Testing the effects of genetic diversity on population survival

using mice from the Great Lakes region

Project Description

I am proposing to study the genetics of endangered animal populations by taking advantage of a “natural experiment” in progress in the Great Lakes region. Two species of mice, the white-footed mouse (Peromyscus leucopus) and the deer mouse (Peromyscus maniculatus), are physically very similar and share an ecological role in many areas of North America (Wolff 1985). Prior to about 1980, the subspecies Peromyscus maniculatus gracilis occupied the northern Midwest, including the Upper Peninsula and northern half of the Lower Peninsula of Michigan, and the Canadian side of the Great Lakes (Fig. 1), while Peromyscus leucopus noveboracensis occupied the southern part of the Lower Peninsula and southern Wisconsin (Hall 1981). During the last several decades, however, P. leucopus has become the predominant mouse species throughout most of Michigan and Wisconsin (Myers et al. in press; Long 1996), replacing P. m. gracilis. South of Lake Superior, the latter is now found only in the middle to eastern Upper Peninsula of Michigan, with a few small populations remaining on the Lower Peninsula (Fig. 2; Myers et al. in press) and on some small islands in Lake Michigan.

This system of ongoing species turnover provides an exceptional opportunity to examine many important biological questions, in fields ranging from ecology to population genetics to conservation biology. The two species have populations that are in a diversity of states—there are populations that are isolated or contiguous, large or small, expanding or declining or stable, or on the verge of disappearing. I plan to assess the genetic makeup of the mouse populations, since genetic variability is a basic characteristic of any population that both reflects its recent history and profoundly affects its future survival.

For this initial project, I will focus on the levels of genetic variability in the P. m. gracilis populations in Michigan. The isolated populations in the Lower Peninsula will be compared to stable populations in the eastern Upper Peninsula and to populations in the western Upper Peninsula that are just now being infiltrated by P. leucopus. Information on the age structure, reproduction, and genetic diversity of the populations will provide baseline data for comparisons in future years. I am especially interested in using the remnant P. m. gracilis populations to test some of the hypotheses that have been published in recent years relating genetic variability to extinction.

Small, isolated populations are believed to be more vulnerable to inbreeding and to genetic drift (loss of variation due to chance) than are larger, more continuous populations (Vucetich et al. 2001). Both inbreeding and drift are forces that can homogenize the genetic makeup of a population, which in turn makes the population less able to respond to changing conditions. There is currently considerable debate over whether genetic homogenization is an important cause of the “crashes” often seen in populations of rare species, which typically decline gradually until they fall below some critical threshold size, and then rapidly disappear (Finke and Jetschke 1999; Bataillon and Kirkpatrick 2000).

The population losses suffered by P. m. gracilis in Michigan mirror both the gradual declines and the severe population crashes experienced by endangered species, so this species can serve as an excellent model system for testing hypotheses that relate genetics to extinction. We will test two specific hypotheses: 1) isolated P. m. gracilis populations have less genetic variability than contiguous northern populations of P. m. gracilis, and 2) losses of genetic variability in P. m. gracilis populations can be used to predict which populations will crash in future. The ongoing turnover of mouse species allows us to observe the loss of genetic variability and the decline of populations as they occur, rather than trying to retroactively reconstruct what has happened to an already rare species (Amos and Harwood 1998; Finke and Jetschke 1999). A great advantage of testing hypotheses using these mice is that, though P. m. gracilis has now disappeared from most of Michigan, it is not actually an endangered species or subspecies, because it remains common north of the Great Lakes. This allows us to experiment with the remnant mouse populations, using them as models of critically endangered species, without threatening the survival of a species that is itself at risk of extinction.

Context

Since they are the most widespread and abundant small mammals in North America, mice in the genus Peromyscus have been studied extensively; their basic biology is well known, and their distributions have historically been observed to be quite stable (Hall 1981). It was therefore a surprise when, about 15 years ago, Dr. Philip Myers (University of Michigan) and I first observed the replacement of P. m. gracilis by P. leucopus in northern Michigan. At about the same time, Dr. Charles Long was noting the same phenomenon in northern Wisconsin (Long 1996). Since that time, Dr. Myers has periodically trapped mice from locations in lower Michigan, and he has documented the rapid disappearance of P. m. gracilis from much of its former range. While he and Dr. Barbara Lundrigan (Michigan State University) are studying morphological (body shape) changes in the changing mouse populations, I plan to study the genetic diversity of the same populations in collaboration with them.

