DISTINGUISHING COEVOLUTION FROM COVICARIANCE IN AN OBLIGATE POLLINATION MUTUALISM: ASYNCHRONOUS DIVERGENCE IN JOSHUA TREE AND ITS POLLINATORS

150 Years of Coevolution Research: Evolution and Ecology of Yucca Moths (Prodoxidae) and Their Hosts

Yucca moths (Tegeticula and Parategeticula) are specialized pollinators of yucca plants, possessing unique, tentacle-like mouthparts used to actively collect pollen and deposit it onto the flowers of their hosts. The moths' larvae feed on the developing seeds and fruit tissue. First described in 1873, the yucca-yucca moth pollination system is now considered the archetypical example of a coevolved intimate mutualism. Research conducted over the past three decades has transformed our understanding of yucca moth diversity and host plant interactions. We summarize the current understanding of the diversity, ecology, and evolution of this group, review evidence for coevolution of the insects and their hosts, and describe how the nature of the interaction varies across evolutionary time and ecological contexts. Finally, we identify unresolved questions and areas for future research.

To explore strange new worlds - The diversification in Tremella caloplacae was linked to the adaptive radiation of the Teloschistaceae

Lichenicolous fungi are a heterogeneous group of organisms that grow exclusively on lichens, forming obligate associations with them. It has often been assumed that cospeciation has occurred between lichens and lichenicolous fungi, but this has been seldom analysed from a macroevolutionary perspective. Many lichenicolous species are rare or are rarely observed, which results in frequent and large gaps in the knowledge of the diversity of many groups. This, in turn, hampers evolutionary studies that necessarily are based on a reasonable knowledge of this diversity. Tremella caloplacae is a heterobasidiomycete growing on various hosts from the lichen-forming family Teloschistaceae, and evidence suggests that it may represent a species complex. We combine an exhaustive sampling with molecular and ecological data to study species delimitation, cophylogenetic events and temporal concordance of this association. Tremella caloplacae is here shown to include at least six distinct host-specific lineages (=putative species). Host switch is the dominant and most plausible event influencing diversification and explaining the coupled evolutionary history in this system, although cospeciation cannot be discarded. Speciation in T. caloplacae would therefore have occurred coinciding with the rapid diversification - by an adaptive radiation starting in the late Cretaceous - of their hosts. New species in T. caloplacae would have developed as a result of specialization on diversifying lichen hosts that suddenly offered abundant new ecological niches to explore or adapt to.

Pollinator and host sharing lead to hybridization and introgression in Panamanian free-standing figs, but not in their pollinator wasps

Obligate pollination mutualisms, in which plant and pollinator lineages depend on each other for reproduction, often exhibit high levels of species specificity. However, cases in which two or more pollinator species share a single host species (host sharing), or two or more host species share a single pollinator species (pollinator sharing), are known to occur in current ecological time. Further, evidence for host switching in evolutionary time is increasingly being recognized in these systems. The degree to which departures from strict specificity differentially affect the potential for hybridization and introgression in the associated host or pollinator is unclear. We addressed this question using genome-wide sequence data from five sympatric Panamanian free-standing fig species (Ficus subgenus Pharmacosycea, section Pharmacosycea) and their six associated fig-pollinator wasp species (Tetrapus). Two of the five fig species, F. glabrata and F. maxima, were found to regularly share pollinators. In these species, ongoing hybridization was demonstrated by the detection of several first-generation (F1) hybrid individuals, and historical introgression was indicated by phylogenetic network analysis. By contrast, although two of the pollinator species regularly share hosts, all six species were genetically distinct and deeply divergent, with no evidence for either hybridization or introgression. This pattern is consistent with results from other obligate pollination mutualisms, suggesting that, in contrast to their host plants, pollinators appear to be reproductively isolated, even when different species of pollinators mate in shared hosts.

From monocots to dicots: host shifts in Afrotropical derelomine weevils shed light on the evolution of non-obligatory brood pollination mutualism

Weevils from the tribe Derelomini (Curculionidae: Curculioninae) are specialized brood pollinators engaged in mutualistic relationships with several angiosperm lineages. In brood pollination systems, reproductive plant tissues are used for the development of insect larval stages, whereas adult insects pollinate their plant hosts as a reward. The evolutionary history of derelomines in relationship to their hosts is poorly understood and potentially contrasts with other brood pollination systems, wherein a pollinator lineage is usually associated with a single host plant family. In the case of Afrotropical Derelomini, host records indicate a diverse host repertoire consisting of several families of monocot and dicot plants. In this study, we investigate their phylogenetic relationships, timing of diversification and evolution of host use. Our results suggest that derelomine lineages started their diversification ~40 Mya. Reconstructions of host use evolution support an ancestral association with the monocotyledonous palm family (Arecaceae), followed by several shifts towards other plant families in Afrotropical lineages, especially to dicotyledonous plants from the family Ebenaceae (on the genus Euclea L.). Some level of phylogenetic conservatism of host use is recovered for the lineages associated with either palms or Euclea. Multiple instances of sympatric weevil assemblages on the same plant are also unravelled, corresponding to either single or independent colonization events. Overall, the diversity of hosts colonized and the frequency of sympatric assemblages highlighted in non-obligatory plant–derelomine brood pollination systems contrast with what is generally expected from plant–insect brood pollination systems.

