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On Kelleys Island, Ohio, in Lake Erie, are found bisexual Ambystoma tigrinum and Ambystoma texanum as well as five different combinations of diploid and polyploid hybrid female salamanders. Genome composition and ploidy of salamanders from five breeding sites on the island were examined using starch gel electrophoresis, erythrocyte area measurements, and chromosome counts. All of the hybrids contained at least one Ambystoma laterale genome, yet pure individuals of this species were not encountered. Embryonic mortality was severe among eggs deposited by 42 hybrid females. The few resulting offspring, when compared electrophoretically with their mothers, showed no evidence of being the product of parthenogenesis. Recently described Ambystoma nothagenes Kraus is not a valid species as this trihybrid is demonstrated to be genetically heterogeneous and independently derived from diploid A. laterale × texanum hybrids.

Polyploid hybrid salamanders of the genus Ambystoma tend to produce unreduced eggs. At higher temperatures (15"C), the sperm often contributes genetically, leading to tetraploid offspring from triploid mothers and pentaploid offspring from tetraploid mothers. At lower temperatures (5"C), the sperm nucleus usually is lost, and the eggs develop gynogenetically. We examined the cytology of fertilization at 5°C in order to determine the nature of the temperature-dependent nuclear loss. As in other urodeles, fertilization is polyspermic. The sperm nuclei decondense to form pronuclei, and one sperm pronucleus associates with the egg pronucleus. At metaphase, a clump of chromatin appears along with the chromosomes at the metaphase plate. This chromatin clump fails to segregate at anaphase and is left at the site of the cleavage furrow. The chromatin clump is likely the discarded sperm pronucleus, so its loss is due to its failure to condense properly into chromosomes.

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It is hypothesized that coexistence of bisexual and unisexual salamanders depends upon various forms of ecological partitioning, as in analogous complexes of fishes. The mixture of clonal and nonclonal, obligately sperm-dependent reproduction occurring in unisexual mole salamanders of the genus Ambystoma suggests that some of this partitioning may be manifest in the variable reproductive attributes of different biotypes and in community structure. Within biotypes, ecological differences related to genomic composition (genomotype) and specific origin (genotype) of individuals are also expected. Using data gathered over a 4-year period from a large population of the A. laterale–jeffersonianum complex from Haliburton, Ontario, I examined four areas of reproductive ecology that may contribute to ecological partitioning: (1) fluctuating sex, hybrid, and ploidy ratios; (2) differential migratory response by triploid and tetraploid genomotypes; (3) ontogenetic differences among triploid and tetraploid genomotypes; and (4) genotypic differences within unisexual genomotypes. For each case I was able to reject a null hypothesis ascribing ecological equivalency to those biotypes involved. The most germane ecological differences among unisexuals were imbedded in the broad syndrome of delays in larval development, metamorphosis, and sexual maturity displayed by tetraploids. These findings parallel those reported in a large body of literature generated from laboratory studies of polyploid salamanders. Using these corresponding data, I conducted a broad review of potential developmental, physiological, and biotypic causes and consequences of the observed delays. The results indicated that several effects of polyploidy and hybridity directly influence community ecology and contribute to stable coexistence of bisexuals and unisexuals. Based on these findings, a strategic model summarizing interaction in these communities was constructed, from which testable hypotheses may be derived for future research. The findings of this study confirmed the ontological status of the biotypic distinctions (genomotype versus genotype) advanced here.

We describe here, for the first time, the existence of an Ambystoma tigrinum tigrinum larval morph characterized by fast growth rates and early metamorphosis and triggered by cannibalism. This new morph does not have the anatomical specializations of true A. tigrinum cannibal morphs, i.e., enlarged vomerine teeth and a wider head described previously by several workers. Functionally, however, this new morph and true cannibal morphs achieve the same end; high growth rates and early metamorphosis may facilitate survival in individuals inhabiting temporary and unpredictable wetlands.