Genetic variation in a gradient of environmental variability: marine bivalvia (Mollusca)

Dominance reversals: the resolution of genetic conflict and maintenance of genetic variation

Beneficial reversals of dominance reduce the costs of genetic trade-offs and can enable selection to maintain genetic variation for fitness. Beneficial dominance reversals are characterized by the beneficial allele for a given context (e.g. habitat, developmental stage, trait or sex) being dominant in that context but recessive where deleterious. This context dependence at least partially mitigates the fitness consequence of heterozygotes carrying one non-beneficial allele for their context and can result in balancing selection that maintains alternative alleles. Dominance reversals are theoretically plausible and are supported by mounting empirical evidence. Here, we highlight the importance of beneficial dominance reversals as a mechanism for the mitigation of genetic conflict and review the theory and empirical evidence for them. We identify some areas in need of further research and development and outline three methods that could facilitate the identification of antagonistic genetic variation (dominance ordination, allele-specific expression and allele-specific ATAC-Seq (assay for transposase-accessible chromatin with sequencing)). There is ample scope for the development of new empirical methods as well as reanalysis of existing data through the lens of dominance reversals. A greater focus on this topic will expand our understanding of the mechanisms that resolve genetic conflict and whether they maintain genetic variation.

Protein variation in the marine bivalve Scrobicularia plana

Seventeen enzyme loci have been assayed for electrophoretically detectable variation in a population of the marine bivalve Scrobicularia plana. Mean heterozygosity is 0.120 +/- 0.033. In a comparison involving thirteen enzymes there is a significant correlation between heterozygosity in S. plana and Mytilus edulis and a suggestion of lower mean heterozygosity in S. plana. These findings are discussed in relation to current theories concerning the selective significance of protein variation.

Population differences of aspartate aminotransferase and peptidase in the bay mussel Mytilus edulis

This investigation has demonstrated considerable heterogeneity among populations and some heterogeneity within populations in the distribution of alleles at two variant loci of Mytilus edulis. Although the causes of this variation remain obscure, some speculations have been made on the basis of available data. A cline for aspartate aminotransferase (AAT) alleles has been observed on the Pacific Coast. An immigration model has been proposed to explain the atypical ecological and genetic characteristics of large mussels found on Amchitka Island, Alaska. Marked differences were found in the distribution of peptidase alleles among collections from Southern California, the North Pacific Ocean, and New Jersey. Deviations from random distribution of phenotypes observed in comparisons made between large and small mussels from the New Jersey collection may reflect selection operating on these loci in this population.

Electrophoretic variation in intertidal and subtidal organisms in Puget Sound, Washington

This paper presents information on the biochemical properties (via starch gel electrophoresis) of 26 marine species (15 fish and 11 invertebrates). Evidence is presented on 48 polymorphisms found in the 26 species. Estimates of the proportion of the genome heterozygous per individual per species were calculated and ranged from 0.000 to 0.176 for the invertebrates and from 0.000 to 0.074 for the fish species. Estimates of the percentage of loci polymorphic per species for invertebrates were 0 to 54%; for the fish species, the range was 0 to 20%.

Population genetics of Collisella subrugosa (Patellogastropoda: Acmaeidae): evidence of two scales of population structure

Marine invertebrate populations usually show high levels of genetic variability that has frequently been associated with spatial and temporal environmental heterogeneity. One of the most heterogeneous marine environments is the intertidal zone, the habitat of Collisella subrugosa, the most widespread and abundant Brazilian limpet. C. subrugosa has planktonic larvae that can disperse over long distances, what can promote gene flow among shores, working against interpopulational differentiation. In this study we investigated the genetic variability and populational substructure of C. subrugosa through analysis of 24 allozyme loci in 14 samples (590 individuals) collected along 2,700 km of the Brazilian coast. The genetic variability was high ([Formula: see text] and [Formula: see text]), as expected for intertidal species. Genetic differentiation among samples was low (F (ST) = 0.03) what may reflect intensive gene flow associated with larval dispersal. However, we detected an isolation-by-distance pattern of population substructure in one sampled region. High levels of heterozygote deficiency were also observed for many loci in each sample. Alternative hypothesis are discussed, and the "breeding groups" is suggested to explain these pattern, indicating the main cause as environmental heterogeneity.

