Individual Heterozygosity Research Articles

Genetic differentiation and speciation inStomion(Coleoptera: Tenebrionidae): flightless beetles of the Galápagos Islands, Ecuador

Stomion is a swarm of 13 tenebrionid beetle taxa endemic to the Galápagos Islands. The taxa show marked morphological variation, and all are flightless. They can be divided into three divergent morphological groups based on external sculpturing of the elytra. Eleven taxa were examined electrophoretically at 20 enzyme loci to measure the extent of genetic differentiation which accompanied morphological differentiation and speciation. Stomion are shown to be ideal candidates for allopatric and founder event speciation, with their limited dispersal ability, narrow distributions, and incidence of single-island endemics. Accordingly, individual heterozygosities were low in most taxa, and gene pool divergence was apparent among populations of single taxa occupying different islands. Both interspecific and intraspecific genetic identities were high, with an average I=0.88 and I=0.96, respectively. The relatively young age of the genus and the allopatric mode of speciation may account for high genetic relatedness. No clear relationship was found between allozyme phenotypes and the three morphological groups, thus, the evolution of these traits may have been facilitated by similar selection among islands of similar age and geography. The geographical distributions of the morphological groups can be used to suggest a general pattern of speciation for the group.

Developmental stability and environmental stress in Salmo salar (Atlantic salmon).

We have studied the developmental stability (measured as fluctuating asymmetry of five meristic characters) of three populations of Atlantic salmon Salmo salar (rivers Imsa, Lone and Ogna, western Norway). All three populations were both sampled in the wild, and hatched and reared in a common environment in a hatchery (with water from the river Imsa) from fertilization until smoltification. Both the Imsa and Lone hatchery populations have been sea-ranched in the Imsa for 10 years, whereas the Ogna populations is novel to the hatchery environment. Individual biochemical heterozygosity was scored at 50 loci, of which 11 were polymorphic. There was no correlation between biochemical heterozygosity and fluctuating asymmetry at the individual level, neither when tested within groups nor when tested between groups. There were no differences in fluctuating asymmetry between wild and hatchery Imsa and Lone fish, indicating that the hatchery environment did not disrupt early developmental homeostatic processes. However, the Ogna hatchery fish had significantly elevated levels of fluctuating asymmetry compared to the wild Ogna fish, indicating that the hatchery environment was hostile. The Ogna hatchery fish also had significantly higher fluctuating asymmetry than the Imsa hatchery and the Lone hatchery fish. Maladaptation to the hatchery environment is the most likely explanation for the increased asymmetry in river Ogna fish.

A theory of mate choice based on heterozygosity

In theories of mate choice that rely on genetic benefits, the nature of the good genes involved has received little attention. A review of genetic studies of mate choice in a variety of species and situations suggests that individual heterozygosity is more important than previously realized. Females are predicted to value heterozygosity in their offspring and under some conditions in their males. The expression of vigor, condition-sensitive ornaments, and symmetry in males may be a direct reflection not of good genes but of individual heterozygosity at key loci or at many loci. Like sexuality itself, mate choice based on heterozygosity and genie diversity may be an adaptation that favors the production of diverse and superior competitors. Female choice is made meaningful by sexuality, and the adaptive value of choice probably depends on some of the same factors that maintain sexuality.

Genetic variation and hybridization in SwedishSchoenus (Cyperaceae)

Schoenus ferrugineus andS. nigricans have restricted distributions in Sweden and are almost exclusively confined to calcareous fen habitats. AtS. nigricans sites,S. ferrugineus is usually also present, and hybrids are frequently found. In this report, I used allozymes to estimate the amount of gene flow between the two species, and to compare the partitioning of genetic diversity in each of them. Thirteen loci were analysed at eight different enzyme systems. Seven loci were variable between or within the species. The two species had completely different alleles at two of the seven variable loci, whereas there was overlap at five loci. In all, 22 different alleles were found. Six of these alleles were confined toS. nigricans, and five alleles were confined toS. ferrugineus. Nei's genetic identity was 0.55.—InS. ferrugineus, three loci (23%) were polymorphic, and the average number of alleles per polymorphic locus was 2.0 (each polymorphic locus had two alleles). InS. nigricans, three loci (23%) were polymorphic, and the average number of alleles per polymorphic locus was 2.3.—The proportion of genetic diversity due to variation among sites (GST) was fairly similar in the two species, mean over loci = 0.12 inS. ferrugineus and 0.15 inS. nigricans. However, the proportion of genetic diversity due to variation among individuals within sites (GIS) differed markedly between the two species, mean over loci = 0.54 inS. ferrugineus and 0.17 inS. nigricans. Accordingly, there was a much higher individual heterozygosity inS. nigricans than inS. ferrugineus. — Most hybrids were interpreted as F1 hybrids. However, a small proportion, 0.5–1.6 %, were Fn hybrids or back-crosses.—On the Swedish mainland, all former occurrences ofS. nigricans are extinct, but viable hybrids are still present at a few sites in southernmost Sweden.

