Parental assignment in fish using microsatellite genetic markers with finite numbers of parents and offspring

Deterministic predictions for the proportion of offspring assigned to different numbers of parent-pairs are developed in order to investigate the power of microsatellite loci for parental assignment in fish species. Comparisons with stochastic simulation results show that predictions based on exclusion probabilities are accurate, provided that the number of parents involved in the crosses is large. Accounting for sampling of parents gave very accurate predictions for a small number of parents and a single biallelic locus. For large numbers of loci or large numbers of alleles per locus stochastic simulations are, however, the only available method to predict the power of assignment of a particular set of loci when the number of parents is small. Nine 5-allele loci or six 10-allele loci with equifrequent alleles, are sufficient for assigning, with certainty, parents to 99% of the fish resulting from either 100 or 400 crosses. Results simulating a set of highly polymorphic microsatellites developed for Atlantic salmon show that the four most informative loci are sufficient to assign at least 99% of the offspring to the correct pair with 100 crosses involving 100 males and 100 females. An additional locus is required for correctly assigning 99% of the offspring when the 100 crosses are produced with 10 males and 10 females.     

Genic heterozygosity, chromosomal interchanges and fitness in rye: any relationship?

Relationship between heterozygosity at allozyme loci, chromosomal interchanges and fitness was analyzed in a rye cultivar showing a polymorphism for such rearrangements. Nine allozyme systems (ACO, ACPH, GOT, GPI, LAP, MDH, PER, PGD and PGM) and five components of fitness (number of fertile tillers, total offspring, egg cell fertility, flowers/ear and seeds/ear) were studied. The estimated selection coefficients against interchange heterozygotes ranged from s = 0.12 to s = 0.34. A significant effect of the genic heterozygosity on some fitness components was observed in interchange heterozygotes (tillering and total offspring), in their standard homozygous sibs (flowers/ear and seeds/ear) and in the descendants of the crosses between standard karyotypes (flowers/ear, seeds/ear and egg cell fertility). However, the main effect was linked to genetic background associated to different crosses. Significant differences for Acph-1, Gpi-1, Lap-1, Mdh-1, Mdh-4, Pgd-2 and Pgm-1 loci were also found in some of these crosses although these differences were inconsistent. This suggests that probably the allozyme loci analyzed were not directly contributing to the fitness and that they are linked, in some cases, to different deleterious alleles depending on both cross and locus. This fact could support the local effect hypothesis as explanation although we do not discard the existence of some inbreeding level (general effect hypothesis) since all crosses and loci studied show a overall consistent trend of increased fitness with increased heterozygosity.     

Offspring sex ratio under inbreeding and outbreeding in a gynodioecious plant

Silene vulgaris is a gynodioecious plant native to Eurasia and now found throughout much of North America. Using hermaphrodite plants from three geographic regions (Stamford, NY; Broadway,VA; and Giles Co., VA) and four local populations within each region, we employed a hierarchical crossing design to explore the geographic structure of sex determining genes. Sex determination in this species is cytonuclear involving multiple cytoplasmic male sterility and nuclear restorer loci. Due to dominance effects within nuclear restorer loci, self-fertilization of hermaphrodites heterozygous at restorer loci should produce some homozygous recessive female offspring. Female offspring may also result from outcrossing among related individuals. At greater geographic and genetic distances, mismatches between cytoplasmic and nuclear sex determining genes should also produce high frequencies of female offspring if coevolution between cytoplasmic and nuclear sex determining alleles occurs independently among widely separated populations. We found evidence of dominance effects among nuclear restorer loci but no evidence of nuclear-cytoplasmic mismatches at the regional level. Of 63 maternal lines, 55 produced at least one female offspring when self-fertilized. Outcrossing within populations produced significantly fewer female offspring than self-fertilization. Outcrossing among regions produced the lowest proportion of female offspring, significantly fewer than outcrossing among populations within regions. Regions responded differently to among-region outcrossing with pollen donors from the two Virginia regions producing far fewer female offspring with New York dams than crosses among New York populations. These results indicate that nuclear restoration is complex, involving multiple loci with epistatic interactions and that most hermaphrodites in nature are heterozygous at one or more restorer locus. Further, regional differences in restorer frequencies indicate significant genetic structure for sex determining genes at large geographic scales, perhaps reflecting invasion history.     

