The Relationship between Allozyme Variation and Life History: Non-Transforming Salamanders Are Less Variable

Abstract

Several recent papers have reported low levels of genetic variation in nontransforming, paedomorphic urodeles. Here, we report the results of an electrophoretic survey of 21 allozyme loci in two populations of the Ozark Hellbender, Cryptobranchus alleganiensis, confirming the extraordinarily low levels of genetic variation previously reported in a different laboratory. We then summarize the literature on 102 species of salamanders to compare genetic variation in transforming and non-transforming taxa. On average, non-transforming salamanders are significantly less variable than transforming ones, supporting the idea that low levels of allozyme variation in Cryptobranchus are associated with larval reproduction. We interpret this association between life history mode and genetic variation as reflecting a fundamental difference in levels of population extinction and recolonization between aquatic and terrestrial species. N 1977, Merkle et al. reported the results of an extensive survey of electrophoretic variation in the non-transforming salamander Cryptobranchus alleganiensis. Their survey demonstrated an extraordinarily low level of genetic variation: among the products of 24 loci examined from a total of 137 specimens representing 12 geographic samples from the four major river systems inhabited by hellbenders, only two loci showed any genetic variation, and nine of the 12 populations were completely monoallelic. Merkle et al. (1977) also pointed out that several other non-transforming salamanders (the mudpuppy, Necturus maculosus; the dwarf siren, Pseudobranchus striatus; and the twotoed amphiuma, Amphiuma means) all appear to have low levels of genetic variation and large amounts of nuclear DNA, and suggested a possible causal relationship. A prediction from the work of Merkle et al. (1977) is that there should be a reasonably strong correlation between life history variation (that is, metamorphosis vs larval reproduction) and genetic variation. However, the relationship between life history pattern and genetic variation has never been evaluated systematically. In this paper, we first report the results of a reexamination of genetic variation in two additional populations of the hellbender, C. alleganiensis, confirming the results of Merkle et al. (1977) in a different laboratory using a somewhat different set of loci. To interpret these results, we present a literature survey of electrophoretic variation in over 100 species of salamanders from six families and 19 genera. We conclude that there is a strong relationship between larval reproduction and genetic variation: larval reproducers are less variable, on average, than metamorphosing salamander species. However, we interpret this relationship as a difference in the population structures of transforming and non-transforming salamanders, rather than representing a causal relationship between structural genetic variation and components of fitness as has been previously suggested (Pierce and Mitton, 1980; Mitton and Grant, 1984).