopsis, brown escovopsis from g2-apterostigma colonies is equally attracted to g <dig> and g <dig> cultivars.
it is possible that this host-attraction would make it easier for brown escovopsis to switch between g <dig> and g <dig> hosts than it would be for yellow escovopsis to switch between hosts, because brown escovopsis would be equally likely to move through g <dig> and g <dig> fungal gardens, find healthy cultivar and establish infection.
this is consistent with the phylogenetic results here, where is seems that brown escovopsis has switched between g <dig> and g <dig> hosts.
microbial bioassays have also revealed that cultivars can defend themselves against some escovopsis but not others  <cit> .
g3-apterostigma cultivars can inhibit isolates of both brown escovopsis and g2-associated yellow escovopsis but cannot inhibit isolates of pink escovopsis, possibly explaining why brown and g2-associated yellow escovopsis do not attack g <dig> cultivars in nature, while pink escovopsis does.
brown and yellow escovopsis are not, however, inhibited by most isolates of g <dig> and g <dig> cultivars, explaining why natural infection is possible in these host-parasite combinations.
overall, both cultivar defense against escovopsis and escovopsis' attraction to host cultivars may maintain escovopsis' specialization and prevent rampant host-switching.
because escovopsis species are host-specific, we hypothesized that wider sampling of escovopsis would reveal four escovopsis clades that correspond to four cultivar and ant clades .
however, contradictory to our hypothesis , g4-associated escovopsis are not monophyletic .
furthermore, there is a lack of congruence of cultivar and escovopsis phylogenies at deeper nodes .
whereas previous analyses  <cit>  have indicated that apterostigma ants, their pterulaceous cultivars and their associated escovopsis are distantly-related to the highly-derived leafcutter ants and their associated microbes , some apterostigma-associated escovopsis lineages, namely brown escovopsis, are sister to the escovopsis attacking g <dig> cultivars .
this suggests that an escovopsis lineage switched between these two distantly-related, ecologically-distinct fungal hosts .
discordance of host and pathogen phylogenies suggests that escovopsis lineages have switched hosts over the evolutionary history of their host association, but the available evidence does not allow inference regarding the frequency at which switching occurs.
it is also unclear whether switching involves the acquisition of novel escovopsis strains by the ants from their environment, or whether it involves the direct transmission of escovopsis between colonies by some unknown mechanism.
further research on the exact mechanism of escovopsis transmission would be helpful in revealing the likelihood of pathogen exchange between colonies containing distantly-related cultivars.
CONCLUSIONS
phylogenetic analyses coupled with extensive sampling of host and parasites reveal a more complete picture of the complexity of the escovopsis-cultivar association in colonies of fungus-growing ants, which consists of specialized pathogen species that occasionally switch between distantly-related hosts.
clades of closely-related escovopsis attack specific cultivar groups, causing the matching of cultivar and escovopsis phylogenies at some scales.
discordance of host-parasite phylogenies, however, arises due to host-switching .
these results reveal the need for additional sampling across the fungus-growing ant microbial symbiosis as a whole.
to date, there has not been extensive sampling and analysis of the pathogens that attack the diverse g <dig> cultivars grown by many fungus-growing ant species  <cit> .
there are also few published genetic analyses of the cultivars of the leafcutter ants, agricultural pests in much of the neotropics, and the leafcutter-associated escovopsis.
broad sampling and genetic analyses across the symbiosis will give insight into how labile these associations are over both ecological and evolutionary time.

