Disequilibrium linkage (gametic disequilibrium) of structural genes in the bovine population

In the population of Kholmogor cattle (n=824), gametic disequilibrium was studied for alleles of the polymorphic loci β-Lg (LGB), β-Cn (CSN2) and æ-Cn (CSN3). In purebred Kholmogor cattle, the β-casein polymorphism is determined by three alleles with the frequencies of CSN2А 1 – 0.368±0.0076, CSN2А 2 – 0.497±0.0078, CSN2B – 0.136±0.0056 and the frequencies of acasein of CSN3А – 0.689 ±0.0061, CSN3B – 0.311±0.0061, respectively. The beta-lactoglobulin polymorphism is controlled by two alleles LGBA and LGBB with a frequency of 0.237±0.0048 and 0.763±0.0048, respectively. In the population of breeding plants, the gametic disequilibrium was revealed according to the alleles of loci CSN2 and LGB; CSN2 and CSN3. The factor that caused the gametic disequilibrium was probably the initial difference between native Russian cattle and black-and-white cattle of Western Europe on the basis of which the Kholmogor breed gene pool was developed. The population showed an excess of haplotypes β-CnВæ-CnВ, β-CnВβ-LgВ (the "attraction" phase), apparently inherited mainly from native Russian cattle, and β-CnА 1æ-CnА, β-CnА 2β-LgА characteristic mainly for black-and-white cattle of Western Europe and a lack of β-CnВæ-CnА, β-CnВβ-LgА (the "repulsion" phase), probably due to the low frequency of these haplotypes in both initial breeds. The role of various factors in the occurrence and maintenance of gamete disequilibrium is discussed. It is suggested that for alleles of non-linked loci, longtime retention of gametic disequilibrium in population is most likely supported by selection

Two-Locus Local Adaptation by Additive or Epistatic Gene Combinations in Autotetraploids Versus Diploids

In this article, we present a theoretical comparison of local adaptation between diploid and autotetraploid populations when fitness is determined by either additive or epistatic interactions between alleles at 2 loci. A continent-island model of local adaptation is derived, with 1-way migration from the continent to the island and distinct genotypes adaptive on the continent versus the island. The meiotic component of the model accounts for multivalent formation and the processes of chromosomal gametic disequilibrium and double reduction, which are unique to autotetraploids. Both the adaptability and efficiency of adaptation are investigated, where adaptability asks whether a population adapts and efficiency is the rate of adaptation. With an additive genetic basis to fitness, diploids experience better adaptability and efficiency than autotetraploids. With epistasis, our results indicate a limited parameter space in which autotetraploids have greater adaptability than diploids, but results indicate an interesting difference between adaptability and efficiency of adaptation. Oftentimes, diploids exhibit greater adaptability whereas autotetraploids exhibit greater efficiency of adaptation. These findings provide evidence for the advantage of epistasis within autotetraploids when efficiency of adaptation is of interest. Although autotetraploids are more efficient, under the same conditions and at equilibrium, diploid populations often have higher mean local fitness. Overall, the most ideal situation for autotetraploid local adaptation compared to diploids is when epistasis is strong, mutation is weak, recombination is high, selection is strong, deleterious selection is additive, chromosomal gametic disequilibrium is present, and double reduction is absent.

Signatures of Recombination in Clonal Lineages of the Citrus Brown Spot Pathogen, Alternaria alternata sensu lato

Most Alternaria spp. are considered asexual but recent molecular evolution analyses of Alternaria mating-type genes show that the mating locus is under strong purifying selection, indicating a possible role in sexual reproduction. The objective of this study was to determine the mode of reproduction of an Alternaria alternata sensu lato population causing citrus brown spot in central Florida. Mating type of each isolate was determined, and isolates were sequenced at six putatively unlinked loci. Three genetically distinct subpopulations (SH1, SH4A, and SH4B) were identified using network and Bayesian population structure analyses. Results demonstrate that most subpopulations of A. alternata associated with citrus are clonal but some have the ability to extensively recombine through a cryptic sexual cycle or parasexual cycle. Although isolates were sampled in close physical proximity (≈2,500-m2 area), we were able to reject a random mating model using multilocus gametic disequilibrium tests for two subpopulations, SH1 and SH4B, suggesting that these subpopulations were predominantly asexual. However, three recombination events were identified in SH1 and SH4B and localized to individuals of opposite mating type, possibly indicating meiotic recombination. In contrast, in the third subpopulation (SH4A), where only one mating type was present, extensive reticulation was evident in network analyses, and multilocus gametic disequilibrium tests were consistent with recombination. Recombination among isolates of the same mating type suggests that a nonmeiotic mechanism of recombination such as the parasexual cycle may be operating in this subpopulation. The level of gene flow detected among subpopulations does not appear to be sufficient to prevent differentiation, and perhaps future speciation, of these A. alternata subpopulations.