Genome Size Evolution in Invading Yellow Starthistle (Centaurea solstitialis)

Abstract

Plant species demonstrate extensive genome size variation, but the underlying processes driving this variation and its phenotypic consequences are uncertain, especially in non-model systems. Recognition of genome size variation within species is particularly novel, and sometimes contested, because it has been difficult to ascertain. If present, then genome size variation within species can function as a potential source of genetic variation that could exhibit dramatic changes on very short evolutionary time scales. To understand the ecological influence of genome size variation and its evolution under different environmental and demographic scenarios, my dissertation examines intraspecific genome size variation in the invasive plant, yellow starthistle (Centaurea solstitialis), hereafter YST. YST is native to Eurasia, but was introduced to North America in the mid-19th century, where it has expanded into a severe invasion of the western United States. Its well-documented expansion history has been confirmed by genetic studies. The invading populations in California have rapidly evolved increased growth and reproduction relative to their native source populations, with contemporary range expansions crossing large elevational and environmental gradients. In Appendix A, I document widespread intraspecific genome size variation in YST and its associations with ecologically important traits in an ongoing range expansion in California. In Appendix B, I compare genome size variation across the native and invaded ranges, and find a reduction in genome size associated with the invasion, as well as trait and environment correlations that are absent from the native range. I also describe a methodological concern for flow cytometry, in which estimation date and increasing plant age contribute to underestimation of genome size, with the magnitude of underestimation being more severe in native range samples. In Appendix C, I examine a potential mechanism of genome size variation by de novo prediction and annotation of transposable element (TE) content from a recent genome assembly of YST. To quantify TE variation, I compare genomic libraries from one native and one invasive genotype, using sequencing coverage as a proxy for element number. I find significant differences in abundance between genotypes for 245 TE families, with the majority of families classified as Long Terminal Repeat retrotransposons (LTR-RT). Differences in mean coverage between genotypes across TE families was not significant; however, families with the greatest differences in coverage tended to be more abundant in the native range genotype, and amplification of only a small number of TE families had dramatic genome size effects in other plant systems.