Phylogenetic Analysis and Flower Color Evolution of the Subfamily Linoideae (Linaceae)

The taxonomy of the subfamily Linoideae at the intergeneric and section levels has been questioned throughout the years, and the evolution of floral characters remains poorly understood. In particular, the evolution of flower color is still uncertain, despite its ecological importance and being one of the most variable and striking traits in Angiospermae. We evaluated the phylogenetic relationships of the genera and sections and used the phylogeny to reconstruct the ancestral state of flower color. The results suggest reevaluating the taxonomic status of segregated genera and re-incorporating them into Linum. Four of the five sections currently accepted were recovered as monophyletic (Cathartolinum, Dasylinum, Linum, and Syllinum). We propose accepting the section Stellerolinon and reevaluating Linopsis, whose representatives were recovered in three separate clades. The ancestral flower color for Linoideae was yellow-white. The flower colors purple and yellow-white were recovered at the deepest nodes of the two main clades. Pink, blue, and red colors were the most recent to evolve. These results appear to be related to diversification events, biogeographical history, and ecological aspects of the subfamily. Our reconstruction constitutes the first plausible scenario that explores the evolution of flower color, leading to new testable hypotheses for future research on the flax group.

Genome-scale data resolves the timing of divergence in Joshua trees

PREMISE

Joshua trees (Yucca brevifolia and Y. jaegeriana) and their yucca moth pollinators (Tegeticula synthetica and T. antithetica) are a model system for studies of plant-pollinator coevolution and, they are thought to be one of the only cases in which there is compelling evidence for cospeciation driven by coevolution. Previous work attempted to evaluate whether divergence between the plant and their pollinators was contemporaneous. That work concluded that the trees diverged more than 5 million years ago-well before the pollinators. However, clear inferences were hampered by a lack of data from the nuclear genome and low genetic variation in chloroplast genes. As a result, divergence times in the trees could not be confidently estimated.

METHODS

We present an analysis of whole chloroplast genome sequence data and RADseq data from >5000 loci in the nuclear genome. We developed a molecular clock for the Asparagales and the Agavoideae using multiple fossil calibration points. Using Bayesian inference, we produced new estimates for the age of the genus Yucca and for Joshua trees. We used calculated summary statistics describing genetic variation and used coalescent-based methods to estimate population genetic parameters.

RESULTS

We find that the Joshua trees are moderately genetically differentiated, but that they diverged quite recently (~100-200 kya), and much more recently than their pollinators.

CONCLUSIONS

The results argue against the notion that coevolution directly contributed to speciation in this system, suggesting instead that coevolution with pollinators may have reinforced reproductive isolation following initial divergence in allopatry.

Evaluating insect-host interactions as a driver of species divergence in palm flower weevils

Plants and their specialized flower visitors provide valuable insights into the evolutionary consequences of species interactions. In particular, antagonistic interactions between insects and plants have often been invoked as a major driver of diversification. Here we use a tropical community of palms and their specialized insect flower visitors to test whether antagonisms lead to higher population divergence. Interactions between palms and the insects visiting their flowers range from brood pollination to florivory and commensalism, with the latter being species that feed on decaying-and presumably undefended-plant tissues. We test the role of insect-host interactions in the early stages of diversification of nine species of beetles sharing host plants and geographical ranges by first delimiting cryptic species and then using models of genetic isolation by environment. The degree to which insect populations are structured by the genetic divergence of plant populations varies. A hierarchical model reveals that this variation is largely uncorrelated with the kind of interaction, showing that antagonistic interactions are not associated with higher genetic differentiation. Other aspects of host use that affect plant-associated insects regardless of the outcomes of their interactions, such as sensory biases, are likely more general drivers of insect population divergence.

Strong Selection Against Early Generation Hybrids in Joshua Tree Hybrid Zone Not Explained by Pollinators Alone

Coevolution frequently plays an important role in diversification, but the role of obligate pollination mutualisms in the maintenance of hybrid zones has rarely been investigated. Like most members of the genus Yucca, the two species of Joshua tree (Yucca brevifolia and Yucca jaegeriana) are involved in a tightly coevolved mutualism with yucca moths. There is strong evidence of a history of coevolution between Joshua trees and their moth pollinators. We use a geographic clines approach in the Joshua tree hybrid zone to ask if selection by the moths may currently contribute to maintaining separation between these species. We compare genomic, phenotypic, and pollinator frequency clines to test whether pollinators maintain the hybrid zone or follow it as passive participants. The results reveal dramatic overlapping genomic and pollinator clines, consistent with a narrow hybrid zone maintained by strong selection. Wider phenotypic clines and a chloroplast genomic cline displaced opposite the expected direction suggest that pollinators are not the main source of selection maintaining the hybrid zone. Rather, it seems that high levels of reproductive isolation, likely acting through multiple barriers and involving many parts of the genome, keep the hybrid zone narrow.

