Biochemical studies of aminopeptidase polymorphism in Mytilus edulis. I. dependence of enzyme activity on season, tissue, and genotype

The influence of stochastic temperature fluctuations in shaping the physiological performance of the California mussel, Mytilus californianus

Climate change is forecasted to increase temperature variability and stochasticity. Most of our understanding of thermal physiology of intertidal organisms has come from laboratory experiments that acclimate organisms to submerged conditions and steady-state increases in temperatures. For organisms experiencing the ebb and flow of tides with unpredictable low tide aerial temperatures, the reliability of reported tolerances and thus predicted responses to climate change requires incorporation of environmental complexity into empirical studies. Using the mussel Mytilus californianus, our study examined how stochasticity of the thermal regime influences physiological performance. Mussels were acclimated to either submerged conditions or a tidal cycle that included either predictable, unpredictable or no thermal stress during daytime low tide. Physiological performance was measured through anaerobic metabolism, energy stores and cellular stress mechanisms just before low tide, and cardiac responses during a thermal ramp. Both air exposure and stochasticity of temperature change were important in determining thermal performance. Glycogen content was highest in the mussels from the unpredictable treatment, but there was no difference in the expression of heat shock proteins between thermal treatments, suggesting that mussels prioritise energy reserves to deal with unpredictable low tide conditions. Mussels exposed to fluctuating thermal regimes had lower gill anaerobic metabolism, which could reflect increased metabolic capacity. Our results suggest that while thermal magnitude plays an important role in shaping physiological performance, other key elements of the intertidal environment complexity such as stochasticity, thermal variability, and thermal history are also important considerations for determining how species will respond to climate warming.

EXCLUSION OF THE ROLE OF SECONDARY CONTACT IN AN ALLELE FREQUENCY CLINE IN THE MUSSEL MYTILUS EDULIS

We examined the hypothesis that secondary contact generates an allele-frequency cline at the aminopeptidase-I locus (Lap) in the marine mussel, Mytilus edulis. It has been proposed that variation at the Lap locus is neutral and that the cline results from secondary contact between differentiated oceanic and estuarine populations (Levinton, 1980). We tested this hypothesis by comparing the genotypic distributions in samples from the cline to distributions that incorporate mixing effects. We employed a statistical model that determines the degree of contact using a maximum likelihood estimator and then incorporates the mixing estimates into an expected distribution of genotypes. Wahlund effects resulting from possible admixture are thereby incorporated into the expected distribution. Failure of the model to reconcile the observed with the expected distribution of genotypes indicates that the observed population structure does not result from admixture. The null hypothesis of mixing was unable to explain about 33% of the samples. Combined tests demonstrated the general departure from the mixing model to be highly significant. The distribution of heterozygote discrepancies across the cline was inconsistent with the expectations of a mixing model. Therefore we reject explanations for the structure of the Lap cline that involve secondary contact. Selection directed at the Lap locus appears necessary to explain the genotypic structure of clinal populations.

