Allozyme Heterozygosity Research Articles

Hybrid vigor in Pacific oysters: an experimental approach using crosses among inbred lines

Two competing genetic hypotheses for heterosis, dominance and overdominance, have been championed to explain positive correlations between allozyme heterozygosity and fitness-related traits for bivalve molluscs. To begin to test these hypotheses, we made controlled crosses among inbred lines of the Pacific oyster Crassostrea gigas. In such mating experiments, heterosis ( h p ) can be defined and quantified through ANOVA as Q L > 1.0 or < − 1.0 , where L is the trait-difference between the two parental inbred lines and Q is twice the deviation of the hybrid from the mid-parent value (Griffing, 1990). Inbred lines of the Pacific oyster were made by selfing hermaphrodites; brood stock pedigrees and a grand mean fixation index of F = 0.5 were confirmed by allozymes. In two separate experiments pairs of inbred lines were crossed in 2 X 2 fashion to produce two inbred and two hybrid progeny genotypes. Each genotype was replicated by pairwise matings and replicates were grown in multiple containers (8 1 plastic bags), from which data on larval mortality (proportional decrement in population number per day) and size (shell-height) were obtained. In the first experiment, the two inbred genotypes differed significantly from each other in daily larval mortality (−0.115 vs. −0.881) and both hybrids had lower mortalities than inbred genotypes (−0.034 and −0.078); mean h p , 1.15, was significantly greater than 1.0. By Day 14, larvae of one hybrid group were significantly smaller than those of the inbred genotypes (241.0 μm vs. 253.5 and 259.2 μm, respectively); only a very small part of this difference could be attributed to negative correlation between larval density and growth. Negative heterosis for larval size was highly significant ( h p = −5.36, P < 0.001) and persisted to the juvenile stage (mean h p = −1.34, ns, for shell-height at Day 154 and h p = −7.77, P < 0.001, for live-weight at Day 163). In the second experiment, inbred genotypes again differed significantly in daily larval mortality (−0.341 vs. −0.654). One hybrid group had significantly less daily mortality than either parent (−0.110), but the other was similar to the best parent (−0.347), so that for one hybrid, h p = 2.47, P < 0.01, and for the other, h p = 0.96, ns). Variance among genotypes for larval shell-length was highly significant on Days 2, 7, and 14, with heterosis evident on Days 2 and 7 ( h p = 1.04 and h p = 3.84, respectively). At 11 months of age, hybrid shell-height averaged 150% of the shell-height for the best inbred parent and h p ≈ 7.7. These measurements of heterosis, both positive and negative, implicate a third explanation of heterosis, epistasis. Intercrosses of F 1 hybrids can be used to discriminate among dominance, overdominance and epistasis hypotheses for quantitative trait loci (QTL) causing hybrid vigor.

Analysis of the relationship between allozyme heterozygosity and fitness in the rare Gentiana pneumonanthe L.

AbstractEspecially for rare species occurring in small populations, which are prone to loss of genetic variation and inbreeding, detailed knowledge of the relationship between heterozygosity and fitness is generally lacking. After reporting on allozyme variation and fitness in relation to population size in the rare plant Gentiana pneumonanthe, we present a more detailed analysis of the association between heterozygosity and individual fitness. The aim of this study was to test whether increased fitness of more heterozygous individuals is explained best by the ‘inbreeding’ hypothesis or by the ‘overdominance’ hypothesis. Individual fitness was measured during 8 months of growth in the greenhouse as the performance for six life‐history parameters. PCA reduced these parameters to four main Fitness Components. Individual heterozygosity was scored for seven polymorphic allozyme loci.For some of these loci (e.g. Aat3, Pgm1 and 6Pgdh2) heterozygotes showed a significantly higher relative fitness than homozygotes. To test the inbreeding model, regression analyses were performed between each Fitness Component and the number of heterozygous loci per individual. Multiple regressions with the adaptive distance of five loci as independent variables were used to test the overdominance model. Only the inbreeding model was a statistically significant explanation for the relationship between heterozygosity and fitness in G. pneumonanthe.The number of heterozygous loci was significantly negatively correlated with the coefficients of variation of three of the six initially measured fitness parameters. This suggests a lower developmental stability among more homozygous plants and may explain the higher phenotypic variation in small populations of the species observed earlier. The importance of the results for conservation biology is discussed.

