School of BioSciences - Theses

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    The evolutionary significance of parthenogenesis and sexual reproduction in the Australian spiny leaf insect, Extatosoma tiaratum
    Alavi, Yasaman ( 2016)
    The costs and benefits of sexual reproduction has long been a subject of debate in biology. The paradox arises from the fact that theoretically, sex is associated with many costs, yet it is the most prevalent mode of reproduction in the tree of life. Facultative parthenogenetic systems, in which females can reproduce both sexually, and in the absence of sperm, parthenogenetically, provide suitable systems to compare costs and benefits of reproductive modes, while minimizing confounding effect that are not directly related to reproductive modes. In this thesis, I used the Australian Phasmatid, Extatosoma tiaratum, to investigate the evolutionary significance of facultative parthenogenesis, and compare fitness consequences of sex and parthenogenesis. The evolutionary significance of facultative parthenogenesis is unknown but male or sperm limitations are potential factors. I investigate male mating frequency and variation in ejaculate size and quality in E. tiaratum. I show that most, but not all, males are able to mate multiply, but ejaculate size decreases with increased number of matings. In addition, ejaculate size increased with increasing time interval between matings, suggesting that E. tiaratum males require time to replenish ejaculate reserves. These findings suggest male sperm limitation may be an important factor influencing the evolution of parthenogenesis in this system. The cytological mechanism of parthenogenesis determines the genetic diversity and heterozygosity levels of the offspring and is thus an important component of the comparison between reproductive modes. Using microsatellite markers that I developed for E. tiaratum, I investigated the cytological mechanism of parthenogenesis in this species. I demonstrated that the most likely mechanism of parthenogenesis in E. tiaratum is automixis with terminal fusion, resulting in substantial loss of heterozygosity in the first generation parthenogenetic offspring. Based on these results I predicted parthenogenesis to be associated with fitness reduction in E. tiaratum. I then investigated the fitness consequences of sex and parthenogenesis in E. tiaratum, in terms of offspring immunity and reproductive success. I show that parthenogenesis is associated with fitness reduction both in terms of immune function and reproductive success. Females derived from sexually conceived mothers have higher immune response compared to females derived from parthenogenetically conceived mothers. A female’s reproductive success is substantially higher if she is conceived sexually and produces sexual offspring. In fact, the cost of parthenogenesis on reproductive success is high enough to eliminate the two-fold cost of sex in E. tiaratum. Nevertheless, the cost of parthenogenesis can be compensated in the next generation, if parthenogenetically females produce daughters sexually. The results of this thesis offer a new perspective in understanding the evolution and maintenance of facultative parthenogenesis in E. tiaratum, and suggest that females enjoy the benefit of parthenogenesis allowing them to reproduce in the absence of sufficient or preferred sperm, while mediating the costs of homozygosity by reproducing sexually.