Strong Selection Against Early Generation Hybrids in Joshua Tree Hybrid Zone Not Explained by Pollinators Alone

150 Years of Coevolution Research: Evolution and Ecology of Yucca Moths (Prodoxidae) and Their Hosts

Yucca moths (Tegeticula and Parategeticula) are specialized pollinators of yucca plants, possessing unique, tentacle-like mouthparts used to actively collect pollen and deposit it onto the flowers of their hosts. The moths' larvae feed on the developing seeds and fruit tissue. First described in 1873, the yucca-yucca moth pollination system is now considered the archetypical example of a coevolved intimate mutualism. Research conducted over the past three decades has transformed our understanding of yucca moth diversity and host plant interactions. We summarize the current understanding of the diversity, ecology, and evolution of this group, review evidence for coevolution of the insects and their hosts, and describe how the nature of the interaction varies across evolutionary time and ecological contexts. Finally, we identify unresolved questions and areas for future research.

Genome-scale data resolves the timing of divergence in Joshua trees

PREMISE

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, they 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.

METHODS

We present an analysis of whole chloroplast genome sequence data and RADseq data from >5000 loci in the nuclear genome. We developed a molecular clock for the Asparagales and the Agavoideae using multiple fossil calibration points. Using Bayesian inference, we produced new estimates for the age of the genus Yucca and for Joshua trees. We used calculated summary statistics describing genetic variation and used coalescent-based methods to estimate population genetic parameters.

RESULTS

We find that the Joshua trees are moderately genetically differentiated, but that they diverged quite recently (~100-200 kya), and much more recently than their pollinators.

CONCLUSIONS

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.