Variation in Fitness-Related Characters Among Small and Large Populations of Salvia Pratensis

1 The threatened perennial Salvia pratensis is restricted to a few isolated populations in the Netherlands, which vary in size from 10 to 1500 flowering individuals. Small populations are known to have significantly lower allozyme diversity than the large populations, probably as a consequence of genetic erosion. We test the prediction that small populations of this species will have lower fitness than large populations. 2 Seed from two small and two large populations was grown in a common environment. There was significant variation among populations in mean seed weight, seed size, percentage germination, plant biomass and growth, and in number of flowers per individual, but none of these differences could be attributed to differences in population size or level of allozyme diversity. 3 Differences in seed set, seed size and seedling size were probably largely influenced by nongenetic maternal effects. Differences in plant biomass at harvest after 5 months were independent of initial differences in seedling size, indicating that characters later in plant life are probably largely independent of maternal effects. 4 The present results, together with the known lack of difference in population growth rate and inbreeding depression in large and small populations, suggest that the small populations are in an early phase of the genetic erosion process, where their allozyme diversity has decreased, but where the quantitative genetic variation underlying fitness traits has not (yet) been affected. 5 Environmental differences and population history may be no less important than allozyme diversity for predicting differences in fitness among populations. Genetic erosion may be a major cause of viability problems in the future. [KEYWORDS: Allozymes; conservation biology; extinction; genetic erosion; population size Inbreeding depression; growth-rate; allozyme heterozygosity; genetic consequences; scabiosa-columbaria; seed production; rumex-crispus; size; extinction; plants]

Alternative models for allozyme-associated heterosis in the marine bivalve Spisula ovalis.

Correlations between allozyme heterozygosity and fitness-related traits, especially growth, have been documented in natural populations of marine bivalves. However, no consistent pattern has been exhibited, because heterotic effects on size vary with age and individual growth parameters are generally unknown. No consensus has emerged on the genetic basis of allozyme-associated heterosis. The species studied here, Spisula ovalis, displays annual shell growth lines, which allows us to compute individual age and growth dynamics over the whole life span. Our morphological study was coupled to a protein electrophoresis study at seven polymorphic loci. While the maximum size gained is not related to heterozygosity, the age at half maximum size, t1/2, is significantly negatively correlated with heterozygosity, indicating an heterotic effect on initial growth. The correlation between heterozygosity and size is expected to vanish when age increases, due to the form of the growth function. This decreasing correlation is consistent with previous studies. We compare the relative performances of five linear models to analyze the genetic basis of heterosis. Surprisingly, the largest part of variance in t1/2 is due to additive effects, the overdominant components being much weaker. Heterosis is therefore due to general genomic effects rather than to local overdominance restricted to allozymes or small neighboring chromosomal segments. A significant dependence of individual heterotic contributions of the enzyme loci upon expected heterozygosities, rather than metabolic function, further supports the hypothesis of enzymes acting as markers. General genomic effects can hold only if allozyme heterozygosity is positively correlated with heterozygosity at fitness-related genes scattered throughout the genome. This hypothesis is supported here by heterozygosity correlations between enzymatic loci.

