Nathalie Tessier

Comparative estimation of effective population sizes and temporal gene flow in two contrasting population systems

Estimation of effective population sizes (Ne) and temporal gene flow (Nem, m) has many implications for understanding population structure in evolutionary and conservation biology. However, comparative studies that gauge the relative performance of Ne, Nem or m methods are few. Using temporal genetic data from two salmonid fish population systems with disparate population structure, we (i) evaluated the congruence in estimates and precision of long‐ and short‐term Ne, Nem and m from six methods; (ii) explored the effects of metapopulation structure on Ne estimation in one system with spatiotemporally linked subpopulations, using three approaches; and (iii) determined to what degree interpopulation gene flow was asymmetric over time. We found that long‐term Ne estimates exceeded short‐term Ne within populations by 2–10 times; the two were correlated in the system with temporally stable structure (Atlantic salmon, Salmo salar) but not in the highly dynamic system (brown trout, Salmo trutta). Four temporal methods yielded short‐term Ne estimates within populations that were strongly correlated, and these were higher but more variable within salmon populations than within trout populations. In trout populations, however, these short‐term Ne estimates were always lower when assuming gene flow than when assuming no gene flow. Linkage disequilibrium data generally yielded short‐term Ne estimates of the same magnitude as temporal methods in both systems, but the two were uncorrelated. Correlations between long‐ and short‐term geneflow estimates were inconsistent between methods, and their relative size varied up to eightfold within systems. While asymmetries in gene flow were common in both systems (58–63% of population‐pair comparisons), they were only temporally stable in direction within certain salmon population pairs, suggesting that gene flow between particular populations is often intermittent and/or variable. Exploratory metapopulation Ne analyses in trout demonstrated both the importance of spatial scale in estimating Ne and the role of gene flow in maintaining genetic variability within subpopulations. Collectively, our results illustrate the utility of comparatively applying Ne, Nem and m to (i) tease apart processes implicated in population structure, (ii) assess the degree of continuity in patterns of connectivity between population pairs and (iii) gauge the relative performance of different approaches, such as the influence of population subdivision and gene flow on Ne estimation. They further reiterate the importance of temporal sampling replication in population genetics, the value of interpreting Neor m in light of species biology, and the need to address long‐standing assumptions of current Ne, Nem or m models more explicitly in future research.

Climate change and habitat fragmentation drive the occurrence of Borrelia burgdorferi, the agent of Lyme disease, at the northeastern limit of its distribution

Lyme borreliosis is rapidly emerging in Canada, and climate change is likely a key driver of the northern spread of the disease in North America. We used field and modeling approaches to predict the risk of occurrence of Borrelia burgdorferi, the bacteria causing Lyme disease in North America. We combined climatic and landscape variables to model the current and future (2050) potential distribution of the black‐legged tick and the white‐footed mouse at the northeastern range limit of Lyme disease and estimated a risk index for B. burgdorferi from these distributions. The risk index was mostly constrained by the distribution of the white‐footed mouse, driven by winter climatic conditions. The next factor contributing to the risk index was the distribution of the black‐legged tick, estimated from the temperature. Landscape variables such as forest habitat and connectivity contributed little to the risk index. We predict a further northern expansion of B. burgdorferi of approximately 250–500 km by 2050 – a rate of 3.5–11 km per year – and identify areas of rapid rise in the risk of occurrence of B. burgdorferi. Our results will improve understanding of the spread of Lyme disease and inform management strategies at the most northern limit of its distribution.

Genetic structure of the white-footed mouse in the context of the emergence of Lyme disease in southern Québec.

