Timing of rapid diversification and convergent origins of active pollination within Agavoideae (Asparagaceae)

Abstract

Premise of the study: Yucca species are ideal candidates for the study of coevolution due to the obligate mutualism they form with yucca moth pollinators (genera Tegeticula and Parategeticula). Yuccas are not the only species to exhibit a mutualism with yucca moths; the genus Hesperoyucca is pollinated by the California yucca moth (Tegeticula maculata). Relationships among yuccas, Hesperoyucca, and other members of subfamily Agavoideae are necessary to understand the evolution of this unique pollination syndrome. Here, we investigate evolutionary relationships of yuccas and closely related genera looking at the timing and origin of yucca moth pollination.

Methods: In this study, we sequenced the chloroplast genomes of 20 species in the subfamily Agavoideae (Asparagaceae) and three confamilial outgroup taxa to resolve intergeneric phylogenetic relationships of Agavoideae. We estimated divergence times using protein-coding genes from 67 chloroplast genomes sampled across monocots to determine the timing of the yucca moth pollination origin.

Key results: We confidently resolved intergeneric relationships in Agavoideae, demonstrating the origin of the yucca-yucca moth mutualism on two distinct lineages that diverged 27 million years ago. Comparisons of Yucca and Hesperoyucca divergence time to those of yucca moths (Tegeticula and Parategeticula, Prodoxidae) indicate overlapping ages for the origin of pollinating behavior in the moths and pollination by yucca moths in the two plant lineages.

Conclusion: Whereas pollinating yucca moths have been shown to have a single origin within the Prodoxidae, there were independent acquisitions of active pollination on lineages leading to Yucca and Hesperoyucca within the Agavoideae.

Keywords: Agavoideae; Hesperoyucca; Yucca; active pollination; chloroplast genome; convergent evolution; divergence time; obligate mutualism; yucca moth.