RAD-SEQ DATA: GENOME-SCALE DATA RESOLVES THE TIMING OF DIVERGENCE IN JOSHUA TREES.

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 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. We analyzed whole chloroplast genome sequence data and RADseq data from more than 5000 loci in the nuclear genome. We developed a molecular clock for the Asparagales and for the Agavoideae using multiple fossil calibration points. Using Bayesian inference we produced new estimates for the age of Yucca and for Joshua trees. We used a variety of methods to calculate summary statistics describing genetic variation and coalescent-based methods to estimate population genetic parameters. We found that the Joshua trees are moderately genetically differentiated, but that they diverged quite recently (100-200 KYA), and much more recently than their pollinators. 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.