Genome-wide quantification of contributions to sexual fitness identifies genes required for spore viability and health in fission yeast

Abstract

Numerous genes required for sexual reproduction remain to be identified even in simple model species like Schizosaccharomyces pombe. To address this, we developed an assay in S. pombe that couples transposon mutagenesis with high-throughput sequencing (TN-seq) to quantitatively measure the fitness contribution of nonessential genes across the genome to sexual reproduction. This approach identified 532 genes that contribute to sex, including more than 200 that were not previously annotated to be involved in the process, of which more than 150 have orthologs in vertebrates. Among our verified hits was an uncharacterized gene, ifs1 ( important for sex), that is required for spore viability. In two other hits, plb1 and alg9, we observed a novel mutant phenotype of poor spore health wherein viable spores are produced, but the spores exhibit low fitness and are rapidly outcompeted by wild type. Finally, we fortuitously discovered that a gene previously thought to be essential, sdg1 ( social distancing gene), is instead required for growth at low cell densities and can be rescued by conditioned medium. Our assay will be valuable in further studies of sexual reproduction in S. pombe and identifies multiple candidate genes that could contribute to sexual reproduction in other eukaryotes, including humans.

Author summary

Sex is absolutely required for many organisms, including humans, to reproduce. However, we still lack a complete understanding of the genetic contributions to sexual reproduction, even in many model organisms. Here we use a high-throughput insertional mutagenesis approach to measure the consequences during sexual reproduction of gene disruption across the genome in fission yeast. As a result, we identified hundreds of novel genes as contributing to sexual reproduction. While we identified a mutant with the expected loss of spore viability phenotype, we also demonstrated that disruption of some of these genes resulted in viable but low-quality spores (analogous to gametes like sperm in humans). In addition, we also identified a gene that was not required for sex itself but was instead required for cells to grow at low density. We propose that this mutant is a representative of a new class of genes that we refer to as “social distancing genes” because they are unable to grow without the presence of neighbors. In sum, this work presents genome-wide measurement of the genetic contributions to sex in fission yeast.