Consequences of chromosome structure polymorphism in sexual populations
Year: 2014
Type: dissertation
Abstract: It has been postulated that speciation can occur without geographical isolation. Chromosome rearrangements, such as inversions and translocations, have been proposed as drivers for speciation without isolation (sympatry). In sexual populations, meiosis between structurally different genomes would have high lethality. Recombination between non-linear genomes may produce progeny with imbalanced DNA content. Therefore, only meiotic products which maintain the breakpoints are viable. This causes linkage between the breakpoints, and with alleles of nearby genes. The latest models propose that rearrangements can entrap mutations which accumulate over time near the breakpoints. If recombination leads to epistasis between mutations in linkage with the breakpoints and others which are not, then selection for suppression of recombination can appear. Suppression of recombination will cause further divergence between karyotypes, and ultimately speciation. An experimental evolution setup was designed to test the chromosome speciation model, using Schizosaccharomyces pombe. Cells containing an inversion in one chromosome were mixed with wild type (WT) cells to create sympatric circumstances. Long periods of mitotic adaptive evolution were intercalated with controlled cycles of meiosis, for a total of 500 generations. We observed that the evolution of sterility was a recurrent phenotype under direct selection caused by sympatric sexual reproduction, appearing in half of the populations. In some populations sterility was fixed. In the remaining, it was maintained as a polymorphism with mating-proficient cells. In this is case, polymorphism for structure and mating type was present, unlike in the former. In one of the polymorphic populations, we also observed an increase in the frequency of healthy asci (a proxy for meiotic success) produced by heterozygous crosses. Interestingly, this was uncoupled from spore viability, which suffered no changes relatively to the ancestral crosses. Additionally, we did not observe any change in recombination rates. Our results suggest that the most common solution to meiotic incompatibility in microbial sexual populations in sympatry is sterility. This phenomenon abolishes meiotic fitness depression by completely eliminating sex, leaving karyotypes’ fates under exclusive control of adaptive forces. We thus found a new mechanism that may lead to genetic divergence between different karyotypes in facultative sexual populations.
Language: English
Subfield: Genetics
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2284509308