Genomic and physiological adaptation to temperature in the invasive golden star tunicate (Botryllus schlosseri)

Zachary John Corey Tobias, Ph.D., 2024
Carolyn Tepolt, Advisor

Because non-indigenous species (NIS) often encounter novel environments during colonization and expansion, species invasions present useful opportunities to investigate the mode and pace of adaptive change in natural populations. In this dissertation, I use the range expansion of the invasive golden star tunicate, Botryllus schlosseri, as a natural experiment to study how a pernicious NIS adapts its thermal physiology on contemporary time scales. In Chapter 2, I applied low-coverage whole genome sequencing (lcWGS) to investigate patterns of population genetic structure and signatures of local adaptation to temperature. In addition to illustrating the potential for rapid adaptation of thermal tolerance at the genomic level, this chapter demonstrated that the molecular basis of thermal adaptation on either coast is distinct, providing valuable evidence for parallel adaptation being driven by divergent molecular means. In Chapter 3, I performed a physiological study to investigate differentiation of post-larval heat tolerance across five populations across a major biogeographic break on the east coast of North America. I found that northern populations are more susceptible to heat stress than their southern, warm-exposed counterparts, providing evidence for adaptive shifts of thermal tolerance. Further, by taking advantage of natural temporal variability in temperature, I demonstrated that temperature during development positively affects heat tolerance at later life stages, establishing developmental plasticity of thermal tolerance. In Chapter 4, I extended my physiological investigation to the west coast of North America, comparing post-larval heat tolerance across three populations spanning a 24.3° latitudinal gradient while expanding to include differentiation of cold tolerance in adults. Similar to the east coast, I observed that the two northern populations were more susceptible to heat stress than their southern counterpart. For cold tolerance, I observed a pattern of countergradient variation whereby northern populations were better able to maintain cardiac function in the cold than southern populations. This suggests compensatory genetic adaptation to the colder water temperatures at higher latitudes. Overall, my work furthers our understanding of how NIS are able to rapidly shift their thermal physiology in response to novel environments, shedding light on the potential of species more generally to adapt to environmental change on contemporary timescales.