A new paper in Science generated a lot of buzz and has an interesting combination of nifty molecular genetics and evolution.
Sex determination is wildly variable in evolution, and that is worthy of its own theme/thread here at PS for people interested in evolution. The new work looked hard at the gene (called Sry) that is most influential in this process, in mice. (Those last two words are more important than usual.) They found a previously unknown exon, that when spliced into the final transcript generates a different version of the protein. This version of the protein is required for sex determination; the other version, which has always been thought to be the only version, contains a degradation signal and is unstable.
But there’s more. Most (70%) of the new exon is a retrotransposon sequence, specifically a LINE element called L3. In fact, all of the coding sequence in the exon is from the L3 element, and the poly-A sequences (this is @Art territory) come from the LTRs of the L3 element. The mostly jargon-free version of that: this newly discovered piece of this interesting and important gene comes completely from a retrovirus-like piece of DNA.
The history of this setup is not emphasized in the paper but their data show that the mechanism (turning the LINE element into a part of the protein) is specific to mice and not even to other rodents, even though all of the rodents have the LINE element. Most (maybe all?) other rodents avoid degradation of the Sry protein by avoiding the degradation signal that is present in the gene. (The other rodent lineages have a stop codon before the degradation sequence, so that sequence is not included in the protein. Without this, there would be no males, admittedly a tempting outcome in 2020.) Mice, instead, use this other version of the protein to overcome the fact that the “main version” is in fact quickly degraded and therefore worthless. This is shown in Figure S9 in the supplement, and they provide this summary diagram of their proposed evolutionary trajectory in Figure S10:
Here is the journal’s summary of the paper. Link to the full paper is below. PDF cheerfully sent on request.
Two rather than one
For several decades, it has been believed that the mammalian sex-determining gene Sry contains a single exon. Miyawaki et al. have now identified a cryptic second exon of mouse Sry . Loss- and gain-of-function analyses revealed that the two-exon SRY (SRY-T), not the canonical single exon–encoded SRY (SRY-S), is the bona fide testis-determining factor. Sry exon2 is composed of retrotransposon-derived sequences. The SRY-S carboxyl terminus contains a degradation sequence (degron), whereas the SRY-T carboxyl terminus encoded in the Sry exon2 is degron free, thereby conferring protein stability on SRY-T.