Genetic Diversity in Two Avian Species Formerly Endemic to Guam

-We examine genetic diversity within endemic island avian species that recently went extinct in the wild and compare results with nonendangered, nonendemic, and nonisland species. The Micronesian Kingfisher (Halcyon cinnamomina cinnamomina) and flightless Guam Rail (Rallus owstoni) once were endemic to the Pacific island of Guam and now survive only in captivity. Horizontal starch-gel electrophoresis and minisatellite DNA profiles were used to measure genetic diversity within and among the rails, kingfishers and closely related species. Allozyme analyses of Micronesian Kingfishers revealed no genetic diversity (29 enzymes screened). DNA profiles were variable, although similarity (S) among founders to the captive kingfisher population was high relative to nonendangered birds (S = 0.56 ? SE of 0.02; 89 bands scored/individual). Three other Halcyon species examined had average allozyme heterozygosity and DNA similarity. Guam Rails had average to high allozyme heterozygosity as measured by four polymorphic loci (H = 0.03 to 0.05) relative to four other rail species (3 Rallus spp., 1 Porzana sp.). However, S (0.62 ? 0.02) was higher than in Micronesian Kingfishers. Further analyses indicated a significant deviation from Hardy-Weinberg equilibrium in two of three polymorphic loci for captive Guam Rails. Our results suggest that caution be taken in making generalizations about genetic diversity in island species. The type and number of genetic techniques used, number of species or populations examined, and variance in life-history traits among species must be considered. Received 14 March 1994, accepted 25 January 1995. ONE PREDICTION regarding island or small populations suggests that both founder effects and inbreeding due to population bottlenecks ought to reduce genetic diversity. The magnitude of the effect depends on factors such as severity of the bottleneck, population growth rate, and mutation rate (e.g. Wright 1931, 1940, Mayr 1963, Nei et al. 1975, Chakraborty and Nei 1977). This prediction has been studied in some detail for other taxa, but surprisingly few avian data are available (Boag 1986). Conclusions from avian research vary depending on the number of populations and measures of genetic diversity examined (Corbin et al. 1974, Yang and Patton 1981, Gyllensten et al. 1985, Parkin and Cole 1985, Baker and Moeed 1987, Baker et al. 1990a, b, Fleischer et al. 1991a, b, Triggs et al. 1992, Browne et al. 1993, Fleischer et al. 1994, Rave et al. 1994). Furthermore, many studies of island organisms have examined genetic diver3Present address: National Biological Service, Oregon State University, 3200 SW Jefferson Way, Corvallis, Oregon, USA 97331. sity for species that were introduced by man and do not reflect natural founding events. Meanwhile, avian species on islands continue to decline. For instance, 93% of avian species that went extinct between 1600 and 1980 were island endemics (King 1980) and 73% of birds that went extinct in North America and U.S. territories from 1492-1987 were island species (Williams and Nowak 1987). Thus, understanding processes contributing to loss of population viability takes on immediate importance. Introduction of the brown tree snake (Boiga irregularis) to the Northern Marianas island of Guam during World War II caused a precipitous decline or extinction for all of Guam's native forest birds, including Guam Rails (Rallus owstoni) and Micronesian Kingfishers (Halcyon cinnamomina cinnamomina; Savidge 1987, U.S. Fish and Wildlife Service 1984a, b). In this paper, we examine genetic diversity in these two recently extirpated endemic species. We then compare genetic diversity found in these island species with levels in mainland forms of similar species. Guam Rails are flightless and have life-his-

Evidence for Selection Against Heterozygotes: Post-Settlement Excess of Allozyme Homozygosity in a Cohort of the Chilean Oyster, Ostrea chilensis Philippi, 1845.

Reports of heterozygote deficiencies in electrophoretic surveys carried out in marine bivalves abound in the literature (1-6) but the mechanism or mechanisms producing this phenomenon have not been well defined. We report that, in the Chilean oyster (Ostrea chilensis), heterozygote deficiencies in a cohort obtained by mass spawning in the laboratory are not randomly distributed in time among genotypes. The eggs of the Chilean oyster are internally fertilized, and the larvae, which are brooded within the mantle cavity, have limited dispersal capabilities because of their extremely short pelagic stage (7). These features could allow mechanisms such as inbreeding or Wahlund effect to produce heterozygote deficiencies. However, we observed no significant heterozygote deficiencies in juveniles at 6 months of age; instead allozyme heterozygosity decreased over time. Inbreeding, Wahlund effect, aneuploidy, and null alleles are unlikely to be main causes of the heterozygosity deficiency in this cohort; if they were, the deficiency should be evident from the juvenile stage and would not necessarily increase over time (2, 5, 8, 9, 10). We suggest that selection against heterozygotes is the most probable cause of the increasing degree of heterozygote deficiency with age in this cohort of O. chilensis, a proposition that accords with data for other marine bivalve species (2, 4, 11).