Genetic diversity and divergence in Chinese yak (Bos grunniens) populations inferred from blood protein electrophoresis.

In 6 Chinese yak (Bos. grunniens) populations including 177 yaks, 34 blood protein loci were studied by horizontal starch gel electrophoresis, four of these loci (AKP, ALB, LDH-1, TF) were found to be polymorphic. The percentage of polymorphic loci(P) is 0.118, the mean individual heterozygosity(H) is 0.015, which means a low level of genetic diversity in the whole Chinese yak population. The coefficient of gene differentiation (Gst) is 0.0625, which indicated an almost-indistinguishable divergence among different populations at the level of blood protein electrophoresis.

Associations of Individual Genomic Heterozygosity, Estimated by Molecular Fingerprinting, and of Dam Major Histocompatibility Complex with Growth and Egg Production Traits in Layer Chickens

Growth and egg production data from F2 layer chickens produced from a mating of MHC-heterozygous B15B44 sires to B15B44 and B15B43 dams were analyzed for associations with the level of the individual's genomic heterozygosity and dam MHC. The F1 sires and dams were produced from matings of birds from two genetically distinct, highly inbred lines. Genomic heterozygosity (expressed as composite band frequency, CBF) was estimated based on the average proportion of shared bands of individual birds compared to a composite pool of Hinf I-digested DNA samples from the two inbred lines. Birds from dams of genotype B15B43 had significantly greater BW than progeny from B15B44 dams at 6 wk and thereafter. Dam MHC genotype differences were also significant for number of eggs laid from 20 to 28 wk of age, with B15B43 progeny being superior. Negative regression coefficients at hatch, 32, and 44 wk were observed for the effect of CBF on BW of progeny from B15B43 dams and at hatch and 32 wk from progeny of pooled dam genotypes. Similarly, negative regression coefficients were observed for the association of CBF with number of eggs within progeny from the B15B44 dam MHC genotypes at 20 to 28 wk and total period recorded, and at 28 to 36 wk for progeny from B15B43 dams. The CBF regression coefficients and the differences among progeny based on dam MHC genotypes suggest that individual heterozygosity and MHC genotype may be associated with BW and egg production in chickens.

Effects of inbreeding on skeletal development of Rattus villosissimus.

In a species of Australian native rat, Rattus villosissimus, which experiences extreme population fluctuations and possibly episodes of local inbreeding in the wild, generations of inbreeding in a laboratory colony led to altered skull shape and increased fluctuating asymmetry in some skeletal characters. Although inbreeding was closely associated with the number of generations in captivity, the effect of inbreeding after controlling statistically for the generation number was to decrease skull length and width. The effect of generations in captivity after controlling for inbreeding was to increase skull length and width. The joint effect of these confounded determinants was to produce rats with increasingly broad and short skulls through generations of inbreeding. Developmental anomalies of the incisors and feet appeared in the population. The average effects of inbreeding on fluctuating asymmetry were not strong, as most inbred and noninbred rats were not detectably asymmetric. Asymmetry appeared to be a threshold phenomenon, as significantly asymmetric animals were more prevalent among the more highly inbred rats of later generations. We detected no trend for a lessening impact of inbreeding after many generations, as would have been expected if selection had been purging the population of its genetic load. Individual heterozygosity across five polymorphic allozyme loci was correlated with size and asymmetry metrics, but only in so far as could be accounted for by the correlation of sample heterozygosity with the level of inbreeding and the effect of inbreeding on size and asymmetry.