A symbiotic balancing act: arbuscular mycorrhizal specificity and specialist fungus gnat pollination in the mycoheterotrophic genus Thismia (Thismiaceae)

BACKGROUND AND AIMS

Mycorrhizal associations in mycoheterotrophic plants are generally more specialized than in autotrophs. Mycoheterotrophs typically bear small, inconspicuous flowers that often self-pollinate to maximize seed set, although some have structurally complex flowers indicative of xenogamy. A trade-off has previously been proposed between specialization in these above- and below-ground symbioses, although empirical data are lacking.

METHODS

We used next-generation DNA sequencing to compare the mycorrhizal communities from the roots of a mycoheterotrophic species, Thismia tentaculata (Thismiaceae), and its neighbouring autotrophs. We furthermore conducted detailed assessments of floral phenology and pollination ecology, and performed artificial pollination experiments to determine the breeding system.

KEY RESULTS

Thismia tentaculata maintains a symbiotic association with a single arbuscular mycorrhizal Rhizophagus species. The flowers are pollinated by a single species of fungus gnats (Corynoptera, Sciaridae), which are attracted by the yellow pigments and are temporarily restrained within the perianth chamber before departing via apertures between the anthers. The plants are self-compatible but predominantly xenogamous.

CONCLUSIONS

Our findings demonstrate that T. tentaculata maintains highly specialized associations with pollinators and mycorrhizal fungi, both of which are widely distributed. We suggest that specialization in multiple symbiotic interactions is possible in mycoheterotrophs if redundant selective pressures are not exerted to further restrict an already constrained suite of life-history traits.

Whole-genome data reveal the complex history of a diverse ecological community

How widespread ecological communities assemble remains a key question in ecology. Trophic interactions between widespread species may reflect a shared population history or ecological fitting of local pools of species with very different population histories. Which scenario applies is central to the stability of trophic associations and the potential for coevolution between species. Here we show how alternative community assembly hypotheses can be discriminated using whole-genome data for component species and provide a likelihood framework that overcomes current limitations in formal comparison of multispecies histories. We illustrate our approach by inferring the assembly history of a Western Palearctic community of insect herbivores and parasitoid natural enemies, trophic groups that together comprise 50% of terrestrial species. We reject models of codispersal from a shared origin and of delayed enemy pursuit of their herbivore hosts, arguing against herbivore attainment of "enemy-free space." The community-wide distribution of species expansion times is also incompatible with a random, neutral model of assembly. Instead, we reveal a complex assembly history of single- and multispecies range expansions through the Pleistocene from different directions and over a range of timescales. Our results suggest substantial turnover in species associations and argue against tight coevolution in this system. The approach we illustrate is widely applicable to natural communities of nonmodel species and makes it possible to reveal the historical backdrop against which natural selection acts.

First Recorded Observations of Pollination and Oviposition Behavior in Tegeticula antithetica (Lepidoptera: Prodoxidae) Suggest a Functional Basis for Coevolution With Joshua Tree (Yucca) Hosts

Yucca moths (Tegeticula spp.) are the exclusive pollinators of Joshua trees (Yucca brevifolia s. l.). The moths actively pollinate the Joshua tree flowers and lay their eggs in the style. Recent studies have revealed that the plants commonly known as Joshua trees include two distinct, sister-species of plant: Yucca brevifolia Engelm. and Yucca jaegeriana McKelvey, each pollinated by two sister-species of yucca moth Tegeticula synthetica Riley and Tegeticula antithetica Pellmyr, respectively. A number of studies have argued that the moths have coevolved with their hosts, producing a pattern of phenotype matching between moth ovipositor length and floral style length. However, the only known descriptions of yucca moth pollination and oviposition behavior on Joshua trees are observations of T. synthetica made in 1893. The behavior of T. antithetica has never been observed before. We produced the first video recordings of the behavior of T. antithetica, and measured the points of oviposition and egg placement within the floral style. We found a number of differences between the behaviors of T. antithetica and T. synthetica, which appear to be a consequence of differences in floral morphology between Y. jaegeriana and Y. brevifolia. We also found that variation in floral style length strongly influences the placement of eggs within the flower, which may explain patterns of phenotype matching described previously. However, unlike in other yucca moths, we find that the mode of oviposition is unlikely to wound the floral ovules, and thus that oviposition by T. antithetica is unlikely to prompt floral abscission.