Lack of association between allozyme heterozygosity and juvenile traits in Eucalyptus

Genetic variability for juvenile traits, which included basal diameter, height, biomass accumulation, and growth increment, was studied in eight provenances involving four species, Eucalyptus grandis, E. saligna, E. camaldulensis and E. urophylla, under uniform greenhouse conditions. The species differed significantly for all juvenile traits including individual mean allozyme heterozygosity, and high levels of variability were observed within and among species, which indicated that scope for selection exists for these traits. There was considerable reduction in genetic variability for all traits at 6 months of age when compared to that observed at 4 months. Eucalyptus saligna recorded the highest mean for basal diameter and biomass accumulation, while E. camaldulensis exhibited the highest mean for height. The two E. camaldulensis provenances exhibited the highest individual mean heterozygosities (0.425 and 0.417), followed by E. saligna (0.296 and 0.258), E. urophylla (0.254 and 0.233), and E. grandis (0.300 and 0.163). The juvenile traits were correlated with individual mean allozyme heterozygosity to examine the possibility of developing a genotypic basis for early selection of potentially superior individuals in a tree improvement program.

Developmental Noise, Phenotypic Plasticity, and Allozyme Heterozygosity in Daphnia

Developmental Noise, Phenotypic Plasticity, and Allozyme Heterozygosity in Daphnia

DEVELOPMENTAL NOISE, PHENOTYPIC PLASTICITY, AND ALLOZYME HETEROZYGOSITY IN DAPHNIA.

Previous theories and studies have postulated negative correlations between allozyme heterozygosity and developmental noise and between heterozygosity and phenotypic plasticity. We examined these relationships for morphological and life-history traits of Daphnia magna in four independent experiments using two different Moscow populations and one German population. Clones were raised under a range of food levels or individual densities. Heterozygosity was scored at five allozyme loci in two experiments and at three loci in two others. Relative differences in developmental noise among clones with different heterozygosity levels were estimated as the pooled residual variation from an analysis of variation that removed the effects of macroenvironment, clones, and their interaction. Plasticity was measured as the amount of macroenvironmental variation plus genotype-by-environment interaction variation. We found a positive correlation between developmental noise and heterozygosity, although this correlation varied among traits and experiments. This result contradicts most previous claims about these relationships. In contrast, we found that phenotypic plasticity and heterozygosity were negatively correlated for some traits. Developmental noise and phenotypic plasticity were correlated for only two traits in two different experiments. This trait-specific relationship is in concordance with previous studies. Our results could not be explained by effects of developmental time, a previously hypothesized mechanism. We propose several explanations for our results and the disparate results of others that do not require that heterozygosity be the actual cause of variation in developmental noise.

Modelling problems in conservation genetics using Drosophila: consequences of fluctuating population sizes.

Many natural populations fluctuate widely in population size. This is predicted to reduce effective population size, genetic variation, and reproductive fitness, and to increase inbreeding. The effects of fluctuating population size were examined in small populations of Drosophila melanogaster of the same average size, but maintained using either fluctuating (FPS) or equal (EPS) population sizes. FPS lines were maintained using seven pairs and one pair in alternate generations, and EPS lines with four pairs per generation. Ten replicates of each treatment were maintained. After eight generations, FPS had a higher inbreeding coefficient than EPS (0.60 vs. 0.38), a lower average allozyme heterozygosity (0.068 vs. 0.131), and a much lower relative fitness (0.03 vs. 0.25). Estimates of effective population sizes for FPS and EPS were 3.8 and 7.9 from pedigree inbreeding, and 4.9 vs. 7.1 from changes in average heterozygosities, as compared to theoretical expectations of 3.3 vs. 8.0. Results were generally in accordance with theoretical predictions. Management strategies for populations of rare and endangered species should aim to minimize population fluctuations over generations.

