A great variety of genetic systems exist in filamentous ascomycetes. The transmission of genetic material does not only occur by (sexual or asexual) reproduction, but it can also follow vegetative fusion of different strains. In this thesis the evolution of this variability is studied, using theoretical population genetic models.
First the evolution of different reproductive systems is studied. It is found that homothallism (allowing selfing) most probably evolved from heterothallism. (with two mating types), and that a polymorphism of homo- and heterothallism can be evolutionary stable. A variable fitness of ascospore production is predicted as an explanation for hermaphroditism in heterothallic species and the formation of both asexual and sexual spores by homothallic species.
Secondly the evolution of vegetative incompatibility (VI) is studied. VI prevents vegetative fusion of different strains, and is very common between different natural isolates. In many species a large number of Vegetative Compatibility Groups (VCGs) is found, that only show vegetative fusion within and not between groups. After a comparison of different selective regimes, it is concluded that a harmful cytoplasmic element offers the most plausible selective explanation for the evolution of VI. However, the effects of genetic drift appear to be important in generating large numbers of VCGs and can override the effects of selection.
Next, attention is focused on spore killing. This is a form of segregation distortion (or meiotic drive), causing the death of half the number of spores in an ascus. In a model it is found that the evolution of spore killing can only be explained if 'Spore killers' have some additional advantage during the process of killing.
Finally, a model is presented for the evolution of sexual incompatibility (SI) in Podospora anserina. As the existence of SI cannot be explained on its own, a hypothesis is studied, that explains SI as an anti meiotic drive device. Although the model shows that this hypothesis could be true, experimental evidence is needed to confirm this.
|Qualification||Doctor of Philosophy|
|Award date||12 Jan 1994|
|Place of Publication||S.l.|
|Publication status||Published - 1994|