Diversification and coevolution in brood pollination mutualisms: Windows into the role of biotic interactions in generating biological diversity

Brood pollination mutualisms-interactions in which specialized insects are both the pollinators (as adults) and seed predators (as larvae) of their host plants-have been influential study systems for coevolutionary biology. These mutualisms include those between figs and fig wasps, yuccas and yucca moths, leafflowers and leafflower moths, globeflowers and globeflower flies, Silene plants and Hadena and Perizoma moths, saxifrages and Greya moths, and senita cacti and senita moths. The high reciprocal diversity and species-specificity of some of these mutualisms have been cited as evidence that coevolution between plants and pollinators drives their mutual diversification. However, the mechanisms by which these mutualisms diversify have received less attention. In this paper, we review key hypotheses about how these mutualisms diversify and what role coevolution between plants and pollinators may play in this process. We find that most species-rich brood pollination mutualisms show significant phylogenetic congruence at high taxonomic scales, but there is limited evidence for the processes of both cospeciation and duplication, and there are no unambiguous examples known of strict-sense contemporaneous cospeciation. Allopatric speciation appears important across multiple systems, particularly in the insects. Host-shifts appear to be common, and widespread host-shifts by pollinators may displace other pollinator lineages. There is relatively little evidence for a "coevolution through cospeciation" model or that coevolution promotes speciation in these systems. Although we have made great progress in understanding the mechanisms by which brood pollination mutualisms diversify, many opportunities remain to use these intriguing symbioses to understand the role of biotic interactions in generating biological diversity.

Floral scent of Joshua trees (Yucca brevifolia sensu lato): Divergence in scent profiles between species but breakdown of signal integrity in a narrow hybrid zone

PREMISE OF THE STUDY

The role of floral scent in facilitating reproductive isolation between closely related plants remains poorly understood. Yucca brevifolia and Yucca jaegeriana are pollinated by different moth species in allopatry, but in a narrow contact zone, pollinator-host specificity breaks down, resulting in hybridization between species. We explored the chemical basis for reproductive isolation and hybridization in these Joshua trees by characterizing the floral scent of each species in allopatry, analyzing scent profiles from trees in the contact zone, and matching these data with genotypic and phenotypic data.

METHODS

We analyzed floral volatiles using gas chromatography-mass spectrometry, tested for species divergence of scent profiles and classified trees in the contact zone as hybrid or either parental species. We used floral and vegetative morphological data and genotypic data to classify trees and analyzed whether certain trait combinations were more correlated than others with respect to assignment of trees and whether frequencies of classified tree types differed depending on which data set was used.

KEY RESULTS

The Joshua tree floral scent included oxygenated 8-carbon compounds not reported for other yuccas. The two species differed (P < 0.001) in scent profiles. In the contact zone, many hybrids were found, and phenotypic traits were generally weakly correlated, which may be explained by extensive gene flow between species or by exposure to different selection pressures.

CONCLUSIONS

Although the two Joshua tree species produce distinct floral scent profiles, it is insufficient to prevent attraction of associated pollinators to both hosts. Instead, floral morphology may be the key trait mediating gene flow between species.

Population genomics of divergence within an obligate pollination mutualism: Selection maintains differences between Joshua tree species

PREMISE OF THE STUDY

Speciation is a complex process that can be shaped by many factors, from geographic isolation to interspecific interactions. In Joshua trees, selection from pollinators on style length has been hypothesized to contribute to the maintenance of differentiation between two hybridizing sister species. We used population genomics approaches to measure the extent of genetic differentiation between these species, test whether selection maintains differences between them, and determine whether genetic variants associated with style length show signatures of selection.

METHODS

Using restriction-site-associated DNA (RAD)-sequencing, we identified 9516 single nucleotide polymorphisms (SNPs) across the Joshua tree genome. We characterized the genomic composition of trees in a narrow hybrid zone and used genomic scans to search for signatures of selection acting on these SNPs. We used a genome-wide association study to identify SNPs associated with variation in phenotypic traits, including style length, and asked whether those SNPs were overrepresented among the group under selection.

KEY RESULTS

The two species were highly genetically differentiated (F_ST = 0.25), and hybrids were relatively rare in the hybrid zone. Approximately 20% of SNPs showed evidence of selection maintaining divergence. While SNPs associated with style length were overrepresented among those under selection (P << 0.0001), the same was true for SNPs associated with highly differentiated vegetative traits.

CONCLUSIONS

The two species of Joshua tree are clearly genetically distinct, and selection is maintaining differences between them. We found that loci associated with differentiated traits were likely to be under selection. However, many traits other than style length appeared to be under selection. Together with the dearth of intermediate hybrids, these findings reveal that these taxa are more strongly diverged than previously suspected and that selection, likely on many targets, is maintaining separation where the two species meet and hybridize.

