Multiple losses of aKRAB from PRDM9 coincide with a teleost-specific intron size distribution

Primary transcripts are largely comprised of intronic sequences that are excised and discarded shortly after synthesis. In vertebrates, the shape of the intron size distribution is largely constant; however, most teleost fish have a diverged log-bimodal 'teleost distribution' (TD) that is seen only in teleosts. How the TD evolved and to what extent this was affected by adaptative or non-adaptive mechanisms is unknown.

Here, we show that the TD has evolved independently at least six times and that its appearance is linked to the loss of the aKRAB domain from PRDM9. We determined intron size distributions and identified PRDM9 orthologues from annotated genomes in addition to scanning 1193 teleost assemblies for the aKRAB domain. We show that a diverged form of PRDM9 ( $\beta$ ) is predominant in teleosts whereas the $\alpha$ version is absent from most species. Only a subset of PRDM9- $\alpha$ proteins contain aKRAB, and hence, it is present only in a small number of teleost lineages. Almost all lineages lacking aKRAB (but no species with) had TDs.

In mammals, PRDM9 defines the sites of meiotic recombination through a mechanism that increases structural variance and depends on aKRAB. The loss of aKRAB is likely to have shifted the locations of both recombination and structural variance hotspots. Our observations suggest that the TD evolved as a side-effect of these changes and link recombination to the evolution of intron size illustrating how genome architectures can evolve in the absence of selection.