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On the Origin of Parthenogenesis in Vertebrates: The Cytogenetic Factors

The main features of the hybrid theory can best be summarized by some of its principal exponents: "The hypothesis entails two to three major steps. The initial step is the same as in all cases--the hybridization of diploid bisexual species to produce a diploid parthenoform, Second, if this diploid parthenoform is adequately adapted and an available habitat is present, it colonizes the habitat as a diploid parthenospecies .... The diploid parthenoform backcrosses to one or the other, or both, of the generating diploid bisexual species... to form an allotriploid parthenoform. Again, if this form is able to invade and capture a habitat, then the result is an allotriploid parthenogenetie species" (Wright and Lowe 1968). Thus: "Hybridization diploid unisexuality and then polyploidy are the natural sequence of events, with unisexual populations sustaining themselves by one of the three types of clonal reproduction" (hultz 1969). Finally: "It is further speculated that the formation of unreduced eggs in a diploid hybrid and the stabilization of triploidy depend on a single mechanism: genetic imbalance or difficulties in pairing preventing cytokinesis at the last premeiotic chromosomal division" (Uzzell and Goldblatt 1967). On the other hand, according to the spontaneous method of origin, hybridization does not result directly in parthenogenesis but only favors it through heterosis. Allotriploids can only arise after the ability to produce consistently unreduced ova has been genetically acquired by the original unisexual autodiploids, Thus, in the spontaneous method, diploid parthenogenesis and then polyploidy are the natural sequence of events. The fact that two such highly contradictory viewpoints exist implies that the origin of parthenogenesis in vertebrates is far more complicated than is currently recognized and that multiple methods of origin may be involved. To be sure, existing facts lend some support to both, although overall evidence presented here leans toward the spontaneous side. Clearly, both possible methods of origin, as well as their supportive evidence, must be kept in mind in future studies in order to elucidate best this complex and enigmatic problem.