The spermatozoan genome and fertility.

The strategy for effective reproduction by eliminating genetically unbalanced gametes during spermatogenesis and transport varies in degree of success within as well as among species, but in no animal has it been reported to be completely effective. In the human subject, for example, it is estimated that one in every 50 ejaculated spermatozoa is genetically abnormal. The causal basis of these anomalies is poorly understood. Meiotic accidents, environmental mutagens, and gamete senescence in utero are all implicated. However, many of these abnormal cells are fertile. This fact plus the weight of the evidence reviewed suggest that fertility differences among males which cannot be ascribed to measurable differences in semen characteristics reflect, in large part, the increased opportunity of nuclear defective gametes in the semen of some males to effect fertilization. The elimination of embryos arising from eggs fertilized by genetically defective spermatozoa through spontaneous abortions, although biologically costly, must be viewed as the final check for the elimination of genetic detritus of the species.