Both sympatric and allopatric scenarios of animal speciation typically envision slow and gradual genetic transformations of populations,
even when vicariant events in the physical environment are sudden. But unisexual vertebrate taxa break this evolutionary rule because each biotype emerges quickly (in one or a few generations) from the two (or sometimes more) sexual species that had hybridized to produce PLX4032 solubility dmso it (Dawley & Bogart, 1989; Vrijenhoek, 1994). Thus, in a temporal sense, the emergence of many parthenogenetic animal species parallels the rapid emergence of many allopolyploid plant species that also have arisen following interspecific hybridization events. Conventional wisdom holds that genetic recombination (typically via sexual reproduction in multicellular organisms) is necessary for continued adaptability to changing environments and for the long-term evolutionary persistence of any species. To assess the evolutionary ages of vertebrate clones, researchers have generated and provisionally
dated phylogenetic Z-VAD-FMK research buy trees (typically from mtDNA sequences and molecular-clock calibrations) for many unisexual taxa and their sexual relatives. Results proved generally consistent with the standard thesis that asexual lineages have short evolutionary durations, but there do seem to be some exceptions. For example, Quattro, Avise & Vrijenhoek (1992b) used a large geographic range and high post-formational cytonuclear genetic diversity to estimate that a monophyletic biotype of the unisexual fish Poeciliopsis monacha-occidentalis is about 60 000 years old. Although Maynard Smith (1992) rightly noted in a commentary that 60 000 years ‘is but an evening gone’ in evolutionary time, it does seem clear that at least some vertebrate clones are far more persistent than formerly realized. In any event, this and other longevity estimates for various unisexual vertebrate lineages all pale in comparison MCE with the ancient origins suspected for some invertebrate parthenogenetic lineages that seem to have survived without sex for tens of
millions of years (Mark Welch, Mark Welch & Meselson, 2004; Domes et al., 2007; Heethoff et al., 2007). Female parthenogens truly are sexually chaste, but females in gynogenetic and hybridogenetic vertebrate taxa might be deemed only ‘semichaste’. As under parthenogenesis, a gynogenetic female reproduces clonally except that sperm from males of a related sexual species are required to initiate cellular divisions in her unreduced ova. A sperm cell does not actually fertilize an egg but merely stimulates it to begin dividing. Thus, a gynogenetic female in effect ‘sexual parasitizes’ a foreign male who receives no genetic payoff for his sexual services. Hybridogenesis is another peculiar mode of reproduction with elements of both clonality and sexuality.