Geographical matching of volatile signals and pollinator olfactory responses in a cycad brood-site mutualism

Brood-site mutualisms represent extreme levels of reciprocal specialization between plants and insect pollinators, raising questions about whether these mutualisms are mediated by volatile signals and whether these signals and insect responses to them covary geographically in a manner expected from coevolution. Cycads are an ancient plant lineage in which almost all extant species are pollinated through brood-site mutualisms with insects. We investigated whether volatile emissions and insect olfactory responses are matched across the distribution range of the African cycad Encephalartos villosus. This cycad species is pollinated by the same beetle species across its distribution, but cone volatile emissions are dominated by alkenes in northern populations, and by monoterpenes and a pyrazine compound in southern populations. In reciprocal choice experiments, insects chose the scent of cones from the local region over that of cones from the other region. Antennae of beetles from northern populations responded mainly to alkenes, while those of beetles from southern populations responded mainly to pyrazine. In bioassay experiments, beetles were most strongly attracted to alkenes in northern populations and to the pyrazine compound in southern populations. Geographical matching of cone volatiles and pollinator olfactory preference is consistent with coevolution in this specialized mutualism.

Coevolution and the diversification of life

Coevolution, reciprocal adaptation between two or more taxa, is commonly invoked as a primary mechanism responsible for generating much of Earth's biodiversity. This conceptually appealing hypothesis is incredibly broad in evolutionary scope, encompassing diverse patterns and processes operating over timescales ranging from microbial generations to geological eras. However, we have surprisingly little evidence that large-scale associations between coevolution and diversity reflect a causal relationship at smaller timescales, in which coevolutionary selection is directly responsible for the formation of new species. In this synthesis, we critically evaluate evidence for the often-invoked hypothesis that coevolution is an important process promoting biological diversification. We conclude that the lack of widespread evidence for coevolutionary diversification may be best explained by the fact that coevolution's importance in diversification varies depending on the type of interaction and the scale of the diversification under consideration.

Phylogeny, systematics and biogeography of the genus panolis (lepidoptera: noctuidae) based on morphological and molecular evidence

The genus Panolis is a small group of noctuid moths with six recognized species distributed from Europe to East Asia, and best known for containing the widespread Palearctic pest species P. flammea, the pine beauty moth. However, a reliable classification and robust phylogenetic framework for this group of potentially economic importance are currently lacking. Here, we use morphological and molecular data (mitochondrial genes cytochrome c oxidase subunit I and 16S ribosomal RNA, nuclear gene elongation factor-1 alpha) to reconstruct the phylogeny of this genus, with a comprehensive systematic revision of all recognized species and a new one, P. ningshan sp. nov. The analysis results of maximum parsimony, maximum likelihood and Bayesian inferring methods for the combined morphological and molecular data sets are highly congruent, resulting in a robust phylogeny and identification of two clear species groups, i.e., the P. flammea species group and the P. exquisita species group. We also estimate the divergence times of Panolis moths using two conventional mutation rates for the arthropod mitochondrial COI gene with a comparison of two molecular clock models, as well as reconstruct their ancestral areas. Our results suggest that 1) Panolis is a young clade, originating from the Oriental region in China in the Late Miocene (6-10Mya), with an ancestral species in the P. flammea group extending northward to the Palearctic region some 3-6 Mya; 2) there is a clear possibility for a representative of the Palearctic clade to become established as an invasive species in the Nearctic taiga.

Host switching promotes diversity in host-specialized mycoparasitic fungi: uncoupled evolution in the Biatoropsis-usnea system

Fungal mycoparasitism-fungi parasitizing other fungi-is a common lifestyle in some basal lineages of the basidiomycetes, particularly within the Tremellales. Relatively nonaggressive mycoparasitic fungi of this group are in general highly host specific, suggesting cospeciation as a plausible speciation mode in these associations. Species delimitation in the Tremellales is often challenging because morphological characters are scant. Host specificity is therefore a great aid to discriminate between species but appropriate species delimitation methods that account for actual diversity are needed to identify both specialist and generalist taxa and avoid inflating or underestimating diversity. We use the Biatoropsis-Usnea system to study factors inducing parasite diversification. We employ morphological, ecological, and molecular data-methods including genealogical concordance phylogenetic species recognition (GCPSR) and the general mixed Yule-coalescent (GMYC) model-to assess the diversity of fungi currently assigned to Biatoropsis usnearum. The degree of cospeciation in this association is assessed with two cophylogeny analysis tools (ParaFit and Jane 4.0). Biatoropsis constitutes a species complex formed by at least seven different independent lineages and host switching is a prominent force driving speciation, particularly in host specialists. Combining ITS and nLSU is recommended as barcode system in tremellalean fungi.

Non-congruent colonizations and diversification in a coevolving pollination mutualism on oceanic islands

A challenge for coevolutionary theory is how different types of interaction influence the diversification of coevolving clades. Reciprocal specialization is characteristic of certain coevolving, mutualistic interactions, but whether this specialization seen in ecological time constrains changes in patterns of interaction over evolutionary time remains unclear. Here, we examine the co-radiation of Glochidion trees (Phyllanthaceae: Phyllanthus s. l.) and pollinating, seed-predatory Epicephala moths (Lepidoptera: Gracillariidae) on young (mostly later than 5 Ma) oceanic islands in southeastern Polynesia. Epicephala are the sole known pollinators of Glochidion trees, and show extreme reciprocal specialization in continental Asia. We find that Glochidion and Epicephala diversified across these islands through repeated, non-congruent colonizations, and that one recently colonizing Epicephala lineage has spread across 12 host species in three archipelagos in less than 1 Myr. These results indicate that reciprocal specialization and coadaptation do not prevent dramatic changes in associations between intimately associated taxa over short evolutionary time scales. Not only are these host associations more dynamic than previously recognized, but these changes in patterns of interaction may play an important role in the diversification of coevolving taxa.

