Inducible Anisogamy and the Evolution of Oogamy from Isogamy

The initial and decisive step in the evolution of oogamy from isogamy involves the generation of size different gamete types in isogamous ancestors. Recent data with isogamous dioecious Chlamydomonas species reveal a potential for the evolution of anisogamy which can be demonstrated experimentally. These species possess, in each sex, two different pathways of gametogenesis. A vegetative cell may produce just one large gamete by intracellular differentiation or may produce four small gametes by means of two gametogenic mitoses. Combination of sexually complementary gametes of different production modes creates phenotypically a distinctly anisogamous copulation. At this developmental potential, any mutation which fixes one or the other mode of gametogenesis will establish micro- or macrogamete producers. Such genetically anisogamous lines will then be subjected to selection for increasingly divergent evolution of the gametic differentiation.