 most parasites are intimately dependent on one or a few hosts. because of this host fidelity, parasites are expected to track speciating hosts by speciating themselves. this process, known as cospeciation, will lead to cocladogenesis, the topological matching of symbiont phylogenies. parasite and host phylogenies are rarely identical, however; forces such as duplication, sorting events, and host-switching can generate discordance between the phylogenies of hosts and their symbionts. despite these complications, congruent phylogenies are known in host-parasite associations and in host-mutualist associations as well. the fungus-growing ant-microbe symbiosis is a novel example of a system in which cocladogenesis occurs between a host and both its mutualistic and parasitic symbionts. research over the last decade has demonstrated the congruence of the phylogenies of fungus-growing ants, the fungi that they cultivate and the cultivar-attacking pathogen escovopsis at ancient phylogenetic levels. genetic analyses of more recently diverged, younger lineages demonstrate discrepancies between ant and cultivar associations, which are likely due to a combination of lateral transfer of cultivars between colonies and occasional domestication of free-living fungi by the ants. to date, the two published phylogenetic studies of the escovopsis-cultivar association indicate no discordance between the phylogenies of the cultivars and escovopsis. ancient codiversification of fungus-growing ants and their cultivars is driven by the intimate dependence of the ants on fungus as their primary food source and the intimate dependence of the fungus on ants for protection, nutrition and dispersal. in ants, the ability to cultivate fungi for food arose only once, about 50– million years ago, and gave rise to roughly described, extant species of fungus-growing ants. the long coevolutionary history of these mutualists has led to the specialization of each ant species on the cultivation of a unique, narrow range of cultivated fungi, most of which are in the family lepiotaceae. as depicted in figure 1a, these lepiotaceous cultivars form two morphologically and molecularly distinct groups. there has been one switch to a distantly related cultivar; most ants in the genus apterostigma now cultivate fungi in the family pterulaceae, which is distantly related to the family lepiotaceae. the pterulaceous cultivars fall into two monophyletic, morphologically distinct cultivar groups. one apterostigma species, a. auriculatum, has retained the ancestral state of growing lepiotaceaous cultivars. currie et al. demonstrated that, at ancient levels, the phylogeny of escovopsis, a genus of specialized, highly pathogenic microfungi that attack the ants' fungal cultivars, matches that of the ants' diverse cultivars and consequently that of the ants themselves. escovopsis has only been found associated with nests of attine ants. upon establishing infection, escovopsis consumes the ants' cultivated fungi and can devastate attine colonies. though infection rates vary across host species, infections are prevalent in colonies of many attine genera throughout their geographic ranges. escovopsis is thought to track the cultivars because of the coevolutionary specialization of each escovopsis lineage on attacking and overcoming defenses of only a narrow range of cultivar hosts. ancient phylogenetic congruence between cultivars and escovopsis suggests that these pathogens may be tightly tracking their speciating hosts by speciating themselves, and that escovopsis lineages have not switched to novel cultivar hosts over evolutionary time. to test for host-switching, however, it is necessary to include extensive sampling across the diversity of both hosts and symbionts. previous studies of escovopsis host-fidelity have included few samples of apterostigma-associated escovopsis despite the fact that they are an extremely diverse group of fungus-growing ant pathogens. currie et al., the most extensive phylogenetic analysis of escovopsis to date, included only two apterostigma-associated escovopsis, which were morphologically similar and were isolated from ant colonies that raised closely-related fungi. not surprisingly, these isolates formed a single monophyletic "apterostigma escovopsis" clade. however, unlike the other fungus-growing ant genera, which each raise cultivars in a single cultivar group, apterostigma ants raise cultivars in three groups, which are each attacked by morphologically distinct escovopsis types. more extensive sampling of these diverse apterostigma pathogens, therefore, can reveal the extent to which escovopsis species are host-faithful, tracking their particular hosts without host-switching. through extensive geographic sampling and phylogenetic analysis of apterostigma-associated escovopsis, we ask whether host and pathogen phylogenies are still congruent when genetic analyses are extended to include the diversity of the apterostigma-associated escovopsis. first, do apterostigma-associated escovopsis form a monophyletic clade as the apterostigma ants do, or are apterostigma-associated escovopsis polyphyletic like their cultivars? second, do the apterostigma escovopsis form three distinct clades that correspond to the three cultivar groups raised by the different species of apterostigma ants? based on earlier findings that escovopsis is highly cultivar-type specific, we hypothesize that more extensive sampling will reveal that the apterostigma-associated escovopsis are not monophyletic like their associated ant-hosts, because the escovopsis that infects lepiotaceous apterostigma cultivars will be more closely-related to escovopsis isolated from lepiotaceous gardens of non-apterostigma ant species than to escovopsis isolated from pterulaceous apterostigma gardens. this would support findings of currie et al., depicted in figure 1b, that the pterulaceous-attacking escovopsis form a monophyletic clade distinct from the escovopsis that infects lepiotaceous cultivars, but would contradict their findings of complete congruence between ant, cultivar, and escovopsis phylogenies. we further hypothesize that more extensive sampling will reveal that pterulaceous-attacking escovopsis will fall into two clades associated with the two pterulaceous cultivar groups. overall, in looking at the fungus-growing ant-microbe symbiosis as a whole, we predict four host-specific escovopsis clades that are each specialized at attacking one of the four known fungus-growing ant cultivar groups.  