Small-mouthed salamander (Ambystoma texanum) COSEWIC assessment and status report: chapter 8

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General Biology

The general biology of Ambystoma texanum was outlined in the 1991 Status Report but some of that information requires amending based on more recent observations and genetic data. It is now known that genomic hybrids maintain a cytoplasm that is unlike that found in the bisexual species (Hedges et al. 1992; Bogart 2003). Therefore, the genomic hybrids have some unknown female ancestor that is unlike any of the known bisexual species that mate with the genomic hybrids. We also know that sperm from sympatric males of the bisexual species can be incorporated in the eggs of genomic hybrids (Bogart et al. 1989) and that the eggs can also develop without sperm incorporation (gynogenesis) (Elinson et al. 1992).

Unusual sex ratios in Ambystoma texanum

It is evident (Table 1) that fewer male Ambystoma texanum were found in the populations than would be expected. The sex ratio of the bisexual species is expected to be about 1 : 1 and the Quarry A. laterale fit this expectation. But there is a deficiency of male A. texanum in all of the sites, with the possible exception of the Pond site where three of the five A. texanum were males. In some sites (especially the North End Woods), many of the nuclear genotypes were electrophoretically determined from larvae and juveniles whose sex was not known. But at the Stone Road and Girl Guide Pond sites many larvae were raised through metamorphosis to a size that the gonads could be used to identify males and females. The only male A. texanum found at the Girl Guide Pond was an adult collected in 1989 and 13 individuals that transformed from two egg masses were all females. No males of 51 A. texanum were found at the Stone Road site and there are fewer males than expected at Mosquito Point Woods. It is also curious that the female A. texanum were raised from discrete egg masses that were collected in 1987 from the Stone Road and Girl Guide Pond sites. Ambystoma texanum normally lays single eggs or small groups of eggs that are attached to leaves and sticks on the substrate of the pond. This pattern is observed in Mosquito Point Woods. Egg masses, laid higher in the water column, are not usually found on Pelee Island and are more reminiscent of eggs laid by A. jeffersonianum or nuclear hybrids that include an A. jeffersonianum genome. One possible explanation would be that some A. texanum on Pelee Island are “hybrids” and may be derived from genomic hybrid females. For example, if an LT female that normally mated with A. laterale produced reduced T eggs, a laterale genome could be incorporated to maintain an LT genomic constitution. If, however, the same female were to mate with A. texanum and incorporated that genome, the offspring would be TT (A. texanum) but such an individual would possess a very recognizable “hybrid” mtDNA sequence. So far, no A. texanum individuals have been found that have a hybrid mtDNA sequence. Another possibility is that some A. texanum on Pelee Island are normally gynogenetic.

Genetics

The two individuals of Pelee Island Ambystoma texanum that were sequenced and included in the study by Hedges et al. (1992) came from different populations on the Island. One (catalogue number 17572; Genbank Accession # 12751) was a female that was raised in the laboratory after being collected as a larva in the spring of 1989 from Mosquito Point Woods. The other (catalogue number 15640; Genbank Accession #12757) was an adult male that was collected on March 27^th, 1989 from the Girl Guide Pond area. Five Pelee Island genomic hybrids that were also sequenced in that study aligned with mainland hybrids. Additional samples were sequenced using frozen tissues from specimens that were previously identified using isozyme electrophoresis. For comparative purposes, we used the same primers that amplified 307 bases of the cytochrome b gene by Hedges et al. (1992). In addition, we also used another set of primers that amplified 660 bases in order to see if additional phylogenetically informative sites existed that might improve resolution of possible relationships of A. texanum both on Pelee Island and between mainland populations and those found on Pelee. The sequence data confirm that the Mosquito Point A. texanum and the specimens from the eastern populations (Girl Guide pond and Stone Road) have separate common ancestors that are more closely related to mainland populations in Ohio and Indiana. These data suggest that the two areas on Pelee are isolated from each other and individuals in those areas were derived from invasions of the two haplotypes from the United States.

Movement and dispersal

Other than the possible isolation of the mtDNA haplotypes (above), we have no new information on the movement and dispersal of Ambystoma texanum. All collections of adults, eggs, larvae and newly transformed juveniles were made at the breeding sites. Non-breeding adults are subterranean and have not been encountered after the breeding season away from the vicinity of their presumed breeding sites. Based on the co-occurrence of A. texanum with LTT triploid and LTTT tetraploid nuclear hybrids, A. texanum may rarely disperse to the Quarry. The Pond locality is the only site that was found to contain both A. texanum and A. laterale and that site had an equal number of LLT and LTT nuclear hybrids. We have no information on the history of that site that might provide a time scale for immigration of both species. Ambystoma texanum may have migrated to the Stone Road, Girl Guide Pond, and the North End Woods sites from a more southern population or provide evidence for remnants of a wider distribution for that species in the past. The mitochondrial data show that individuals from those eastern sites probably are derived from independant mainland ancestors. It is expected that additional genetic data will shed some more light on these alternate hypotheses.