Dominant cataract and recessive specific locus mutations in offspring of X-irradiated male mice

Male mice were X-irradiated with 3.0 + 3.0 Gy or 5.1 + 5.1 Gy (fractionation interval 24 h). The offspring were screened for dominant cataract and recessive specific locus mutations. In the 3.0 + 3.0-Gy spermatogonial treatment group, 3 dominant cataract mutations were confirmed in 15 551 offspring examined and 29 specific locus mutations were recovered in 18 139 offspring. In the post-spermatogonial treatment group, 1 dominant cataract mutation was obtained in 1120 offspring and 1 recessive specific locus mutation was recovered in 1127 offspring. The induced mutation rate per locus, per gamete, per Gy calculated for recessive specific locus mutations is 2.0 X 10(-5) in post-spermatogonial stages and 3.7 X 10(-5) in spermatogonia. For dominant cataract mutations, assuming 30 loci, the induced mutation rate is 5.0 X 10(-6) in the post-spermatogonial stages and 1.1 X 10(-6) in spermatogonia. In the 5.1 + 5.1-Gy spermatogonial treatment group, 3 dominant cataract mutations were obtained in 11 205 offspring, whereas in 13 201 offspring 27 recessive specific locus mutations were detected in the spermatogonial group. In the post-spermatogonial treatment group no dominant cataract mutation was observed in 425 offspring and 2 recessive specific locus mutations were detected in 445 offspring. The induced mutation rate per locus, gamete and Gy in spermatogonia for recessive specific locus mutations is 2.8 X 10(-5) and for dominant cataract mutations 0.9 X 10(-6). In post-spermatogonial stages, the mutation rate for recessive specific locus alleles is 6.2 X 10(-5). In the concurrent untreated control group, in 11 036 offspring no dominant cataract mutation and in 23 518 offspring no recessive specific locus mutation was observed. Litter size and the number of carriers at weaning have been determined in the confirmation crosses of the obtained dominant cataract mutants as indicators of viability and penetrance effects. Two mutants had a statistically significantly reduced litter size and one mutant had a statistically significantly reduced penetrance.     

Population-genetic structure and optimal sampling of land races of barley from Iran

Allozyme variation at 25 genetic loci was assayed in twelve indigenous cultivars (land races) of barley from Iran. For these loci the average probability that two gametes drawn at random from one population would differ at a locus was 0.082. In the collection as a whole, this measure of genetic diversity was 0.161, so that about half the total diversity was distributed between populations. Of the total of 31 allozyme variants, about half were common (frequency >0.10) in only one or two regions. The results were compared with diversity estimates based on spike morphological polymorphisms; with allozyme polymorphism in two composite crosses (CC21 and CC34) of cultivated barley; and with previous results for allozyme polymorphism in Israel populations of wild barley. In the land race samples, the diversity of spike types was a poor index of allozyme diversity. The total allozyme diversity in this collection of land races was intermediate between the moderate levels in composite crosses and the high levels in Hordeum spontaneum from Israel. These results emphasize the role of land races as valuable genetic resources for plant breeding. They support sampling strategies which, by taking samples of moderate size from many sites, emphasize the collection of locally common alleles, as against strategies framed to capture the rare, conspicuous morphological variant by intensive or biased sampling, or strategies based on collecting the extreme ecotypes from a cline. The chromosomal location of 20 of the allozyme loci as deduced from wheat-barley addition lines, is listed in the appendix.     

On the evolution of genetic incompatibility systems. V. Origin of sporophytic self-incompatibility in response to overdominance in viability

Conditions for the origin of partial sporophytic self-incompatibility (SSI) are obtained from two quantitative models, which differ with respect to the determination of offspring viability. Offspring viability depends solely on the source (self or nonself) of the fertilizing pollen in the first model, which describes changes only at a primitive S-locus itself. Two loci evolve in the second model: overdominant viability selection maintains an arbitrary number of alleles at one locus, with SSI under the control of a separate locus. In both cases, the origin of SSI requires that the relative change in the numbers of offspring derived by the two reproductive modes compensate for the twofold cost of outcrossing. In the first model studied, the viability of inbred offspring fully determines the relative change in the numbers of inbred and outbred offspring produced. In the second model, the relative change in offspring numbers depends in addition on associations between the S-locus and the viability locus. Because these two-locus associations are comparable in magnitude to the differences between the viabilities of inbred and outbred offspring, SSI can arise under less restrictive conditions than expected from the one-locus model. Greater allelic multiplicity at the viability locus facilitates the origin of SSI by reducing the relative viability of inbred offspring. Tight linkage between the S-locus and the viability locus and high rates of receipt of self-pollen promote the generation and maintenance of associations between the S-locus and the viability locus. In populations in which more than two viability alleles are maintained, the active S-allele can invade even in the absence of linkage with the viability locus. The present study establishes that incompatibility systems can arise in response to identity disequilibrium between a modifier of incompatibility and a locus subject to overdominant viability selection; in particular, compensation for the twofold cost of outcrossing does not require preexisting gametic level disequilibria.     

