Robb F. Leary

The problems with hybrids: setting conservation guidelines

Abstract Rates of hybridization and introgression are increasing dramatically worldwide because of translocations of organisms and habitat modifications by humans. Hybridization has contributed to the extinction of many species through direct and indirect means. However, recent studies have found that natural hybridization has played an important role in the evolution of many plant and animal taxa. Determining whether hybridization is natural or anthropogenic is crucial for conservation, but is often difficult to achieve. Controversy has surrounded the setting of appropriate conservation policies to deal with hybridization and introgression. Any policy that deals with hybrids must be flexible and must recognize that nearly every situation involving hybridization is different enough that general rules are not likely to be effective. We provide a categorization of hybridization to help guide management decisions

Fluctuating asymmetry as an indicator of stress: Implications for conservation biology

Extinction can be attributed broadly to environmental or genetic stress. The ability to detect such stresses before they seriously affect a population can enhance the effectiveness of conservation programs. Recent studies have shown that within-individual morphological variability may provide a valuable early indicator of environmental and genetic stress.

Superior Developmental Stability of Heterozygotes at Enzyme Loci in Salmonid Fishes

We examined the association between heterozygosity at 42 enzyme loci and fluctuating asymmetry at five bilateral meristic characters in 14 population samples of rainbow trout, cutthroat trout, and brook trout. There is a significant negative correlation between heterozygosity and the proportion of asymmetric characters per individual in two populations of rainbow trout and two populations of cutthroat trout. This correlation is negative in 13 of the 14 population samples (sign test, P < .001). Thus, individuals that are more heterozygous at isozyme loci have reduced fluctuating asymmetry; this appears to be general among salmonid fishes. We also found that individuals with obvious morphological deformities in three populations have increased fluctuating asymmetry. Thus, fluctuating asymmetry appears to be a reliable indicator of overall developmental stability and is therefore negatively correlated with fitness. The association between heterozygosity and asymmetry and between asymmetry and morphological deformities suggest that heterozygosity is positively correlated with fitness in these fishes.

Hybridization rapidly reduces fitness of a native trout in the wild

Human-mediated hybridization is a leading cause of biodiversity loss worldwide. How hybridization affects fitness and what level of hybridization is permissible pose difficult conservation questions with little empirical information to guide policy and management decisions. This is particularly true for salmonids, where widespread introgression among non-native and native taxa has often created hybrid swarms over extensive geographical areas resulting in genomic extinction. Here, we used parentage analysis with multilocus microsatellite markers to measure how varying levels of genetic introgression with non-native rainbow trout (Oncorhynchus mykiss) affect reproductive success (number of offspring per adult) of native westslope cutthroat trout (Oncorhynchus clarkii lewisi) in the wild. Small amounts of hybridization markedly reduced fitness of male and female trout, with reproductive success sharply declining by approximately 50 per cent, with only 20 per cent admixture. Despite apparent fitness costs, our data suggest that hybridization may spread due to relatively high reproductive success of first-generation hybrids and high reproductive success of a few males with high levels of admixture. This outbreeding depression suggests that even low levels of admixture may have negative effects on fitness in the wild and that policies protecting hybridized populations may need reconsideration.

DEVELOPMENTAL INSTABILITY AND HIGH MERISTIC COUNTS IN INTERSPECIFIC HYBRIDS OF SALMONID FISHES

Interspecific hybrids have been proposed to have reduced developmental stability in comparison to their parental species because the parental genomes have not undergone selection for the maintenance of developmental stability when they occur together. We present data from four interspecific hybrids of salmonid fishes that support this view. Natural hybrids of bull trout (Salvelinus confluentus) with brook trout (Salvelinus fontinalis) and laboratory hybrids of rainbow trout (Salmo gairdneri) with Yellowstone (Salmo clarki bouvieri), westslope (S. c. lewisi), and coastal (S. c. clarki) cutthroat trout all have higher levels of fluctuating asymmetry than either of their parental species raised in the same environment. Thus, the hybrids have reduced developmental stability.

