Evidence Of Strong Selection Research Articles

Population genetic structure of a nonmigratory estuarine fish (Fundulus heteroclitus) across a strong gradient of polychlorinated biphenyl contamination

Populations of the estuarine fish Fundulus heteroclitus indigenous to contaminated sites exhibit heritable resistance to some of the toxic effects of early life-stage exposure to polychlorinated biphenyls (PCBs). This evolved tolerance provides evidence of strong selection by PCBs, and it suggests other potential genetic effects of these stressors on resident populations. Environmental contaminants have the potential to affect the genetic structure of populations and to reduce genetic diversity, but species life-history traits, particularly patterns of migration and dispersal, also influence the distribution of genetic variation among populations. Therefore, the present was conducted to determine whether genetic diversity or genetic structure is altered in populations of F. heteroclitus indigenous to 18 sites in Massachusetts (USA) and Rhode Island (USA), representing a steep gradient of sediment PCB contamination and culminating in a Superfund site at New Bedford Harbor (NBH; MA, USA). Allele frequencies at enzymatic loci were used to assess genetic structure and diversity. Selection experiments using a highly toxic PCB congener (3,3',4,4',5-pentachlorobiphenyl) were conducted to determine if genetic patterns at field sites could be associated with contaminant exposures. Although allele frequencies clearly reflected a pattern of isolation by distance, the results indicated neither significant loss of genetic diversity nor alteration of allele frequencies for populations of F. heteroclitus in NBH.

Divergent selection during speciation of Lake Malawi cichlid fishes inferred from parallel radiations in nuptial coloration.

Repeated evolution of the same phenotypic difference during independent episodes of speciation is strong evidence for selection during speciation. More than 1,000 species of cichlids, >10% of the world's freshwater fish species, have arisen within the past million years in Lakes Malawi and Victoria in eastern Africa. Many pairs of closely related sympatric species differ in their nuptial coloration in very similar ways. Nuptial coloration is important in their mate choice, and speciation by sexual selection on genetically or ecologically constrained variation in nuptial coloration had been proposed, which would repeatedly produce similar nuptial types in different populations, a prediction that was difficult to test in the absence of population-level phylogenies. We measured genetic similarity between individuals within and between populations, species, and lake regions by typing 59 individuals at >2,000 polymorphic genetic loci. From these data, we reconstructed, to our knowledge, the first larger species level phylogeny for the most diverse group of Lake Malawi cichlids. We used the genetic and phylogenetic data to test the divergent selection scenario against colonization, character displacement, and hybridization scenarios that could also explain diverse communities. Diversity has arisen by replicated radiations into the same color types, resulting in phenotypically very different, yet closely related, species within and phenotypically highly similar yet unrelated sets of species between regions, which is consistent with divergent selection during speciation and is inconsistent with colonization and character displacement models.

Genetic diversity and selection in the maize starch pathway.

Maize is both phenotypically and genetically diverse. Sequence studies generally confirm the extensive genetic variability in modern maize is consistent with a lack of selection. For more than 6,000 years, Native Americans and modern breeders have exploited the tremendous genetic diversity of maize (Zea mays ssp. mays) to create the highest yielding grain crop in the world. Nonetheless, some loci have relatively low levels of genetic variation, particularly loci that have been the target of artificial selection, like c1 and tb1. However, there is limited information on how selection may affect an agronomically important pathway for any crop. These pathways may retain the signature of artificial selection and may lack genetic variation in contrast to the rest of the genome. To evaluate the impact of selection across an agronomically important pathway, we surveyed nucleotide diversity at six major genes involved in starch metabolism and found unusually low genetic diversity and strong evidence of selection. Low diversity in these critical genes suggests that a paradigm shift may be required for future maize breeding. Rather than relying solely on the diversity within maize or on transgenics, future maize breeding would perhaps benefit from the incorporation of alleles from maize's wild relatives.

Do pheromone binding proteins converge in amino acid sequence when pheromones converge?

Convergence in amino acid sequences between proteins can be strong evidence for selection. Here, I look for evidence of convergence in the amino acid sequences of pheromone binding protein (PBP) in response to convergence in pheromones. PBPs are involved in sex pheromone reception by the antennae of male moths. In this role PBPs may selectively bind pheromone components and experience convergent selection in response to convergence in pheromone components. However, examination of the PBPs of the taxa that have converged upon the use of (E)- or (Z)-11-tetradecenyl acetate as their major pheromone component reveals little evidence for convergence in the PBPs identified from these taxa. A few sites show a pattern consistent with convergence or parallelism; however, it cannot be ruled out that these sites share the ancestral state. Two of these sites fall within the proposed binding region of PBPs. These results suggest that PBPs either have not converged in sequence or have converged at very few sites in response to convergence on the same pheromone component.