This project represents a new area of enquiry for my lab: the study of the distribution and genetic variability of wild mammal populations, and how these factors in turn influence the extinction of animal species. Besides providing a model system for the study of extinction processes, these mice also illustrate a possible link between species turnover and climate change. A clear warming trend that may be part of a larger global warming phenomenon has been evident throughout much of North America over the past several decades (Houghton 1995). Though the specific mechanisms driving this climate change are as yet poorly understood, it is having significant effects on populations of native plants and animals, (e.g. Wang et al. 2002; Gaston et al. 2002) and it may be accelerating the extinction of some endangered species by making their previous habitats unsuitable, or by allowing other species to invade their historic ranges.

P. leucopus is known to be less well adapted to cold weather than P. m. gracilis (Pierce and Vogt, 1993), and this factor may have set the range boundary between the species in the past. We are exploring the possibility that earlier spring warming in Michigan (Myers et al. in press) may be driving the turnover of mouse species. A Master’s student (L. Rowland) in my group is testing the hypothesis that during mild springs the juveniles of P. leucopus, which begins reproducing early in the year, establish themselves in the best territories and thus competitively exclude the later-developing P. m. gracilis juveniles. Though her study is separate from the genetics project, some of the data collected by Rowland can be used in this new study. Also, tissue samples that have been taken from all mice involved in the reproduction study will be used in later genetic analyses.

There are many additional questions that we hope to address in the future using this exceptional model system. P. m. gracilis from unaffected northern populations can be reintroduced into isolated Michigan populations to increase variability; we can then assess whether direct manipulation of the levels of genetic variability is practical as a strategy for managing endangered populations. Also, my collaborators have been examining the “invading” populations of P. leucopus to see whether they differ physically from their source populations to the south; in the future, I plan to do a genetic analysis of these populations and see whether “invader” versus “stay-at-home” mice have distinct genetic characteristics.

Experimental Approach

As mentioned above, a graduate student in my lab (L. Rowland) is currently studying the remaining isolated populations of P. m. gracilis found in Lower Michigan. Five field sites in Cheboygan County, Michigan were selected; these sites, plus another nearby, are the only places in the Lower Peninsula known to have overlapping populations of P. leucopus and P. m. gracilis. During the spring and summer of 2002, these sites were periodically trapped. All mice caught were individually marked, age and sex were determined, and small saliva and tissue samples were taken for genetic testing and to confirm the species identifications (all immature Peromyscus look very similar). The mice were then released back into their home territories.

In addition, I made one trapping trip (July 2002) to the Upper Peninsula, and will make one more trip before winter; I have found at least two sites that have both mouse species, and expect to find more. During the spring and summer of 2003, I plan to return to all of the sites in both Upper and Lower Michigan and to trap them again, marking and taking samples from all new mice. Saliva and tissue samples from other Upper Peninsula sites will be supplied by Dr. Myers.

Saliva samples from each mouse will be run on cellulose acetate gels to assay salivary amylase, an enzyme used to confirm the species identifications (Bruseo et al. 1999). DNA will be extracted from all mouse tissue samples using standard protocols (Kass and Hoffman 1993). The polymerase chain reaction (PCR) technique will be used to isolate small stretches of highly variable DNA, called microsatellites, from the DNA samples (Chirhart et al. 2000). By sizing and sequencing microsatellite DNAs, we can estimate both relatedness between individuals and the total amount of genetic variability within and between each group of mice (Vucetich 2001). Comparisons will be made among the isolated populations of P. m. gracilis, and between those populations and the mice that remain in their historical range in the eastern Upper Peninsula. Standard analyses that have worked well with similar data sets (e.g. Lacey 2001; Goossens et al. 2001) will be used for the comparisons.

