The role of genetic structure in the adaptive divergence of populations experiencing saltwater intrusion due to relative sea-level rise.

Saltwater intrusion into estuaries creates stressful conditions for nektonic species. Previous studies have shown that Gambusia affinis populations with exposure to saline environments develop genetic adaptations for increased survival during salinity stress. Here, we evaluate the genetic structure of G. affinis populations, previously shown to have adaptations for increased salinity tolerance, and determine the impact of selection and gene flow on structure of these populations. We found that gene flow was higher between populations experiencing different salinity regimes within an estuary than between similar marsh types in different estuaries, suggesting the development of saline-tolerant phenotypes due to local adaptation. There was limited evidence of genetic structure along a salinity gradient, and only some of the genetic variation among sites was correlated with salinity. Our results suggest limited structure, combined with selection to saltwater intrusion, results in phenotypic divergence in spite of a lack of physical barriers to gene flow.

Adaptation to sea level rise: does local adaptation influence the demography of coastal fish populations?

This study compared the growth of two western mosquitofish Gambusia affinis populations that were previously demonstrated to have genetic adaptations that increased survival under lethal salinity exposures. The objective was to evaluate how genetic adaptations to lethal salinity stress affect population demography when exposed to sublethal salinity stress. Results indicate that chronic salinity exposure had a generally negative impact on population size, but fish originating from one of the two populations established with fish from a brackish site exhibited an increase in population size. Saltwater intrusion seems to result in reduced population size for most populations. Some populations inhabiting more saline sites, however, may develop localized adaptations, mitigating the consequences of increased salinity on population productivity.

Estimating allelic richness: effects of sample size and bottlenecks.

Although differences in sampling intensity can bias comparisons of allelic richness (A) among populations, investigators often fail to correct estimates of A for differences in sample size. Methods that standardize A on the basis of the size of the smallest number of samples in a comparison are preferable to other approaches. Rarefaction and repeated random subsampling provide unbiased estimates of A with the greatest precision and thus provide greatest statistical power to detect differences in variation. Less promising approaches, in terms of bias or precision, include single random subsampling, eliminating very small samples, using sample size as a covariate or extrapolating estimates obtained from small samples to a larger number of individuals.

Relationship between allozyme genotype and sensitivity to stressors in the western mosquitofish Gambusia affinis detected for elevated temperature but not mercury.

Previous studies with Gambusia holbrooki have found associations of allozyme genotype with tolerance to metals, pesticides, heat, and salinity. To examine the generality of these relationships, we looked for similar associations of mercury and heat tolerance with allozyme genotype at the GPI-2, MDH-1 and MDH-2 loci in its sister species Gambusia affinis. This was done to assess if the loci themselves or closely linked loci were associated with mercury tolerance, because weaker linkage associations would be unlikely to persist across species boundaries. Moreover, the use of two very different types of stress allowed us to determine if the higher tolerance of particular allozyme genotypes is specific to a certain stress or reflects a higher tolerance to stress in general. Associations between genotype and tolerance to mercury and heat were determined in laboratory exposures of about 875 fish in each of two exposures where tolerance was measured as time-to-death, followed by electrophoresis on cellulose acetate gels. For none of the three loci did we find an association of genotype with tolerance to mercury. This contrasts with reports of such an association for GPI-2 and MDH-1 in G. holbrooki, so our results do not support the hypothesis that observed associations between allozyme genotypes and mercury tolerance are due to the allozymes or closely linked loci. However, our comparison was weakened by a scarcity of the GPI-2 genotypes reported to be mercury sensitive in G. holbrooki. Furthermore, rapid mortality in our mercury exposure may have affected the ability to detect genotypic differences in survival. The MDH-1 heterozygote showed higher tolerance to heat stress compared to homozygotes, although this difference was only significant for the most common homozygous genotype. No such relationship between MDH-1 and heat stress has been reported in G. holbrooki. We found no evidence that associations between allozyme genotype and tolerance are similar for different types of stresses, which could be an advantage for using allozymes as an indicator of exposure history to a stressor. Our study also showed that G. affinis sex and weight influence tolerance to mercury and heat.

Experimental evaluation of the usefulness of microsatellite DNA for detecting demographic bottlenecks.

