ABSTRACT
What if there were a parasitic entity that existed through time by stealing and incorporating genomes from a number of successful vertebrates? That parasite could use its host's genetic machinery to produce a phenotype and behavioral attributes that would serve as a successful disguise to enable it to pursue its own evolutionary agenda. Allowing some teleological 'shorthand', if one host was deemed to be inferior to an alternate host, the parasite could opt for the superior genome. The successful parasite may even adopt and incorporate genomes from more than one host species. Such an evolutionary strategy might best describe the "hybrid" mole salamanders in the genus Ambystoma and help to address the chronic confusion that has plagued the "complex" since Piersol's observation of high mortality within egg masses of Ambystoma jeffersonianum (Green) in 1910. The most compelling evidence that would support a parasitic entity hypothesis is based on the disparity observed by comparing mitochondrial DNA (mtDNA) of the "hybrids" and their "host species" with their nuclear genotypes that are revealed and assessed by isozyme electrophoresis (allozymes). Allozymes show the "hybrids" to be mostly fixed heterozygotes that possess alleles also found in contemporary species. The allozyme data support recent hybridization events that involve four bisexual species in the genus Ambystoma, i.e., A. jeffersonianum (Green), A. laterale Hallowell, A. texanum (Matthus), and A. tigrinum (Green). However, a phylogeny derived from mtDNA sequences, places all "hybrids" in a deeply rooted monophyletic clade that shows no paraphyly with any of the four species. Indeed, based on the mtDNA phylogeny and a phylogenetic species concept, the hybrid clade would qualify as a distinct and separate species. Other than the mtDNA sequences and the fact that the "hybrids" are virtually
all female, there are no obvious synapomorphic characters that could be used to define such a species. The phenotypes of the "hybrids" reflect an expected intermediate phenotype that is dictated by their nuclear genomic content that can be estimated from allozymes. In this chapter, I attempt to address the paradox that the "hybrids" posses nuclear genomes that seem to result from crosses between contemporary species but these same hybrids have a common mtDNA that is unlike any of the putative hybridizing species.