Molecular evolution of two linked genes, Est-6 and Sod, in Drosophila melanogaster.

We have obtained 15 sequences of Est-6 from a natural population of Drosophila melanogaster to test whether linkage disequilibrium exists between Est-6 and the closely linked Sod, and whether natural selection may be involved. An early experiment with allozymes had shown linkage disequilibrium between these two loci, while none was detected between other gene pairs. The Sod sequences for the same 15 haplotypes were obtained previously. The two genes exhibit similar levels of nucleotide polymorphism, but the patterns are different. In Est-6, there are nine amino acid replacement polymorphisms, one of which accounts for the S-F allozyme polymorphism. In Sod, there is only one replacement polymorphism, which corresponds to the S-F allozyme polymorphism. The transversion/transition ratio is more than five times larger in Sod than in Est-6. At the nucleotide level, the S and F alleles of Est-6 make up two allele families that are quite different from each other, while there is relatively little variation within each of them. There are also two families of alleles in Sod, one consisting of a subset of F alleles, and the other consisting of another subset of F alleles, designed F(A), plus all the S alleles. The Sod F(A) and S alleles are completely or nearly identical in nucleotide sequence, except for the replacement mutation that accounts for the allozyme difference. The two allele families have independent evolutionary histories in the two genes. There are traces of statistically significant linkage disequilibrium between the two genes that, we suggest, may have arisen as a consequence of selection favoring one particular sequence at each locus.     

Sex-restricted non-Mendelian inheritance of mouse Chromosome 11 in the offspring of crosses between C57BL/6J and (C57BL/6J × DBA/2J)F1 mice

We report on the observation of sex-restricted, non-Mendelian inheritance over a region of mouse Chromosome (Chr) 11, occurring in the offspring of crosses between two commonly used Mus musculus-derived inbred strains, C57BL/6J and DBA/2J. In the surviving backcross progeny of reciprocal matings between (C57BL/6J × DBA/2J)F₁ hybrids and the C57BL/6J parental strain, we observed the preferential appearance of C57BL/6J alleles along a region of Chr 11. The deviation from Mendelian predictions was observed only in female offspring from both reciprocal backcrosses, and not in males from either cross. The sex-specificity of the observed non-Mendelian inheritance points to an explanation based on embryonic or neonatal lethality. Our data add to previously obtained evidence for a Chr 11 locus or loci with sex-specific and allele-specific effects on viability. Received: 19 December 1997 / Accepted: 10 June 1998     

Fitness differences associated with Pgi SNP genotypes in the Glanville fritillary butterfly (Melitaea cinxia)

Allozyme variation at the phosphoglucose isomerase (PGI) locus in the Glanville fritillary butterfly (Melitaea cinxia) is associated with variation in flight metabolic rate, dispersal rate, fecundity and local population growth rate. To map allozyme to DNA variation and to survey putative functional variation in genomic DNA, we cloned the coding sequence of Pgi and identified nonsynonymous variable sites that determine the most common allozyme alleles. We show that these single‐nucleotide polymorphisms (SNPs) exhibit significant excess of heterozygotes in field‐collected population samples as well as in laboratory crosses. This is in contrast to previous results for the same species in which other allozymes and SNPs were in Hardy–Weinberg equilibrium or exhibited an excess of homozygotes. Our results suggest that viability selection favours Pgi heterozygotes. Although this is consistent with direct overdominance at Pgi, we cannot exclude the possibility that heterozygote advantage is caused by the presence of one or more deleterious alleles at linked loci.     

HETEROZYGOSITY‐FITNESS CORRELATIONS IN RAINBOW TROUT: EFFECTS OF ALLOZYME LOCI OR ASSOCIATIVE OVERDOMINANCE?

