Genome complexity: protections for genome stability (Introduction)

by David Turell , Friday, February 17, 2023, 20:37 (435 days ago) @ David Turell

Germ cell mechanisms at work to protect the integrity of DNA sequences:

https://ecoevocommunity.nature.com/posts/genetic-instability-also-protects-genome-integ...

"Non-coding RNAs (ncRNAs) include small RNAs and long non-coding RNAs. They do not encode protein; however, they could be involved in the post-transcriptional regulation of genes. Among the classes of small ncRNAs, PIWI-interacting RNAs (piRNAs) are germ cell-specific and longer than other types of small RNAs, such as endogenous small interfering RNAs (siRNAs) and micro RNAs (miRNAs). Most piRNAs in mammalian testes are derived from long non-coding RNAs, which are fragmented into pre-piRNAs, and the pre-piRNAs loaded onto PIWI proteins at a 1:1 ratio undergo further trimming and 2’-O-methylation at the 3’end, which generates mature piRNAs (Sun et al., 2022). The primary function of mature piRNAs is to silence transposable elements (TEs) across bilateral animals.

"In mammalian testes, germ cells express a low amount of piRNAs during the pre-pachytene stages of germ cell development. In contrast, germ cells express highly abundant piRNAs during the pachytene stage of germ cell development. Because over 80 percent of pachytene piRNAs in adult mammalian testes lack obvious targets, earlier studies proposed that pachytene piRNAs regulate mRNAs or stabilize PIWI proteins essential for spermatogenesis. However, the exact function of pachytene piRNAs and the mechanism promoting rapid evolution and divergence of piRNAs and piRNA-producing loci still remains to be clearly understood.

"In our new paper, “Amniotes co-opt intrinsic genetic instability to protect germ-line genome integrity” (Sun et al., 2023), we revealed three fundamental evolutionary forces driving the rapid evolution of piRNA loci across amniotes. The first force is a high local mutation rate of structural variations (SVs), the second force is a positive selection to suppress young and actively mobilizing TEs (all identified TEs belong to retrotransposons) after meiosis, and the third force is a negative selection to eliminate deleterious SV hotspots. To reveal the above findings, we performed comprehensive comparative studies of amniotes, including six different chicken breeds, ducks (Pekin ducks), mice, and humans. Our new paper collectively indicates that genetic instability at the pachytene piRNA loci also protects germ-line genome integrity against TE mobilization by driving the formation of rapidly-evolving piRNA sequences."

Comment: An amazingly precise way germ cells protect DNA sequential integrity. Once again, not by chance but by precise design.