Distinct sources of gene flow produce contrasting population genetic dynamics at different range boundaries of a Choristoneura budworm.

Populations are often exposed to multiple sources of gene flow, but accounts are lacking of the population genetic dynamics that result from these interactions or their effects on local evolution. Using a genomic clines framework applied to 1,195 single nucleotide polymorphisms, we documented genomewide, locus-specific patterns of introgression between Choristoneura occidentalis biennis spruce budworms and two ecologically divergent relatives, C. o. occidentalis and Choristoneura fumiferana, that it interacts with at alternate boundaries of its range. We observe contrasting hybrid indexes between the two hybrid zones, no overlap in "gene-flow outliers" (clines showing relatively extreme extents or rates of locus-specific introgression) and variable linkage disequilibrium among those outliers. At the same time, correlated genomewide rates of introgression between zones suggest the presence of processes common to both boundaries. These findings highlight the contrasting population genetic dynamics that can occur at separate frontiers of a single population, while also suggesting that shared patterns may frequently accompany cases of divergence-with-gene-flow that involve a lineage in common. Our results point to potentially complex evolutionary outcomes for populations experiencing multiple sources of gene flow.

Life-stage differences in spatial genetic structure in an irruptive forest insect: implications for dispersal and spatial synchrony.

Dispersal determines the flux of individuals, energy and information and is therefore a key determinant of ecological and evolutionary dynamics. Yet, it remains difficult to quantify its importance relative to other factors. This is particularly true in cyclic populations in which demography, drift and dispersal contribute to spatio-temporal variability in genetic structure. Improved understanding of how dispersal influences spatial genetic structure is needed to disentangle the multiple processes that give rise to spatial synchrony in irruptive species. In this study, we examined spatial genetic structure in an economically important irruptive forest insect, the spruce budworm (Choristoneura fumiferana) to better characterize how dispersal, demography and ecological context interact to influence spatial synchrony in a localized outbreak. We characterized spatial variation in microsatellite allele frequencies using 231 individuals and seven geographic locations. We show that (i) gene flow among populations is likely very high (Fst  ≈ 0); (ii) despite an overall low level of genetic structure, important differences exist between adult (moth) and juvenile (larvae) life stages; and (iii) the localized outbreak is the likely source of moths captured elsewhere in our study area. This study demonstrates the potential of using molecular methods to distinguish residents from migrants and for understanding how dispersal contributes to spatial synchronization. In irruptive populations, the strength of genetic structure depends on the timing of data collection (e.g. trough vs. peak), location and dispersal. Taking into account this ecological context allows us to make more general characterizations of how dispersal can affect spatial synchrony in irruptive populations.

Utility of microsatellites and mitochondrial DNA for species delimitation in the spruce budworm (Choristoneura fumiferana) species complex (Lepidoptera: Tortricidae).

Species identifications have been historically difficult in the Choristoneura fumiferana group, an important insect pest complex. We examined the utility of simple sequence repeats (SSRs, also referred to as microsatellites) and mitochondrial DNA (mtDNA) for delimiting and identifying eight currently recognized species sampled across North America. Four of these species formed discrete clusters using SSRs, while only two species were delimited with mtDNA. There was evidence for hybridization or incomplete lineage sorting between several species pairs. An integrative approach, using both phenotypic traits and molecular markers, allowed for the discrimination of more biologically relevant species units than did the use of molecular markers alone. As species are currently identified using putatively adaptive phenotypic traits, the differences observed between recognized species and neutral SSRs or mtDNA suggests that these species (or evolutionary significant units) have diverged via natural selection in spite of some gene flow.

A new species of rake-legged mite, Caeculus cassiopeiae (Prostigmata, Caeculidae), from Canada and a systematic analysis of its genus.

The genus Caeculus Dufour (Prostigmata, Caeculidae) contains 19 previously described species, most of which are found in North America, and for which no comprehensive phylogenetic treatment exists. Here, one new species from Alberta, Canada, is described: Caeculus cassiopeiae Bernard & Lumley, sp. nov., and another caeculid known to be present in Canada is documented. The new species is characterized within the genus with a character state matrix, from which an updated key is produced. A systematic analysis of all 20 species based on morphological and geographical distribution traits obtained from literature represents the first phylogenetic review of the genus.

Continent-wide population genomic structure and phylogeography of North America's most destructive conifer defoliator, the spruce budworm (Choristoneura fumiferana).

The spruce budworm, Choristoneura fumiferana, is presumed to be panmictic across vast regions of North America. We examined the extent of panmixia by genotyping 3,650 single nucleotide polymorphism (SNP) loci in 1975 individuals from 128 collections across the continent. We found three spatially structured subpopulations: Western (Alaska, Yukon), Central (southeastern Yukon to the Manitoba-Ontario border), and Eastern (Manitoba-Ontario border to the Atlantic). Additionally, the most diagnostic genetic differentiation between the Central and Eastern subpopulations was chromosomally restricted to a single block of SNPs that may constitute an island of differentiation within the species. Geographic differentiation in the spruce budworm parallels that of its principal larval host, white spruce (Picea glauca), providing evidence that spruce budworm and spruce trees survived in the Beringian refugium through the Last Glacial Maximum and that at least two isolated spruce budworm populations diverged with spruce/fir south of the ice sheets. Gene flow in the spruce budworm may also be affected by mountains in western North America, habitat isolation in West Virginia, regional adaptations, factors related to dispersal, and proximity of other species in the spruce budworm species complex. The central and eastern geographic regions contain individuals that assign to Eastern and Central subpopulations, respectively, indicating that these barriers are not complete. Our discovery of previously undetected geographic and genomic structure in the spruce budworm suggests that further population modelling of this ecologically important insect should consider regional differentiation, potentially co-adapted blocks of genes, and gene flow between subpopulations.

Life-history traits maintain the genomic integrity of sympatric species of the spruce budworm (Choristoneura fumiferana) group on an isolated forest island.

Identification of widespread species collected from islands can be challenging due to the potential for local ecological and phenotypic divergence in isolated populations. We sought to determine how many species of the spruce budworm (Choristoneura fumiferana) complex reside in Cypress Hills, an isolated remnant coniferous forest in western Canada. We integrated data on behavior, ecology, morphology, mitochondrial DNA, and simple sequence repeats, comparing Cypress Hills populations to those from other regions of North America to determine which species they resembled most. We identified C. fumiferana, C. occidentalis, C. lambertiana, and hybrid forms in Cypress Hills. Adult flight phenology and pheromone attraction were identified as key life-history traits involved in maintaining the genomic integrity of species. Our study highlights the importance of extensive sampling of both specimens and a variety of characters for understanding species boundaries in biodiversity research.