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Assistant Professor of Biology
Office: 324B BBIowa City, IA 52242
Email: firstname.lastname@example.orgWeb: More About Dr. Neiman - Related Websites and Resources
BA, Biology, Carleton CollegePhD, Evolutionary Biology, Indiana University
Post Doctorate, Evolutionary Biology, UNIV OF MINNESOTAPost Doctorate, Evolutionary Biology, UNIV OF VIRGINIAPost Doctorate, Evolutionary Biology, UNIV OF ST THOMAS
Why sexual reproduction is so common is one of the greatest mysteries in evolutionary biology. All else being equal, asexual females will produce twice as many daughters as sexual females, because sexual females allocate half their resources to producing male offspring. The production of males by sexual females therefore creates a two-fold cost of sexual reproduction that should logically result in the selective elimination of sex. Despite this cost, sex is ubiquitous in nature to the extent that many of the fundamental features of the eukaryotes (e.g. distinct chromosomes, meiosis, the union of gametes to form a unicellular zygote) revolve around sex and recombination.
The predominance of sexual reproduction is only paradoxical if sex does not confer advantages relative to asexual reproduction. This means that the two-fold cost of sex can be mitigated if asexuals experience fitness disadvantages directly linked to asexuality. Accordingly, our research into why sex is so common is focused around manifold ways in which asexuals and sexuals might differ in nature.
Much of our research utilizes Potamopyrgus antipodarum, a freshwater snail native to New Zealand. This snail is of particular interest because populations vary in the frequency of obligately sexual and obligately asexual individuals, which sets the stage for empirical investigation into the benefits of sex. Potamopyrgus antipodarum has been the focus of research into the maintenance of sex for nearly 20 years, and is now the best-characterized natural system available for studying why sexual reproduction is so common.
Mutation accumulation. Theory predicts that asexual lineages are doomed to extinction via the inevitable accumulation of deleterious mutations. This is because genetic recombination is necessary for organisms to produce offspring that have fewer mutations than they do themselves. We are particularly interested in whether parasite pressure can accelerate this process by increasing the strength of genetic drift and thus the rate of mutation accumulation. We are using DNA sequence data to address these questions in P. antipodarum.
Mitochondrial performance. We are using assays of mitochondrial function to determine whether mutation accumulation in asexual P. antipodarum lineages has detectable negative effects. Mitochondrial genomes are an appropriate focus for this study because 1) they are important to organismal fitness, 2) they can easily be isolated and their performance assayed, and 3) the proteins they produce interact with proteins produced by the nuclear genome, so mutations afflicting the nuclear genome can be indirectly assayed along with mutations in mtDNA. We are comparing mitochondrial performance in sexual and asexual lineages as well as in asexual lineages of varying time since derivation from sexual ancestors, since mutational degradation of asexual lineages is expected to increase with time since recombination (and mutational clearance) last occurred.
Disadvantages of polyploidy. Like many mixed sexual/asexual systems, sexual P. antipodarum are diploid, while asexual P. antipodarum are triploid. Differences in ploidy level could be relevant to considering the distribution and maintenance of sex because polyploidy can confer both advantages and disadvantages relative to diploidy. One reason that higher ploidy level could provide a disadvantage to many asexual taxa is that polyploid organisms contain more DNA and RNA than diploid counterparts. Nucleic acids make up a substantial fraction of organismal dry mass, and RNA production in particular can limit organismal growth. Nucleic acids also contain a lot more phosphorus than other biomolecules, and recent research suggests phosphorus content in organisms can mediate key ecological and evolutionary processes. Moreover, phosphorus availability is often a key limiting nutrient, especially in aquatic ecosystems. Thus, the “paradox” of sex could be mitigated by the substantive ecological implications of an additional set of chromosomes in asexual competitors. I am investigating this possibility in P. antipodarum by comparing RNA and phosphorus content in sexual and asexual snails, and studying the dynamics of mixed sexual/asexual populations under nutrient-limited conditions.
Population dynamics of asexuals. We have used laboratory experiments to demonstrate that asexual females have a negative impact on one another's reproduction. This finding is interesting in light of the fact that P. antipodarum is an invasive species in Europe, Australia, and North America. Almost all of these invading populations are comprised entirely of asexual females. We are conducting research into snail behavior to better understand why and how asexual females affect one another's reproduction may provide insight into invasion dynamics and control.
Mutation accumulation and recombination in mitochondrial genomes
Mitochondrial mutations have been implicated in a variety of genetic disorders and age-related syndromes. Mitochondrial genomes are also models for studying the evolution of non-recombining genomes and thus the selective advantages of sex and recombination. A fundamental observation across many eukaryotic taxa is that mitochondrial genomes accumulate deleterious mutations at a much higher rate than nuclear genomes. Recent studies have asserted that the asexuality of mitochondrial genomes is responsible for this pattern. However, population genetic theory shows that this pattern of molecular evolution could also result from high mutation rates and/or small effective population size due to haploid/uniparental inheritance. We are addressing this question by using comparisons of patterns of mutation accumulation in closely-related snail species that vary in mating system to disentangle the contributions of recombination and uniparental inheritance to mutation accumulation in mitochondrial genomes.
I will have space for new graduate students starting in the fall of 2008:
Prospective graduate students with interests in evolutionary biology and especially the evolution and ecology of mating systems and sex who would like to consider joining my lab should email me (email@example.com) to discuss the possibility of applying to the graduate program.
Date Last Modified: 06/06/2016 -
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