a well - established theoretical framework for the evolution of cooperative interactions contrasts  partner fidelity  and  partner choice  as the most important mechanisms promoting and maintaining cooperation . previous studies have empirically shown that partner choice via host sanctions or differential rewards can stabilize cooperation in environmentally transmitted symbioses like mycorrhizal fungi or nitrogen - fixing rhizobia of plants , while the specialized intracellular symbioses of insects are generally assumed to be stabilized by partner fidelity . it remains unknown , however , which factors contribute to the maintenance and specificity of the vast majority of symbiotic associations in animals that involve facultative microbial associates , which contribute significantly to the ecological success of insects as well as many other organisms . beewolves are solitary digger wasps of the genera philanthus , trachypus , and philanthinus ( hymenoptera , crabronidae ) that hunt other hymenoptera and provision them as prey for their developing offspring in subterranean brood cells . female beewolves cultivate symbiotic bacteria in specialized gland reservoirs in the antennae and secrete them into the brood cells prior to oviposition . the larvae later transfer the symbionts to the cocoon silk , where they provide protection during subsequent development  which is often not completed until the following year  by producing a mixture of at least 9 different antimicrobial substances . recent phylogenetic analyses indicated that the symbionts are descendants of soil - dwelling streptomycetes that were acquired by the insects at least 68 million years ago ( fig . the external route of vertical symbiont transmission from mothers to daughters resulted in host - symbiont co - diversification , but also allowed for horizontal exchange of symbionts among hosts . artificial infection of beewolf females with opportunistic soil bacteria revealed that these bacteria can grow in the antennal reservoirs but are not transmitted to the offspring , providing strong evidence for partner choice via host control over symbiont transmission . we previously discussed the importance of partner choice for the long - term stability of the beewolf - streptomyces mutualism and will focus here on the implications of its biogeographic history . node ages in the host phylogeny ( left ) are shown in million years ago ( mya ) with 95% highest posterior density ( hpd ) interval bars . values at the nodes of the symbiont phylogeny ( right ) are local support values from the fasttree analysis ( gtr model ) , bootstrap values from phyml , and bayesian posteriors , respectively . branches are color - coded according to the geographic distribution of the host species ( see world map , hatched yellow and red branches indicate occurrence in africa and/or eurasia ) . colored boxes around host and symbiont names denote host genera ( green = philanthinus , blue = philanthus , red = trachypus ) . node ages in the host phylogeny ( left ) are shown in million years ago ( mya ) with 95% highest posterior density ( hpd ) interval bars . values at the nodes of the symbiont phylogeny ( right ) are local support values from the fasttree analysis ( gtr model ) , bootstrap values from phyml , and bayesian posteriors , respectively . branches are color - coded according to the geographic distribution of the host species ( see world map , hatched yellow and red branches indicate occurrence in africa and/or eurasia ) . colored boxes around host and symbiont names denote host genera ( green = philanthinus , blue = philanthus , red = trachypus ) . the reconstruction of the beewolf phylogeny and a calibration based on the fossil record allows for proposing hypotheses on the biogeographic history of these solitary wasps and their symbiotic association with streptomyces . however , as the fossil record underlying the dating analyses is rather sparse ( 2 philanthini , one cercerini , and one bembicinae fossil ) , age estimates are characterized by broad confidence intervals , so the exact timing of biogeographic events remains speculative . nevertheless , the phylogeny of philanthinae reveals eurasia or africa as the probable origin of beewolves , because philanthinus and the oldest nodes in philanthus all have palearctic or paleotropical distributions ( fig . the radiation of beewolves likely followed those of angiosperms and their most important pollinators , the bees , as adult beewolves feed on the nectar of angiosperms and predominantly use bees as larval provisions . bees originated during the early to middle cretaceous in the southern hemisphere ( gondwana ) , most likely in africa , and the majority of extant philanthus species are african ( 77 of 137 ) , so an african origin of beewolves seems likely . interestingly , the south indian philanthus species ( philanthus pulcherrimus , philanthus sp . in - e010 , and philanthus cf . basalis ) are interspersed among asian and african taxa in the phylogeny , suggesting that the indian subcontinent was colonized from both africa and asia . after spreading across the paleotropics and palearctic , beewolves colonized the americas about 37 mya ( 95% ci : 2551 mya ) . as the de geer and thulean bridges broke up around 63 and 56 mya , respectively , colonization via beringia during the warm climate of the late eocene seems most likely , which has previously been suggested for other insect taxa ( e.g. aphids ) . the colonization of south america may have occurred via the aves ridge or island arc ( 34 mya ) , which existed roughly around the time of the estimated split between the south american trachypus and the north american philanthus clade ( about 30 mya , 95% ci : 1941 mya , see fig . 