Small-mouthed salamander (Ambystoma texanum) COSEWIC assessment and status report: chapter 3

Introduction

Pelee Island (Fig. 1) is the largest (4,262 ha) Island in Lake Erie and is inhabited by two species of mole salamanders (Ambystoma laterale and A. texanum) as well as diploid, triploid, and tetraploid genomic hybrids (Bogart et al. 1985; Bogart and Licht, 1986). Ambystoma texanum and hybrids also occur on other Islands in Lake Erie (Downs 1978; Bogart et al. 1987; King et al. 1997) and on the mainland in Ohio and Illinois (Downs 1978; Kraus 1985). Pelee Island is the only Canadian locality for Ambystoma texanum, the Small-mouthed Salamander (Bogart et al. 1985). In light of the previous Status Report on Ambystoma texanum (Bogart and Licht 1991), the species was assigned a status of Vulnerable because of its very restricted range in Canada. At the time the status report was written, the species was considered to be abundant on the Island.

The identification of the species of Ambystoma and the hybrids on the island was determined by allozyme electrophoresis and karyotypes by Bogart et al. (1985). At that time, only one of 34 collected specimens was A. texanum and only diploid and triploid hybrids were identified. Subsequent collecting yielded more A. texanum and documented the presence of tetraploid hybrids. It is now known that there exist diploid (A. laterale x texanum or LT), triploid (LLT and LTT) and tetraploid hybrids (LLLT, LLTT, and LTTT) on Pelee Island. The Island’s salamanders later served to test various hypotheses related to the origin of the hybrids and consequences of polyploidy by Bogart and Licht (1986) and Licht and Bogart (1987; 1989). Although crosses of Pelee Island A. texanum and A. laterale have not been done in the laboratory nor observed in nature, it was generally assumed that the hybrids were produced through such a mechanism and that backcrosses were responsible for the various polyploid classes. But, in an examination of mtDNA from putative parental and hybrid Ambystoma over much of their known range, Hedges et al. (1992) showed that the hybrids possess a cytoplasmic genome that differed from any possible sperm-donating parent in eastern North America. Hybrids, irrespective of their nuclear genome or possible parental associations, have a similar “hybrid” cytochrome b sequence. They found that Pelee Island diploid, triploid, and tetraploid hybrids clustered with mainland hybrids that had nuclear contributions from A. laterale and A. jeffersonianum (not A. texanum). Additionally, Hedges et al. found that the two Pelee Island A. texanum used in their study were not homosequential and represented two distinctive haplotypes or mtDNA clones.

The new molecular data clearly show that the hybrids on Pelee Island were not produced through crosses involving the two parental species on the Island or, indeed, elsewhere in the range of the hybrids that have been examined. Apparently, the hybrids were isolated on the Island at the same time as the two diploid bisexual species and presently exchange nuclear genomes with those extant species (A. laterale and A. texanum). The significance of these findings is dealt with in detail by Hedges et al. (1992), Bogart and Klemens (1997), and Bogart (2003). The hybrids are virtually all females and the hybrids on Pelee Island must obtain a spermatophore from a male (A. texanum or A. laterale) for reproductive success (Bogart and Licht 1986; Bogart 2003). It is of interest that the two specimens of Pelee Island A. texanum that were sequenced by Hedges et al. demonstrated greater sequence divergence than all A. laterale samples which included individuals from several Ontario populations as well as individuals from Prince Edward Island, Connecticut, Illinois, and Vermont.

Figure 1. Pelee Island, the only locality for Ambystoma texanum in Canada, with localities on the Island as mentioned in the text.

Figure 1.  PeleeIsland, the only locality for Ambystoma texanum in Canada, with localities on the Island as mentioned in the text.

Other than the molecular study by Hedges et al. (1992), no additional information on A. texanum from Pelee Island has been published subsequent to the 1991 Status Report and there are relatively few new studies that include A. texanum from elsewhere in its range. Petranka (1998) included a few new references and provided some additional detail concerning the ecology and ethology of A. texanum and A. barbouri (which was split taxonomically from A. texanum by Kraus and Petranka in 1989). The new references include southern range extensions, discovery of an albanistic individual, and the inclusion of A. texanum in some community ecological studies. Ambystoma texanum from Ohio and Texas were included in a mitochondrial sequence study by McKnight and Shaffer (1997) that compared 17 species of Ambystoma. A further resolution of the reproductive process used by nuclear hybrids and additional mitochondrial DNA sequences that show the unisexual lineage to be monophyletic were provided by Bogart (2003).

A trip was made to Pelee Island on the 1st and 2nd of April, 2000. Historic population sites were examined during the night and day to assess the conditions of these populations and to make observations with respect to successful breeding at those sites. Because individuals and/or egg masses were collected from these sites at the same time of year in the 1980s, the presence of individuals and/or egg masses would confirm that the habitat was suitable for current populations. Observations were also made with respect to any changes in the habitat that might have an effect on the current status of Ambystoma texanum.

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