diversity of apterostigma-associated escovopsis of colonies from which microbes were sampled, at least one fungal symbiont was isolated from each of colonies. for the purpose of this study, based on field identification of the ants, garden architecture and growth form of cultivar isolates, each colony was classified as either a g g or g colony, which raise respectively g g and g cultivars. escovopsis infection of these colonies was common and pathogen phenotypes were diverse. g colonies and g colonies had much higher infection rates than g colonies. more than 50% of g and g colonies were infected with at least one escovopsis type, whereas only 11% of g colonies were infected. escovopsis samples isolated from infected colonies were classified into four morphotypes based on spore-color: brown, yellow, white and pink. these types have different micromorphological conidiophore structures and likely represent different escovopsis lineages. in the absence of proper species descriptions, we will refer to the different escovopsis lineages by their characteristic spore-color. while white and pink escovopsis were each specific to a single cultivar group, brown escovopsis infected both g cultivars and g cultivars, and yellow escovopsis infected both g cultivars and g cultivars. the yellow escovopsis isolates associated with these two clades, however, are micromorphologically distinct from one another and likely are two separate species. a small percentage of colonies were infected by multiple escovopsis morphotypes. escovopsis types are identified here according to spore-color. for the g2-cultivar colonies, the number of infections based on spore-color exceeds the number of total infections because many colonies were infected with two or three escovopsis types. though sharing a similar spore-color, the g2-attacking and g3-attacking yellow escovopsis have distinct micromorphologies. phylogenetic relationships of apterostigma-associated escovopsis the results of parsimony, likelihood and bayesian analyses were highly concordant. three well supported clades were identified that correspond to brown, white and pink escovopsis. yellow escovopsis is not monophyletic; g2-attacking yellow escovopsis is genetically distinct from the single isolate of g3-attacking yellow escovopsis. overall, as predicted, these diverse apterostigma-associated escovopsis do not form a monophyletic clade. both the yellow and pink escovopsis isolated from apterostigma colonies with g cultivars are nested within other g3-attacking escovopsis and are distinct from other apterostigma-associated escovopsis. contradictory to our predictions, g2-attacking and g4-attacking escovopsis do not form separate, monophyletic clades. within the brown escovopsis, there are two clades of g4-associated escovopsis. parametric-bootstrapping verified the polyphyly of isolates of g4-associated escovopsis. the null hypothesis of a single origin of g4-associated escovopsis was rejected at p <. this implies that brown escovopsis has switched multiple times between g and g hosts. discussion phylogenetic patterns of the fungus-growing ant microbe symbiosis reveal a coevolutionary history of host-fidelity punctuated by occasional host-shifts. all known escovopsis lineages have some limitation to their host-range. for example, we here show that pink escovopsis attacks only lepiotaceous g cultivars, white escovopsis attacks only g cultivars, and though escovopsis with yellow spores attacks both g and g cultivars, the yellow escovopsis lineages associated with each of these host groups are morphologically and genetically distinct. despite this specificity, however, there is not complete congruence of host and pathogen phylogenies as suggested by previous studies, indicating that escovopsis host ranges have shifted and may continue to shift. this complex history parallels that of other symbiotic associations in which extensive sampling reveals that codiversification is interrupted often by host-switches. in fact, it appears that the cases where cocladogenesis persists over evolutionary time are mostly vertically-transmitted endosymbionts, whereas most ectosymbionts, such as escovopsis and the fungal cultivar, show patterns of switching and absence of strict cocladogenesis with their hosts. adaptive processes may explain the host-fidelity of most escovopsis types, which leads to the host-specific escovopsis clades revealed here. gerardo et al. demonstrated through microbial bioassays that escovopsis lineages are attracted to chemical signals released by their host cultivars. for example, in microbial bioassays, isolates of yellow escovopsis from g2-apterostigma colonies grow more rapidly towards chemical signals produced by g than by g and g cultivars, which is consistent with the host-range of yellow escovopsis. unlike yellow escovopsis, brown escovopsis from g2-apterostigma colonies is equally attracted to g and g cultivars. it is possible that this host-attraction would make it easier for brown escovopsis to switch between g and g hosts than it would be for yellow escovopsis to switch between hosts, because brown