Backround: Data on taxonomic diversity, effective population size and movement of individuals among populations (connectivity and gene flow) are critical to the development of a comprehensive species management plan. This data can now be more readily assessed with the use of recently developed molecular genetic techniques.
The Siskiyou Mountains salamander (Plethodon stormi) occurs in about a 400 km2 area of northwestern California and southwestern Oregon. This species is federally designated as a Survey and Manage species and protection of occupied sites has been recommended.
Despite the critical need for information on population size and movement of individuals among populations, this data is difficult to obtain directly for secretive animals such as the Siskiyou Mountains salamander with restricted daily and seasonal activity. Molecular genetic techniques can now be used to estimate population size and gene flow indirectly by analyzing genetic diversity within and among populations. The purpose of this research is to determine the genetic diversity in the Siskiyou Mountains salamander in order to estimate 1) the potential for independently evolving lineages, and 2) effective population sizes and migration rate of individuals among populations. This will enable us to learn more about the potential genetic diversity among these populations, which has implications for management.
The number of independently evolving lineages and potential for gene flow among populations is estimated by analyzing the genetic variation within a single gene that codes for ATPase 6 in mitochondrial DNA (mtDNA). MtDNA is a small, rapidly evolving, maternally inherited genome that has been effectively used in numerous studies of genetic diversity within species. Because most genetic diversity in mtDNA is found among populations, we selected sites that were widely distributed within the range and more or less isolated from one another. Twenty individuals from 6 populations distributed throughout the range of the Siskiyou Mountains salamander were sampled and tissue from the tail tip of each animal was used for the analysis.
Findings: Preliminary analysis from five of the six populations shows that the populations are very closely related and show no variation in the mtDNA. There are three possible explanations for these findings. The first is that this species may have gone through a recent (<5,000 yrs.), genetic bottleneck and the populations are the result of a very few individuals that survived and went on to reproduce and rebuild those populations. The second possibility is that there is consistent genetic flow between these populations which would explain the low level of genetic diversity in the mtDNA. It is unlikely that the former is the case as these populations are for the most part quite large and consist of rather large areas of suitable habitat and many individuals. But, this species is considered to be an extremely limited and non-vagile species, ie., they have very small home ranges and are not known to be good dispersers across the landscape. So, these widely distributed populations may be connected genetically in some way we do not understand. Another remote possibility may be that this single gene loci is somehow very conservative and does not evolve as rapidly in this species and therefore this analysis may not show the true genetic diversity of these populations.
How do we attempt to resolve this apparent genetic puzzle and find out what is really going on with the species? In order to test the mtDNA analysis, independent evidence of genetic diversity will be obtained from allozyme gene loci analysis. Approximately 20 gene loci can be effectively analyzed from whole individuals, which will allow us to look at a much finer resolution at the genetic diversity between the populations. This may confirm that these populations are indeed very closely related or it may show us some sort of genetic diversity at an entirely different level.
Application: Discovery of independent evolutionary lineages using genetic analysis may indicate the need to protect all distinct populations in order to ensure the future viability of these evolutionarily significant populations. Extreme genetic fragmentation may also indicate that dispersal corridors for maintenance of connectivity among these populations are unnecessary. In contrast, if genetic diversity is low, indicating that some level of migration or gene flow among populations is occurring, connectivity between populations by providing dispersal corridors for that migration may be needed. Combining population genetic data with information on the distribution, abundance, and habitat associations of the Siskiyou Mountains salamander, derived from ongoing the habitat associations project by the FS, USFWS, BLM, ODFW, CA F&G, and various private landowners, will provide the groundwork for a comprehensive species and habitat management plan for this species which is scheduled to begin in 1999.
Follow-up: The final analysis and report will be available in the spring of 1999 and will detail all data analyses and findings of the project.
Submitted by: David Clayton, Applegate RD