Ecological drivers of coexistence and lethal variant evolution in the <i>t</i> haplotype

One of the most extensively studied segregation distorters is the t haplotype in house mice (Mus musculus), which can have transmission rates as high as 99% in heterozygous males. Numerous t-haplotype variants exist that induce male sterility or cause embryonic lethality when homozygous. Lethal t variants are thought to have evolved through the accumulation of lethal mutations on a sterile background, but theoretical work indicates that this process is plausible only under substantial inbreeding and reproductive compensation. Here, we use a spatially explicit and stochastic individual-based model to investigate how sperm competition and polyandry, pre-copulatory mate choice, and male-specific fitness costs influence coexistence of sterile and lethal t-haplotype variants. Polyandry, pre-copulatory mate choice, male-specific fitness costsand immigration reduced t-haplotype frequencies, but there was substantial variation in coexistence patterns. Coexistence between two sterile variants was rare unless their transmission rates were nearly identical, whereas coexistence between two non-complementing lethal variantswas more common even when their transmission rates varied widely. The coexistence of sterile and lethal variants was highly asymmetric; thelethal variant was able to persist only when its transmission rate greatly exceeded that of the sterile variant. Notably, pre-copulatory mate choice exhibited by t-carrying females against t-carrying males impacted only the sterile variant negatively and allowed the lethal variant to persist without a transmission ratio advantage. Our results reconcile longstanding discrepancies between theoretical predictions and observations in natural populations and provide new insights into the evolution of lethal variants.