Cospeciation vs host-shift speciation: methods for testing, evidence from natural associations and relation to coevolution

Hosts and their symbionts are involved in intimate physiological and ecological interactions. The impact of these interactions on the evolution of each partner depends on the time-scale considered. Short-term dynamics - 'coevolution' in the narrow sense - has been reviewed elsewhere. We focus here on the long-term evolutionary dynamics of cospeciation and speciation following host shifts. Whether hosts and their symbionts speciate in parallel, by cospeciation, or through host shifts, is a key issue in host-symbiont evolution. In this review, we first outline approaches to compare divergence between pairwise associated groups of species, their advantages and pitfalls. We then consider recent insights into the long-term evolution of host-parasite and host-mutualist associations by critically reviewing the literature. We show that convincing cases of cospeciation are rare (7%) and that cophylogenetic methods overestimate the occurrence of such events. Finally, we examine the relationships between short-term coevolutionary dynamics and long-term patterns of diversification in host-symbiont associations. We review theoretical and experimental studies showing that short-term dynamics can foster parasite specialization, but that these events can occur following host shifts and do not necessarily involve cospeciation. Overall, there is now substantial evidence to suggest that coevolutionary dynamics of hosts and parasites do not favor long-term cospeciation.

Nuclear and chloroplast DNA phylogeography of Ficus hirta: obligate pollination mutualism and constraints on range expansion in response to climate change

This study uses a phylogeographic approach to investigate how interspecific interactions in an obligate pollination mutualism enhance or constrain dispersal and the range distributions of species through time. Fifteen populations of Ficus hirta, a bird-dispersed fig pollinated by a species-specific fig wasp, were sampled from Thailand to the northern limits of the tropical forest in China. These populations were assayed for six nuclear microsatellite loci and two intergenic chloroplast DNA sequences. Analyses of range expansion and genetic clustering indicated a relatively slow rate of range expansion from two or more southern glacial refugia. Low nuclear differentiation, combined with high interpopulation differentiation, and phylogeographic structuring of chloroplast variation indicated that seed dispersal has had a greater constraint than obligate interactions with fig wasps on the rate of post-glacial range expansion. This study is the first to investigate the phylogeographic history of a widely distributed southeast Asian tropical plant whose distribution extends to the northern limits of tropical forest habitat in China. It is also the first study of Ficus utilizing molecular data to evaluate whether species-specific pollination is a limitation or an aid to range expansion in response to climate change.

Homoploid hybrid origin of Yucca gloriosa: intersectional hybrid speciation in Yucca (Agavoideae, Asparagaceae)

There is a growing appreciation for the importance of hybrid speciation in angiosperm evolution. Here, we show that Yucca gloriosa (Asparagaceae: Agavoideae) is the product of intersectional hybridization between Y. aloifolia and Y. filamentosa. These species, all named by Carl Linnaeus, exist in sympatry along the southeastern Atlantic coast of the United States. Yucca gloriosa was found to share a chloroplast haplotype with Y. aloifolia in all populations sampled. In contrast, nuclear gene-based microsatellite markers in Y. gloriosa are shared with both parents. The hybrid origin of Y. gloriosa is supported by multilocus analyses of the nuclear microsatellite markers including principal coordinates analysis (PCO), maximum-likelihood hybrid index scoring (HINDEX), and Bayesian cluster analysis (STRUCTURE). The putative parental species share only one allele at a single locus, suggesting there is little to no introgressive gene flow occurring between these species and Y. gloriosa. At the same time, diagnostic markers are segregating in Y. gloriosa populations. Lack of variation in the chloroplast of Y. aloifolia, the putative maternal parent, makes it difficult to rule out multiple hybrid origins of Y. gloriosa, but allelic variation at nuclear loci can be explained by a single hybrid origin of Y. gloriosa. Overall, these data provide strong support for the homoploid hybrid origin of Y. gloriosa.

