De novo assembly of the selfish t supergene reveals a deleterious evolutionary trajectory

Supergenes are linked clusters of DNA that are transmitted together due to rare or absent recombination. They undergo co-adaptation, allowing evolution to work on several genes to refine complex phenotypes, giving supergenes a competitive edge. Yet, due to their lack of recombination, supergenes are susceptible to deterioration as they cannot efficiently purge deleterious DNA. Few examples outside of sex chromosomes have been available for study. Here, we present the first assembly of the t haplotype, a 33.4 Mb supergene in house mice that 'selfishly' transmits itself at non-Mendelian frequencies. We characterize the four large non-overlapping inversions that make up the t haplotype. We compare in a t/t individual two different t variants with different recessive lethal phenotypes (age at death). Despite that difference, they differ much less from each other than the rest of the chromosome. However, the differences that they have were much more likely to be deleterious than the differences between the two variants of the rest of the chromosome. We interpret this marked difference as evidence of the accumulation of deleterious variants, a hallmark of deterioration. The t region of chromosome 17 is more distant to the reference than the rest of the chromosome, and has a higher fraction of impactful differences here as well. Thus, we conclude that the t appears as a quickly spreading and deteriorating selfish supergene, a rare example of Muller's ratchet outside of sex chromosomes. Furthermore, we aim for our assembly to provide a resource for comparative work on the t haplotype, such as its evolutionary history.