Allozyme and mitochondrial DNA variation in orange roughy, Hoplostethus atlanticus (Teleostei: Trachichthyidae): little differentiation between Australian and North Atlantic populations

Allozyme and mitochondrial DNA (mtDNA) genetic variation was compared in orange roughy (Hoplostethus atlanticus Collett) collected from waters off southern Australia and from waters about 22 000 km away in the North Atlantic west of Scotland. Samples were screened for 11 polymorphic allozyme loci and with 9 restriction enzymes. Significant heterogeneity between the two areas was detected for three allozyme loci (ADA *, CK * and GPI-1 *), and the overall G ST (gene-diversity statistic) value of ∼1% was small but significant. Significant mtDNA haplotype heterogeneity was observed after χ2- of haplotype frequencies but not after a G ST analysis. Nucleotide sequence-diversity analysis showed very low net divergence (0.0023%) between the two samples. The Australian orange roughy had a lower allozyme heterozygosity and a lower mitochondrial DNA nucleon diversity than the North Atlantic sample. The very limited, although significant, allozyme and mitochondrial DNA heterogeneity between these areas suggests that there is some gene flow between these two populations. The species appears to be widespread, with its presence reported from the southern Pacific, southern Indian, and northern and southern Atlantic Oceans, and it is likely that gene flow between the antipodes is mediated by stepping-stone exchange between adjacent populations rather than by direct migration.

Allozyme and RFLP heterozygosities as correlates of growth rate in the scallop Placopecten magellanicus: a test of the associative overdominance hypothesis.

Several studies have reported positive correlations between the degree of enzyme heterozygosity and fitness-related traits. Notable among these are the correlations between heterozygosity and growth rate in marine bivalves. Whether the correlation is the result of intrinsic functional differences between enzyme variants at the electrophoretic loci scored or arises from non-random genotypic associations between these loci and others segregating for deleterious recessive genes (the associative overdominance hypothesis) is a matter of continuing debate. A prediction of the associative overdominance hypothesis, not shared by explanations that treat the enzyme loci as causative agents of the correlation, is that the correlation is not specific to the type of genetic marker used. We have tested this prediction by scoring heterozygosity at single locus nuclear restriction fragment length polymorphisms (RFLPs) in a cohort of juvenile scallops (Placopecten magellanicus) in which growth rate was known to be positively correlated with an individual's degree of allozyme heterozygosity. A total of 222 individuals were scored for their genotypes at seven allozyme loci, two nonspecific protein loci of unknown function and eight nuclear RFLPs detected by anonymous cDNA probes. In contrast to the enzyme loci, no correlation was observed between growth rate and the degree of heterozygosity at the DNA markers. Furthermore, there was no relationship between the magnitude of heterozygote deficiency at a locus and its effect on the correlation. The differences observed between the effects of allozyme and RFLP heterozygosity on growth rate provide evidence against the associative overdominance hypothesis, but a strong case against this explanation must await corroboration from similar studies in different species.

Does hunting affect the demography and genetic structure of the greywing francolin Francolinus africanus?

Hunted and unhunted populations of greywing francolin Francolinus africanus have been studied in the eastern Cape Province of South Africa in order to understand the effects of hunting on the demography and genetic structure of these populations. Greywing population density cycled annually for both hunted and unhunted populations. However, there was an apparent ‘pulse’ of immigration of sub-dominant birds, and earlier reproduction, in the hunted populations immediately after the winter hunting season. Average levels of allozyme heterozygosity (H) for hunted and unhunted populations were both 0.076, and although the proportion of polymorphic loci per sample and the mean number of alleles per locus for each sample were lower for the hunted populations than for the unhunted populations, these differences were not significant. However, the hunted populations displayed higher levels of outbreeding (lower F IS and F IT values) than those for unhunted populations. Therefore, it is concluded that although greywing francolin populations contain relatively high levels of genetic heterogeneity, it is probably the increased levels of local immigration following hunting which reduces the effects of any reduction in genetic variation due to a decrease in local population size from hunting.

Resource allocation and population genetics of the bay scallop, Argopecten irradians irradians: effects of age and allozyme heterozygosity on reproductive output