Comparative phylogeography of a coevolved community: concerted population expansions in Joshua trees and four yucca moths

Comparative phylogeographic studies have had mixed success in identifying common phylogeographic patterns among co-distributed organisms. Whereas some have found broadly similar patterns across a diverse array of taxa, others have found that the histories of different species are more idiosyncratic than congruent. The variation in the results of comparative phylogeographic studies could indicate that the extent to which sympatrically-distributed organisms share common biogeographic histories varies depending on the strength and specificity of ecological interactions between them. To test this hypothesis, we examined demographic and phylogeographic patterns in a highly specialized, coevolved community--Joshua trees (Yucca brevifolia) and their associated yucca moths. This tightly-integrated, mutually interdependent community is known to have experienced significant range changes at the end of the last glacial period, so there is a strong a priori expectation that these organisms will show common signatures of demographic and distributional changes over time. Using a database of >5000 GPS records for Joshua trees, and multi-locus DNA sequence data from the Joshua tree and four species of yucca moth, we combined paleaodistribution modeling with coalescent-based analyses of demographic and phylgeographic history. We extensively evaluated the power of our methods to infer past population size and distributional changes by evaluating the effect of different inference procedures on our results, comparing our palaeodistribution models to Pleistocene-aged packrat midden records, and simulating DNA sequence data under a variety of alternative demographic histories. Together the results indicate that these organisms have shared a common history of population expansion, and that these expansions were broadly coincident in time. However, contrary to our expectations, none of our analyses indicated significant range or population size reductions at the end of the last glacial period, and the inferred demographic changes substantially predate Holocene climate changes.

When does coevolution promote diversification?

Coevolutionary interactions between species are thought to be an important cause of evolutionary diversification. Despite this general belief, little theoretical basis exists for distinguishing between the types of interactions that promote diversification and those types that have no effect or that even restrict it. Using analytical models and simulations of phenotypic evolution across a metapopulation, we show that coevolutionary interactions promote diversification when they impose a cost of phenotype matching, as is the case for competition or host-parasite antagonism. In contrast, classical coevolutionary arms races have no tendency to promote or inhibit diversification, and mutualistic interactions actually restrict diversification. Together with the results of recent phylogenetic and ecological studies, these results suggest that the causes of diversification in many coevolutionary systems may require reassessment.

Comparative dating of attine ant and lepiotaceous cultivar phylogenies reveals coevolutionary synchrony and discord

The mutualistic symbiosis between fungus-gardening ants and their cultivars has made fundamental contributions to our understanding of the coevolution of complex species interactions. Reciprocal specialization and vertical symbiont cotransmission are thought to promote a pattern of largely synchronous coevolutionary diversification in attines. Here we test this hypothesis by inferring the first time-calibrated multigene phylogeny of the lepiotaceous attine cultivars and comparing it with the recently published fossil-anchored phylogeny of the attine ants. While this comparison reveals some possible cases of synchronous origins of ant and fungal clades, there were a number of surprising asynchronies. For example, leaf-cutter cultivars appear to be significantly younger than the corresponding ant genera. Similarly, a clade of fungi interacting with primitive fungus-gardening ants--thought to be ancestral to the more derived leaf-cutter symbionts--appears instead to be a more recent acquisition from free-living stock. These macroevolutionary patterns are consistent with recent population-level studies suggesting occasional acquisition of novel cultivar types from environmental sources and horizontal transmission of cultivars between different ant species. Horizontal transmission events, even if rare, appear to form loose ecological connections between diffusely coevolving ant and fungus lineages that permit punctuated changes in the topology of the mutualistic ant-fungus interaction network.

Mutualism favours higher host specificity than does antagonism in plant-herbivore interaction

Coevolved mutualisms often exhibit high levels of partner specificity. Obligate pollination mutualisms, such as the fig-fig wasp and yucca-yucca moth systems, represent remarkable examples of such highly species-specific associations; however, the evolutionary processes underlying these patterns are poorly understood. The prevailing hypothesis suggests that the high degree of specificity in pollinating seed parasites is the fortuitous result of specialization in their ancestors because these insects are derived from endophytic herbivores that are themselves highly host-specific. Conversely, we show that in the Glochidion-Epicephala obligate pollination mutualism, pollinators are more host-specific than are closely related endophytic leaf-feeding taxa, which co-occur with Epicephala on the same Glochidion hosts. This difference is probably not because of shifts in larval diet (i.e. from leaf- to seed-feeding), because seed-eating lepidopterans other than Epicephala do not show the same degree of host specificity as Epicephala. Species of a tentative sister group of Epicephala each attack several distantly related plants, suggesting that the evolution of strict host specificity is tied to the evolution of pollinator habit. These results suggest that mutualists can attain higher host specificity than that of their parasitic ancestors and that coevolutionary selection can be a strong promoter of extreme reciprocal specialization in mutualisms.

Inference of population history by coupling exploratory and model-driven phylogeographic analyses

Understanding the nature, timing and geographic context of historical events and population processes that shaped the spatial distribution of genetic diversity is critical for addressing questions relating to speciation, selection, and applied conservation management. Cladistic analysis of gene trees has been central to phylogeography, but when coupled with approaches that make use of different components of the information carried by DNA sequences and their frequencies, the strength and resolution of these inferences can be improved. However, assessing concordance of inferences drawn using different analytical methods or genetic datasets, and integrating their outcomes, can be challenging. Here we overview the strengths and limitations of different types of genetic data, analysis methods, and approaches to historical inference. We then turn our attention to the potentially synergistic interactions among widely-used and emerging phylogeographic analyses, and discuss some of the ways that spatial and temporal concordance among inferences can be assessed. We close this review with a brief summary and outlook on future research directions.

