Divergent Directional Selection Research Articles

Red and blond Mangalitza pigs display a signature of divergent directional selection in the SLC45A2 gene.

The Mangalitza lard-type pig breed is well known for its fat appearance and curly hair, and it is mainly distributed in Eastern Europe. Four main lines were created in the nineteenth century by artificial selection: Blond Mangalitza, Black Mangalitza, Swallow-Belly Mangalitza and Red Mangalitza. The Swallow-Belly line has a black coat combined with yellow-blond throat and underbelly. In the current work, we aimed to investigate if the colourations of Mangalitza pigs are genetically determined by one or a few loci whose frequencies have been modified by artificial selection. The results of selection scans, with HapFLK and BayeScan, and of a GWAS for coat colour highlighted the existence of one region on SSC16 (18-20Mb) with potential effects on hair pigmentation (Red vs. Blond contrast). The analysis of the gene content of this region allowed us to detect the solute carrier family 45 member 2 (SLC45A2) locus as a candidate gene for this trait. The polymorphism of the SLC45A2 locus has been associated with reduced levels or the absence of melanin in several mammalian species. The genotyping of four missense polymorphisms evidenced that rs341599992:G>A and rs693695020:G>A SNPs are strongly but not fully associated with the red and blond coat colours of Mangalitza pigs, a result that was confirmed by performing a haplotype association test. The near fixation of alternative SLC45A2 genotypes in Red and Blond Mangalitza pigs provides a compelling example of the consequences of a divergent directional selection for coat colour in a domestic species.

Hybridization and genome evolution II: Mechanisms of species divergence and their effects on evolution in hybrids

Abstract Recent genomic studies have highlighted the importance of hybridization and gene exchange in evolution. We ask what factors cause variation in the impact of hybridization, through adaptation in hybrids and the likelihood of hybrid speciation. During speciation, traits that diverge due to both divergent and stabilizing selection can contribute to the buildup of reproductive isolation. Divergent directional selection in parent taxa should lead to intermediate phenotypes in hybrids, whereas stabilizing selection can also produce extreme, transgressive phenotypes when hybridization occurs. By examining existing theory and empirical data, we discuss how these effects, combined with differences between modes of divergence in the chromosomal distribution of incompatibilities, affect adaptation and speciation in hybrid populations. The result is a clear and testable set of predictions that can be used to examine hybrid adaptation and speciation. Stabilizing selection in parents increases transgression in hybrids, increasing the possibility for novel adaptation. Divergent directional selection causes intermediate hybrid phenotypes and increases their ability to evolve along the direction of parental differentiation. Stabilizing selection biases incompatibilities towards autosomes, leading to reduced sexual correlations in trait values and reduced pleiotropy in hybrids, and hence increased freedom in the direction of evolution. Directional selection causes a bias towards sex-linked incompatibilities, with the opposite consequences. Divergence by directional selection leads to greater dominance effects than stabilizing selection, with major but variable impacts on hybrid evolution.

Extensive Functional Diversification of thePopulusGlutathioneS-Transferase Supergene Family

Identifying how genes and their functions evolve after duplication is central to understanding gene family radiation. In this study, we systematically examined the functional diversification of the glutathione S-transferase (GST) gene family in Populus trichocarpa by integrating phylogeny, expression, substrate specificity, and enzyme kinetic data. GSTs are ubiquitous proteins in plants that play important roles in stress tolerance and detoxification metabolism. Genome annotation identified 81 GST genes in Populus that were divided into eight classes with distinct divergence in their evolutionary rate, gene structure, expression responses to abiotic stressors, and enzymatic properties of encoded proteins. In addition, when all the functional parameters were examined, clear divergence was observed within tandem clusters and between paralogous gene pairs, suggesting that subfunctionalization has taken place among duplicate genes. The two domains of GST proteins appear to have evolved under differential selective pressures. The C-terminal domain seems to have been subject to more relaxed functional constraints or divergent directional selection, which may have allowed rapid changes in substrate specificity, affinity, and activity, while maintaining the primary function of the enzyme. Our findings shed light on mechanisms that facilitate the retention of duplicate genes, which can result in a large gene family with a broad substrate spectrum and a wide range of reactivity toward different substrates.

Assaying polymorphism at DNA level for genetic diversity diagnostics of the safflower (Carthamus tinctorius L.) world germplasm resources

Carthamus tinctorius (2n = 2x = 24), commonly known as safflower, is widely cultivated in agricultural production systems of Asia, Europe, Australia, and the Americas as a source of high quality vegetable and industrial oil. Twenty-two RAPD primers, 18 SSR primers, and 10 AFLP primer combinations were used to assess: (1) the genetic diversity of 85 accessions (originating from 24 countries) representing global germplasm variability of safflower and (2) the interrelationships among safflower 'centers of similarity' or 'regional gene pools' proposed earlier. The RAPD and SSR primers and AFLP primer combinations revealed 57.6, 68.0, and 71.2% polymorphism, respectively, among 111, 72, and 330 genetic loci amplified from the accessions. The sum of effective number of alleles (66.44), resolving power (59.16), and marker index (51.3) explicitly revealed the relative superiority of AFLP as a marker system in uncovering variation in safflower. Overall, AFLP markers could recognize 'centers of similarity' or 'regional gene pools'. Analysis of molecular variance and Shannon's information index provided corroborating evidences for the present and previous studies that concluded fragmentation of safflower gene pool into many gene pools. Divergent directional selection is likely to have played an important role in shaping the diversity. From the practical applications standpoint, the diversity of Iran-Afghanistan gene pool is very high, equivalent to the total diversity of the species. The Far East gene pool is the least diverse. The present comprehensive input, first of its own kind in safflower, will assist marker based improvement programmes in the crop.

