Sex In Natural Populations Research Articles

Exploring potential drivers of brain size variation in the electric fish Brachyhypopomus occidentalis

Characterizing the factors that shape variation in brain size in natural populations is crucial to understanding the evolution of brain size in animals. Here, we explore how relative brain size and brain allometry vary with drainage, predation risk and sex in natural populations of the electric knifefish Brachyhypopomus occidentalis. Fish were sampled from high and low predation risk sites within two independent river drainages in eastern and central Panamá. Overall, we observed low variation in brain-body size allometric slopes associated with drainage, predation risk and sex category. However, we observed significant differences in allometric intercepts between predation risk sites. We also found significant differences in relative brain mass associated with drainage, as well as significant differences in absolute brain mass associated with drainage, predation risk and sex category. Our results suggest potential constraints in brain-body allometry across populations of B. occidentalis. However, both drainage and predation risk may be playing a role in brain mass variation among populations. We suggest that variation in brain mass in electric fishes is affected by multiple extrinsic and intrinsic factors, including geography, environmental complexity, social interaction and developmental or functional constraints.

Natural populations of the entomopathogenic fungus Beauveria bassiana in Chinese forest ecosystems are diverse and reveal equal frequencies of mating types within phylogenetic species

Beauveria bassiana is an important entomopathogenic fungus with widespread application in the biological control of harmful insect pests. This species is widely distributed as an anamorph while only two teleomorph specimens have been found in eastern China. However, little is known about the ecological conditions for sexual reproduction in natural populations of B. bassiana . Here, we collected 488 isolates of Chinese B. bassiana sensu stricto from five sites, in which teleomorph or anamorph occurred, and used molecular phylogenetic and haplotype information to determine phylogenetic diversity, mating types, and sexual reproductive potential in these populations. Molecular identification based on denaturing gradient gel electrophoresis (DGGE) and combined data of the nuclear intergenic region Bloc and translation elongation factor-1a (TEF) assemblage resolved five B. bassiana s.s phylogenetic species labeled according to their geographic origin: Europe/N. Africa 1, Asia 3, Asia 4, AFNEO_1, and N. America 2. In Guniujiang and Manshuihe collection sites, teleomorph isolates RCEF 0771 and RCEF 0382 were both identified as Europe/N. Africa 1 phylogenetic species. In addition, more than half of the isolates in five representative sites belonged to Europe/N. Africa 1. However, the teleomorph of B. bassiana s.s . was not detected in Kuankuoshui while isolates within Europe/N. Africa 1 were present at this site, and isolates belonging to Europe/N. Africa 1 were not found in either Jingyuetan or Dinghushan collection sites. Distribution of MAT1 and MAT2 mating type idiomorphs in Europe/N. Africa 1 were 51:69, 37:24, and 15:15 in Guniujiang, Manshuihe, and Kuankuoshui, respectively. The presence of teleomorph and roughly equal frequencies of opposite mating types indicate regular sexual reproduction in B. bassiana natural populations. The data offer a better understanding of the ecological conditions of sexual reproduction in natural populations of B. bassiana. These results also yield insights into the potential for sexual reproduction in other supposedly ‘asexual’ fungi. • Five phylogenetic species of B. bassiana sensu stricto have been recognized in China. • Two teleomorph isolates are both identified as Europe/N. Africa 1 phylogenetic species. • The equal frequencies of mating types indicate regular sex in natural populations of B. bassiana.

Detecting sex-linked genes using genotyped individuals sampled in natural populations.

We propose a method, SDpop, able to infer sex-linkage caused by recombination suppression typical of sex chromosomes. The method is based on the modeling of the allele and genotype frequencies of individuals of known sex in natural populations. It is implemented in a hierarchical probabilistic framework, accounting for different sources of error. It allows statistical testing for the presence or absence of sex chromosomes, and detection of sex-linked genes based on the posterior probabilities in the model. Furthermore, for gametologous sequences, the haplotype and level of nucleotide polymorphism of each copy can be inferred, as well as the divergence between them. We test the method using simulated data, as well as data from both a relatively recent and an old sex chromosome system (the plant Silene latifolia and humans) and show that, for most cases, robust predictions are obtained with 5 to 10 individuals per sex.

Extremely Widespread Parthenogenesis and a Trade-Off Between Alternative Forms of Reproduction in Mayflies (Ephemeroptera).

