Diverse mating phenotypes impact the spread of wtf meiotic drivers in Schizosaccharomyces pombe

J. F. L. Hernandez, R. M. Helston, J. J. Lange, R. B. Billmyre, S. H. Schaffner, M. T. Eickbush, S. McCroskey and S. E. Zanders,  Elife,  10:30. 2021.

Meiotic drivers are genetic elements that break Mendel’s law of segregation to be transmitted into more than half of the offspring produced by a heterozygote. The success of a driver relies on outcrossing (mating between individuals from distinct lineages) because drivers gain their advantage in heterozygotes. It is, therefore, curious that Schizosaccharomyces pombe, a species reported to rarely outcross, harbors many meiotic drivers. To address this paradox, we measured mating phenotypes in S. pombe natural isolates. We found that the propensity for cells from distinct clonal lineages to mate varies between natural isolates and can be affected both by cell density and by the available sexual partners. Additionally, we found that the observed levels of preferential mating between cells from the same clonal lineage can slow, but not prevent, the spread of a wtf meiotic driver in the absence of additional fitness costs linked to the driver. These analyses reveal parameters critical to understanding the evolution of S. pombe and help explain the success of meiotic drivers in this species. eLife digest The fission yeast, Schizosaccharomyces pombe, is a haploid organism, meaning it has a single copy of each of its genes. S. pombe cells generally carry one copy of each chromosome and can reproduce clonally by duplicating these chromosomes and then dividing into two cells. However, when the yeast are starving, they can reproduce sexually. This involves two cells mating by fusing together to create a ‘diploid zygote’, which contains two copies of each gene. The zygote then undergoes ‘meiosis’, a special type of cell division in which the zygote first duplicates its genome and then divides twice. This results in four haploid spores which are analogous to sperm and eggs in humans that each contain one copy of the genome. The spores will grow and divide normally when conditions improve. The genes carried by each of the haploid spores depend on the cells that formed the zygote. If the two ‘parent’ yeast had the same version or ‘allele’ of a gene, all four spores will have it in their genome. However, if the two parents have different alleles, only 50% of the offspring will carry each version. Although this is usually the case, there are certain alleles, called meiotic drivers, that are transmitted to all offspring even in situations where it is only carried by one parent. Meiotic drivers can be found in many organisms, including mammals, but their behavior is easiest to study in yeast. Meiotic drivers known as killers achieve this by disposing of any ‘sister’ spores that do not inherit the same allele of this gene. This ‘killing’ can only happen when only one of the ‘parents’ carries the driver. This scenario is thought to rarely occur in species that inbreed, as inbreeding leads to both gene copies being the same. However, this does not appear to be the case for S. pombe, which contain a whole family of killer meiotic drivers, the wtf genes, despite also being reported to mainly inbreed. To investigate this contradiction, Lopez Hernandez et al. isolated several genetically distinct populations of S.pombe. These isolates were grown together to determine how often the each one would outcross (mate with an individual from a different population) or inbreed. The results found that levels of inbreeding varied between isolates. Next, Lopez Hernandez et al. used mathematical modelling and experimental evolution analyses to study how wtf drivers spread amongst these populations. This revealed that wtf genes spread faster in populations with more outcrossing. In some instances, the wtf driver was linked to a gene that could harm the population. In these cases, Lopez Hernandez et al. found than inbreeding could purge these drivers and stop them from spreading the dangerous alleles through the population. Lopez Hernandez et al. establish a simple experimental system to model driver evolution and experimentally demonstrate how key parameters, such as outcrossing rates, affect the spread of these genes. Understanding how meiotic drivers spread is important, as these systems could potentially be used to modify populations important to humans, such as crops or disease vectors.

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