The white-footed mouse (Peromyscus leucopus) has expanded its northern limit into southern Québec over the last few decades. P. leucopus is a great disperser and colonizer and is of particular interest because it is considered a primary reservoir for the spirochete bacterium that causes Lyme disease. There is no current information on the gene flow between mouse populations on the mountains and forest fragments found scattered throughout the Montérégie region in southern Québec, and whether various landscape barriers have an effect on their dispersal. We conducted a population genetics analysis on eleven P. leucopus populations using eleven microsatellite markers and showed that isolation by distance was weak, yet barriers were effective. The agricultural matrix had the least effect on gene flow, whereas highways and main rivers were effective barriers. The abundance of ticks collected from mice varied within the study area. Both ticks and mice were screened for the presence of the spirochete bacterium Borrelia burgdorferi, and we predicted areas of greater risk for Lyme disease. Merging our results with ongoing Lyme disease surveillance programs will help determine the future threat of this disease in Québec, and will contribute toward disease prevention and management strategies throughout fragmented landscapes in southern Canada.

Canid hybridization: contemporary evolution in human‐modified landscapes

Contemporary evolution through human-induced hybridization occurs throughout the taxonomic range. Formerly allopatric species appear especially susceptible to hybridization. Consequently, hybridization is expected to be more common in regions with recent sympatry owing to human activity than in areas of historical range overlap. Coyotes (Canis latrans) and gray wolves (C. lupus) are historically sympatric in western North America. Following European settlement gray wolf range contracted, whereas coyote range expanded to include eastern North America. Furthermore, wolves with New World (NW) mitochondrial DNA (mtDNA) haplotypes now extend from Manitoba to Québec in Canada and hybridize with gray wolves and coyotes. Using mtDNA and 12 microsatellite markers, we evaluated levels of wolf-coyote hybridization in regions where coyotes were present (the Canadian Prairies, n = 109 samples) and absent historically (Québec, n = 154). Wolves with NW mtDNA extended from central Saskatchewan (51°N, 69°W) to northeastern Québec (54°N, 108°W). On the Prairies, 6.3% of coyotes and 9.2% of wolves had genetic profiles suggesting wolf-coyote hybridization. In contrast, 12.6% of coyotes and 37.4% of wolves in Québec had profiles indicating hybrid origin. Wolves with NW and Old World (C. lupus) mtDNA appear to form integrated populations in both regions. Our results suggest that hybridization is more frequent in historically allopatric populations. Range shifts, now expected across taxa following climate change and other human influence on the environment, might therefore promote contemporary evolution by hybridization.

Molecular identification of two species of myiasis-causing Cuterebra by multiplex PCR and RFLP.

Abstract.  The myiasis‐causing flies Cuterebra grisea (Coquillet) and Cuterebra fontinella (Clark) (Diptera: Oestridae) are normally parasites of mice, predominantly of the genus Peromyscus. The morphological similarities of these species and the existence of intermediate morphotypes bearing characters of both species make the identification of adults problematic; furthermore the identification of larvae is apparently not possible. This study presents two molecular approaches to discriminate between these species using specific band patterns: (i) species‐specific primers designed in the cytochrome oxidase II (COII) region used in multiplex polymerase chain reaction (PCR) and (ii) restriction fragment length polymorphism (RFLP) on amplified segments of cytochrome oxidase I (COI) gene. Both methods were tested on Cuterebra larvae and on adult museum specimens. The two techniques showed a clear difference between C. grisea and C. fontinella, although species‐specific primers were more successful than RFLP for degraded DNA. No intraspecific variation in RFLP and species‐specific amplifications were detected for the two species of Cuterebra. The results exhibit discrepancies between molecular and morphological identification, suggesting that some of the adults were misidentified.

Phylogeographic Structure of the White-Footed Mouse and the Deer Mouse, Two Lyme Disease Reservoir Hosts in Québec