Studying alternative forms of reproduction in natural populations is of fundamental importance for understanding the costs and benefits of sex. Mayflies are one of the few animal groups where sexual reproduction co-occurs with different types of parthenogenesis, providing ideal conditions for identifying benefits of sex in natural populations. Here, we establish a catalog of all known mayfly species capable of reproducing by parthenogenesis, as well as species unable to do so. Overall, 1.8% of the described species reproduce parthenogenetically, which is an order of magnitude higher than reported in other animal groups. This frequency even reaches 47.8% if estimates are based on the number of studied rather than described mayfly species, as reproductive modes have thus far been studied in only 17 out of 42 families. We find that sex is a more successful strategy than parthenogenesis (associated with a higher hatching success of eggs), with a trade-off between the hatching success of parthenogenetic and sexual eggs. This means that improving the capacity for parthenogenesis may come at a cost for sexual reproduction. Such a trade-off can help explain why facultative parthenogenesis is extremely rare among animals despite its potential to combine the benefits of sexual and parthenogenetic reproduction. We argue that parthenogenesis is frequently selected in mayflies in spite of this probable trade-off because their typically low dispersal ability and short and fragile adult life may frequently generate situations of mate limitation in females. Mayflies are currently clearly underappreciated for understanding the benefits of sex under natural conditions.

Sexual reproduction in a natural Trypanosoma cruzi population.

BackgroundSexual reproduction provides an evolutionary advantageous mechanism that combines favorable mutations that have arisen in separate lineages into the same individual. This advantage is especially pronounced in microparasites as allelic reassortment among individuals caused by sexual reproduction promotes allelic diversity at immune evasion genes within individuals which is often essential to evade host immune systems. Despite these advantages, many eukaryotic microparasites exhibit highly-clonal population structures suggesting that genetic exchange through sexual reproduction is rare. Evidence supporting clonality is particularly convincing in the causative agent of Chagas disease, Trypanosoma cruzi, despite equally convincing evidence of the capacity to engage in sexual reproduction.Methodology/ Principle FindingsIn the present study, we investigated two hypotheses that can reconcile the apparent contradiction between the observed clonal population structure and the capacity to engage in sexual reproduction by analyzing the genome sequences of 123 T. cruzi isolates from a natural population in Arequipa, Peru. The distribution of polymorphic markers within and among isolates provides clear evidence of the occurrence of sexual reproduction. Large genetic segments are rearranged among chromosomes due to crossing over during meiosis leading to a decay in the genetic linkage among polymorphic markers compared to the expectations from a purely asexually-reproducing population. Nevertheless, the population structure appears clonal due to a high level of inbreeding during sexual reproduction which increases homozygosity, and thus reduces diversity, within each inbreeding lineage.Conclusions/ SignificanceThese results effectively reconcile the apparent contradiction by demonstrating that the clonal population structure is derived not from infrequent sex in natural populations but from high levels of inbreeding. We discuss epidemiological consequences of this reproductive strategy on genome evolution, population structure, and phenotypic diversity of this medically important parasite.

Habitat heterogeneity favors asexual reproduction in natural populations of grassthrips.

Explaining the overwhelming success of sex among eukaryotes is difficult given the obvious costs of sex relative to asexuality. Different studies have shown that sex can provide benefits in spatially heterogeneous environments under specific conditions, but whether spatial heterogeneity commonly contributes to the maintenance of sex in natural populations remains unknown. We experimentally manipulated habitat heterogeneity for sexual and asexual thrips lineages in natural populations and under seminatural mesocosm conditions by varying the number of hostplants available to these herbivorous insects. Asexual lineages rapidly replaced the sexual ones, independently of the level of habitat heterogeneity in mesocosms. In natural populations, the success of sexual thrips decreased with increasing habitat heterogeneity, with sexual thrips apparently only persisting in certain types of hostplant communities. Our results illustrate how genetic diversity‐based mechanisms can favor asexuality instead of sex when sexual lineages co‐occur with genetically variable asexual lineages.

