Genetic drift and mutational hazard in the evolution of salamander genomic gigantism
Date
2016
Authors
Mohlhenrich, Erik, author
Mueller, Rachel, advisor
Sloan, Dan, committee member
Black, William, committee member
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Abstract
Salamanders have the largest nuclear genome sizes among tetrapods and, with the exception of lungfishes, among vertebrates as a whole. Lynch and Conery (2003) have proposed the mutational hazard hypothesis to explain variation in genome size and complexity. Under this hypothesis, non-coding DNA imposes a selective cost by increasing the target for degenerative mutations, i.e. the mutational hazard. Expansion of non-coding DNA, and thus genome size, is expected to be driven by increased levels of genetic drift and/or decreased mutation rates; the former determines the efficiency with which excess non-coding DNA can be selected against, while the latter determines the level of mutational hazard. Here, we test the hypothesis that salamanders have experienced stronger long-term, persistent genetic drift than frogs, a clade with more typically sized vertebrate genomes. To test this hypothesis, we compared dN/dS and Kr/Kc values between these clades. Our results reject this hypothesis; we find that salamanders have not experienced stronger genetic drift than frogs. Additionally, we find evidence consistent with a lower nucleotide substitution rate in salamanders. This result, along with previous work showing lower rates of small deletions and ectopic recombination in salamanders, suggests that a lower mutational hazard may contribute to genome expansion in this clade. Taken together, these results further underscore the importance of studying large genomes and indicate that salamanders provide an important model system for the study of how non-drift processes (i.e. mutation, natural selection) shape the evolution of genome size.
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Subject
genetic drift
mutation
evolution
salamanders
genome