Catching the Spore killers : Genomic conflict and genome evolution in Neurospora

Abstract: A genome is shaped by many different forces. Recombination can for instance both create and maintain genetic diversity, but the need to locally reduce recombination rates will also leave specific signatures. Genetic elements can act selfishly and spreading at the expense of the rest of the genome can leave marks of their activity, as can mechanisms that suppresses them, in a phenomenon known as genomic conflict. In this thesis, I have studied the forces driving genome evolution, using modern genome sequencing techniques and with a special focus on a class of selfish genetic elements known as Spore killers found in the fungus Neurospora. First, we show novel findings on large-scale suppression of recombination by non-structural means in the N. tetrasperma genomes. In contrary, in the genomic region harbouring the spore killer elements Sk-2 and Sk-3 of N. intermedia, a dense set of inversions that are interspersed with transposable elements have accumulated. The inversions are unique for each killer type, showing that they have a long separated evolutionary history and likely have established themselves independently. For the Sk-2 haplotype, where we have polymorphism data, we see signs of relaxed selection, which is consistent with the hypothesis that recombination suppression reduces the efficacy of selection in this region. These results show the strong effects the divergent selective forces of genomic conflicts can have on chromosome architecture. Furthermore, we investigate the hypothesis that spore killing can drive reproductive isolation, by comparing the fertility of crosses between N. metzenbergii and either killer or non-killer N. intermedia strains. We show that crosses with spore killer strains have lower fertility, which cannot be explained by the killing itself, but is potentially caused by an incompatibility gene captured in the non-recombining region. Finally, we identified the genetic element responsible for causing spore killing in the Sk-1 spore killer strains found in N. sitophila. Unlike the Sk-2 and Sk-3 elements, Sk-1 is not connected to a large, non-recombining region, but is caused by a single locus, and we also find indications that this locus was introgressed from N. hispaniola.