Biochemical studies of aminopeptidase polymorphism in Mytilus edulis. II. Dependence of reaction rate on physical factors and enzyme concentration

Enzymatic parameters of aminopeptidase-I that may be sensitive to temperature an solute variations were investigated to provide a functional explanation for specific activity differences among genotypes in natural populations. The effect of temperature on the apparent Km of L-leucyl-4-methoxy 2-naphthylamide and th dipeptide phenylalanyl-glycine was small, especially between 10 and 25 C. The apparent Km varied only between 36.7 and 49.8 microM at these temperatures and the six common genotypes did not differ in temperature-dependent substrate affinities. While pH had a significant effect on Km, no differences among genotypes were observed. Activation enthalpies were also identical among genotypes. Thermal inactivation was slowest at 15 C and the same for all genotypes. Of 18 tested amino acids, only phenylalanine inhibited aminopeptidase-I; K1 values ranged from 1.2 to 0.8 mM and were the same for all genotypes. Small differences among genotypes were detected in the inhibitory effect of zinc. The concentration of aminopeptidase-I enzyme was the same for all genotypes in a population exposed to oceanic salinity, but the concentration of Lap 94/94 was 15% lower than that of other genotypes in a population experiencing estuarine salinity. Genotypes with the Lap 94 allele exhibited higher apparent Kcat values in all population samples.The probable genotype-dependent effects of enzyme concentration and Kcat differences are discussed with regard to maintenance of the polymorphism and genetic differences among populations.

Biochemical characterization of "LAP," a polymorphic aminopeptidase from the blue mussel, Mytilus edulis

A genetically variable naphthylamidase enzyme, previously described as "leucine aminopeptidase," was purified approximately fiftyfold, and its biochemical properties were investigated. The enzyme was renamed "aminopeptidase I." Substrate affinities demonstrate that it is an alpha-aminoacyl peptide hydrolase (E.C. 3.4.11.-). Aminopeptidase I had a monomer molecular weight of 65--68,000, average of pI of pH 4.88, and broad pH optima between 6.5 and 8.0. The enzyme was inactivated rapidly between 40 and 50 C. Antibodies from purified enzyme did not cross-react with other naphthylamidases, but aminopeptidase I activity was inhibited by immune serum. The enzyme exhibited highest naphthylamidase activity for aromatic and hydrophobic aminoacyl naphthylamides. Aminopeptidase activity was highest for aromatic and hydrophobic N-terminal residues of tripeptides. Certain divalent metal cations, p-OH-mercuribenzoate, and N-ethylmaleimide were strongly inhibitory while chelating agents activated the enzyme.

An estimate of the amount of genetic variation in the common mussel Mytilus edulis

Allozyme variation in a population of the common mussel Mytilus edulis in Mumbles, South Wales, has been studied by starch gel electrophoresis. On the basis of data obtained for 34 loci, we estimate the proportion of loci polymorphic to be 30%. Using only the 29 loci for which individual genotypes can be accurately typed, the average heterozygosity is estimated to be 9.5 +/- 3.6%. The calculated expected average heterozygosity based on Hardy-Weinberg expectations is identical with the observed value. Allele frequency data at six polymorphic loci are given for several other British populations. There is no significant geographic heterogeneity. The results are discussed in relation to genetic adaptive strategies and are shown to be inconsistent with the predictions of the neutral hypothesis.

Genetic variation in constant environments

Allozymic variation in proteins encoded by 32 gene loci was analyzed electrophoretically in 64 specimens from 6 localities representing 2 species of the spadefoot toads Pelobates syriacus and P. cultripes from Israel and Portugal, respectively. Out of the 32 loci examined, Esterase-1 was the only locus that proved strongly polymorphic in all 6 localities and in the 2 species. The pattern of genetic variation in Pelobates is best explained by the environmental variability model. Selection for homozygosity as an adaptive strategy seems to operate in the relatively constant and narrow subterranean niche.