Discrete genetic variation in mate choice and a condition-dependent preference function in the side-blotched lizard: implications for the formation and maintenance of coadapted gene complexes

Variation in female preference functions, both genotypic and phenotypic, has been largely ignored in the literature, despite its implications to the maintenance of genetic variation in populations and the resolution of the ‘‘Lek paradox.’’ Polymorphic populations, such as in the side-blotched lizard, provide ideal study systems for its investigation, especially in the context of incipient processes of sympatric speciation. Females of the side blotch lizard exist in 2 genetically distinct morphs, yellow throated and orange throated, that experience disruptive selection for life history traits. Males express 3 throat color morphs, blue, orange, and yellow, that exhibit alternative strategies in intrasexual competition. We experimentally tested for female preference in triadic mate choice trials to identify the presence of discrete genetic and condition-dependent variation in female preference function. We found that females did in fact show genetic variation in preference for males but that females also operate a multicondition preference function dependent upon the genotype of the female and her state (number of clutches laid). Females exhibited positive assortative mating prior to the first clutch. However, prior to later clutches, orange females switched choice, preferring yellow males. These findings are discussed in relation to the maintenance of coadapted gene complexes within populations and the prevention of divergent directional selection (population bifurcation) by conditiondependent variation in mate choice. Key words: coadapted gene complex, condition dependent, disruptive selection, genetic variation, lek paradox, mate choice, polymorphism, side-blotched lizard, sympatric speciation. [Behav Ecol]

Modes of selection and recombination response in Drosophila melanogaster.

A selection experiment for sternopleural bristle number in Drosophila melanogaster was undertaken to analyze the correlated effects on recombination. Replicate lines were subjected to divergent directional selection and to stabilizing selection. Recombination rates for markers on chromosomes 2 (dp-cn-bw) and 3 (se-ss-ro) were compared to those from a control. All lines responded as predicted for bristle number. Lines selected for both increased and decreased bristle number exhibited significantly increased recombination rates. The predicted recombination response from stabilizing selection is suggested by our data, but only one comparison is statistically significant. These results, taken with other studies, support the proposal that genetic recombination enhances individual fitness when populations are experiencing environmental change. Less conclusively, our results suggest that populations undergoing stabilizing selection may respond by reducing their rates of crossing over.

Selection for pupation height in Drosophila melanogaster

Divergent directional selection for high and low pupation height was practiced in D. melanogaster. A quick response was observed in the two directions of selection. This is the first time selection for low pupation sites was successful. Realized heritabilities were 18% and 13% for the high and low lines. Reciprocal crosses between divergent lines showed little or no dominance for low pupation sites. The need for a strict control of environmental factors when measuring pupation height is emphasized.

Directional selection in lines founded from different parts of the phenotypic distribution of sternopleural chaetae number in Drosophila melanogaster

Divergent directional selection lines were initiated from base populations founded from parents taken from different parts of the sternopleural chaetae distribution in a cage population of Drosophila melanogaster. Lines founded from parents taken from the central part of the distribution showed greater response and higher realised heritability than lines derived from parents with extreme high or extreme low chaetae number. The results suggest that centrally derived phenotypes have higher heterozygosity for chaetae factors than extreme phenotypes and that these factors have a large effect on the character.

Association of allozyme heterozygosity and sternopleural chaetae number in Drosophila melanogaster.

Variation at two polymorphic allozyme loci (Adh and alpha Gpdh) has been assayed in relation to sternopleural chaetae number in flies derived from a cage population of D. melanogaster known to be under stabilising selection with respect to chaetae number. Central phenotypes were significantly more heterozygous at these loci than more extreme phenotypes. Analysis of the base population and of divergent directional selection lines derived from it suggested that alleles at Adh and alpha Gpdh have a pleiotropic effect on chaetae number or are in strong linkage disequilibrium with chaetae factors. In addition, homozygotes at Adh and alpha Gpdh were observed to have a higher variance in chaetae number than heterozygotes. It is concluded that additive and homeostatic models of gene action might both partly explain the higher heterozygosity of central phenotypes.

Divergent selection on locomotor activity in Drosophila melanogaster. I. Selection response.

Selection for high and low locomotor activity has been applied in two base populations of Drosophila melanogaster of distinct geographical origin. From each base population a high and a low line were selected, in which anesthesia was performed with ether. In addition, from one of the base populations a high line and a low line were selected under CO2 narcosis. Locomotor activity was measured in an apparatus consisting of rows of 20 tubes in a line. Heritabilities in the base populations determined in progeny tests were approximately 10%. Divergent directional selection was successful with realized heritabilities of similar value.

Divergent selection on locomotor activity in Drosophila melanogaster. III. Genetic analysis.

Divergent directional selection produced three pairs of lines each consisting of a line with high and a line with low locomotor activity. Reciprocal crosses between the high and low lines of one of these pairs showed that a considerable part of the activity differences was contributed by differences between the X chromosomes. This was confirmed by a substitution of the three large chromosomes, between the low and the high lines. The two large autosomal chromosomes had only minor effects. Interactions between chromosomes were sometimes significant. Low-activity alleles tended to be dominant over alleles for high activity.

Alcohol dehydrogenase activity in Drosophila melanogaster: a quantitative character.

SUMMARYAlcohol dehydrogenase activity inDrosophila melanogastermay be considered as a quantitative character, since it shows many features typically associated with such traits. Although strains with the electrophoretically fast phenotype generally have activities greater than those with the slow phenotype, presumably reflecting differences in the nucleotide sequences of the structural alleles, within each electrophoretic class there is considerable variation in activity. The expression of the structural gene, in terms of ADH activity, is to some extent regulated by its genetic background. Strains homozygous for particular structural alleles respond to divergent directional selection for ADH activity. Modifiers have been located to theX, second and third chromosomes.