Review of Genetic Studies ofDreissenaspp.

Synopsis. Temperate fresh waters of eastern North America were invaded in the late 1980s by two bivalve species, the zebra mussel (Dreis? sena polymorpha) and the quagga mussel (D. bugensis). The point of origin of zebra mussels is unknown because they are widespread through? out western and eastern Europe, but the quagga mussel is confined to the Dnieper and Bug rivers in the Ukraine. Both species are highly variable in shell morphology. Zebra mussel color and pattern morphs defy clas? sification, but at least one distinct pattern is found only in the Aral-Caspian region and North American populations. An extremely pale, laterally compressed quagga morph is found in eastern Lake Erie and southern Lake Ontario, but has not been noted in the Ukraine. Allozyme electro? phoresis has revealed extraordinarily high levels of genetic variability in the zebra mussel. Average heterozygosities range from 27.0 to 43.5%, compared to 9.7 to 14.5% in the quagga mussel. Two surveys of North American and European populations indicated that little variability was lost when zebra mussels were transported to North America. Therefore, either the invader population was comprised of a large number of indi? viduals, and/or more than one source population contributed to the in? vasion. No genetic differentiation among populations has occurred in North America in the nine years since the first invasion. Zebra mussels, like many bivalves, demonstrate heterozygote deficiencies at some loci. A weak correlation has been found between individual heterozygosity and shell length. The need for identification of enzymes, loci, and allelic mobilities to allow comparison of genetic data among studies is stressed, as is the need for vouchering specimens. Fruitful areas for future research include elucidation of dreissenid taxonomy and status of putative sub? species, monitoring for the potential invasion of the saline tolerant D. rostriformis grimmi into North American estuarine waters, and ecological and physiological comparison of extreme morphs such as the normal and profundal quagga mussels.

Multiple-locus heterozygosity, physiology and growth at two different stages in the life cycle of the Chilean oyster Ostrea chilensis

A random sample of 150 individuals of a laboratory-produced cohort of Ostrea chilensis Philippi, 1845 was taken at 10 and 36 mo of age to estimate physiological variables and individual heterozygosity using 4 loci (Lap, Pgi, Pgm and Ca). Juveniles of 10 mo of age showed a mean D value of 0.134 (p > 0.05) and a positive correlation between oyster size and multiple-locus heterozygosity (MLH) (p 0.05), oxygen consumption rate (p < 0.05) and MLH was found. The K2 value (standardized net growth efficiency) was positively correlated (p < 0.05) with MLH. At 36 mo a heterozygote deficiency was present with a mean value D = -0.431 (p < 0.05). No relationship between standard dry weight and MLH and also a negative correlation between the scope for growth and MLH were found. The oxygen consumption and excretion rates also showed an increase in large size individuals. The slopes for filtration and excretion rates against MLH were negative and not statistically significant. However, ingestion and absorption rates showed significant (p < 0.05) decrease with an increase in heterozygosity. The results seem to indicate that within sexually immature individuals of O. chilensis, a positive correlation between growth rate and MLH can be found, while in adults the higher energy allocation for reproduction precludes the detection of this relationship.

Limitations in the genetic variation of hatchery produced batches of the giant clam, Tridacna gigas

Eight production batches of juvenile giant clams ( Tridacna gigas) were sampled from three different hatcheries where some batches were derived from single-pair matings and some derived from mixtures of gametes from multiple adults. Allozyme electrophoresis was used to assess parentage and genetic diversity of the batches. Genetic diversity measures were lower, and gene frequencies markedly skewed, in all the batches relative to those in the wild populations from which they were derived. Genotype frequencies in the batches recording single-pair matings did not fit those expected from a pairwise mating of only two individuals. Selfing of at least one parent must have occurred. Contamination by gametes from other clams at fertilization, or by later accidental introduction of individuals from other batches, was also suggested by the presence of additional alleles. Excesses of heterozygotes were frequent, suggesting selection for heterozygotes, either at fertilization, or in the production process. However, there was no consistent correlation of individual heterozygosity and growth rate within batches.