Evolutionary and conservation biologists often use molecular markers to evaluate whether populations have experienced demographic bottlenecks that resulted in a loss of genetic variation. We evaluated the utility of microsatellites for detection of recent, severe bottlenecks and compared the amounts of genetic diversity lost in bottlenecks of different sizes. In experimental mesocosms, we established replicate populations by releasing 1, 2, 4 or 8 pairs of the western mosquitofish, Gambusia affinis (Poeciliidae). Using eight polymorphic microsatellite loci, we quantified seven indices of genetic diversity or change that have been used to assess the effects of demographic bottlenecks on populations. We compared indices for the experimentally bottlenecked populations to those for the source population and examined differences between populations established with different numbers of founders. Direct count heterozygosity and the proportion of polymorphic loci were not very sensitive to genetic changes that resulted from the experimental bottlenecks. Heterozygosity excess and expected heterozygosity were useful to varying degrees in the detection of bottlenecks. Allelic diversity and temporal variance in allele frequencies were most sensitive to genetic changes that resulted from the bottlenecks, and the temporal variance method was slightly more correlated with bottleneck size than was allelic diversity. Based on comparisons to a previous study with allozymes, heterozygosity, temporal variance in allele frequencies and allelic diversity, but not proportion of polymorphic loci, appear to be more sensitive to demographic bottlenecks when quantified using microsatellites. We found that analysis of eight highly polymorphic loci was sufficient to detect a recent demographic bottleneck and to obtain an estimate of the magnitude of bottleneck severity.

Genetic diversity in black howler monkeys (Alouatta pigra) from Belize.

To assess the level of genetic variation in a threatened black howler monkey (Alouatta pigra) population, we examined 36 allozyme loci and restriction fragment profiles of mitochondrial DNA (mtDNA). Mean heterozygosity at allozyme loci was only 0.021 and 5.6 percent of the loci were polymorphic. Analyses of mtDNA also revealed low genetic diversity compared with other primates. F-statistics revealed no significant genetic heterogeneity among troops within the Bermudian Landing preserve, but did indicate a deficiency of heterozygotes at one of the two loci. We explore several explanations for this result, which is unexpected in a socially structured primate. Low genetic diversity in this population may reflect its history of demographic bottlenecks.

Lipid and water depletion in migrating passerines following passage over the Gulf of Mexico.

Lipid depletion is currently believed to be the primary factor limiting flight duration of migrating birds in North America, while the influence of water loss is thought to be small. Three migrating species of passerines, wood thrush (Hylocichla mustelina), Swainson's thrush (Catharus ustulatus), and summer tanager (Piranga rubra) were captured during the 1993 spring migration just after crossing the Gulf of Mexico and examined for lipid and water depletion. The redwinged blackbird (Agelaius phoeniceus), a winter resident, was examined for comparison. Although some migrants had low levels of fat, most were not seriously fat-depleted and had much higher lipid levels than red-winged blackbirds. Migrants appeared dehydrated, usually having less than 60% body water and significantly less water than the blackbirds. Recaptured wood thrushes had significantly higher mass than when initially captured. Many of these birds were replenishing significant amounts of water, while the acquisition of lipid mass was dependent on the amount of time the birds spent on the study site. Migrants were significantly fatter but had significantly less water mass when captured during unfavorable weather than under favorable conditions. It appears that both dehydration and lipid depletion are major physiological constraints on migrating passerines.

COMPARATIVE POPULATION GENETIC STRUCTURE OF A PARASITE (FASCIOLOIDES MAGNA) AND ITS DEFINITIVE HOST.

The population genetic structure of the American liver fluke, Fascioloides magna, and its definitive host the white-tailed deer, Odocoileus virginianus, was examined in South Carolina. Flukes were significantly more common in deer from river-swamp habitat than upland areas and prevalence increased with host age. The distribution of flukes among deer occurred as a negative binomial with the mean dispersion parameter, k, equal to 0.17 and the range from 0.10 to 1.11 within local areas. Significant spatial genetic differentiation was observed for flukes and deer. Patterns of genetic distance in flukes were not concordant with those of the definitive host nor were they related to geographic distance between sample locations. Spatial genetic differentiation among flukes reflected the tendency for individual hosts to harbor multiple individuals from a limited number of parasite clones. The large population size of the parasite and movements of the definitive host tend to counteract factors that lead to spatial differentiation.