The essence of fertilization is the mingling of female and male genomes to create a new individual with a genomic combination that never existed before. In the female germ cell, the plasma membrane becomes fusion-competent first, and then maturation of the cytoplasm and nucleus follows. This order of maturation is reversed in the male germ cell: the male germ cell completes first its nuclear maturation, then its cytoplasmic maturation; the plasma membrane becomes fusion-competent last. The nuclei of polar bodies can be used as substitutes for female pronuclei to produce live offspring. At least in the mouse, the nuclei of spermatocytes can participate in embryo development after completion of meiosis within the oocytes. The nuclei of deformed spermatozoa can participate in embryo development as long as they are genomically normal. By ICSI, men with defective Y chromosomes transmit their infertility to their sons, but not to their daughters. In the future, it may be possible for defective Y chromosomes in spermatozoa and prespermatozoal cells to be repaired or replaced by the normal Y chromosomes of other individuals. Cloning using adult somatic cells is an entirely new reproduction method. Its efficiency is rather low at present, regardless of the species and cell types tested. In the future, cloning may become as efficient as natural reproduction, but exclusive use of cloning would not benefit long-term survival of the species.
