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