The maintenance of sexually reproducing organisms is one of the long-standing puzzles in evolutionary biology. Why would a sexual population not be replaced by an all-female producing, asexual lineage? One potential explanation for the persistence of sex is provided by the Red Queen Hypothesis, which relies on coevolving hosts and parasites. Parasites may evolve to be able to infect locally common genotypes and thus decrease their frequency. This would give advantage to rare genotypes, which can be generated through sexual reproduction.
Sexual reproduction may generate genetic diversity and increase the chance of producing a rare genotype by at least two different processes: recombination and chromosomal segregation. It is known that these processes generate genetic diversity, and thus are thought to be an advantage to sex. However, within sexual populations, multiple reproductive behaviors may lead to even greater genetic diversity within broods. For example, multiple paternity occurs when multiple males sire offspring within a single female, which may result in increased genetic diversity within broods. Thus, multiple paternity is predicted to occur where selection for rare genotypes exists.
In my dissertation, I investigated female reproductive behaviors, as well as male genital structures and spermatogenesis in Potamopyrgus antipodarum, a freshwater snail from New Zealand. In my first chapter, I document the sequence of behavioral events leading to mating. I used a series of behavioral experiments to investigate whether assortative mating for sympatric males occurs in female P. antipodarum. I found that females have the ability to reject male mating attempts, and that females reject allopatric males and mate with sympatric males significantly more. In my second chapter, I investigated whether multiple males sire P. antipodarum broods. I used field collected female P. antipodarum from two lake populations with a high frequency of infection by coevolving parasites. I found multiple paternity occurred in females from both lakes.
In my final chapter, I investigated the reproductive capacity of recently discovered triploid males. I first determined whether triploid male P. antipodarum had testes and produced sperm. After documenting the presence of sperm in triploid males, I then investigated the amount of DNA carried in sperm produced by triploid males and mostly found aneuploid sperm. I used electron microscopy to investigate how chromosomes synapse during the meiotic process and found aneuploid sperm are likely due to abnormal chromosomal pairing during prophase I of meiosis. These results indicate that triploid males have the capability of producing sperm, although most sperm appear to be aneuploid. Aneuploid sperm is generally considered not viable; however, in some plants and a few animal species fertile aneuploid sperm are known to produce viable offspring. Thus, dismissing the reproductive capability of triploid male P. antipodarum may be premature.
Overall, my dissertation focuses on different aspects of the reproductive biology of the freshwater snail from New Zealand, Potamopyrgus antipodarum. I found that sexual females exhibit mate choice and that multiple males can fertilize them. These two reproductive behaviors may interact to yield a selective advantage to sexual females. In populations that are coevolving with virulent parasites, females that exhibit polyandry and mate choice may increase the chance of producing offspring with rare genotypes, which may confer an advantage. In addition, I investigated the reproductive potential of triploid males. Triploid males were found to have reproductive tissues that undergo meiosis, but due to the extra chromosomes in triploid males, chromosomal pairing is aberrant. This means it is possible that triploid males are contributing to the creation of new polyploid linages, although it is likely to be a rare event.
|Advisor:||Lively, Curtis M., Delph, Lynda F.|
|Commitee:||Janssen, Erick, Smith, G. Troy|
|School Location:||United States -- Indiana|
|Source:||DAI-B 74/02(E), Dissertation Abstracts International|
|Subjects:||Cellular biology, Microbiology, Evolution and Development, Behavioral Sciences|
|Keywords:||Evolution of sex, Mating behavior, Potamopyrgus antipodarum, Reproductive biology, Spermatogenesis|
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