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

Area of Science:

• Ecology
• Evolutionary Biology
• Genomics

Background:

• Joshua trees and yucca moths are a model for plant-pollinator coevolution and cospeciation.
• Previous studies suggested Joshua trees diverged significantly before their pollinators, but lacked robust genetic data.

Purpose of the Study:

• To re-evaluate divergence times between Joshua trees and their yucca moth pollinators.
• To investigate the role of coevolution in the speciation of this system.

Main Methods:

• Analysis of whole chloroplast genome and nuclear genome RADseq data (>5000 loci).
• Development of a molecular clock for Asparagales and Agavoideae using fossil calibrations.
• Bayesian inference for estimating divergence times and coalescent-based methods for population genetics.

Main Results:

• Joshua trees are moderately genetically differentiated and diverged recently (~100-200 kya).
• Joshua tree divergence is significantly more recent than that of their yucca moth pollinators.

Conclusions:

• Coevolution likely reinforced reproductive isolation after initial allopatric divergence, rather than directly driving speciation.
• The findings challenge the hypothesis of direct coevolutionary contribution to speciation in the Joshua tree-yucca moth system.