Quality vs. quantity: apoptosis maintains oocyte viability during aging
Posted Dec 20 2008 5:47pm
Evolution generally rewards early reproduction, but there is some advantage to maintaining reproductive capacity into later life — e.g., if a young organism encounters conditions that would be adverse for offspring viability, it makes sense to delay reproduction or at least to have the capacity to try again when that organism is older. How best to maintain the viability of gametes over the course of the lifespan? The answer appears to be stringent quality control: Andux and Ellis report that in C. elegans, apoptosis in the germ line helps guarantee that high-viability oocytes keep getting the resources they need:
Apoptosis Maintains Oocyte Quality in Aging Caenorhabditis elegans Females
In women, oocytes arrest development at the end of prophase of meiosis I and remain quiescent for years. Over time, the quality and quantity of these oocytes decreases, resulting in fewer pregnancies and an increased occurrence of birth defects. We used the nematode Caenorhabditis elegans to study how oocyte quality is regulated during aging. To assay quality, we determine the fraction of oocytes that produce viable eggs after fertilization. Our results show that oocyte quality declines in aging nematodes, as in humans. This decline affects oocytes arrested in late prophase, waiting for a signal to mature, and also oocytes that develop later in life. Furthermore, mutations that block all cell deaths result in a severe, early decline in oocyte quality, and this effect increases with age. However, mutations that block only somatic cell deaths or DNA-damage–induced deaths do not lower oocyte quality. Two lines of evidence imply that most developmentally programmed germ cell deaths promote the proper allocation of resources among oocytes, rather than eliminate oocytes with damaged chromosomes. First, oocyte quality is lowered by mutations that do not prevent germ cell deaths but do block the engulfment and recycling of cell corpses. Second, the decrease in quality caused by apoptosis mutants is mirrored by a decrease in the size of many mature oocytes. We conclude that competition for resources is a serious problem in aging germ lines, and that apoptosis helps alleviate this problem.