Curtis Strobeck

Microsatellite analysis of population structure in Canadian polar bears

Attempts to study the genetic population structure of large mammals are often hampered by the low levels of genetic variation observed in these species. Polar bears have particularly low levels of genetic variation with the result that their genetic population structure has been intractable. We describe the use of eight hypervariable microsatellite loci to study the genetic relationships between four Canadian polar bear populations: the northern Beaufort Sea, southern Beaufort Sea, western Hudson Bay, and Davis Strait ‐ Labrador Sea. These markers detected considerable genetic variation, with average heterozygosity near 60% within each population. Interpopulation differences in allele frequency distribution were significant between all pairs of populations, including two adjacent populations in the Beaufort Sea. Measures of genetic distance reflect the geographic distribution of populations, but also suggest patterns of gene flow which are not obvious from geography and may reflect movement patterns of these animals. Distribution of variation is sufficiently different between the Beaufort Sea populations and the two more eastern ones that the region of origin for a given sample can be predicted based on its expected genotype frequency using an assignment test. These data indicate that gene flow between local populations is restricted despite the long‐distance seasonal movements undertaken by polar bears.

Undesirable evolutionary consequences of trophy hunting.

Phenotype-based selective harvests, including trophy hunting, can have important implications for sustainable wildlife management if they target heritable traits. Here we show that in an evolutionary response to sport hunting of bighorn trophy rams (Ovis canadensis) body weight and horn size have declined significantly over time. We used quantitative genetic analyses, based on a partly genetically reconstructed pedigree from a 30-year study of a wild population in which trophy hunting targeted rams with rapidly growing horns, to explore the evolutionary response to hunter selection on ram weight and horn size. Both traits were highly heritable, and trophy-harvested rams were of significantly higher genetic ‘breeding value’ for weight and horn size than rams that were not harvested. Rams of high breeding value were also shot at an early age, and thus did not achieve high reproductive success. Declines in mean breeding values for weight and horn size therefore occurred in response to unrestricted trophy hunting, resulting in the production of smaller-horned, lighter rams, and fewer trophies.

Microsatellite analysis of genetic variation in black bear populations

Measuring levels of genetic variation is an important aspect of conservation genetics The informativeness of such measurements is related to the variability of the genetic markers used; a particular concern in species, such as bears, which are characterized by low levels of genetic variation resulting from low population densities and small effective population sizes We describe the development of microsatellite analysis in bears and its use in assessing interpopulation differences in genetic variation in black bears from three Canadian National Parks These markers are highly variable and allowed identification of dramatic differences in both distribution and amount of variation between populations Low levels of variation were observed in a population from the Island of Newfoundland The significance of interpopulation differences in variability was tested using a likelihood ratio test of estimates of θ= 4Neu.

Gene flow between insular, coastal and interior populations of brown bears in Alaska

The brown bears of coastal Alaska have been recently regarded as comprising from one to three distinct genetic groups. We sampled brown bears from each of the regions for which hypotheses of genetic uniqueness have been made, including the bears of the Kodiak Archipelago and the bears of Admiralty, Baranof and Chichagof (ABC) Islands in southeast Alaska. These samples were analysed with a suite of nuclear microsatellite markers. The ‘big brown bears’ of coastal Alaska were found to be part of the continuous continental distribution of brown bears, and not genetically isolated from the physically smaller ‘grizzly bears’ of the interior. By contrast, Kodiak brown bears appear to have experienced little or no genetic exchange with continental populations in recent generations. The bears of the ABC Islands, which have previously been shown to undergo little or no female‐mediated gene flow with mainland populations, were found not to be genetically isolated from mainland bears. The data from the four insular populations indicate that female and male dispersal can be reduced or eliminated by water barriers of 2–4 km and 7km in width, respectively.

