Resampling the pool of genotypic possibilities: an adaptive function of sexual reproduction

Background Natural populations harbor significant levels of genetic variability. Because of this standing genetic variation, the number of possible genotypic combinations is many orders of magnitude greater than the population size. This means that any given population contains only a tiny fraction of all possible genotypic combinations. Results We show that recombination allows a finite population to resample the genotype pool, i.e., the universe of all possible genotypic combinations. Recombination, in combination with natural selection, enables an evolving sexual population to replace existing genotypes with new, higher-fitness genotypic combinations that did not previously exist in the population. This process allows the sexual population to gradually increase its fitness far beyond the range of fitnesses in the initial population. In contrast to this, an asexual population is limited to selection among existing lower fitness genotypes. Conclusions The results provide an explanation for the ubiquity of sexual reproduction in evolving natural populations, especially when natural selection is acting on the standing genetic variation.

The adaptive function of sexual reproduction: resampling the pool of genotypic possibilities.

BackgroundNatural populations harbor significant levels of genetic variability. Because of this standing genetic variation, the number of possible genotypic combinations is many orders of magnitude greater than the population size. This means that any given population contains only a tiny fraction of all possible genotypic combinations.ResultsWe show that recombination allows a finite population to resample the genotype pool, i.e., the universe of all possible genotypic combinations. Recombination, in combination with natural selection, enables an evolving sexual population to replace existing genotypes with new, higher-fitness genotypic combinations that did not previously exist in the population. Gradually the selected sexual population approaches a state where the optimum genotype is produced by recombination and where it rises to fixation. In contrast to this, an asexual population is limited to selection among existing lower fitness genotypes.ConclusionsThe significance of the result is two-fold. First, it provides an explanation for the ubiquity of sexual reproduction in evolving populations. Secondly, it shows that recombination serves to remove concerns about the cost of natural selection acting on the naturally occurring standing genetic variation. This means that classic population genetics theory is applicable to ecological studies of natural selection acting on standing genetic variation.

Two new species of Cypricercinae McKenzie, 1971 (Crustacea: Ostracoda) from Thailand

Two new species of the subfamily Cypricercinae McKenzie, 1971 are described from the Western part of Thailand: Pseudostrandesia ratchaburiensis sp. nov. and Strandesia prachuapensis sp. nov. Pseudostrandesia ratchaburiensis sp. nov. is mainly characterized by a flange on the antero-ventral part of the left valve (LV), a markedly large β seta on the mandibular (Md) palp, serrated bristles on the third endite of the maxillula (Mx1), a slender caudal ramus (CR) with a long claw Ga (length ca half that of the ramus) and a relatively low number (13) of spiny whorls in the Zenker’s organ. The discovery of both males and females of Pseudostrandesia ratchaburiensis sp. nov. in the present study constitutes the first report of a sexual population in this genus, thereby allowing for a comparison of the male reproductive organs (hemipenis and Zenker’s organ) from a new species with those of other genera of Cypricercinae. Strandesia prachuapensis sp. nov. is most closely related to Strandesia odiosa (Moniez, 1892) and Strandesia flavescens Klie, 1932 as they bear similar anterior flanges on the right valve (RV). The key diagnostic features of the new Strandesia species are a large carapace (ca 1.5 mm), an angulated antero-ventral part of the LV, a weak and small anterior inner list on the LV, an anterior flange on the RV, a markedly small aesthetasc Y on the second antenna, a large β seta on the Md-palp, smooth bristles on the third endite of the Mx1 and a slender CR with a short claw Ga (length ca ⅓ of the ramus). In addition, Pseudostrandesia complexa (Victor & Fernando, 1981) comb. nov. is here proposed.

