Coevolution driving speciation? Evidence from a Joshua tree hybrid zone

The idea that biotic interactions foster diversification is supported by substantial circumstantial evidence, but is challenging to test. We use a uniquely suitable system to ask whether putative coevolution in a plant-pollinator interaction is contributing to speciation. Two subspecies of Joshua tree (Yucca brevifolia brevifolia and Y. b. jaegeriana) have reciprocally obligate relationships with their pollinators, two sister species of yucca moth (Tegeticula synthetica and T. antithetica). The plantpollinator pairs exhibit trait matching suggestive of coevolution in key characters involved in the interaction – style length in the trees, ovipositor length in the moths. Although speciation in the moths is complete, the trees hybridize in a narrow contact zone in southern Nevada. We hypothesize that coevolution is contributing to reproductive isolation in the trees. If this is true, we expect to find disruptive selection on style length in and around the hybrid zone. To test this prediction, we used RAD sequencing to identify 9516 SNPs in the Joshua tree genome. We then performed a GWAS to find loci associated with variation in style length. Fst-outlier analysis identified 1797 loci putatively under selection. Finally, we performed Bayesian Genomic Clines analyses to better characterize the nature of the selection acting within the hybrid zone. Together, these analyses suggest that loci associated with style length are overrepresented among those under differential selection in the two subspecies, and are also likely to experience disruptive selection in the hybrid zone. These results support the Joshua tree-pollinator interaction as a source of reproductive isolation, potentially contributing to speciation in this group.