The survival of a species, humans included, relies on healthy offspring that can produce healthy offspring of their own. In nature, only the strong survive while the weaker members of a species die young, usually before they can bear offspring that will pass their genetics onto the next generation.
In humans and other mammals, this purging of the weakest links begins in the womb, where the budding eggs of a female fetus are eliminated to maintain reproductive quality years in the future. Researchers at the Carnegie Institute for Science in Washington, DC, use jumping genes and photocopying to describe the phenomenon.
The Research Team
Alex Bortvin and Safia Malki conducted the research that delved deep into the body’s reproductive quality control mechanism. They knew that a female child’s massive supply of budding eggs is depleted by about 80% before she’s even born. More eggs are purged as she matures. Bortvin and Malki wanted to know how this purging process happens and how an egg is selected for purging.
The researchers discovered segments of DNA they called “jumping genes” (transposons) due to their ability to leapfrog along an egg’s DNA. The mutations created by these jumping genes can be especially damaging since the DNA in these extremely important cells are half the blueprint for the next generation.
If the DNA in an egg (or any cell) was compared to a large stack of papers, every page would represent an exact location of genetic sequences within the DNA. The genetic material reads just like the highly structured, well-organized words and sentences on each page.
As jumping genes leapfrog through DNA, the mutations they cause upset the order of the DNA. If the pages in the stack of paper get out of order, damaged, or lost, the story on the pages becomes distorted and impossible to understand.
Previous studies on sperm germ (immature) cells indicate the jumping genes are stopped as sperm matures. By ending the action of the jumping genes, the potential for genetic damage stops, too.
The Carnegie researchers discovered a different mechanism in egg germ cells (oocytes). The jumping gene mutations continue throughout egg development but the body purges defective eggs, leaving only the highest-quality survivors in the ovaries.
In cases where defective eggs do survive purging and become fertilized, the researchers speculate the purging continues as spontaneous termination of pregnancy. If the pregnancy continues to childbirth, the child is likely to have genetic defects affecting physical and/or mental health.
The researchers suggest the body selects the most perfect eggs first for ovulation but as a woman ages and her egg supply is depleted, defective eggs are released during ovulation. This could be one reason why babies born to older mothers are more prone to chromosomal abnormalities such as Down syndrome but, according to Botvin, further study is needed to confirm this theory.
Source: Malki, Safia, et al. “A Role for Retrotransposon LINE-1 in Fetal Oocyte Attrition in Mice.” Developmental Cell. Elsevier Inc. Jun 9, 2014. Web. Jun 15, 2014.