Date of Award
Open Access Dissertation
Thomas J Hilbish
Rising ocean temperatures are a severe and ever-present threat to marine life. With environmental temperature having such a large impact on organismal performance, understanding the mechanisms which contribute to the ability to survive at higher temperatures is a crucial research focus. Significant progress has been made in discovering these mechanisms on the cellular, biochemical, and physiological levels, but is it much less common for those to be examined together. Using the Mytilus edulisspecies complex as a model system, this dissertation takes a closer look at how prolonged exposure to sub-lethal high temperatures impacts marine organisms on multiple levels of biological organization. The M. edulis species complex is ideal for this work given the district thermal niches of the congeners, existence of persistent natural hybrid zones, and a reference genome.
Using a combination of molecular, physiological, and bioinformatic approaches, we were able to assess the population structure of the M. edulis x M. galloprovincialis hybrid zone in southwest England, compare the responses to chronic warming between the two parent species and their hybrids, and link gene expression patterns to physiological responses. We found that individuals collected from parent populations were mostly genetically pure, and hybrids showed a gradient of ancestry from mostly M. galloprovincialis-like to mostly M. edulis-like, with a skew toward M. edulis ancestry. Physiologically, our results support previous work that suggests feeding rate limits the cool water M. edulis at high temperatures. Our work is the first to study this response in M. edulis x M. galloprovincialis hybrids, and we show that unexpectedly, they perform better energetically at high temperatures, and show a more M. galloprovincialis-like response. This translates well to the transcriptomic analysis which showed that in feeding rate (the limiting physiological factor in M. edulis) hybrids were more like the warm tolerant M. galloprovincialis than their cool water parent. Additionally, despite their close phylogenetic relationship, the three genetic groups deployed largely unique transcriptomic responses to chronic warming. This, coupled with the limited number of genes which exhibited a genotype-by-environment interaction, indicates there might be limited genetic variation for plasticity related to thermal stress in this species complex. Lastly, we found significant standing genetic variation in the populations but were unable to corelate it to either of the physiological phenotypes measured. With little genetic variation in plasticity and in traits shown to be limiting at high temperatures, the M. edulis species complex may be especially vulnerable to climate change. However, we did find that hybrids tended to exhibit greater plasticity in gene expression and were better able to cope physiologically with the high temperature treatment. Therefore, hybridization may be able to generate new combinations of genes that are more fit in the novel environments created by climate change and may offer a route to species persistence.
Schwartz, L. C.(2022). From Gene Expression to Physiology: A Study of Chronic Thermal Tolerance in the Mytilus Edulis Species Complex. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/6823