Literature Cited

Amos, W. and J. Harwood. 1998. Factors affecting levels of genetic diversity in natural populations. Philosophical transactions of the Royal Society of London. 353(1366): 177-186.

Bataillon, T. and M. Kirkpatrick. 2000. Inbreeding depression due to mildly deleterious mutations in finite populations: size does matter. Genetical Research. 75(1): 75-81.

Bruseo, J.A., Vessey, S.H., and Graham, J.S. 1999. Discrimination between Peromyscus leucopus noveboracensis and Peromyscus maniculatus nubiterrae in the field. Acta Theriologica. 44(2): 151-160.

Chirhart, S.E., Honeycutt, R.L., and I.F. Greenbaum. 2000. Microsatellite markers for the deer mouse Peromyscus maniculatus. Molecular Ecology. 9(10):1669-71.

Finke, E. and G. Jetschke. 1999. How inbreeding and outbreeding influence the risk of extinction—a genetically explicit model. Mathematical Biosciences. 156(1-2): 309-314.

Gaston, A.J., Hipfner, J.M., and Campbell, D. 2002. Heat and mosquitoes cause breeding failures and adult mortality in an Arctic-nesting seabird. Ibis 144(2):185-191.

Goossens, B., Chikh, L., Taberlet, P., Waits, L.P., and D. Allaine. 2001. Microsatellite analysis of genetic variation among and within Alpine marmot populations in the French Alps. Molecular Ecology. 10(1): 41-52.

Hall, E.R. 1981. The Mammals of North America. John Wiley & Sons, New York, NY.

Hoffman, S.M.G. and Brown, W.M. 1995. The molecular mechanism underlying the "rare allele phenomenon" in a subspecific hybrid zone of the California mouse (Peromyscus californicus). Journal of Molecular Evolution. 41:1165-1169.

Houghton, J.T., Meria, Filho, L.G., Callender, B., and N. Harris. 1995. The Science of Climate Change. Cambridge University Press, Cambridge, U.K.

Kass, D.H. and S.M.G. Hoffman. 1993. Unusual patterns of susceptibility to degradation of DNA isolated from tissues in Peromyscus californicus. Laboratory Animals. 27:273-277.

Lacey, E.A. 2001. Microsatellite variation in solitary and social tuco-tucos: molecular properties and population dynamics. Heredity. 86(5): 628-637.

Long, C.A. 1996. Ecological replacement of the deer mouse, Peromyscus maniculatus, by the white-footed mouse, P. leucopus, in the Great Lakes region. Canadian Field-Naturalist. 110(2): 271-277.

Pierce, S.S., and F.D. Vogt. 1993. Winter acclimatization in Peromyscus maniculatus gracilis, P. leucopus noveboracensis, and P. l. leucopus. Journal of Mammalogy. 74(3): 665-677.

Vucetich, L.M., Vucetich, J.A., Joshi, C.P., Waite, T.A., and R.O. Peterson. 2001. Genetic (RAPD) diversity in Peromyscus maniculatus populations in a naturally fragmented landscape. Molecular Ecology. 10: 35-40.

Wang, G., Hobbs, N.T., Singer, F.J., Ojima, D.S., and Lubow, B.C. 2002. Impacts of climate changes on elk population dynamics in Rocky Mountain National Park, Colorado, U.S.A. Climatic Change 54(1-2):205-223.

Wolff, J.O. 1985. The effects of density, food, and interspecific interference on home range size in Peromyscus leucopus and Peromyscus maniculatus. Canadian Journal of Zoology. 63: 2657-2662.

Wolff, J.O. 1996. Coexistence of white-footed mice and deer mice may be mediated by fluctuating environmental conditions. Oecologia. 108: 529-533.

Figure 1. Distribution of P. leucopus and P. m. gracilis in the Great Lakes region,

circa 1970 (adapted from Hall 1981).

Figure 2. Present distribution of P. m. gracilis in Michigan. Shading represents well-established, widespread populations, while dots indicate small isolated populations. P. leucopus (not shown) is present everywhere in Michigan except the eastern third of the Upper Peninsula and the Lake Michigan islands.