David's theory of evolution: Shapiro's view; sperm controls (Evolution)

by David Turell @, Saturday, January 25, 2020, 19:46 (243 days ago) @ David Turell

There is a newly found mechanism that manages sperm to carefully mange how DNA is transmitted to the egg, undamaged and with DNA mutation controls:


A summary:

"The testis expresses the largest number of genes of any mammalian organ, a finding that has long puzzled molecular biologists. Our single-cell transcriptomic data of human and mouse spermatogenesis provide evidence that this widespread transcription maintains DNA sequence integrity in the male germline by correcting DNA damage through a mechanism we term transcriptional scanning. We find that genes expressed during spermatogenesis display lower mutation rates on the transcribed strand and have low diversity in the population. Moreover, this effect is fine-tuned by the level of gene expression during spermatogenesis. The unexpressed genes, which in our model do not benefit from transcriptional scanning, diverge faster over evolutionary timescales and are enriched for sensory and immune-defense functions. Collectively, we propose that transcriptional scanning shapes germline mutation signatures and modulates mutation rates in a gene-specific manner, maintaining DNA sequence integrity for the bulk of genes but allowing for faster evolution in a specific subset."

a commentary with more explanation:
"Xia and colleagues show that heritable mutations are kept in check in the male germline partly by‘‘transcriptional scanning,’’ wherein the majority of genes are transcribed and therefore subject to transcription-coupled repair. They provide a new model for understanding the mechanisms of genome surveillance and evolution. Evolution, genetics, and cell biology collide in the male germline, with outcomes that can be both spectacular and puzzling. Male germ cells are tasked with shepherding the heritable genome through an array of assaults on genome integrity, including programmed double-strand breaks, homologous recombination between chromosomes, de-repression of transposable elements, and a near-complete repackaging of nuclear chromatin. At the same time, these cells must coordinate their own differentiation program to generate sperm, a highly specialized cell type whose function is absolutely required for reproductive fitness. Germline-specific mechanisms that shield the genome during these events include piRNAs, specialized components of the DNA damage machinery,and a lower threshold to activate apoptotic pathways. There are, however, many aspects of genome regulation in male germ cells that remain mysterious. One such phenomenon is the extreme complexity of the transcriptome during spermatogenesis: nearly the whole genome is expressed in testes, more than in any other cell type. In this issue of Cell, Xia et al. (2020) propose a new evolutionary explanation for this phenomenon: correction of heritable DNA damage by ‘‘transcriptional scanning.’’ The model they propose provides a new framework for understanding genome regulation in the germline and for evaluating patterns of genome variation, mutation, and selection at the evolutionary and population levels.

Comment: This degree of protection of sperm transmission (the vagina is a particularly hostile place) implies thought behind the design of the processes, in which the dangers to the sperm have to recognized beforehand, and then planned for with appropriate protections built in. Not by chance: the creation of these protections is well beyond simple germ cellular abilities to foresee, design and produce.

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