Date of Degree
MS (Master of Science)
Although genomes are perhaps the single most important element of living systems, why they feature such striking variation and how this variation is maintained within and across natural populations remains unclear. One of the most common and important means by which genomic variation is generated is ploidy elevation. While polyploidy has been implicated in the remarkably successful radiations of angiosperms, teleost fish, and amphibians, the phenotypic consequences of changes in ploidy level are poorly understood, especially in animals. I use a large, multi-year common garden experiment to identify potential life history costs and benefits of polyploidy and asexual reproduction, a trait often associated with polyploidy, in Potamopyrgus antipodarum. This snail is well suited for studying ploidy variation and sex because diploid sexuals and triploid and tetraploid asexuals frequently coexist, allowing us to use comparisons of sexuals to asexuals and triploid to tetraploid asexuals to study both the effects of ploidy elevation and sex. I detected a strong negative correlation between growth rate and time to maturity and found that sexual P. antipodarum grew and matured significantly more slowly than the polyploid asexuals. Sexual P. antipodarum were also more likely to die before achieving reproductive maturity than their asexual counterparts. By contrast, there were no apparent life history differences between triploid and tetraploid asexuals, indicating that direct phenotypic benefits of ploidy elevation are unlikely to explain the relatively rapid growth and maturation of asexuals. My results suggest that ploidy elevation does not inevitably confer phenotypic consequences, that reproductive mode influences life history trait expression, and that sexual P. antipodarum persist in many natural populations in spite of substantial life history disadvantages.
Why nuclear DNA content varies so much within and among species remains one of the major unanswered questions in biology. One of the most common means by which DNA content can change is ploidy elevation, the addition of one complete set of chromosomes relative to the normal number of chromosome sets in a species. Even though ploidy elevation is common, we still know very little about how extra chromosome sets affect organismal biology. Here, I studied the effects of ploidy elevation in a snail system, Potamopyrgus antipodarum. These snails are ideally suited for studying the effects of ploidy elevation because snails with 2, 3, or 4 sets of chromosomes (“2x”, “3x”, “4x”) coexist in nature, allowing me to use direct comparisons between otherwise similar snails that differ in ploidy to identify effects of ploidy elevation. I focused on three traits, growth rate, age at reproduction and adult body size, that are all main determinants of reproductive fitness in P. antipodarum. I found that 2x snails grow more slowly and reach reproductive maturity later than 3x and 4x snails and that 2x snails were more likely to die before reproducing. I did not detect any differences in any of these traits between 3x and 4x snails. One explanation for the lack of differences between 3x and 4x snails is that the differences between the 2x and 3x/4x snails were driven by reproductive mode (2x snails reproduce sexually while 3x and 4x snails reproduce asexually) rather than ploidy elevation.
publicabstract, asexual reproduction, growth rate, ploidy level, Potamopyrgus antipodarum, reproductive maturity, sexual reproduction
Copyright 2015 Katelyn Larkin