Genetic evidence for environment-dependent hybrid incompatibilities in threespine stickleback

Hybrid incompatibilities occur when interactions between opposite-ancestry alleles at different loci reduce the fitness of hybrids. Most work on incompatibilities has focused on those that are 'intrinsic', meaning they affect viability and sterility in the laboratory. Theory predicts that ecological selection can also underlie hybrid incompatibilities, but tests of this hypothesis are scarce. In this article, we compiled genetic data for F2 hybrid crosses between divergent populations of threespine stickleback fish (Gasterosteus aculeatus L.) that were born and raised in either the field (semi-natural experimental ponds) or the laboratory (aquaria). We tested for differences in excess heterozygosity between these two environments at ancestry informative loci—a genetic signature of selection against incompatibilities. We found that excess ancestry heterozygosity was elevated by approximately 3% in crosses raised in ponds compared to those raised in aquaria. Previous results from F1 hybrids in the field suggest that pond-specific (single-locus) heterosis is unlikely to explain this finding. Our study suggests that, in stickleback, a coarse signal of environment-dependent hybrid incompatibilities is reliably detectable and that extrinsic incompatibilities have evolved before intrinsic incompatibilities.

Excess heterozygosity and scarce genetic differentiation in the populations of Phoenix dactylifera L.: human impact or ecological determinants

Extensive research has been conducted on the characterization of hundreds of date palm (Phoenix dactyliferaL.) cultivars worldwide. However, the population genetics of date palms has never been studied. In this study, we collected 200 individuals from 19 populations of different geographical locations in Sudan. The collection sites were grouped according to the type of dates (date palm fruits) that dominates in the area. Ten microsatellite markers were used to investigate the genetic diversity within and among populations and the correlation between the genetic and geographical distances. The tested microsatellite markers showed a high level of polymorphism. A total of 261 alleles were detected at the ten loci. The overall mean value of fixation indices equalled − 0.163, which shows the presence of excess heterozygosity. However, the χ2tests conducted for every locus in each population indicated no significant deviation from the Hardy–Weinberg equilibrium. The analyses of molecular variance exhibited that about 95% of the total genetic variation existed within populations, while significant differentiation within the type groups could be detected. Although significant isolation by distance (r2 = 0.552,P < 0.05) was detected by a Mantel test, it seems that the spatial effect has become complicated as a result from the exchange and introduction of different kinds of plant materials by date palm growers and traders, as well as seed dispersal. This complexity was clearly apparent in the weak clustering relationships among most of the tested populations.

Excess Heterozygosity Contributes to Genetic Map Expansion in Pea Recombinant Inbred Populations

Several plant genetic maps presented in the literature are longer than expected from cytogenetic data. Here we compare F2 and RI maps derived from a cross between the same two parental lines and show that excess heterozygosity contributes to map inflation. These maps have been constructed using a common set of dominant markers. Although not generally regarded as informative for F2 mapping, these allowed rapid map construction, and the resulting data analysis has provided information not otherwise obvious when examining a population from only one generation. Segregation distortion, a common feature of most populations and marker systems, found in the F2 but not the RI, has identified excess heterozygosity. A few markers with a deficiency of heterozygotes were found to map to linkage group V (chromosome 3), which is known to form rod bivalents in this cross. Although the final map length was longer for the F2 population, the mapped order of markers was generally the same in the F2 and RI maps. The data presented in this analysis reconcile much of the inconsistency between map length estimates from chiasma counts and genetic data.

Compensatory Aspects of Allele Diversity at Immunoglobulin Loci: Gene Correlations in Rabbit Populations Devoid of Light Chain Diversity (oryctolagus cuniculus L.; Kerguelen Islands)

Is there a selective advantage of increased diversity at one immunoglobulin locus when diversity at another locus is low? A previous paper demonstrated excess heterozygosity at the rabbit light chain b locus when heterozygosity was low at the heavy chain constant region e locus. Here we consider the reverse situation by analyzing allele distributions at heavy chain loci in populations fixed for the light chain b locus. We analyzed the a locus that encodes the predominantly expressed heavy chain variable region, and the d and e loci that control different parts of the Ig gamma class constant region. While there was excess heterozygosity, genetic differentiation between localities was extensive and was most pronounced for females. This was in marked contrast with observations in areas where b-locus diversity was important and confirms a negative correlation between e- and b-locus heterozygosity. Trigenic disequilibria corresponded to a significant negative correlation between e- and a-locus heterozygosity due mainly to strong variation among localities within the context of pronounced (digenic) linkage disequilibria. Although substantial, the average increase in a/e-locus single heterozygosity implemented by higher order disequilibria within localities was not significant.

Gene diversity and female philopatry.

The effect of female philopatry on the apportionment of gene diversity within a population is evaluated. Even with random mate selection, the apportionment of gene diversity within and among social lineages (groups of related females) is inherently different than in classically defined demic groups. Considerable excess heterozygosity occurs within lineages without substantial changes in total or population heterozygosity. The proportion of genetic variance among lineages within the population was dependent on the lineage size and the number of male breeders per lineage. The greatest genetic differentiation among lineages was evident when there was one polygynous male breeding within a lineage of philopatric females, a common breeding tactic in mammalian social systems. The fixation indices depicting the genetic structure of the population were found to attain constant values after the first few generations despite the continuous loss of gene diversity within the population by genetic drift. Additionally, the change of gene correlations within individuals relative to the change within the population attains a state of dynamic equilibrium, as do the changes of gene correlations within lineages relative to the total and within individuals relative to within lineages. Comparisons of coancestries and fixation indices for philopatric versus randomly dispersing females indicate that philopatry and polygyny have probably not evolved independently and that promotion of gene correlations among adults rather than offspring has been of primary importance.

DYNAMICS OF CORRELATED GENETIC SYSTEMS. I. SELECTION IN THE REGION OF THE GLUED LOCUS OF DROSOPHILA MELANOGASTER

ABSTRACT The dynamical behavior of chromosomal segments undergoing strong selection was investigated in four replicate populations of Drosophila melanogaster. This was accomplished by following the joint behavior of allozyme markers at the loci phosphoglucomutase and esterase C, adjacent to the recessive lethal locus Glued, during and following the course of selection against Glued. The results show strong selection at other loci in the region of the marked segment. Examination of the joint dynamic of the two markers indicates that there must be more than one, and probably several, selected loci in the region under observation, with large epistatic effects. The mode of selection on the segment often results in excess heterozygosity at the markers, but does not appear to be constant in time. It is concluded that the density of selective effects in the region under study is substantial.

INHERITED BIOCHEMICAL VARIATION IN DROSOPHILA MELANOGASTER: NOISE OR SIGNAL? I. SINGLE-LOCUS ANALYSES

ABSTRACT A study was conducted using small effective population size as an experimental design to test selective neutrality of seven isozyme polymorphisms. Loci varied as to the degree to which the decay of heterozygosity over 21 generations was retarded. Selection for heterozygotes, overdominance, is implicated for at least four of seven loci. Of these ADH gave the largest heterozygote excess in the presence of inbreeding. An interaction between the small population size treatment and excess heterozygosity suggests that (1) the loci studied may be selectively neutral and linked to other loci which are under the influence of selection or (2) the selection coefficients for the loci studied are not independent of the background genotype. In either case four of the seven enzymes studied are signaling the operation of selection. The problem of distinguishing the effect of a single marker from that of a chromosome segment is emphasized. The identification of the genetic unit of selection is crucial to any interpretation of the meaning of enzyme polymorphisms.