The relationships among multiple-locus heterozygosity, age, reproduction and growth were examined over the ca. 2-yr lifespan of a cohort of northern bay scallops, Argopecten irradians irradians (Lamarck), from the Niantic River estuary, Connecticut, USA. Electrophoretic analyses revealed a relatively low proportion of polymorphic loci (=0.35) and low level of heterozygote deficiency ( D. across 6 loci=-0.05) in this population. Allele frequency distributions showed a high degree of temporal stability within and among cohorts. No significant correlation was found between multiple-locus heterozygosity and either somatic or reproductive growth. Our results lend support to the postulated linkage between low heterozygote deficit and lack of a heterozygosity/growth association in marine bivalves, and in the pectinid group in particular. Partial reproductive senility occurs in these scallops in their second year of life. Somatic tissue weight increased exponentially with increasing shell height, whereas pre-spawning gonad mass attained asymptotic values in 2-yr-old scallops. Thus, weight-specific reproductive effort was significantly lower in second- than first-year scallops (24 and 33% respectively). This pattern could be generated if high individual reproductive effort in the first year were correlated with accelerated (early) post-reproductive senescence. This kind of resource allocation, in which somatic growth is given precedence over reproductive growth in older individuals, has been previously reported in only one other bivalve, a pectinid (Chlamys islandica), in which the decline in reproductive effort occurs only after the scallops reach an age of ca. 20 yr.

EFFECTS OF POPULATION BOTTLENECKS ON GENETIC DIVERSITY AS MEASURED BY ALLOZYME ELECTROPHORESIS.

Low levels of allozyme heterozygosity in populations are often attributed to previous population bottlenecks; however, few experiments have examined the relationship between heterozygosity and bottlenecks under natural conditions. The composition and number of founders of 55 experimental populations of the eastern mosquitofish (Gambusia holbrooki), maintained under simulated field conditions, were manipulated to examine the effects of bottlenecks on three components of allozyme diversity. Correlations between observed and expected values of allozyme heterozygosity, proportions of polymorphic loci, and numbers of alleles per locus were 0.423, 0.602, and 0.772, respectively. The numbers of polymorphic loci and of alleles per locus were more sensitive indicators of differences in genetic diversity between the pre-bottleneck and post-bottleneck populations than was multiple-locus heterozygosity. In many populations, single- and multiple-locus heterozygosity actually increased as a result of the founder event. The weak relationship between a population's heterozygosity and the number and composition of its founders resulted from an increase in the variance of heterozygosity due to drift of allele frequencies. There was little evidence that selection influenced the loss of allozyme variation. When it is not possible to estimate heterozygosity at a large number of polymorphic loci, allozyme surveys attempting to detect founder events and other types of bottlenecks should focus on levels of locus polymorphism and allelic diversity rather than on heterozygosity.

Relationships among allozyme heterozygosity, morphology and lipid levels in house sparrows during winter

House sparrows (Passer domesticus) were collected during cold (43 individuals) and warm (31 individuals) periods in February 1982 from farms near Lawrence, Kansas. Fat scores (from flank, rump and furcula) and body mass were measured from the carcasses, which were then reduced to skeletons. Heterozygosity was determined by electrophoresis of three polymorphic allozymes. Pectoral lipid content was determined by petroleum ether extraction. Principal component (PC) analysis was conducted on 14 skeletal measurements. Summed fat score, body mass and pectoral lipid content served as dependent variables in three multiple regressions. Sex, period (cold versus warm), number of heterozygous loci (0–3), and scores of PCI, PC2 and PC3 were independent variables. Each of the dependent variables was also included as an independent variable in regressions in which they were not the dependent variable. Pectoral lipid content was significantly related only to the number of heterozygous loci (R2= 0.18, P= 0.008). Summed fat score differed between sexes and periods (P= 0.012 and 0.001, respectively) when body mass was included in the regression (P &lt; 0 001). Body mass was related to summed fat score as above, and also to body size (PCI, P &lt; 0.0001) and period (P= 0.014). Females exhibited a positive regression between body size and summed fat score (P= 0.007). Body size (PCI) was greater for both sexes in the sample from the warm period (P= 0.022), suggesting that selection for increased body size may have occurred. Increased metabolic efficiency of heterozygous enzymes or individuals is suggested as an explanation for the observed relationship between heterozygosity and intramuscular lipid level.