High genetic variability in an ecologically variable vertebrate, Bufo viridis

Allozymic variation in proteins encoded by 26 loci was analyzed electrophoretically in 517 specimens of green toads from 11 populations from Israel and one population from Vis Island in the Adriatic Sea. Genetic variation in this toad is the highest yet reported in any vertebrate. All three genetic parameters, mean number of alleles per locus (A), mean proportion of loci polymorphic per population (P), and mean number of heterozygous loci per individual (H), are very high (A = 1.65, range 1.38-2.04; P = 0.423, range 0.346-0.615; H = 0.133, range 0.108-0.159). Central and marginal mainland populations are only slightly more variable than desert isolates, but much more variable than the Vis Island population. Genetic similarity is very high between mainland populations (S = 0.951, range 0.93-0.97). Frequencies of two alleles (Icd-lc and Tfa) are correlated with an ecological gradient of increasing aridity. Regulatory enzymes appeared to contribute more to overall polymorphism than non-regulatory enzymes. The genetic variation observed suggests that selection for heterozygosity as an adaptive strategy is operating in the ecologically variable environment in which green toads live.

Genetic variation as a test of natural selection

Allozymic variation encoded by 26 loci was analyzed electrophoretically in 507 specimens representing 12 populations of green toads, Bufo viridis, in Israel and the Vis Adriatic Island. Genetic variation in Bufo viridis is higher than in any vertebrate yet studied. Mean heterozygosity per locus per individual (H) is 0.133 (range, 0.105 to 0.159). H is higher in central populations as compared with isolates, and varies among four major protein classes, being highest in transferases and hydrolases and lowest in oxidoreductases and nonenzymatic proteins. Differential gene frequencies among polymorphisms was tested as an indicator of natural selection. Significant heterogeneity between loci in their apparent inbreeding coefficients Fe=S-2p/P(1-p) was found for all alleles and for each of the four major classes of proteins tested, which may be taken as evidence of selection. Both uniform and diversifying selection are suggested by the low and high Fe values, respectively. The general pattern of high heterozygosity in Bufo viridis is best explained as an adaptive strategy in heterogeneous environments.

Environmental and Evolutionary Stability in Bivalve Mollusks

There is no relationship between environmental stability [as indicated by infaunal (stable) versus epifaunal (unstable) habits] and the generic duration of extinct marine bivalve mollusks when the effects of cosmopolitanism (which is associated with long generic durations) and other paleontological "noise" are excluded. This is contrary to the "depauperate gene pool" hypothesis of extinctions.

Genetic variation in a heterogeneous environment. I. Temporal heterogeneity and the absolute dominance model

The conditions for a stable polymorphism and the equilibrium gene frequency in an infinite population are compared when there is spatial or temporal environmental heterogeneity for the absolute dominance model. For temporal variation the conditions for stability are more restrictive and the equilibrium gene frequency is often at a low gene frequency. In a finite population, temporal environmental heterogeneity for the absolute dominance model was found to be quite ineffective in maintaining genetic variation and is often less effective than no selection at all. For comparison, the maximum maintenance for temporal variation is related to the overdominant model. In general, cyclic environmental variation was found to be more effective at maintaining genetic variation than where the environment varies stochastically. The importance of temporal environmental variation and the maintenance of genetic variation is discussed.

A general model to account for enzyme variation in natural populations

Approximate conditions for genetic polymorphism in temporally and spatially varying environments are presented for loci which are intermediate at the level of fitness or at the level of gene function. The conditions suggest that polymorphism will be more likely in more variable environments while unlikely in constant environments. Biochemical evidence is presented to justify the assumption of heterozygote intermediacy. Observations on natural populations are cited which substantiate the claim that allozymic polymorphism is primarily due to selection acting on environmental variation in gene function.

Taxonomic and Environmental Stability in the Paleozoic

Available paleontological data show no relationship between environmental stress (proximity to the ancient shoreline) and duration of marine benthic macroinvertebrate genera in the Paleozoic. This is contrary to inferences from fossil community lineages, but in accord with genetic and physiological data on living taxa.