No abstract

We analyzed 13 microsatellite loci to estimate gene flow among westslope cutthroat trout, Oncorhynchus clarkii lewisi, populations and determine the invasion pattern of hybrids between native O. c. lewisi and introduced rainbow trout, Oncorhynchus mykiss, in streams of the upper Flathead River system, Montana (USA) and British Columbia (Canada). Fourteen of 31 sites lacked evidence of O. mykiss introgression, and gene flow among these nonhybridized O. c. lewisi populations was low, as indicated by significant allele frequency divergence among populations (θST = 0.076, ρST = 0.094, P < 0.001). Among hybridized sites, O. mykiss admixture declined with upstream distance from a site containing a hybrid swarm with a predominant (92%) O. mykiss genetic contribution. The spatial distribution of hybrid genotypes at seven diagnostic microsatellite loci revealed that O. mykiss invasion is facilitated by both long distance dispersal from this hybrid swarm and stepping-stone dispersal between hybridized populations. This study provides an example of how increased straying rates in the invasive taxon can contribute to the spread of extinction by hybridization and suggests that eradicating sources of introgression may be a useful conservation strategy for protecting species threatened with genomic extinction.

Rapid and inexpensive methods for genome-wide SNP discovery and genotyping are urgently needed for population management and conservation. In hybridized populations, genomic techniques that can identify and genotype thousands of species-diagnostic markers would allow precise estimates of population- and individual-level admixture, as well as identification of “super invasive” alleles, which show elevated rates of introgression above the genome-wide background (likely due to natural selection). Techniques like restriction-site associated DNA (RAD) sequencing can discover and genotype large numbers of SNPs, but they have been limited by the length of continuous sequence data they produce with Illumina short-read sequencing. We present a novel approach, overlapping paired-end RAD sequencing, to generate RAD contigs of >300-400bp. These contigs provide sufficient flanking sequence for design of high-throughput SNP genotyping arrays and strict filtering to identify duplicate paralogous loci. We applied this approach in five populations of native westslope cutthroat trout that previously showed varying (low) levels of admixture from introduced rainbow trout. We produced 77,141 RAD contigs and used these data to filter and genotype 3,180 previously identified species-diagnostic SNP loci. Our population-level and individual-level estimates of admixture were generally consistent with previous microsatellite-based estimates from the same individuals. However, we observed slightly lower admixture estimates from genome-wide markers, which might result from natural selection against certain genome regions, different genomic locations for microsatellites versus RAD-derived SNPs, and/or sampling error from the small number of microsatellite loci (n = 7). We also identified candidate adaptive super invasive alleles from rainbow trout that had excessively high admixture proportions in hybridized cutthroat trout populations.

The invasion of nonnative fishes in freshwater systems is often facilitated by the interaction of biotic and abiotic factors operating at multiple spatial and temporal scales. We evaluated the association of local habitat features (width, gradient, and elevation), watershed characteristics (mean and maximum summer water temperatures, the number of road crossings, and road density), and biotic factors (the distance to the source of hybridization and trout density) with the spread of hybridization between native westslope cutthroat trout Oncorhynchus clarkii lewisi and introduced rainbow trout O. mykiss in the upper Flathead River system in Montana and British Columbia. The presence of hybridization and the proportion of rainbow trout admixture were estimated using seven diagnostic microsatellite loci. We defined logistic and linear regression models including various combinations of spatial and environmental factors and used an information-theoretic approach to evaluate the relative plausibility of these models. Models combining measures of water temperature, disturbance, and source connectivity were the best-approximating ones for the presence of hybridization. Hybridization was positively associated with mean summer water temperature and the number of upstream road crossings and negatively associated with the distance to the main source of hybridization. The best-approximating models associated with the level of introgression among hybridized sites included measures of temperature, source connectivity, and the density of trout. The proportion of rainbow trout admixture was negatively related to the distance to the source and positively related to mean summer water temperature and density. Our results suggest that hybridization is more likely to occur and spread in streams with warm water temperatures, increased land use disturbance, and proximity to the main source of hybridization. However, habitat features alone may not limit the spread of hybridization; populations with high proportions of admixture and high densities may have to be reduced or eliminated.

Abstract. The source strength and capability of aerosol particles in the Arctic to act as cloud condensation nuclei have important implications for understanding the indirect aerosolcloud effect within the polar climate system. It has been shown in several Arctic regions that ultrafine particle (UFP) formation and growth is a key contributor to aerosol number concentrations during the summer. This study uses aerosol number size distribution measurements from shipboard expeditions aboard the research icebreaker CCGS Amundsen in the summers of 2014 and 2016 throughout the Canadian Arctic to gain a deeper understanding of the drivers of UFP formation and growth within this marine boundary layer. UFP number concentrations (diameter > 4 nm) in the range of 10 1 -10 4 cm −3 were observed during the two seasons, with concentrations greater than 10 3 cm −3 occurring more frequently in 2016. Higher concentrations in 2016 were associated with UFP formation and growth, with events occurring on 41 % of days, while events were only observed on 6 % of days in 2014. Assessment of relevant parameters for aerosol nucleation showed that the median condensation sink in this region was approximately 1.2 h −1 in 2016 and 2.2 h −1 in 2014, which lie at the lower end of ranges observed at even the most remote stations reported in the literature. Apparent growth rates of all observed events in both expeditions averaged 4.3 ± 4.1 nm h −1 , in general agreement with other recent studies at similar latitudes. Higher solar radiation, lower cloud fractions, and lower sea ice concentrations combined with differences in the developmental stage and activity of marine microbial communities within the Canadian Arctic were documented and help explain differences between the aerosol measurements made during the 2014 and 2016 expeditions. These findings help to motivate further studies of biosphere-atmosphere interactions within the Arctic marine environment to explain the production of UFP and their growth to sizes relevant for cloud droplet activation.