Allozyme heterozygosity and fluctuating asymmetry in the brown hare (Lepus europaeus) : a test of the developmental homeostasis hypothesis

The influence of allozyme heterozygosity on developmental homeostasis as indicated by fluctuating morphological asymmetry (FA) has been a controversial issue in evolutionary studies. In the present investigation, relationships between overall individual heterozygosity (H), calculated over 13 polymorphic enzyme loci, and fluctuating asymmetry (FA) in 27 non-metric and 9 metric bilateral skull and mandible traits were examined in a total of 417 brown hares. The respective tests were performed separately for juveniles and adults within and among five geographic arrays of samples, and among 17 single sampling localities. Within geographic units, neither in metric nor in non-metric characters could a clear relationship between FA and H be detected. Among geographic units and single sampling localities, for metric traits the result remained the same. In non-metric traits, however, a significant negative correlation between overall FA and H became apparent in adults. Thus, fluctuating asymmetry and heterozygosity are inversely related also in a mammalian species. The long lasting dispute as to the existence of a homeotherm-poikilotherm dichotomy may have been the result of a disregard of non-metric traits and of an interpopulation approach for assessing relationships between morphological FA and H in mammals.

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.

Geographic and individual variation in haematozoan infections in the greenfinch, Carduelis chloris

Among- and within-population variation in prevalence and intensity of haematozoan infections was studied in blood samples taken from nine geographically widely separated greenfinch (Carduelis chloris) populations. Within populations, there were no consistent differences in prevalence between sexes or between older age-classes, but prevalence was significantly lower among yearling birds (&lt;5 months old) in one population. Individual heterozygosity, as revealed by analysis of five polymorphic protein coding loci, did not differ between infected and noninfected birds. However, significant differences in prevalence between Fennoscandian (high), central European (low), and Iberian (high) populations is consistent with the hypothesis that destruction of natural habitats has led to a significant decline of vector populations in central Europe. Given the high among-populations variation in prevalence, our results point to the need to consider the possibility of sampling bias in studies that seek to test, using avian blood parasites, Hamilton and Zuk's hypothesis that parasite resistance might be the target of female choice and drive the evolution of bright plumage and elaborate ornamentation.

Detecting heterozygosity — growth relationships: How should growth be computed?

Abstract Individual multilocus heterozygosity (MLH) for eight polymorphic allozyme loci and weekly fresh weight (FW) over four weeks were measured in juvenile earthworrns (Eisenia fetida, n = 85) raised in a moderately stressful environment (25°C, 3 ml H2O/g peat moss). Initial FW of earthworms was 6.6±0.4 mg. The growth curve of heterozygotes (MLH ≥ 4) diverged from that of homozygotes (MLH ≦ 3) throughout the experiment after which weight of heterozygotes and homozygotes was 106.1±6.0 and 71.1±4.6 mg, respectively. MLH-growth curves were compared for several computations of growth. MLH-FW curves were significantly positive four of the five times weight was measured. MLH-Δ FW/week curves were significantly positive in three of four weekly intervals. MLH-standardized Δ FW/week curves (i.e. adjusted for initial FW) were significantly positive in two of four weekly intervals. MLH-instantaneous growth rate curves were significantly positive in only one of four intervals because of contrasting autocorrelations between FW and MLH (+) vs. FW and instantaneous growth (-). The best computations of growth for detecting relationships with MLH in populations with divergent sizes are size adjusted for differences in age and change in size per unit time.

Heterozygosity and fitness: no association in Scots pine.

The association of six quantitative traits related to fitness with heterozygosity at 12 allozyme loci has been examined in three populations of Scots pine, Pinus sylvestris. Because of several characteristics of this organism and of this extensive data set, it appeared that this study would show a positive association between heterozygosity and these traits if indeed heterozygotes had higher values for these quantitative traits. Using several different statistical techniques including analysis of variance, regression with the scaling recommended from the adaptive distance model, and multiple regression, no evidence of an association was found. For example, only between 7 and 8% of the regression tests were significant at the 5% level and half of these showed a positive association and half showed a negative association. Further, the multiple regression analysis explained on average only 5.8% of the variation observed in the six different traits and only 1.5% of this variation was explained by a positive association. Power analysis was carried out (for the first time on these type of data), both for the single locus heterozygous advantage and the association of individual multiple locus heterozygosity and the quantitative traits. For diameter and height, two traits often used in similar studies, the average power to detect a single locus heterozygous advantage of 0.10 was 0.737 and the average power to detect a mean heterozygote advantage of 0.05 per locus for multiple loci was 0.797. As a result of this study and an examination of the published results from other studies, it appears that what positive associations have been observed are probably not, in large part, due to the presence of intrinsic heterozygote advantage.

