Hadjivasiliou, Z; (2014) Theoretical studies on the role and evolution of mating types and two sexes. Doctoral thesis , UCL (University College London).

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Abstract

Why there are two distinct sexes has received little attention compared with that lavished on the value of sexual reproduction. While sex requires two parents, there is no obvious need for these to be of different sexes. Furthermore, self-incompatible gametes seemingly reduce the likelihood of finding a partner. What causes mating types and sexes to predominate in nature remains a conundrum. The uniparental inheritance (UPI) of mitochondria (in which only one sex, usually the female, passes on its mitochondria) is widespread among sexual organisms. Theoretical work suggests that the evolution of two sexes can be understood in the light of mitochondrial inheritance. However, the exact role of UPI is not clearly understood. Part I of this thesis considers the evolution of self-incompatible mating types in relation to this perspective, using probability theory and population genetics. Chapter 2 studies the impact of UPI on interactions between genes in the mitochondria and the nucleus, in an effort to elucidate the role of UPI itself. In Chapter 3, I develop a new, explicit theoretical model that challenges the prominent view that selection for UPI leads to the establishment of self-incompatible mating types and sexes. An alternative hypothesis proposes that mating types evolved as a consequence of selection for asym- metry in gamete attraction and recognition. This idea is based on the assumption that an asymmetry in gamete communication leads to more effective attraction and recognition. In Part II of this thesis, I examine this idea further. In Chapter 4, I perform an extensive literature review of mating type interactions and provide empirical support for the prediction that an asymmetry in signalling is indeed common in nature. The underlying assumptions of this hypothesis are linked to the physical constraints that gametes experi- ence during sex, and the role of polarity in cell-cell interactions. To assess the impact of these constraints rigorously, in Chapter 5 I develop a biophysical model for signaller-detector dynamics based on chemical diffusion, chemotaxis and individual cell movement that can be tested in silico and in vitro. This thesis examines the role and origins of self-incompatible mating types and sexes. The novel theo- retical methods and perspective on the empirical literature presented here place this evolutionary question in a fresh context and encourage further theoretical and empirical work.

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