Previous studies with rainbow trout (Oncorhynchus mykiss) have shown that increased heterozygosity at allozyme loci is correlated with several phenotypic traits associated with fitness. We expected to find a similar effect of heterozygosity at other nuclear loci if these associations are due to loci in linkage disequilibrium with the allozyme loci (i.e., associative overdominance), rather than the allozymes themselves. We examined the association between multiple locus heterozygosity and condition factor at 10 allozyme and 10 microsatellite loci. Individuals that were more heterozygous at allozyme loci had significantly greater condition factor in two hatchery cohorts of rainbow trout (1996 P = 0.006; 1997 P < 0.001). In contrast, there was no evidence at microsatellite loci that increased heterozygosity was associated with greater condition factor. Our results suggest that the observed relationship between heterozygosity at allozyme loci and condition factor in rainbow trout appears to be due to the allozyme loci themselves, rather than associative overdominance. We cannot, however, rule out that differences in the mutation process between allozymes and microsatellites may be responsible for these observations. Regardless of the underlying mechanism, these results support the view that allozymes and microsatellites are differentially affected by natural selection.     

Concordance of genetic divergence among sockeye salmon populations at allozyme, nuclear DNA, and mitochondrial DNA markers

Abstract.— We examined genetic variation at 21 polymorphic allozyme loci, 15 nuclear DNA loci, and mitochondrial DNA in four spawning populations of sockeye salmon ( Oncorhynchus nerka ) from Cook Inlet, Alaska, to test for differences in the patterns of divergence among different types of markers. We were specifically interested in testing the suggestion that natural selection at allozyme loci compromises the effectiveness of these markers for describing the amount and patterns of gene flow among populations. We found concordance among markers in the amount of genetic variation within and among populations, with the striking exception of one allozyme locus ( sAH ), which exhibited more than three times the amount of among-population differentiation as other loci. A consideration of reports of discordance between allozymes and other loci indicates that these differences usually result from one or two exceptional loci. We conclude that it is important to examine many loci when estimating genetic differentiation to infer historical amounts of gene flow and patterns of genetic exchange among populations. It is less important whether those loci are allozymes or nuclear DNA markers.     

CROSS ABILITY OF MIMULUS GUTTATUS IN RELATION TO COMPONENTS OF GENE FIXATION

The joint effects of parental gene fixation and consanguinity of mates upon the fitness of matings was examined in Mimulus guttatus. Plants from four populations were crossed at five levels of genetic relatedness, and five viability characters were scored in progeny. Parental gene fixation at 12 polymorphic allozyme loci was partitioned into local, subpopulation, and population components. A model is proposed wherein parental gene fixation influences distance-dependent crossing success. At a fixed locus, inbreeding is favored if natural selection caused allele fixation, or is disfavored if gene fixation was random. The distance between mates required to eliminate gene fixation depends upon patch size of fixation. When selective fixation and patch size differ among loci, an optimal crossing distance is possible. In M. guttatus, progeny viability generally decreased with greater relatedness between mates, but this decrease was often heterogeneous among populations. The highest pollen viability and the lowest seed set were found at an intermediate relatedness between mates. To determine whether parental gene fixation influences these crossing patterns, a type of mutational-load analysis was performed. Progeny fitness was regressed on parent F and fitness estimated at F = 1. This was done for each component of F, for a) crosses that maintain gene fixation and b) crosses that eliminate gene fixation. A multiplicative, composite measure of fitness indicates that, in M. guttatus, genes fixed during local or population differentiation favor outbreeding, while genes fixed during subpopulation differentiation favor inbreeding. This predicts that random mating within subpopulations confers highest progeny fitness, exclusive of between-population matings. However, predictions did not fit the observed patterns of crossing success very well, perhaps because gene fixation was relatively low or was not adequately measured at loci influencing fitness.     

Microsatellite DNA marker inheritance indicates preferential pairing between two highly homologous genomes in polyploid and hemisexual dog-roses, Rosa L. Sect. Caninae DC

