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.

Asymmetric hybridization and gene flow between Joshua trees (Agavaceae: Yucca) reflect differences in pollinator host specificity.

The angiosperms are by far the largest group of terrestrial plants. Their spectacular diversity is often attributed to specialized pollination. Obligate pollination mutualisms where both a plant and its pollinator are dependent upon one another for reproduction are thought to be prone to rapid diversification through co-evolution and pollinator isolation. However, few studies have evaluated the degree to which pollinators actually mediate reproductive isolation in these systems. Here, we examine evidence for hybridization and gene flow between two subspecies of Joshua tree (Yucca brevifolia brevifolia and Yucca brevifolia jaegeriana) pollinated by two sister species of yucca moth. Previous work indicated that the pollinators differ in host specificity, and DNA sequence data suggested asymmetric introgression between the tree subspecies. Through intensive sampling in a zone of sympatry, a large number of morphologically intermediate trees were identified. These included trees with floral characters typical of Y. b. jaegeriana, but vegetative features typical of Y. b. brevifolia. The opposite combination-Y. b. brevifolia flowers with Y. b. jaegeriana vegetative morphology-never occurred. Microsatellite genotyping revealed a high frequency of genetically admixed, hybrid trees. Coalescent-based estimates of migration indicated significant gene flow between the subspecies and that the direction of gene flow matches differences in pollinator host fidelity. The data suggest that pollinator behaviour determines the magnitude and direction of gene flow between the two subspecies, but that specialized pollination alone is not sufficient to maintain species boundaries. Natural selection may be required to maintain phenotypic differences in the face of ongoing gene flow.

Geographic isolation trumps coevolution as a driver of yucca and yucca moth diversification.

Coevolution is thought to be especially important in diversification of obligate mutualistic interactions such as the one between yuccas and pollinating yucca moths. We took a three-step approach to examine if plant and pollinator speciation events were likely driven by coevolution. First, we tested whether there has been co-speciation between yuccas and pollinator yucca moths in the genus Tegeticula (Prodoxidae). Second, we tested whether co-speciation also occurred between yuccas and commensalistic yucca moths in the genus Prodoxus (Prodoxidae) in which reciprocal evolutionary change is unlikely. Finally, we examined the current range distributions of yuccas in relationship to pollinator speciation events to determine if plant and moth speciation events likely occurred in sympatry or allopatry. Co-speciation analyses of yuccas with their coexisting Tegeticula pollinator and commensalistic Prodoxus lineages demonstrated phylogenetic congruence between both groups of moths and yuccas, even though moth lineages differ in the type of interaction with yuccas. Furthermore, Yucca species within a lineage occur primarily in allopatry rather than sympatry. We conclude that biogeographic factors are the overriding force in plant and pollinator moth speciation and significant phylogenetic congruence between the moth and plant lineages is likely due to shared biogeography rather than coevolution.

Characterization of microsatellite loci in Yucca brevifolia (Agavaceae) and cross-amplification in related species.

PREMISE OF THE STUDY: Microsatellite primers were characterized in Yucca brevifolia for use in population genetic studies and, particularly, analyses of gene flow between varieties. METHODS AND RESULTS: We characterized 12 microsatellite loci polymorphic in Yucca brevifolia by screening primers that were developed using an SSR-enriched library or which were previously described in Yucca filamentosa. Genetic analysis of four populations resulted in the mean number of alleles per locus ranging from 10.25 to 14.58 and mean expected heterozygosity from 0.78 to 0.88. Cross-amplification of all 12 loci was attempted in six additional yucca species. CONCLUSIONS: These loci should prove useful for population genetic research in Yucca brevifolia, and cross-amplification of these loci in related species suggests that they may be useful in studies of hybridization and introgression between species.

Species identification and sibship assignment of sympatric larvae in the yucca moths Tegeticula synthetica and Tegeticula antithetica (Lepidoptera: Prodoxidae).

Ecological interactions between yucca moths (Tegeticula, Prodoxidae) and their host plants (Yucca, Agavaceae) are exemplary of obligate plant-pollinator mutualism and co-evolution. We describe a multiplex microsatellite DNA protocol for species identification and sibship assignment of sympatric larvae from Tegeticula synthetica and Tegeticula antithetica, pollinators of the Joshua tree (Yucca brevifolia). Bayesian clustering provides correct diagnosis of species in 100% of adult moths, with unambiguous identification of sympatric larvae. Sibship assignments show that larvae within a single fruit are more likely to be full-sibs or half-sibs than larvae from different fruit, consistent with the hypothesis that larval clutches are predominantly the progeny of an individual female.