Heterozygosity and developmental stability in gynogenetic diploid and triploid rainbow trout

We have previously reported that rainbow trout (Salmo gairdneri) more heterozygous at enzyme loci generally show increased developmental stability, as measured by reduced fluctuating asymmetry. We experimentally produced gynogenetic diploid and triploid individuals to test the effect of extreme heterozygosities on developmental stability. Gynogenetic diploids are identical by descent at an estimated 34 per cent of all loci and show a 50 per cent increase in the mean proportion of traits asymmetric per individual compared to normal rainbow trout. Triploids from two different strains have an estimated 30 per cent increase in genomic heterozygosity and a 14 per cent decrease in the mean proportion of traits asymmetric per individual compared to normal diploids. These changes in asymmetry induced by gynogenesis and triploidy are not as great as we expected based on the association between heterozygosity and fluctuating asymmetry in random mating populations. The narrow range of mean asymmetry in rainbow trout may result from an upper limit restricted by directional selection for developmental stability and a lower limit determined by inherent randomness in the developmental process.

Developmental Instability as an Indicator of Reduced Genetic Variation in Hatchery Trout

Abstract We present evidence for reduced developmental stability in a hatchery stock of westslope cutthroat trout Salmo clarki lewisi. These fish have a high frequency of two obvious morphological deformities and an unusually high amount of asymmetry at five bilateral meristic traits. We have previously reported that this stock has a reduced amount of genetic variation at 35 isozyme loci compared to the wild population from which it was derived. The loss of genetic variation in this stock apparently has reduced the ability of these fish to develop precisely along genetically determined developmental pathways. We also present the biological rationale and the methods for using fluctuating asymmetry as a diagnostic tool for detecting the effects of the loss of genetic variation in cultured stocks of fish. Received April 7, 1984 Accepted November 8, 1984

Genetic Divergence and Identification of Seven Cutthroat Trout Subspecies and Rainbow Trout

Abstract We estimated the amount of genetic divergence among seven cutthroat trout Salmo clarki subspecies and rainbow trout Salmo gairdneri using electrophoretic data from 46 protein loci. There was little genetic divergence among the Colorado River, finespotted, greenback, and Yellowstone subspecies of cutthroat trout, but a large amount existed among the coastal, Lahontan, and westslope subspecies. These latter three subspecies were electrophoretically as similar to rainbow trout—or more so—as they were to the other four subspecies of cutthroat trout examined. Morphologically, in contrast, the cutthroat trout subspecies were all more similar to each other than to rainbow trout. The data, therefore, suggest that morphological and protein evolution have proceeded at different rates among some of these fishes. The presence of fixed or nearly fixed allele-frequency differences between the subspecies of cutthroat trout and rainbow trout and between many pairs of cutthroat trout subspecies provides a powerfu...

Heterozygosity in gynogenetic diploids and triploids estimated by gene-centromere recombination rates

Abstract We present methods for estimating the effect on heterozygosity of diploid gynogenesis and triploidy induced by suppressing the second meiotic division. The reduction in heterozygosity, as estimated by the fixation index (F), induced by a single generation of diploid gynogenesis is ( 1− y ), where y is the mean frequency of second meiotic division segregation. The fixation index in triploids at a single locus depends upon y and the number and frequencies of alleles present. A simple estimate of the maximum increase in heterozygosity is provided by F = −( y 2 ) ; this estimate assumes that there are only two alleles at each locus. The minimum increase in heterozygosity at a locus with multiple alleles is F = −( y n ) , where n is the number of alleles present.

RAD sequencing yields a high success rate for westslope cutthroat and rainbow trout species-diagnostic SNP assays

Hybridization with introduced rainbow trout threatens most native westslope cutthroat trout populations. Understanding the genetic effects of hybridization and introgression requires a large set of high‐throughput, diagnostic genetic markers to inform conservation and management. Recently, we identified several thousand candidate single‐nucleotide polymorphism (SNP) markers based on RAD sequencing of 11 westslope cutthroat trout and 13 rainbow trout individuals. Here, we used flanking sequence for 56 of these candidate SNP markers to design high‐throughput genotyping assays. We validated the assays on a total of 92 individuals from 22 populations and seven hatchery strains. Forty‐six assays (82%) amplified consistently and allowed easy identification of westslope cutthroat and rainbow trout alleles as well as heterozygote controls. The 46 SNPs will provide high power for early detection of population admixture and improved identification of hybrid and nonhybridized individuals. This technique shows promise as a very low‐cost, reliable and relatively rapid method for developing and testing SNP markers for nonmodel organisms with limited genomic resources.