According to previous cytological evidence, the hemisexual dog-rose species, Rosa sect. Caninae, transmit only seven chromosomes (derived from seven bivalents) through their pollen grains, whereas egg cells contain 21, 28 or 35 chromosomes (derived from seven bivalents and 14, 21 or 28 univalents) depending on ploidy level. Two sets of reciprocal pairwise interspecific crosses involving the pentaploid species pair R. dumalis and R. rubiginosa, and the pentaploid/tetraploid species pair R. sherardii and R. villosa, were analysed for 13 and 12 microsatellite DNA loci, respectively. Single loci were represented by a maximum of three simultaneously occurring alleles in R. villosa, and four alleles in the other three parental plants. In the experimentally derived offspring, the theoretical maximum of five alleles was found for only one locus in the pentaploid progenies. Microsatellite DNA allele composition was identical with that of the maternal parent in 10 offspring plants, which were probably derived through apomixis. Almost all microsatellite DNA alleles were shared with the maternal parent also in the remaining offspring, but 1-4 alleles shared only with the paternal parent, indicating sexual seed formation. Analysis of quantitative peak differences allowed a tentative estimation of allelic configuration in the individual plants, and suggested that bivalent formation preferentially takes place between chromosomes that consistently share the same microsatellite alleles and therefore appear to be highly homologous. Moreover, alleles that were shared between the species in each cross combination comparatively often appear to reside on the bivalent-forming chromosomes, whereas species-specific alleles instead occur comparatively often on the univalent-forming chromosomes and are therefore inherited through the maternal parent only. Recombination then takes place between very similar genomes also in interspecific crosses, resulting in a reproduction system that is essentially a mixture between apomixis and selfing.     

Temporal and Microgeographic Variation in Allozyme Frequencies in a Natural Population of DROSOPHILA BUZZATII

Temporal variation in allozyme frequencies at six loci was studied by making monthly collections over 4 yr in one population of the cactophilic species Drosophila buzzatii. Ten sites were defined within the study locality, and for all temporal samples, separate collections were made at each of these sites. Population structure over microgeographic space and changes in population structure over time were analyzed using F-statistic estimators, and multivariate analyses of allele and genotype frequencies with environmental variables were carried out. Allele frequencies showed significant variation over time, although there were no clear cyclical or seasonal patterns. A biplot analysis of allele frequencies over seasons within years and over years showed clear discrimination among years by alleles at four loci. During the 4 yr, three alleles showed directional changes which were associated with directional changes in environmental variables. Significant associations with one or more environmental variables were found for allele frequencies at every locus and for both expected and observed heterozygosities (except those for Est-1 and Est-2). Thus, variation in allele frequencies over time cannot be attributed solely to drift. Significant linkage disequilibria were detected among three loci (Est-2, Hex and Aldox), but there was no evidence for spatial or temporal patterns. The F-statistic analyses showed significant differentiation among months within years for all loci, but the statistic used (coancestry) was heterogeneous among loci. Estimates of F (inbreeding) for all loci were significantly different from zero, with the loci in four groups, Adh-1 (negative), Pgm(small positive), Est-2 and Hex (intermediate) and Est-1 and Aldox (high positive). The correlation of genes within individuals within populations (f) for each locus in each month by site sample differed among loci, as did the (f) for each locus in each month by site sample differed among loci, as did the patterns of change in f over time (seasons). Heterogeneity in the F-statistic estimates indicates that natural selection is directly or indirectly affecting allele and genotype frequencies at some loci. However, the F-statistic analyses showed essentially no microgeographic structure (i.e., among sites), although there was significant heterogeneity in allele frequencies among flies emerging from individual rots. Thus, microspatial heterogeneity probably is most important at the level of individual rots, and coupled with habitat selection, it could be a major factor promoting diversifying selection and the maintenance of polymorphism.(ABSTRACT TRUNCATED AT 400 WORDS)     

Genetic differentiation between alpine and lowland populations of a butterfly is related to PGI enzyme genotype

Understanding the ecological process of population differentiation and identifying the molecular changes that contribute to adaptation is a central aspect of evolutionary biology. In this study we analyzed the geographic variation in allozyme allele frequencies (based on 15 enzyme systems representing 18 loci) across 18 populations of the butterfly Lycaena tityrus from different altitudes. Population genetic analyses showed that within population genetic diversity (e.g. mean number of alleles per loci: 1.8; expected heterozygosity: 12%) was within the typical value range for the Lepidoptera. The populations of L. tityrus investigated showed a remarkable genetic differentiation (F_ST: 0.065), being clearly divided into an alpine (high-altitude) and a non-alpine (low-altitude) cluster. This differentiation was almost entirely caused by variation at a single enzyme locus, PGI. Although the involvement of historical events cannot be ruled out, several lines of evidence suggest that the specific pattern of allozyme (and in this case particularly PGI) variation found is caused by thermal selection. For example, genetic variation was highly locus-specific rather than relatively uniform, as should be expected for effects of natural selection. Further, the PGI 2–2 genotype dominating in alpine (in contrast to low-altitude) populations is known to exhibit increased cold stress resistance and relatively long development times typical of alpine populations. Thus, PGI (or possibly a closely linked gene) is an obvious target for thermal selection in L. tityrus . This study exemplifies how allozyme analyses can be used to detect candidate loci likely to be under selection.     