escovopsis would be equally likely to move through g and g fungal gardens, find healthy cultivar and establish infection. this is consistent with the phylogenetic results here, where is seems that brown escovopsis has switched between g and g hosts. microbial bioassays have also revealed that cultivars can defend themselves against some escovopsis but not others. g3-apterostigma cultivars can inhibit isolates of both brown escovopsis and g2-associated yellow escovopsis but cannot inhibit isolates of pink escovopsis, possibly explaining why brown and g2-associated yellow escovopsis do not attack g cultivars in nature, while pink escovopsis does. brown and yellow escovopsis are not, however, inhibited by most isolates of g and g cultivars, explaining why natural infection is possible in these host-parasite combinations. overall, both cultivar defense against escovopsis and escovopsis' attraction to host cultivars may maintain escovopsis' specialization and prevent rampant host-switching. because escovopsis species are host-specific, we hypothesized that wider sampling of escovopsis would reveal four escovopsis clades that correspond to four cultivar and ant clades. however, contradictory to our hypothesis, g4-associated escovopsis are not monophyletic. furthermore, there is a lack of congruence of cultivar and escovopsis phylogenies at deeper nodes. whereas previous analyses have indicated that apterostigma ants, their pterulaceous cultivars and their associated escovopsis are distantly-related to the highly-derived leafcutter ants and their associated microbes, some apterostigma-associated escovopsis lineages, namely brown escovopsis, are sister to the escovopsis attacking g cultivars. this suggests that an escovopsis lineage switched between these two distantly-related, ecologically-distinct fungal hosts. discordance of host and pathogen phylogenies suggests that escovopsis lineages have switched hosts over the evolutionary history of their host association, but the available evidence does not allow inference regarding the frequency at which switching occurs. it is also unclear whether switching involves the acquisition of novel escovopsis strains by the ants from their environment, or whether it involves the direct transmission of escovopsis between colonies by some unknown mechanism. further research on the exact mechanism of escovopsis transmission would be helpful in revealing the likelihood of pathogen exchange between colonies containing distantly-related cultivars.  phylogenetic analyses coupled with extensive sampling of host and parasites reveal a more complete picture of the complexity of the escovopsis-cultivar association in colonies of fungus-growing ants, which consists of specialized pathogen species that occasionally switch between distantly-related hosts. clades of closely-related escovopsis attack specific cultivar groups, causing the matching of cultivar and escovopsis phylogenies at some scales. discordance of host-parasite phylogenies, however, arises due to host-switching. these results reveal the need for additional sampling across the fungus-growing ant microbial symbiosis as a whole. to date, there has not been extensive sampling and analysis of the pathogens that attack the diverse g cultivars grown by many fungus-growing ant species. there are also few published genetic analyses of the cultivars of the leafcutter ants, agricultural pests in much of the neotropics, and the leafcutter-associated escovopsis. broad sampling and genetic analyses across the symbiosis will give insight into how labile these associations are over both ecological and evolutionary time. 
the fungus-growing ant-microbe symbiosis consists of coevolving microbial mutualists and pathogens. here, a more extensive phylogenetic analysis of escovopsis lineages infecting the gardens of apterostigma ants demonstrates that these pathogens display patterns of phylogenetic congruence with their fungal hosts. though there is clear evidence of host-specificity on the part of diverse escovopsis lineages, these pathogens have switched occasionally to novel host fungi.
 the fungus-growing ant-microbe symbiosis consists of coevolving microbial mutualists and pathogens. the diverse fungal lineages that these ants cultivate are attacked by parasitic microfungi of the genus escovopsis. previous molecular analyses have demonstrated strong phylogenetic congruence between the ants, the ants-cultivated fungi and the garden pathogen escovopsis at ancient phylogenetic levels, suggesting coevolution of these symbionts. however, few studies have explored cophylogenetic patterns between these symbionts at the recent phylogenetic levels necessary to address whether these parasites are occasionally switching to novel hosts or whether they are diversifying with their hosts as a consequence of long-term host fidelity.  here, a more extensive phylogenetic analysis of escovopsis lineages infecting the gardens of apterostigma ants demonstrates that these pathogens display patterns of phylogenetic congruence with their fungal hosts. particular clades of escovopsis track particular clades of cultivated fungi, and closely-related escovopsis generally infect closely-related hosts. discordance between host and parasite phylogenies, however, provides the first evidence for occasional host-switches or acquisitions of novel infections from the environment.  the fungus-growing ant-microbe association has a complex coevolutionary history. though there is clear evidence of host-specificity on the part of diverse escovopsis lineages, these pathogens have switched occasionally to novel host fungi. such switching is likely to have profound effects on how these host and parasites adapt to one another over evolutionary time scales and may impact how disease spreads over ecological time scales. 