Ryan Bracewell led the discussion about Dixon et al (2018) study on the origin of new sex chromosome by introgression. We tend to think sex chromosomes are less likely to introgress than the rest of the genome due to the ‘large-X effect’ (Charlesworth et al. 1987). However in this case, it is the sex determination region introgressed between species that started a new sex chromosome. This paper was an impressive synthesis of population genetic and molecular analyses to show that the sex chromosome of Pungitius pungitius (pun) arose from introgression of an 15Mb inverted region in P. sinensis (sin). Even more interestingly, this introgressed region served as sex determination region (SDR) as soon as (if not before) it landed onto pun, turning the Z-W into an X-Y sex determination system in pun. Such new sex chromosome is currently under positive selection, in the early stage of degeneration.
How did Dixon et al figure out that the introgression served as a sex chromosome as soon as (if not before) the arrival in pun? The demonstration of relative timing of the events was clever. The authors rationalized that had it been an autosome and became the sex chromosome after the introgression, both male and female pun SDR would cluster with sin. However, they found only the male pun SDR clustered with sin (see figure below), suggesting that this region was a sex chromosome as soon as (if not before) the introgression. This introgressed SDR has to be dominant over the pre-existing Z-W sex determination system to maintain such clustering patterns. Otherwise, not only the male SDR would cluster with sin, the female copy should as well.
The discussion about the evolutionary driver of such introgression was fascinating. The introgression could have been direct sexual selection favoring “sexy” genes in this SDR block, selection favoring introgressed sex chromosome at the degradation of pre-existing sex chromosomes, or meiotic drive for driving allele to escape from unlinked coevolved repressors in sin. Unfortunately, we could not parse out the possibilities.
Another fun speculation following this observation is that the rapid evolution of sex chromosomes resulted from the previously porous species boundary (between pun and sin) might in turn lead to strengthening of species boundary, contributing to the current cessation of gene flow. Once the SDR introgressed into chromosome 12 in pun, and turn chromosome 12 into a new Y chromosome in pun, could this prevent pun from interbreeding with sin (which has a ZW system with chromosome 12 being an autosome)? The new sex chromosome started from introgression might later "destroy the bridge" that it arrived from.
Reference
Charlesworth B, Coyne JA, Barton NH. 1987. The relative rates of evolu- tion of sex chromosomes and autosomes. Am Nat. 130:113–146.
Dixon, G., Kistano, J., Kirkpatrick, M. 2018. The origin of a new sex chromosome by introgression between two stickleback fishes. Mol. Biol. Evol. 36: 28-38.