MEASUREMENTS OF NATURAL SELECTION ON FLORAL TRAITS IN WILD RADISH (RAPHANUS RAPHANISTRUM). II. SELECTION THROUGH LIFETIME MALE AND TOTAL FITNESS.

It has often been suggested that selection on floral traits in hermaphroditic plants should occur primarily through differences in male fitness. However, measurements of selection on floral traits through differences in lifetime male fitness have been lacking. We measured selection on a variety of wild radish floral traits using lifetime male fitness measures derived from genetic paternity analysis. These male fitness estimates were then combined with estimates of lifetime female fitness of the same plants to produce measurements of selection based on lifetime total fitness. Contrary to the prediction above, there was no strong evidence for selection on floral morphology through male fitness differences in any of the three years of the study, but there was strong selection for increased flower size through female fitness differences in one year. The main determinant of both male and female fitness in all years was flower number; this lead to moderately positive correlations between male and female fitness in all three years.

Relation of Breast Cancer with Passive and Active Exposure to Tobacco Smoke

Studies on passive smoking have consistently shown a tendency toward an increased risk of breast cancer, while studies an active smoking have failed to demonstrate an association. This apparent contradiction may stem from not separating passive smokers from the unexposed when assessing the effect of active smoking. a population based case-control study was conducted in Geneva, Switzerland, between January 1992 and October 1993 to determine the relation of passive and active smoking to breast cancer when the referent unexposed category consisted of women unexposed to active and passive smoke. The 244 patients with breast cancer (cases) were compared with 1.032 women free of breast cancer (controls). The lifetime history of active and passive smoking was recorded year by year, between the age of 10 and the date of the interview. The adjusted odds of breast cancer for ever active smokers, compared with women unexposed to either passive or active smoke, were 2.2 (95% confidence interval (CI) 1.0-4.4) for an average lifetime consumption of 1-9 cigarettes per day, 2.7% (95% CI 1.4-5.4) for 10-19 cigarettes per day, and 4.6 (95% CI 2.2-9.7) for 30 or more cigarettes per day. Among passive smokers, the adjusted odds ratio was 3.2 (95% CI 1.6-6.3) for being exposed for the equivalent of 2 hours per day for 25 years. The odds ratios were adjusted for known or postulated risk factors of breast cancer, including alcohol and saturated fat intake. There was no evidence of strong selection, detection, or recall biases. Active and passive exposure to tobacco smoke may increase the risk of breast cancer. Additional studies are needed to decide whether the association is causal. Further elucidation of this relation would benefit not only the prevention of breast cancer but also the the prevention of other smoking-related diseases in women.

FIFTY YEARS OF INTERSPECIFIC HYBRIDIZATION: GENETICS AND MORPHOMETRICS OF A CONTROLLED EXPERIMENT ON THE LAND SNAIL CERION IN THE FLORIDA KEYS.

An unusually well documented case of secondary hybridization and introgression involving two morphologically dissimilar species of land snails is described. In 1915, 55 Bahamian Cerion casablancae were established on Bahia Honda Key, Florida. The introduced snails thrived, bred true to form, and the colony grew until about 1928 when it began to hybridize with C. incanum, the unrelated Florida species. In 1977, morphological hybrids occupied an area of about 3.5 ha. Spatial and temporal aspects of the interaction were characterized morphologically (18 shell characters were studied in source populations, founders, and hybrids sampled in 1933 and 1977) and genetically (variation at 17 allozyme loci in relevant populations sampled in 1977). In addition, special circumstances permitted us to discern the genetics of the founders: C. incanum is isogenic locally, C. casablancae is variable (Po = 0.29-0.35), and the two species are well-differentiated (Nei's D̄ = 0.27). Hybrid intermediacy of form and continued enhanced variation appeared in characters from three covariance sets, while some unique hybrid morphologies resulted from characters in a fourth set. Comparison of 1933 with 1977 samples showed that the hybrids are gradually approaching C. incanum in phenotype. Regardless of phenotype, the snails on Bahia Honda Key were panmictic and outbreeding. There was no evidence for strong selection against hybrids, and by 1977 introduced alleles had spread over 5 ha. However, no pure C. casablancae remain; low (m = 0.026/generation) but persistent gene flow has brought about a 30% diminution of the introduced genome. These observations are useful in interpreting Cerion's remarkable variability as colonization following hurricane dispersal has undoubtedly played a role in the group's complex evolution. More generally, the results are relevant to the problem of interpreting hybrid zones of unknown origin, and the differences in the generally concordant patterns of morphological and genetic introgression reveal constraints on the way components of different coadapted genomes interact.

