Monday, January 24, 2005

Degradation of human genome?

Evidence for Widespread Degradation of Gene Control Regions in Hominid Genomes

Although sequences containing regulatory elements located
close to protein-coding genes are often only weakly conserved during evolution,
comparisons of rodent genomes have implied that these sequences are subject to
some selective constraints. Evolutionary conservation is particularly apparent
upstream of coding sequences and in first introns, regions that are enriched for
regulatory elements. By comparing the human and chimpanzee genomes, we show here
that there is almost no evidence for conservation in these regions in hominids.
Furthermore, we show that gene expression is diverging more rapidly in hominids
than in murids per unit of neutral sequence divergence. By combining data on
polymorphism levels in human noncoding DNA and the corresponding
human–chimpanzee divergence, we show that the proportion of adaptive
substitutions in these regions in hominids is very low. It therefore seems
likely that the lack of conservation and increased rate of gene expression
divergence are caused by a reduction in the effectiveness of natural selection
against deleterious mutations because of the low effective population sizes of
hominids. This has resulted in the accumulation of a large number of deleterious
mutations in sequences containing gene control elements and hence a widespread
degradation of the genome during the evolution of humans and chimpanzees.
The phrase "low effective population sizes", I assume, refers to the early hominid evolution. But relaxed selection on regulatory regions in many (most, all?) genes does not sound right. Those are the most important parts. It is not that a mutation will produce a low-efficiency enzyme, it will produce a new piece of information: where, when, and how many copies, if any, of a gene to transcribe. This alters the identity of a cell. That would have profound effects on development, as well as physiology.

Are hominids evolving evolvability? Have hominids invented a novel mode of regulation of gene expression in which 5' ends of genes are not that important? Any molecular biologists in the audience who can explain this to me?


PLoS-B has published a commentary which does not answer my questions, or even go any further than the paper itself:

Hominids Lose Control

posted by Bora Zivkovic @ 11:03 PM | permalink | (0 comments) | Post a Comment | permalink