On Evolution

6. Molecular Clones within Organismal Clones

Evolution has produced a rather large number of vertebrate “species” that consist solely of females. For several years, Avise joined forces with several other researchers to study the genetic origins and evolutionary histories of these unisexual taxa, which typically reproduce by asexual mechanisms such as parthenogenesis. These remarkable organisms are of interest in their own right, and they also provide wonderful fodder for addressing the evolutionary ramifications of nonrecombining biological systems. The article on which this chapter is based was a review of genetic studies on asexual mitochondrial lineages within asexual organismal lineages (i.e., of clones within clones). In sharp contrast to the situation in sexually reproducing taxa, the transmission history of mitochondrial DNA (mtDNA) (as well as all nuclear loci) in any asexual species is in principle one-and-the-same as an entire organismal genealogy, and this peculiar fact offers unique advantages for deciphering the evolutionary geneses and properties of unisexual taxa. Some vertebrate “species” exist predominantly or exclusively as females , exhibiting asexual or semisexual reproduction. Examples occur among the fishes, amphibians, and squamate reptiles. Essentially all known unisexual vertebrates carry the nuclear genomes of two or more bisexual species, and thus arose via interspecific hybridization. These all-female “biotypes” reproduce without genetic recombination, by one of three modes: () parthenogenesis, in which the female’s nuclear genome is transmitted intact to the egg, which then develops into an offspring genetically identical with the mother; () gynogenesis , in which the process is the same except that sperm from a related bisexual species is required to stimulate egg development; and () hybridogenesis , in which an ancestral genome from the maternal line is Molecular Clones within Organismal Clones 6 transmitted to the egg without recombination, while paternally derived chromosomes are discarded only to be replaced in each generation through fertilization by sperm from a related sexual species. For the sake of clarity, a note about terminology is required. The usual definitions of biological species do not easily apply to unisexual forms. “Biotype” will be used in this chapter, although its meaning also may be unclear unless the genomic constitution of a hybrid unisexual form is specified. For the sake of continuity with the literature, I will employ traditional Latin binomials where they have been assigned to unisexual biotypes, and use hybrid genomic designations where they have been applied. Conventional wisdom holds that the rarity of unisexual reproduction in higher animals stems from both proximate and evolutionary factors. The window of opportunity for production of unisexual biotypes may be quite narrow—presumably, genetic differences between the hybridizing taxa must be sufficient to disrupt recombinant processes during gametogenesis, but not so great as to severely impair viability , fecundity, or other fitness components. Longer-term evolutionary constraints presumably involve a paucity of genetic variation by which unisexuals might adapt to changing environments, and the accumulation of deleterious mutations and gene combinations that cannot be purged in the absence of genetic recombination. Nonetheless , approximately  unisexual vertebrate biotypes have been identi fied, and they are common in some groups, such as Cnemidophorus lizards. Some unisexuals also have large populations and occupy extensive ranges, suggesting that asexuality can be a successful evolutionary strategy at least in the short term. How frequently do unisexual biotypes arise? What are the mechanics of their origin? Which bisexual species provided the male and female parents in the original hybridizations? How long do unisexual lineages survive? Answers to these and related questions are of interest in their own right, and may offer broader insights into the significance of sexual reproduction.  On Evolution The mitochondrial DNA (mtDNA) molecule is favorable for analyses of unisexual complexes because it evolves rapidly in nucleotide sequence and exhibits maternal, nonrecombining transmission through organismal pedigrees. Thus, mtDNA provides a common genetic yardstick by which to compare the magnitudes and patterns of maternal lineage separation in populations both of unisexual biotypes and their sexual progenitors. Furthermore, unlike the situation in sexually reproducing species, where each gene genealogy represents only a minuscule component of the organismal phylogeny, the transmission pathway of mtDNA within a unisexual biotype is in principle oneand -the-same as the organismal pedigree. To unravel the unisexual complexities mentioned above, genetic analyses of mtDNA have been conducted on more than  unisexual vertebrate biotypes and their sexual relatives. Typically, sequence divergence values between mtDNA haplotypes were calculated and estimates of phylogeny were generated from these genetic distances...