Host specificity, phenotype matching and the evolution of reproductive isolation in a coevolved plant-pollinator mutualism

Coevolutionary interactions between plants and their associated pollinators and seed dispersers are thought to have promoted the diversification of flowering plants (Raven 1977; Regal 1977; Stebbins 1981). The actual mechanisms by which pollinators could drive species diversification in plants are not fully understood. However, it is thought that pollinator host specialization can influence the evolution of reproductive isolation among plant populations because the pollinator's choice of host is what determines patterns of gene flow in its host plant, and host choice may also have important consequences on pollinator and host fitness (Grant 1949; Bawa 1992). In this issue of Molecular Ecology, Smith et al. (2009) present a very interesting study that addresses how host specialization affects pollinator fitness and patterns of gene flow in a plant host. Several aspects of this study match elements of a seminal mathematical model of plant-pollinator codivergence (Kiester et al. 1984) suggesting that reciprocal selection for matched plant and pollinator reproductive traits may lead to speciation in the host and its pollinator when there is strong host specialization and a pattern of geographic subdivision. Smith et al.'s study represents an important step to fill the gap in our understanding of how reciprocal selection may lead to speciation in coevolved plant-pollinator mutualisms.

Host-associated divergence and incipient speciation in the yucca moth Prodoxus coloradensis (Lepidoptera: Prodoxidae) on three species of host plants

A wide range of evolutionary processes have been implicated in the diversification of yuccas and yucca moths, which exhibit ecological relationships that extend from obligate plant-pollinator mutualisms to commensalist herbivory. Prodoxus coloradensis (Lepidoptera: Prodoxidae) is a yucca moth, which feeds on the flowering stalks of three Yucca species as larvae, but does not provide pollination service. To test for evidence of host-associated speciation, we examined the genetic structure of P. coloradensis using mitochondrial (cytochrome oxidase I) and nuclear (elongation factor 1 alpha) DNA sequence data. Multilocus coalescent simulations indicate that moths on different host plant species are characterized by recent divergence and low levels of effective migration, with large effective population sizes and considerable retention of shared ancestral polymorphism. Although geographical distance explains a proportion of the mitochondrial and nuclear DNA variation among moths on different species of Yucca, the effect of host specificity on genetic distance remains significant after accounting for spatial isolation. The results of this study indicate that differentiation within P. coloradensis is consistent with the evolution of incipient species affiliated with different host plants, potentially influenced by sex-biased dispersal and female philopatry.

Coevolution in Rhizobium-legume symbiosis?

Legume nodules, specialized structures for nitrogen fixation, are probably the result of coevolution of plants and ancestral rhizobia. Among the evolutionary processes leading to legume radiation and divergence, coevolution with rhizobia might have occurred. Alternatively, bacteria could have been constantly selected by plants, with bacteria slightly influencing plant evolution (required to fulfill the criteria for a coevolutionary hypothesis). Evidence of bacterial effects on plant evolution is scarce but being searched for. Bacterial genetic plasticity may be indicative of the large capacity of Rhizobium to adapt to legumes. Events such as symbiotic replacement, easy recruitment of symbiotic bacteria by legume plants, and lateral transfer of symbiotic genes seem to erase the coevolutionary or selected relationships in rhizobial-legume symbiosis. In particular, the hypotheses proposed are (1) Rhizobium replaced Bradyrhizobium in a few hosts of the Phaseoleae tribe, Phaseolus vulgaris and P. coccineus; (2) Rhizobium etli as a species did not coevolve with bean; and (3) beta-Proteobacteria replaced alpha-Proteobacteria in South American mimosas. Novel results on symbiosis suggest a more complex evolutionary process for nodulation that may include multiple organisms, such as mycorrhiza, nematodes, and other bacteria in addition to rhizobia.

Divergence in an obligate mutualism is not explained by divergent climatic factors

Adaptation to divergent environments creates and maintains biological diversity, but we know little about the importance of different agents of ecological divergence. Coevolution in obligate mutualisms has been hypothesized to drive divergence, but this contention has rarely been tested against alternative ecological explanations. Here, we use a well-established example of coevolution in an obligate pollination mutualism, Yucca brevifolia and its two pollinating yucca moths, to test the hypothesis that divergence in this system is the result of mutualists adapting to different abiotic environments as opposed to coevolution between mutualists. We used a combination of principal component analyses and ecological niche modeling to determine whether varieties of Y. brevifolia associated with different pollinators specialize on different environments. Yucca brevifolia occupies a diverse range of climates. When the two varieties can disperse to similar environments, they occupy similar habitats. This suggests that the two varieties have not specialized on distinct habitats. In turn, this suggests that nonclimatic factors, such as the biotic interaction between Y. brevifolia and its pollinators, are responsible for evolutionary divergence in this system.