Mothers that produce sons and daughters are genetically different in red deer

Sex ratio theory, and in particular Fisher principle, assumes parental control over the sex of offspring through the action of autosomal genes with Mendelian segregation. In spite of the importance of Fisher's principle in evolutionary biology, the number of studies looking for possible loci involved in sex ratio bias is, at best, very low. Newly developed genetic tools frequently allow evolutionary biologists to manage genetic data. Here we encourage the application of association tools to databases that include genetic information for autosomal loci and offspring sex to improve our knowledge on sex ratio evolution. As an example we use microsatellite markers to scan autosomal chromosomes and look for linked genetic regions associated with offspring sex in red deer (Cervus elaphus). We found a microsatellite marker (CelJP38) mapped in chromosome 27 for which females producing sons and daughters were genetically different. To the best of our knowledge, this is the first study that shows a genetic signal that points out an association between mother genotype and offspring sex in natural populations of a mammal.

The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms.

Amoebae are generally assumed to be asexual. We argue that this view is a relict of early classification schemes that lumped all amoebae together inside the 'lower' protozoa, separated from the 'higher' plants, animals and fungi. This artificial classification allowed microbial eukaryotes, including amoebae, to be dismissed as primitive, and implied that the biological rules and theories developed for macro-organisms need not apply to microbes. Eukaryotic diversity is made up of 70+ lineages, most of which are microbial. Plants, animals and fungi are nested among these microbial lineages. Thus, theories on the prevalence and maintenance of sex developed for macro-organisms should in fact apply to microbial eukaryotes, though the theories may need to be refined and generalized (e.g. to account for the variation in sexual strategies and prevalence of facultative sex in natural populations of many microbial eukaryotes). We use a revised phylogenetic framework to assess evidence for sex in several amoeboid lineages that are traditionally considered asexual, and we interpret this evidence in light of theories on the evolution of sex developed for macro-organisms. We emphasize that the limited data available for many lineages coupled with natural variation in microbial life cycles overestimate the extent of asexuality. Mapping sexuality onto the eukaryotic tree of life demonstrates that the majority of amoeboid lineages are, contrary to popular belief, anciently sexual, and that most asexual groups have probably arisen recently and independently. Additionally, several unusual genomic traits are prevalent in amoeboid lineages, including cyclic polyploidy, which may serve as alternative mechanisms to minimize the deleterious effects of asexuality.

Using Parthenogenetic Lineages to Identify Advantages of Sex

The overwhelming predominance of sexual reproduction in nature is surprising given that sex is expected to confer profound costs in terms of production of males and the breakup of beneficial allele combinations. Recognition of these theoretical costs was the inspiration for a large body of empirical research—typically focused on comparing sexual and asexual organisms, lineages, or genomes—dedicated to identifying the advantages and maintenance of sex in natural populations. Despite these efforts, why sex is so common remains unclear. Here, we argue that we can generate general insights into the advantages of sex by taking advantage of parthenogenetic taxa that differ in such characteristics as meiotic versus mitotic offspring production, ploidy level, and single versus multiple and hybrid versus non-hybrid origin. We begin by evaluating benefits that sex can confer via its effects on genetic linkage, diversity, and heterozygosity and outline how the three classes of benefits make different predictions for which type of parthenogenetic lineage would be favored over others. Next, we describe the type of parthenogenetic model system (if any) suitable for testing whether the hypothesized benefit might contribute to the maintenance of sex in natural populations, and suggest groups of organisms that fit the specifications. We conclude by discussing how empirical estimates of characteristics such as time since derivation and number of independent origins of asexual lineages from sexual ancestors, ploidy levels, and patterns of molecular evolution from representatives of these groups can be used to better understand which mechanisms maintain sex in natural populations.

The ecology and genetics of fitness in Chlamydomonas. XII. Repeated sexual episodes increase rates of adaptation to novel environments.

We investigated the dynamics of adaptation of the unicellular chlorophyte Chlamydomonas reinhardtii to new and hostile conditions of growth provided by novel carbon substrates in the dark. The experiment was designed to contrast perennially asexual lines with lines that had experienced one or more sexual episodes. All lines were capable of adapting to the novel environment. The sexual lines, however, showed greater adaptation over the course of the experiment, especially in more complex environments. Moreover, the effect of sex on adaptation increased with the number of successive sexual episodes. The time-course of adaptation showed that sex initially caused an increase in the standardized variance of fitness and an initial drop in mean fitness, at least after a second or third sexual episode. These short-term effects were followed by a period of recovery during which the fitness of sexual lines eventually exceeded that of asexual lines. The increase in mean fitness was mirrored by a decrease in the standardized variance of fitness relative to asexuals, suggesting that directional selection used up the variation generated by meiotic recombination and thereby conferred a fitness advantage to the sexual lines. These results support the Weismann-Fisher-Muller hypothesis for the maintenance of sex in natural populations.

