Earlier today, Master’s student Mikhail Plaza successfully defended his thesis research, in which he built a linkage map for Joshua tree and used it to reexamine data identifying genetic loci that may play a role in local adaptation to climate and to specialized pollinating yucca moths. Mikhail’s project is among the first fruits of the Joshua Tree Genome Project, taking advantage of a new, exceptionally complete draft genome assembly for Yucca jaegeriana, the eastern Joshua tree.
Over the last few years it’s been widely recognized that eastern and western Joshua trees are genetically differentiated, possibly to the point of being separate species, though they continue to hybridize in a narrow zone where they co-occur in central Nevada. The western and eastern tree types are associated with different species of specialized pollinating yucca moths, and differ in floral traits that impact the interaction with those moths — but also in vegetative traits like leaf length, trunk height, and branching architecture, which should not impact pollination but may play a role in adaptation to climate differences between their different ranges. Recent work by Chris Smith’s lab has found single-nucleotide polymorphism (SNP) markers that show signs of divergent natural selection between eastern and western Joshua trees, and determined that these SNPs may also play roles in the development of floral and vegetative traits that differ between the two tree types. However, these SNPs were “anonymous markers” — their placement within the Joshua tree genome was unknown. That meant that it was impossible to know whether they represented many separate genes experiencing divergent selection, or closely linked regions in which selection on one gene might lead to differentiation in nearby regions, a process known as “divergence hitchhiking”.
Mikhail used genome-wide SNP markers collected for a series of Joshua tree seedlings with known parentage — half-sibling families from six “mother” trees — to construct what is known as a linkage map, then re-analyzed the Smith Lab’s data within the framework of that map to find cases in which regions showing signs of divergent selection coincide with loci associated with floral or vegetative traits. Differentially selected regions containing both floral and vegetative loci would be consistent with divergence hitchhiking — and Mikhail’s preliminary results suggest that this has indeed occurred in the evolutionary divergence of eastern and western Joshua tree. Look for a publication reporting his results in the near future, as Mikhail wraps up his work in the lab and gets started in the doctoral program in Plant Biology and Conservation at Northwestern University.