Genetics of growth rate variation in bivalves: aneuploidy and heterozygosity effects in a Crassostrea gigas family

Enzyme homozygosity and somatic aneuploidy are both known to adversely affect juvenile growth rate in marine bivalves. We have examined the joint effects of these two factors by scoring genotypes at nine segregating allozyme loci and counting the numbers of chromosomes lost in 30 cells in each of 83 full sibs of the Pacific oyster. A highly significant negative correlation was observed between the number of chromosomes missing and shell length in full sibs of the same age. No relationship was seen, however, between allozyme heterozygosity and either shell length or chromosome loss, nor was there any difference in the distribution of aneuploidy among genotypes at any given enzyme locus. Thus, the effects of homozygosity and aneuploidy on growth rate appear to have different genetic bases. Even in the most aneuploid oysters, more than half the cells examined had a complete chromosome complement of 2n = 20. This eliminates somatic aneuploidy as an explanation for the excess of enzyme homozygosity frequently observed in populations of marine molluscs. Significant deviations from Mendelian expectations, favoring homozygotes at some loci and heterozygotes at others, were recorded at eight of the nine allozyme loci, but these occurred independently of the aneuploidy observed. Our results suggest that within families a much larger component of variation in growth rate is due to aneuploidy than to allozyme genotype, but this conclusion cannot, at present, be extended to natural populations.Key words: aneuploidy, heterozygosity, growth rate, oysters.

Evidence for the adaptive significance of allozymes in forest trees

Positive correlations between allozyme heterozygosity and fitness measures, primarily growth and fecundity, have been reported for a number of forest tree species. Because the amount of variation in growth explained by allozyme genotype is usually on the order of only a few percent, there has been little effort made towards using electrophoretic screening of allozymes as a tool in early selection on seedlings in production nurseries. We review the progress made in studies of heterozygosity in forest trees, focusing on how recent studies have utilized careful experimental design to allow testing of hypotheses as to the causative nature of the heterozygosity-fitness phenomena. We discuss evidence suggesting a deleterious nature for rare allozyme alleles, and present a case of apparent balancing selection across life history stages acting to maintain rare alleles in Pinus taeda. We also review the apparently common trend in natural stands toward increasing heterozygosity over time, and suggest how gains might be made through artificial selection based on allozyme survey data.

Allozyme heterozygosity and morphological homeostasis in pink salmon, Oncorhynchus gorbuscha (Walbaum): evidences from family analysis*

In the families produced by diallel 2 × 2 crosses there were positive relationships in two experimental groups between individual heterozygosity (H0) at four allozyme loci (PGM*, G3PDH*, sMEP*, sMDH−3,4*) and the size of the fish, their morphological variability (10 traits) and the viability among families. An integrative H0 influence on complexes of morphological traits was also obtained. On the basis of factor and discriminant analysis it was shown that H0 correlated with morphological homeostasis at least within one experimental group.

THE IMPACT OF ELECTROPHORETIC GENOTYPE ON LIFE HISTORY TRAITS IN PINUS TAEDA.

Reports of positive associations between allozymic heterozygosity and measures of fitness are routine, but it has not been possible to distinguish between the two preeminent explanations of the phenomenon, dominance and overdominance. We tested several of the assumptions of these hypotheses in our study of the relationship between electrophoretic genotype and three life history traits in loblolly pines (Pinus taeda L.). Traits examined included the survival and growth of selfed and outcrossed progeny of 45 maternal trees, and maternal fecundity, measured as the number of surviving progeny per mother tree. Inbreeding depression was severe; the relative fitness of the selfed progeny was only 8% that of the outcrossed progeny. We found a heterozygote fecundity advantage, which should have resulted in an excess of rare alleles in the progeny. Instead, there was evidence of severe survival selection against rare alleles in both heterozygous and homozygous forms. The deficit of rare alleles averaged 69 and 50% in the selfed and outcrossed progeny, respectively. The one allele in the sample that we should have suspected of being maintained by overdominance (a PGI2 mid-frequency allele) appeared to be overdominant for outcrossed height growth and probably for fecundity as well. Multiple-locus genotype explained very little of the variation in growth, however, and rather than seeing evidence for overdominance as a force in maintaining most of the observed polymorphism, we were left to explain, in the face of the severe survival selection, why the rare alleles were present at all. Projection of the stand into the future through computer simulation showed how balancing selection acting on differential growth, fecundity, and mortality among genotypes could, over the life of the stand, account for the maintenance of the rare alleles in the population.