Shannon Scully, of Manawatu College, asks :-
Why do we female humans (and possibly other mammals) have all our eggs present in our ovaries at birth? Are there not advantages in making new ones (like sperm) when needed?
Sam Peterson, a physiologist at Massey University's Institute of Veterinary, Animal and Biomedical Sciences, responded.
Eggs are subject to mutations and to DNA damage by radiation and toxins which could mean the embryo might develop incorrectly, or die. To minimize the time in which DNA damage can occur, eggs are made soon after conception (about 20 days in humans). That leaves a long time (until puberty or long after) before ovulation. But mammals have an extra trick; the egg begins to divide (meiosis), then stops part way through division (at prophase 1) when it has two copies (sister chromatids) of its genes. Cells have mechanisms to detect and repair DNA damage and, using the good copy, eggs are able to repair damage. The extra copies are dumped at the time of ovulation.
Early arrested development may minimise conflict amongst eggs. Most eggs die: of approximately seven million in a human fetus only about 160 are ovulated in the woman’s lifetime. Eggs that die may do so at random, or there may be inter-gamete conflict, since each has different genes. Some may have “killer genes” that destroy eggs that are unlike them. But “killer” eggs may have characteristics unsuitable for survival; arresting development early prevents intergametic conflict.
Finally, early production of eggs may allow epigenetic programming (intrauterine environmental effects masking less-suitable genes in the grand-offspring so they are better suited to the environment).
Males require sperm in numbers too high for long storage. They do have high rates of genetic damage, and many are defective, but there are millions, and competition amongst sperm usually ensures the fittest fertilise the ovum.