THE PHYSIOLOGICAL BASIS OF NATURAL SELECTION AT THE LAP LOCUS

An extensive research program was undertaken to evaluate the contribution of genetic variation at the Lap locus to variation in physiological traits under natural conditions. Rates of carbon and nitrogen metabolism were monitored in a population of the mussel Mytilus edulis near the center of the Lap allele frequency cline on the north shore of Long Island. The goal of this research was to establish whether the previously described genotype-dependent differences in physiological phenotype are meaningful in ecologically relevant circumstances. It was predicted from laboratory studies that, in nature, genotype-dependent differences will exist for rates of nitrogen excretion and that other aspects of the animal's physiology, particularly rates of carbon metabolism, will be unaffected by Lap genotype. Rates of amino acid and ammonia excretion were significantly dependent upon Lap genotype; individuals with the Lap⁹⁴ allele exhibited greater rates of nitrogen loss. These differences among genotypes were most evident in the fall, between September and December. The genotype-dependent component of rates of nitrogen loss were also largest relative to the total rate of excretion during the fall period. As predicted, other aspects of the nitrogen metabolism (acquisition) and rates of carbon metabolism were independent of Lap genotype. There was a striking congruity among a variety of observations that all indicate that phenotypic differences in nitrogen metabolism are the basis of natural selection at the Lap locus in Long Island Sound. Rates of growth were minimal during the fall months (Hilbish, 1985) and mussels are known to lose weight in a genotype-specific manner during this period (Koehn et al., 1980). Rates of elemental gain and loss were summed to produce carbon and nitrogen budgets; these data show the fall to be a period of extended deficit in carbon and nitrogen balance. Genotype-dependent losses of ammonia and amino acids were greatest during the fall months. Finally, selection against the Lap⁹⁴ allele occurs predominantly in the fall (Hilbish, 1985). The data indicate that the depletion of nitrogen resources provides the basis for selection against Lap⁹⁴ genotypes during the fall months.

Differences in allele frequencies of Aat between high- and mid-rocky shore populations of Littorina saxatilis (Olivi) suggest selection in this enzyme locus

Samples of the intertidal prosobranch Littorina saxatilis were collected along vertical transects from high- to mid-store levels at five different geographic locations of western Europe. Electrophoretic screening of ten metabolic enzymes revealed five highly polymorphic loci. Four of these showed no or few significant differences in allele frequencies between high- and mid-shore samples of Littorina saxatilis. The fifth locus, Aat (aspartate aminotransferase, EC 2.6.1.1), showed clinal variation in allele frequencies over the few metres of each transect, suggesting that this locus, or a coupled locus, is under selection with a slow allele (Aat100) favoured in mid-shore habitats and a faster allele (Aat120) selected for in the high littoral fringe.

Rapid enzyme assays investigating the variation in the glycolytic pathway in field-caught populations of Fundulus heteroclitus

Variation in enzyme expression may be important in evolutionary adaptation, yet is seldom studied. Furthermore, no studies have examined the expression of all enzymes in a defined metabolic pathway. Enzyme concentration is a measure of enzyme expression and was ascertained by assaying maximal activity. Presented here is an analysis of variation of maximal enzyme activity for all the enzymes in a single metabolic pathway, glycolysis, from three clinically distributed populations of the fish, Fundulus heteroclitus. Techniques for rapidly analyzing maximal enzyme activity for all the enzymes of an entire metabolic pathway from many individuals are described. The high degree of repeatability (mean coefficient of variation for replicates, 4.4%) and sensitivity (less than 3 mg of tissue is required to measure all 10 enzymes) of these assays demonstrate the utility of such an approach for analyzing variation among populations for a large numbers of enzymes. Results from these studies indicate that (1) the average coefficient of variation for all enzyme determinations within a population is 45.3% and (2) between populations, the activity of 5 of the 10 glycolytic enzymes are significantly different. This considerable variation occurs even in populations where there is little allelic variation. These data demonstrating substantial variation in enzyme expression support the idea that changes in gene regulation may be as important as, or even more important than, changes in biochemical kinetic parameters in evolutionary processes.

Dominance in physiological phenotypes and fitness at an enzyme locus

Aminopeptidase-I allozymes, which are products of the Lap locus in the marine mussel, Mytilus edulis, differ in their catalytic efficiencies. These biochemical differences result in genotype-specific rates of change in the free amino acid pool, that is, cell volume regulation, when mussels are subjected to changes in salinity. A high degree of dominance was found among genotypes for these biochemical and physiological phenotypes. Selection models that incorporate dominance adequately predict observed genotypic properties at the Lap locus among natural populations that exhibit clinical allele frequency. This suggests that a high degree of dominance for fitness must also occur at this locus in natural populations. These results provide additional evidence that the maintenance of an allele frequency cline is operating by natural selection at the Lap locus.

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.