Doctor of Philosophy (PhD)
Streamertails hummingbirds (Trochilus polytmus and T. scitulus) are recently diverged sister taxa that appear to have speciated in situ on the island of Jamaica. They are distinguished by male bill color, a secondary sexual trait that is coral red in T. polytmus and jet black in T. scitulus. They hybridize in a narrow zone where their ranges meet in eastern Jamaica. In Chapter 2, I performed a formal population survey of T. scitulus to determine the size of the population, which was unknown. I determined that the total population contains well over 100,000 individuals despite its limited geographic range. In Chapter 3, I build on previous studies to identify divergent morphological and genetic traits. Additionally, I use geographic cline models to determine the center and widths of individual clines and make inferences about the relative strength of selection acting on each trait. The clines for male bill color (2.2 km) and bill width (13.9 km) were narrow relative to neutral expectations and centered on the Rio Grande Valley. Female values were slightly wider. Consistent with expectations for recently diverged species, I detected little to no neutral genomic differentiation using six microsatellites, but a Radseq dataset containing 6,451 single nucleotide polymorphisms (SNPs) generated by a genotyping-by-sequencing protocol showed modest genetic differentiation between species. A narrow subset of SNPs (n = 23) with high loadings in a discriminant function analysis may be physically or epistatically linked to the divergent morphological traits. A structure analysis based on these discriminant SNPs shows a range of admixture assignments in the hybrid zones and strong differences in assignment between the parental species. A geographic cline analysis based on admixture assignments revealed a narrow cline (6.6 km) also centered on the Rio Grande. Finally, I performed a transcriptomics study to examine baseline differences in gene expression. While gene expression profiles were extremely similar, four genes showed significant gene expression differences. One gene, BLOC-1S1, is a ubiquitously expressed gene that is associated with pigmentation disorders in mice and humans. Its role in avian pigmentation is not well characterized, but is one gene that warrants further investigation as a candidate gene underpinning male bill color. I also looked at sequence divergence across the assembled transcripts. This panel of SNPs showed low divergence and little evidence for positive selection, indicating that divergent genes may be few and potentially located in regulatory regions not captured here.
Judy, Caroline D., "Speciation and hybridization in Jamaican-endemic Streamertail Hummingbirds (Trochilus polytmus and T. scitulus)" (2018). LSU Doctoral Dissertations. 4760.
Robb T. Brumfield