Allelic forms of rat kappa chain genes: evidence for strong selection at the level of nucleotide sequence.

The genes that code for two allotypic forms of the rat kappa light chain constant region (C(kappa)) have been cloned and the nucleotide sequence of 1172 base pairs of coding and flanking sequence has been determined for both alleles. These sequences have been compared to each other and to the corresponding sequences found in the mouse and human. Comparison of the LEW allele with mouse C(kappa) reveals two surprising features: (i) There is an unusually large number of amino acid substitutions (21) relative to the total number of nucleotide changes (37) in the coding region. Comparision among several other mammalian genes reveals a larger proportion of "silent" changes. (ii) The rate of accumulation of base substitutions is the same within the coding region as it is in some 870 base pairs of noncoding sequence (including 3' untranslated, 3' flanking, and 5' intervening sequences). Comparison of the two allelic forms of rat C(kappa) shows the same unusual features in more extreme form. (i) Twelve base substitutions in the coding region determine 11 amino acid differences-only one "silent" change exists. (ii) There are 12 base substitutions in the 318 base pairs of coding sequence (3.7% difference) and only 9 in the remaining 854 base pairs of noncoding DNA (1.1%), a highly significant difference. This degree of conservation of noncoding sequences and of "silent" sites within the coding region is unique among the mammalian genes studied thus far. These patterns suggest that there has been strong selection for conservation of nucleotide sequences, both inside and outside the coding region, independent of the selection required to maintain the function and characteristic structure of the immunoglobulin domain itself. The functions of the nucleotide sequences that account for this selective pressure are unclear at the present time.

Samoan Migrant Fertility: Adaptation and Selection

Samoan women who migrate to traditional communities on Oahu experience less environmental change are exposed to the new environment for less of their reproductive period have relatively few of the other characteristics which would expose them to modern influences and are more severely constrained by their past reproductive history than women who settle in the less traditional communities. It is not surprising that the age-specific fertility rates of the former group are considerably closer to the nonmigrant Samoans than those of women settling in less traditional areas. Migrants to the less traditional communities are exposed to greater environmental change for a longer portion of their reproductive years have markedly lower age-specific fertility rates before migration and have education and labor force characteristics which tend to expose them to modernizing influences. It appears that migrants to the less traditional communities are selected for low age-specific fertility rates before migration. Although there is some evidence of postmigration catching up these women bear 1 less child on average during their reproductive period than women in traditional areas. The strong evidence of selection found in the data reported here highlights the importance of greater emphasis on the premigration characteristics of migrants in future research. (authors)

Allozyme clines and evidence of strong selection in three loci inMytilus edulisL. (Bivalvia) from Danish waters

Conspicuous differences in allozyme frequencies are found between Mytilus edulis populations from Bornholm in the Baltic and from the Kattegat and the North Sea in three allozyme loci (PGM: phospho...

Enzyme Polymorphism and Adaptation in Alpine Butterflies

The high levels of enzyme polymorphism often detected by electrophoresis probably reflect on-going balancing selection. However, hypotheses of single gene heterosis (such as overdominance or environmental balance) do not seem adequate to account for this variation. To investigate this problem further, polymorphism was studied in Colorado butterflies of the genus Colias. Colias are easy to breed and study in the field and occupy a diverse array of habitats. Four species were studied: all were found to be highly polymorphic. Polymorphism at the a-glycerophosphate dehydrogenase locus occurred only when the species in question occupied a montane habitat; no polymorphism was seen in populations occupying alpine or lowland habitats. Three populations of C. meadii were examined in detail which encompass both alpine and montane habitats (crossing timberline); in one of them detailed demographic studies were carried out. In each such population, marked dines were observed in a-GPdH frequency, despite the swamping effects of migration within the population. At thirteen other loci a variety of different patterns of allele frequency are seen (some clinal, some uniform, some discontinuous), despite the fact that all loci were assayed in the same individuals. This result constitutes strong evidence of selection. There is clear evidence that particular alleles at the different loci preferentially associate together at specific locations along the line, different assemblages occurring at different locations. A hypothesis is presented that these represent integrated metabolic phenotypes, and that the enzyme polymorphism is a multi-locuis strategy to preserve that integration in a heterogeneous environment.