THE ECOLOGY AND GENETICS OF FITNESS IN CHLAMYDOMONAS. XII. REPEATED SEXUAL EPISODES INCREASE RATES OF ADAPTATION TO NOVEL ENVIRONMENTS

We investigated the dynamics of adaptation of the unicellular chlorophyte Chlamydomonas reinhardtii to new and hostile conditions of growth provided by novel carbon substrates in the dark. The experiment was designed to contrast perennially asexual lines with lines that had experienced one or more sexual episodes. All lines were capable of adapting to the novel environment. The sexual lines, however, showed greater adaptation over the course of the experiment, especially in more complex environments. Moreover, the effect of sex on adaptation increased with the number of successive sexual episodes. The time-course of adaptation showed that sex initially caused an increase in the standardized variance of fitness and an initial drop in mean fitness, at least after a second or third sexual episode. These short-term effects were followed by a period of recovery during which the fitness of sexual lines eventually exceeded that of asexual lines. The increase in mean fitness was mirrored by a decrease in the standardized variance of fitness relative to asexuals, suggesting that directional selection used up the variation generated by meiotic recombination and thereby conferred a fitness advantage to the sexual lines. These results support the Weismann-Fisher-Muller hypothesis for the maintenance of sex in natural populations.

Selfish element wars in a wasp battleground

Bacteria of the genus Wolbachia are masters in the art of sex-determination manipulation. By exploiting the haplodiploid system of sex determination (in which haploid eggs develop into males and diploid eggs develop into females) in certain Hymenoptera, Wolbachia are able to convert unfertilized eggs, which would normally develop as males, into females. As a consequence, a sexually reproducing wasp population might, on infection with Wolbachia, become parthenogenetic. Furthermore, it is a well-established fact that the life style of many parasitoid wasp species favours a female-biased sex ratio. Therefore, one would expect in Trichogramma kaykai, a minute wasp that parasitizes the eggs of a lycaenid butterfly and which has a typical sex ratio consisting of 22–29% males, that infection with Wolbachia would mean just one thing: switch to parthenogenesis. Alas, this is not the case, with Wolbachia infection levels being stably maintained in frequencies of between 5% and 25%. In a recent paper, Stouthamer et al. 1xSelfish element maintains sex in natural populations of a parasitoid wasp. Stouthamer, R. et al. Proc. R. Soc. London B Biol. Sci. 2001; 268: 617–622Crossref | Scopus (53)See all References1 exploited this system for some elegant experimental and theoretical work.Three hypotheses could explain such infection levels; inefficient transmission of Wolbachia; presence of a suppressor gene that either kills Wolbachia or balances its sex-ratio-distorting effect; or another non-Mendelian balancing factor acting in the opposite direction from Wolbachia (i.e. favouring a male-biased sex ratio). Plotting Wolbachia transmission frequencies versus a range of infection costs is enough to show that, for the range of infection frequencies observed in field populations of T. kaykai, the two possible outcomes are either the fixation or loss of the Wolbachia infection (rather than a maintenance of a stable equilibrium of about 5–25%). Adding to this, the recent demonstration (by the same research group) that Wolbachia might be horizontally transmitted at an appreciable frequency (37%) and inefficient transmission of Wolbachia as the sole explanation for the low levels of infection becomes unlikely.Alternatively, low infection levels of Wolbachia could be explained by the presence of suppressor genes that could either kill the bacteria or eliminate their effect on the host. Modelling carried out by Stouthamer et al., which takes into account the peculiarities of the T. kaykai system, shows that the presence of a suppressor gene would be a plausible theoretical outcome; however, detailed crossing protocols failed to demonstrate its presence.A third scenario invokes the presence of a sex-ratio factor acting in the opposite direction from Wolbachia, favouring male production. Such elements are collectively called paternal sex ratio (PSR) factors and have been identified in previous studies on parasitic wasps. PSRs are paternally transmitted and eggs fertilized with sperm from PSR-bearing males become male instead of female (exactly the opposite effect of that of Wolbachia). By crossing experiments and chromosome karyotypes, Stouthamer et al. present compelling evidence for the major effect of a PSR factor keeping Wolbachia infection levels low.This study is an elegant example of the occurrence of genomic conflict in a natural population. As our knowledge of the characterization and capabilities of these amazing reproductive parasites increases, ever more exciting cases of genomic parasitism will appear.