Estimating population size of grizzly bears using hair capture, DNA profiling, and mark-recapture analysis

We used DNA analysis to estimate grizzly bear (Ursus arctos) population size in a 9.866-km 2 area in southeast British Columbia and a 5,030-km 2 area in southwest Alberta. We sampled bears by removing hair at bait sites surrounded by a single strand of barbed wire. DNA profiling with microsatellites of the root portion of the hair was used to identify individuals. We collected hair from 109 different bears and had 25 recaptures in 5 10-day trapping sessions in British Columbia. In Alberta we collected hair from 37 bears and had 9 recaptures in 4 14-day sessions. A model in program CAPTURE (M h ) that acconmodates heterogeneity in (individual capture probaluilities estimated the population size in British Columbia as 262 (95% CI = 224-313) and in Alberta as 71 (60 100), We believe that hair capture combined with DNA profiling is a promising technique for estimating distribution and abundance of bears and potentially many other species. This approach is of special interest to management biologists because it can be applied at the scale conservation and management decisions are made.

Age-dependent sexual selection in bighorn rams

Although mating systems and sexual selection have been intensively studied in ungulate model systems, very few studies have combined genetic paternity analysis with individual phenotypic data over several breeding seasons. We used microsatellite paternity analysis to determine the parentage of 83 bighorn sheep (Ovis canadensis) born between 1995 and 2000 at Ram Mountain, Alberta, Canada. We could assign the paternity of 64 lambs at a high level of statistical confidence (95%). Within each season, the most successful ram sired an average of 35.5% of the lambs with assigned paternity, and a single ram sired 26.1% of all lambs over the six mating seasons. Although a few large-horned, mature (age 8+ years) rams had very high reproductive success, younger rams sired ca. 50% of the lambs. Mixed–effects models indicated that mating success increases as a nonlinear function of age, with horn length increasingly positive in correlation with mating success in older rams. These results indicate that young or small rams possibly achieve mating success through alternative mating tactics that are less dependent on body and weapon size, such as coursing and blocking. Sexual selection is therefore likely to have age–dependent effects on traits such as agility, body and horn size.

Genetic structure of the world’s polar bear populations

We studied genetic structure in polar bear (Ursus maritimus) populations by typing a sample of 473 individuals spanning the species distribution at 16 highly variable microsatellite loci. No genetic discontinuities were found that would be consistent with evolutionarily significant periods of isolation between groups. Direct comparison of movement data and genetic data from the Canadian Arctic revealed a highly significant correlation. Genetic data generally supported existing population (management unit) designations, although there were two cases where genetic data failed to differentiate between pairs of populations previously resolved by movement data. A sharp contrast was found between the minimal genetic structure observed among populations surrounding the polar basin and the presence of several marked genetic discontinuities in the Canadian Arctic. The discontinuities in the Canadian Arctic caused the appearance of four genetic clusters of polar bear populations. These clusters vary in total estimated population size from 100 to over 10 000, and the smallest may merit a relatively conservative management strategy in consideration of its apparent isolation. We suggest that the observed pattern of genetic discontinuities has developed in response to differences in the seasonal distribution and pattern of sea ice habitat and the effects of these differences on the distribution and abundance of seals.

Influence of landscape on the population genetic structure of the alpine butterfly parnassius smintheus (Papilionidae)

Four microsatellite DNA markers were developed which were used to examine the relationship between landscape and population genetic structure among a set of populations of the butterfly Parnassius smintheus located in the foothills of the Canadian Rockies. Detailed information on the dispersal of adult butterflies among this same set of populations was available. Simple and partial Mantel tests were used to examine the relationships between genetic distances, predicted rates of dispersal, and a number of landscape variables, all measured pairwise for 17 sample sites. Nei’s standard genetic distance was negatively correlated with predicted dispersal. We observed a significant pattern of isolation by distance at a very small spatial scale. The distance between sites that was through forest was a stronger predictor of genetic distance than the distance through open meadow, indicating a significant effect of landscape on population genetic structure beyond that of simple isolation by distance. Our results suggest that rises in the tree‐line in alpine areas, caused by global warming, will lead to reduced gene flow among populations of P. smintheus.