Male Sexual Trait Decay in Two Asexual Springtail Populations Follows Neutral Mutation Accumulation Theory

The transition to asexual reproduction is frequent and widespread across the tree of life and constitutes a major life history change. Without sexual reproduction, selection on sexually selected traits is expected to be weaker or absent, allowing the decay and ultimately loss of sexual traits. In this study, we applied an experimental approach to investigate the decay of reproductive traits under asexuality in two asexual populations of the springtail Folsomia candida. Specifically, we compared several key male sexual traits of a sexual population and two distinct parthenogenetic lines. To allow direct comparisons between sexual and asexual individuals we first determined a suite of life history characteristics in the sexual F. candida population, which performs an indirect transfer of sperm packages (spermatophores).To investigate the decay of male sexual traits under asexuality we measured the size of spermatophores, quantified the amount of sperm DNA material, and tested spermatophore attractiveness to females in all three populations. The amount of sperm DNA material in the sperm droplets and the attractiveness of spermatophores were lower in the asexual lines compared to the sexual population. However, the two asexual lines differed in the extent of decay of these traits. Our results are consistent with predictions from neutral mutation accumulation theory, and thus suggest this to be the main evolutionary process underlying the decay of male traits in F. candida.

An Avirulence Gene Cluster in the Wheat Stripe Rust Pathogen (Puccinia striiformis f. sp. tritici) Identified through Genetic Mapping and Whole-Genome Sequencing of a Sexual Population

ABSTRACT Puccinia striiformis f. sp. tritici, the causal agent of wheat stripe (yellow) rust, is an obligate, biotrophic fungus. It was difficult to study the genetics of the pathogen due to the lack of sexual reproduction. The recent discovery of alternate hosts for P. striiformis f. sp. tritici makes it possible to study inheritance and map genes involved in its interaction with plant hosts. To identify avirulence (Avr) genes in P. striiformis f. sp. tritici, we developed a segregating population by selfing isolate 12-368 on barberry (Berberis vulgaris) plants under controlled conditions. The dikaryotic sexual population segregated for avirulent/virulent phenotypes on nine Yr single-gene lines. The parental and progeny isolates were whole-genome sequenced at >30× coverage using Illumina HiSeq PE150 technology. A total of 2,637 high-quality markers were discovered by mapping the whole-genome sequencing (WGS) reads to the reference genome of strain 93-210 and used to construct a genetic map, consisting of 41 linkage groups, spanning 7,715.0 centimorgans (cM) and covering 68 Mb of the reference genome. The recombination rate was estimated to be 1.81 ± 2.32 cM/10 kb. Quantitative trait locus analysis mapped six Avr gene loci to the genetic map, including an Avr cluster harboring four Avr genes, AvYr7, AvYr43, AvYr44, and AvYrExp2. Aligning the genetic map to the reference genome identified Avr candidates and narrowed them to a small genomic region (<200 kb). The discovery of the Avr gene cluster is useful for understanding pathogen evolution, and the identification of candidate genes is an important step toward cloning Avr genes for studying molecular mechanisms of pathogen-host interactions. IMPORTANCE Stripe rust is a destructive disease of wheat worldwide. Growing resistant cultivars is the most effective, easy-to-use, economical, and environmentally friendly strategy for the control of the disease. However, P. striiformis f. sp. tritici can produce new virulent races that may circumvent race-specific resistance. Therefore, understanding the genetic basis of the interactions between wheat genes for resistance and P. striiformis f. sp. tritici genes for avirulence is useful for improving cultivar resistance for more effective control of the disease. This study developed a high-quality map that facilitates genomic and genetic studies of important traits related to pathogen pathogenicity and adaptation to different environments and crop cultivars carrying different resistance genes. The information on avirulence/virulence genes identified in this study can be used for guiding breeding programs to select combinations of genes for developing new cultivars with effective resistance to mitigate this devastating disease.

The adaptive function of sexual reproduction: resampling the genotype pool

Recombination allows a finite population to resample the genotype pool, i.e., the universe of all possible genotypic combinations. This is important in populations that contain abundant genetic variation because, in such populations, the number of potential genotypes is much larger than the number of individuals in the population. Here, we show how recombination, in combination with natural selection, enables an evolving sexual population to replace existing genotypes with new, higher-fitness genotypic combinations. In contrast to this, an asexual population is limited to selection among existing genotypes. Since it has been shown that most eukaryotic species are genetically polymorphic, our model can explain the ubiquity of sex among such species. The model also indicates that classic population genetics theory is applicable to ecological studies of natural selection acting on standing genetic variation.