1 ) . a plausible scenario assumes a single colonization event with a subsequent radiation of the trachypus clade in south america . however , the current range of trachypus extends north to southern texas , and although it is possible that the northern species have arisen since the emergence of the panamanian land bridge , the phylogenetic relationships of these taxa have yet to be investigated and they may date from an earlier time . despite a monophyletic origin of the beewolf symbiont clade , it shows many discrepancies with the beewolf phylogeny , indicating frequent horizontal transfer of symbiont lineages among host species . such transfer could conceivably occur through interspecific predation or nest reuse , or by infection from an environmental reservoir of symbiont spores , all of which require the co - occurrence of a host and its horizontally acquired symbiont . surprisingly , the symbiont strains show only a moderate degree of clustering according to their hosts ' geographical distribution ( fig . this pattern is unlikely to be explained by poor resolution or phylogenetic errors , because the phylogeny is based on the sequences of 5 different genes and was recently corroborated by an independent analysis using genome - wide aflp markers . four mutually non - exclusive hypotheses may explain the occurrence of closely related symbionts in geographically widely separated host taxa : ( i ) the host taxa colonizing new geographical areas may have carried a mixture of symbionts , which were subsequently exchanged among hosts , and individual symbiont strains were subsequently lost in different host lineages . although possible , this scenario seems unlikely , as recent analyses suggest a high degree of homogeneity in symbiont populations within individual beewolves . furthermore , given the ancient separation of old and new world species , biogeographic patterns would still be expected in the symbiont phylogeny under this scenario . ( ii ) the symbionts may be dispersed via wind or water over large distances ( even across continents and oceans ) and infect novel hosts . although recent studies provide increasing evidence for dispersal limitation and biogeographic patterns in microorganisms , some microorganisms indeed appear to be globally dispersed , and dormancy can be expected to facilitate long - range dispersal and successful colonization of new habitats . in fact , trans - oceanic dispersal events of microorganisms in dust clouds have been well documented , particularly from africa to the americas . previous studies have shown that the beewolf symbionts undergo morphological differentiation on the beewolf cocoon and thereby survive inhospitable conditions as dormant cells for at least 9 months , and possibly much longer . seed banks of ancient symbiont spores in the environment , thereby obscuring apparent biogeographic patterns among supposedly extant symbiont lineages . ( iv ) it is conceivable that the symbionts are only facultatively associated with beewolves and occur as free - living bacteria in the environment , which would provide ample opportunities for horizontal transfer and at least partially obscure biogeographic patterns ( although isolation by distance would still be expected , if dispersal is limited ) . possibly , the symbiotic ancestors were already globally distributed , and subsequent vicariance events and host switches resulted in the diversification of bacterial strains and led to the observed phylogenetic patterns , with closely related symbiont strains occurring in geographically distant host taxa . the recent isolation of closely related streptomyces strains from moroccan soil and from chilli pepper rhizosphere in thailand could provide evidence for the environmental occurrence of the beewolf symbionts , although the phylogenetic affiliation of these isolates with the symbiont clade is currently based on 16s rdna only and needs confirmation by multi - locus sequencing . beewolves and antibiotic - producing streptomyces bacteria participate in a defensive symbiosis involving both vertical and horizontal symbiont transmission . the biogeographic history of beewolves is well defined , but that of the symbionts is only loosely concordant with it , suggesting global dispersal or vicariance . could greatly advance our understanding of the biogeographic history of the ancient association between beewolves and streptomyces , and thereby illuminate larger questions about the relative importance of animal - vectored and free - living dispersal in shaping the distribution of microorganisms in nature . we gratefully acknowledge financial support from the max planck society ( mk ) and the german science foundation ( dfg - str532/2 - 2 [ es / mk ] and dfg - ka2846/2 - 1 [ mk ] ) .
mutualistic microorganisms play important roles in nutrition , reproduction and defense of many insects , yet the factors contributing to their maintenance and dispersal remain unknown in most cases . 
 theory suggests that collaboration can be maintained by repeated interaction of the same partners ( partner fidelity ) or by selective discrimination against non - cooperative partners ( partner choice ) . in the defensive mutualism between solitary beewolf wasps and their antibiotic - producing streptomyces bacteria , partner choice by host control of vertical symbiont transmission reinforces partner fidelity and has helped to maintain this highly specific association since it originated in the late cretaceous . 
 however , co - phylogenetic and biogeographic analyses suggest that there has also been considerable horizontal transmission of the symbionts . 
 while the beewolves clearly have a paleotropic or palearctic origin , with later colonization of the nearctic and neotropics via beringia and the aves ridge , respectively , the bacteria show only weak geographical clustering , implying global dispersal or vicariance within the confines of an otherwise apparently exclusive symbiotic relationship . 
 we discuss several hypotheses that may explain these patterns . 
 future studies investigating the occurrence of beewolf symbionts in the environment could yield broadly applicable insights into the relative impact of animal - vectored and free - living dispersal on the distribution of microorganisms in nature .