Modification of a species range is one of many consequences of climate change and is driving the emergence of Lyme disease in eastern Canada. The primary reservoir host of the bacteria responsible for Lyme disease, Borrelia burgdorferi, is the white-footed mouse (Peromyscus leucopus), whose range is rapidly shifting north into southern Québec. The deer mouse, P. maniculatus, is occurring over most Québec province and is a less competent host for B. burgdorferi. Here, we compared the phylogeographic structure of both Peromyscus species in Québec. Using a combination of multiple mitochondrial DNA markers and phylogeographic methods, we detected an ongoing and rapid expansion of P. leucopus, while P. maniculatus appears more stable. Haplotype and populations networks indicated that populations of P. maniculatus exhibit more genetic structure than P. leucopus across the study area. Furthermore, significant and consistent genetic divergences between populations of the two species on both sides of the St. Lawrence River suggest that distinct lineages of P. leucopus and P. maniculatus with different ancestral origins colonized Southern Québec following the Last Glacial Maximum. The phylogeographic structure of pathogens is expected to mirror the structure observed in their reservoir hosts. As different strains of Borrelia burgdorferi may be associated with different levels of pathogenicity and immune responses of their hosts, our results are helpful at better understanding the pattern of spread of Lyme disease in a zone of emergence, and associated risk for human populations.

Genetic Confirmation of Cougars (Puma concolor) in Eastern Canada

Abstract - This paper presents the results of a long-term study to detect the presence of Puma concolor (Cougar) in eastern Canada. We installed 38 scratching posts to attract wild Cougars and collect hair samples in several national and provincial parks in Québec, New Brunswick, and Nova Scotia. A set of semi-nested primers was used to discriminate Cougar samples from other mammalian species based on variation in the 16S rRNA gene of the mitochondrial DNA. Our analyses performed on 476 hair samples revealed 19 positive identifications of Cougars in Québec and New Brunswick. Sequencing further showed that some specimens were from South America, whereas others had a North American origin. We discuss the implications of these results for the conservation of Cougars in eastern Canada.

Paternity Analysis of Wood Turtles (Glyptemys insculpta) Reveals Complex Mating Patterns

Mating system characteristics are of great importance as they may influence male and female reproductive success and reproductive isolation. The wood turtle (Glyptemys insculpta) is a terrestrial freshwater species listed as endangered by the International Union for Conservation of Nature. Considering its conservation status and the paucity of information currently available on parentage relationship for the species, we performed a microsatellite analysis to study the mating system of wood turtles in the Shawinigan River (Québec). We sampled 38 clutches over 2 years (14 in 2006 and 24 in 2007), for a total of 248 offspring genotyped with 7 microsatellite loci. The reconstructed genotypes of the fathers revealed that reproductive success in the sampled clutches varied greatly between males and are positively correlated with the number of mates and clutches sired. Frequency of multiple paternity was estimated at 37% through a consensus of 3 different estimation methods. Positive correlation was observed between the genetic diversity of clutches and the number of fathers. Repeat paternity, however, was observed in 88% of the clutches by the same female in successive years, which suggests either a frequent use of sperm storage, or remating with the same partner in successive years.

Applying genetic methods to identify northern and southern flying squirrels and determine conservation needs

The two species of flying squirrels found in North America are the northern (Glaucomys sabrinus) and the southern (G. volans) flying squirrel. Both species have wide range distributions across North America, yet differences in their preferred habitat result in few areas of sympatry. Climate change however, has shifted the southern flying squirrel’s range limit further north, thus allowing the two species to encounter each other more frequently. Glaucomys volans is the more competitive of the two, and also carries a nematode parasite (Strongyloides robustus) proven to be deleterious towards its sister species. The northern incursion of the southern flying squirrel may therefore dislodge its sister species from its original distribution. With G. sabrinus considered endangered in different parts of North America, and G. volans listed as “concerned” in its northernmost limit, the two species require close monitoring. Morphological identification is often challenging for incomplete and juvenile specimens, therefore we developed a molecular identification protocol using mitochondrial and nuclear markers in tandem to help distinguish each species. These protocols were tested on Glaucomys individuals located elsewhere in Canada and the USA to ensure the validity of the designed primers. Moreover, employing our methods may detect F1 hybrids. To assess the current genetic portrait of G. sabrinus populations in Québec before the potential invasion of its sister species, a preliminary population genetics study using seven microsatellite loci was also realized. The various conservation and management implications are discussed.