Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

In gonadal parasitism, one of the partners preferentially and sometimes exclusively occupies the reproductive organs and produces all the sex cells of the chimeric organism. This danger is somewhat less in plant chimeras because of the existence of the cell walls and thus the related lack of motility of the cells within the organism; amongst animal chimers, encountered, for example, in a great many marine invertebrates, it is substantially greater. However, even in humans a case has been described of a woman whose somatic tissues were genetically uniform and thus were not of chimeric origin, but genetic tests of her four children showed that the sex cells in her ovaries were derived from her (fraternal) twin. I would happily be wrong, but I suspect that at least part of those convicted of rape, who are currently being set free with great publicity on the basis of DNA tests, could correspond to similar cases.

On a long-term evolutionary scale, possible cases of successful gonadal parasitism are quite common. In a great many taxa of vertebrates, including mammals and birds, the precursors of sex cells are not formed directly in the tissues of the future gonads, but rather travel to these organs from other parts of the embryo or even from extra-embryonic fluid during embryogenesis. Simultaneously, the places where the future precursors of the sex cells are formed differ substantially in the individual taxa. It thus follows, amongst other things, that the sex cells in various groups of vertebrates are not mutually homologous (Davison 1998; Davison 2001).