Paternity assessment in wild groups of toque macaques Macaca sinica at Polonnaruwa, Sri Lanka using molecular markers

Genetic variation at four microsatellite loci in conjunction with that at a highly variable allozyme locus was used to analyse paternity over a 12-year period in 13 social groups of toque macaques Macaca sinica inhabiting a natural forest in Polonnaruwa, Sri Lanka. Paternity exclusion analysis revealed that the set of offspring produced by a female usually consists of half-siblings because few males father more than one offspring with a particular female. No evidence of offspring produced by matings between first degree relatives was found. The social unit in toque macaques was not identical to the reproductive unit and the possibility of paternity by males outside the social group should be considered when estimating male reproductive output. Although it was common for multiple males to father offspring in a social group each year, reproduction within a group during a breeding season tended to be limited to a few males. The mean number of males reproducing per group per year was independent of the number of males in a group. The paternity data suggests that many males may father relatively few offspring during their entire lives and that the effective population size for toque macaques may be much smaller than indicated by demographic data.     

Genetic correlation and response to selection in simulated populations

Summary Effects of truncation selection of a primary trait upon genetic correlation with a secondary trait were examined over 30 generations in genetic populations simulated by computer. Populations were 24 males and 24 females mated randomly with replacement; number of offspring was determined by intensity of selection. Each trait was controlled by 48 loci segregating independently, effects were equal at every locus, and gene frequency was arbitrarily set at 0.5 at each locus in the initial generation. All combinations of three genetic correlations, three intensities of selection, and three environmental variances were simulated. Gene action was additive. Genetic correlation was set by number of loci which affected both traits and was measured each generation as the product-moment correlation of genotypic values and estimated by two methods of combining phenotypic covariances between parent and offspring.Genetic correlations in each offspring generation remained consistently near initial correlations for all environmental variances when fraction of offspring saved as parents was as large as one-half. When the fraction of offspring saved was as small as one-fifth, genetic correlations decreased but most rapidly with heritability high and after the 15th generation of selection. Truncation selection caused genetic correlation to decrease in those offspring selected to become parents of the next generation. Amount of reduction depended on heritability of the selected trait rather than on degree of truncation selection. Estimates of genetic correlation from phenotypic covariances between parent and offspring fluctuated markedly from real correlations in the small populations simulated.Michigan Agricultural Experiment Station Journal Article 4836. Part of North Central Regional Project NC-2.     

Mixed mating strategies in cooperatively breeding apostlebirds Struthidea cinerea

In cooperatively breeding apostlebirds Struthidea cinerea philopatry by both sexes is coupled with low levels of dispersal, resulting in large groups (up to 17 birds) that may include multiple males and females of varying age and relatedness. We investigated mating patterns within 18 groups of apostlebirds using a set of six polymorphic microsatellite loci. Apostlebirds appear to adopt flexible and context-dependent mating strategies, with both monogamy and polygamy detected in separate groups. Most groups (11/18, 17/26 group-seasons) were putatively monogamous, with a single pair matching all typed offspring. Despite the potential for inbreeding, members of breeding pairs were less closely related than other potential within-group mating combinations. Polygamy was inferred in four groups (four group-seasons) from the presence of more than four alleles at one locus among offspring within a brood. Pairwise relatedness was lower among adults in polygamous groups than those in groups that included a monogamous breeding pair. There was no conclusive evidence of extra-group paternity or egg dumping.     

Genetic analysis of shikimate dehydrogenase allozymes in Trifolium repens L.

 A genetic analysis was carried out on progeny families from pair crosses among plants polymorphic for shikimate dehydrogenase (SDH) isozymes in white clover (Trifolium repens L.). SDH was controlled by two independently assorting disomic loci. This result is consistent with the presence of a single gene (Sdh) represented in this putative allotetraploid by one locus in each of the genomes. One of the Sdh loci is linked (6.0±2.0 cM) to the linamarase (Li) locus. There was no evidence for differentiation of the duplicate Sdh loci (both carry common alleles). White clover behaves genetically as a diploidised allotetraploid but the possibility of a low frequency of multivalent formation and homoeologous pairing has not been ruled out. The SDH locus is likely to be useful for the marker-assisted selection of agronomic traits. Received: 5 June 1997 / Accepted: 19 November 1997     

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.