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.

Lack of an association between heterozygosity and growth rate in the wood frog,Rana sylvatica

The effect of both multilocus and single-locus heterozygosity on growth rate was examined in a cohort of larvae of the wood frog, Rana sylvatica, collected from a pond during the later stages of premetamorphic development. Seven electrophoretically detected enzyme loci were used to determine individual heterozygosity, whereas the growth rate was measured as wet mass. In all cases, no significant correlation was found between multilocus heterozygosity and mass among larvae collected at intervals during the developmental period. In addition, multiple regression analyses indicated that no single locus had a demonstrable effect on growth rate. The results of this study, therefore, provide no evidence for a link between enzyme heterozygosity and growth rate during the later stages of premetamorphic development in wood frog larvae.

Selection of breeding stock in pigs favours 6PGD heterozygotes.

Associations between genetic variation at four enzyme polymorphisms and growth rate, feed conversion and economic index were studied in commercial lines of pigs. These lines were also tested for the fit of observed genotypic frequencies to expected genotypic frequencies at the four polymorphic loci. Genotypic distribution fit Hardy-Weinberg expectations in four pure-breeding lines but crosses among lines produced excesses in heterozygosity at the locus coding for 6-phosphogluconate dehydrogenase (6PGD) due to differences in allelic frequencies among lines. Selection of breeding individuals further enhanced the heterozygosity at 6PGD to 73-93 per cent but it had no impact on a linked locus that was sometimes in linkage disequilibrium with 6PGD. The relationship between feed conversion and individual heterozygosity was examined in one population sample; feed conversion increased with individual heterozygosity.

Genetic Variation in the Aquatic Salamander genus Amphiuma

-Individuals representing the three recognized extant species of the salamander family Amphiumidae were collected and subjected to protein analysis by horizontal starch-gel electrophoresis. Products of 24 presumptive genetic loci (22 enzymes and hemoglobins) were resolved for all individuals. Estimates of individual heterozygosity (H = 0.0790.096) and percent polymorphic loci (95% criterion, 29.2% for each species) were higher than those reported for other aquatic salamander species. These results are interpreted with respect to the low levels of genetic variability reported for several other aquatic salamander species (Shaffer and Breden, 1989). Genetic distance estimates suggest a high level of similarity between Amphiuma means and A. tridactylum (Hillis' 1984 D* = 0.118; Nei's 1978 D = 0.121) and a much greater dissimilarity between A. pholeter and A. tridactylum (D* = 0.710; D = 0.729) or A. pholeter and A. means (D* = 0.858; D = 0.902).

Allozymic Variation and Species Diversity in North American Bosminidae

North American members of the genus Bosmina show marked morphological variation and integradation, which have been attributed to the occurrence of both phenotypic plasticity and occasional hybridization between a small number of species. We employed allozyme analysis on 72 bosminid populations from 62 North American habitats to ascertain the validity of species boundaries and to verify the occurrence of hybridization. Results showed that levels of genetic variability (15.8% polymorphic loci, 10% individual heterozygosities) in bosminids were comparable with those reported in other zooplankton groups. Significant Hardy–Weinberg deviations were detected at 38% of polymorphic loci and were usually due to heterozygote deficiency. No clear evidence of interspecific hybridization was obtained, although some cases of heterozygote excess require further investigation. While prior morphological studies indicated the occurrence of only six species, the allozyme data revealed the presence of eight assemblages showing allelic substitutions at one or more loci. Each assemblage corresponded to a different species belonging to one of four subgenera (Bosmina, Eubosmina, Neobosmina, or Sinobosmina). Within two assemblages, individuals could be further partitioned into two morphologically distinct, but genetically similar species, leading to the recognition of 10 species in this study.