Characterization of microsatellite loci in caribou Rangifer tarandus, and their use in other artiodactyls.

useful tool for studying population structure and relatedness in diverse wild animal species (Amos et al. 1993; Morin et al. 1994; Paetkau et al. 1995). Some microsatellite loci have been developed for white-tailed deer (DeWoody et al. 1995), but as yet few are available for other cervids. Large populations of caribou Rangifer tarandus are widely distributed across northern Canada and in the northern regions of Scandinavia and Russia. Genetic analysis of caribou populations using microsatellites will be useful in addressing a number of population issues in this species, including the amount of genetic variation within and between populations, gene flow and herd identity. For this purpose, 13 microsatellites were isolated from caribou. Amplification with the caribou primers was attempted in a number of other artiodactyls, because cross-species amplification has proven to be successful for primers developed from other species in this order (Forbes et al. 1995; Engel et al. 1996). Caribou genomic DNA was isolated from whole blood samples. A genomic library was isolated as previously described (Paetkau & Strobeck 1994). Screening was performed with a synthetic (dT-dG)12 oligonucleotide probe including two biotinylated T residues, and a biotin detection kit (BRL). Approximately 100 positive clones resulted, of which 31 were sequenced as according to Zheng et al. (1995). Primers were designed for 23 clones using the computer program O L I G O (National Biosciences, Version 4.0) and tested for variability on a population sample of 20 caribou from the Selkirk region of south-eastern British Columbia in Canada. PCR amplifications were performed in a final reaction volume of 15 μL containing 60 ng of genomic DNA, 0.16 μM of each primer, 120 μM dNTP, 2 mM MgCl2, 0.3 units of Taq DNA polymerase (purified as described in Engelke et al. 1990) and PCR buffer (10 mM Tris buffer, pH 8.8, 0.1% Triton X100, 50 mM KCl and 0.16 mg/mL BSA). For loci RT 5 and RT 27, the MgCl2 concentration was lowered to 1.5 mM. In addition, for RT 27 the number of units of Taq DNA polymerase was reduced to 0.25. The microsatellite loci were amplified by PCR using a Perkin Elmer 9600 thermal cycler. The cycling conditions were 1 min at 94 °C, followed by three cycles of 30 s at 94 °C, 20 s at 54 °C and 5 s at 72 °C, followed by 33 cycles of 15 s at 94 °C, 20 s at 54 °C and 1 s at 72 °C, and then 30 s at 72 °C. All PCR products were electrophoresed using an ABI 373 A Automated Sequencer, which detects fluorescently labelled primers. The microsatellite loci exhibiting DNA bands having sufficient signal strength, no extra bands, and some degree of variability are listed in Table 1. Locus RT 23 was included in Table 1 as it showed a clean banding pattern in species other than caribou. Cross-species amplification with these primers was performed on four other cervid (white-tailed deer, mule deer, elk and moose) and two bovid (bison and bighorn sheep) species. Conditions for the heterologous amplifications were the same as those used to amplify the loci in caribou. The individuals from each species tested were from the same population. In all species examined, at least some of the loci were heterozygous (Table 2).

Genetic analysis reveals demographic fragmentation of grizzly bears yielding vulnerably small populations

Ecosystem conservation requires the presence of native carnivores, yet in North America, the distributions of many larger carnivores have contracted. Large carnivores live at low densities and require large areas to thrive at the population level. Therefore, if human-dominated landscapes fragment remaining carnivore populations, small and demographically vulnerable populations may result. Grizzly bear range contraction in the conterminous USA has left four fragmented populations, three of which remain along the Canada–USA border. A tenet of grizzly bear conservation is that the viability of these populations requires demographic linkage (i.e. inter-population movement of both sexes) to Canadian bears. Using individual-based genetic analysis, our results suggest this demographic connection has been severed across their entire range in southern Canada by a highway and associated settlements, limiting female and reducing male movement. Two resulting populations are vulnerably small (≤100 animals) and one of these is completely isolated. Our results suggest that these trans-border bear populations may be more threatened than previously thought and that conservation efforts must expand to include international connectivity management. They also demonstrate the ability of genetic analysis to detect gender-specific demographic population fragmentation in recently disturbed systems, a traditionally intractable yet increasingly important ecological measurement worldwide.