PARENTAL EFFECTS ON SEED DEVELOPMENT AND SEED YIELD IN RAPHANUS RAPHANISTRUM: IMPLICATIONS FOR NATURAL AND SEXUAL SELECTION

The possibility that sexual selection operates in angiosperms to effect evolutionary change in polygenic traits affecting male reproductive success requires that there is additive genetic variance for these traits. I applied a half-sib breeding design to individuals of the annual, hermaphroditic angiosperm, wild radish (Raphanus raphanistrum: Brassicaceae), to estimate paternal genetic effects on, or, when possible, the narrow-sense heritability of several quantitative traits influencing male reproductive success. In spite of significant differences among pollen donors with respect to in vitro pollen tube growth rates, I detected no significant additive genetic variance in male performance with respect to the proportion of ovules fertilized, early ovule growth, the number of seeds per fruit, or mean individual seed weight per fruit. In all cases, differences among maternal plants in these traits far exceeded differences among pollen donors. Abortion rates of pollinated flowers and fertilized ovules also differed more among individuals as maternal plants than as pollen donors, suggesting strong maternal control over these processes. Significant maternal phenotypic effects in the absence of paternal genetic or phenotypic effects on reproductive traits may be due to maternal environmental effects, to non-nuclear or non-additive maternal genetic effects, or to additive genetic variance in maternal control over offspring development, independent of offspring genotype. While I could not distinguish among these alternatives, it is clear that, in wild radish, the opportunity for natural or sexual selection to effect change in seed weight or seed number per fruit appears to be greater through differences in female performance than through differences in male performance.     

Condition‐dependent mutation rates and sexual selection

‘Good genes’ models of sexual selection show that females can gain indirect benefits for their offspring if male ornaments are condition‐dependent signals of genetic quality. Recurrent deleterious mutation is viewed as a major contributor to variance in genetic quality, and previous theoretical treatments of ‘good genes’ processes have assumed that the influx of new mutations is constant. I propose that this assumption is too simplistic, and that mutation rates vary in ways that are important for sexual selection. Recent data have shown that individuals in poor condition can have higher mutation rates, and I argue that if both male sexual ornaments and mutation rates are condition‐dependent, then females can use male ornamentation to evaluate their mate’s mutation rate. As most mutations are deleterious, females benefit from choosing well‐ornamented mates, as they are less likely to contribute germline‐derived mutations to offspring. I discuss some of the evolutionary ramifications of condition‐dependent mutation rates and sexual selection.     

Protoplast fusion in Bacillus species produces frequent,unbiased, genome-wide homologous recombination

In eukaryotes, fine-scale maps of meiotic recombination events have greatly advanced our understanding of the factors that affect genomic variation patterns and evolution of traits. However, in bacteria that lack natural systems for sexual reproduction, unbiased characterization of recombination landscapes has remained challenging due to variable rates of genetic exchange and influence of natural selection. Here, to overcome these limitations and to gain a genome-wide view on recombination, we crossed Bacillus strains with different genetic distances using protoplast fusion. The offspring displayed complex inheritance patterns with one of the parents consistently contributing the major part of the chromosome backbone and multiple unselected fragments originating from the second parent. Our results demonstrate that this bias was in part due to the action of restriction–modification systems, whereas genome features like GC content and local nucleotide identity did not affect distribution of recombination events around the chromosome. Furthermore, we found that recombination occurred uniformly across the genome without concentration into hotspots. Notably, our results show that species-level genetic distance did not affect genome-wide recombination. This study provides a new insight into the dynamics of recombination in bacteria and a platform for studying recombination patterns in diverse bacterial species.     

Anisogamy,sexual selection,and the evolution and maintenance of sex

Summary In the present paper we distinguish between two aspects of sexual reproduction. Genetic recombination is a universal features of the sexual process. It is a primitive condition found in simple, single-celled organisms, as well as in higher plants and animals. Its function is primarily to repair genetic damage and eliminate deleterious mutations. Recombination also produces new variation, however, and this can provide the basis for adaptive evolutionary change in spatially and temporally variable environments.The other feature usually associated with sexual reproduction, differentiated male and female roles, is a derived condition, largely restricted to complex, diploid, multicellular organisms. The evolution of anisogamous gametes (small, mobile male gametes containing only genetic material, and large, relatively immobile female gametes containing both genetic material and resources for the developing offspring) not only established the fundamental basis for maleness and femaleness, it also led to an asymmetry between the sexes in the allocation of resources to mating and offspring. Whereas females allocate their resources primarily to offspring, the existence of many male gametes for each female one results in sexual selection on males to allocate their resources to traits that enhance success in competition for fertilizations. A consequence of this reproductive competition, higher variance in male than female reproductive success, results in more intense selection on males.The greater response of males to both stabilizing and directional selection constitutes an evolutionary advantage of males that partially compensates for the cost of producing them. The increased fitness contributed by sexual selection on males will complement the advantages of genetic recombination for DNA repair and elimination of deleterious mutations in any outcrossing breeding system in which males contribute only genetic material to their offspring. Higher plants and animals tend to maintain sexual reproduction in part because of the enhanced fitness of offspring resulting from sexual selection at the level of individual organisms, and in part because of the superiority of sexual populations in competition with asexual clones.     

New colony formation in the "highly inbred" eusocial naked mole-rat: outbreeding is preferred

If the adaptive significance of sexual reproduction derivesfrom genetic recombination, then sexual organisms that severelyinbreed minimize the benefits of sexuality without fully escapingits costs. Local populations of the eusocial naked mole-ratare extremely genetically uniform, and colonies have the highestinbreeding coefficient known for wild mammals. Because non-breedingworkers cooperate to rear the queen's offspring, researchershave speculated that the inclusive fitness benefits of closerelatedness outweigh the benefits of genetic recombination,thus leading to the adaptive avoidance of outbreeding. However,I show that in laboratory colonies in which each individualhad an equal number of familiar siblings and unfamiliar distantkin (UDK) as potential mates, mating pairs were significantlymore likely to consist of UDK. Some form of kin recognitionmust have been used in order for this pattern to result. Aggressionwas not mitigated by relatedness, however, as both fightingand cooperation occurred as frequently between siblings as betweenUDK. By studying the early stages of colony development I wasable to investigate correlations between physical characteristicsof workers and success in attaining breeding status. I alsoobserved allonursing, hitherto unseen in this species, and suggestthat it is less likely to occur later in colony development.     

The evolution and maintenance of monoecy and dioecy in Sagittaria latifolia (Alismataceae)

Plant species rarely exhibit both monoecious and dioecious sexual systems. This limits opportunities to investigate the consequences of combined versus separate sex function on mating patterns and genetic variation and the analysis of factors responsible for the evolution and maintenance of the two sexual systems. Populations of the North American clonal aquatic Sagittaria latifolia are usually either monoecious or dioecious and often grow in close geographic proximity. We investigated mating patterns, genetic structure, and relationships between the two sexual systems using allozyme variation in populations from southern Ontario, Canada. As predicted, selfing rates in monoecious populations (n = 6, mean = 0.41) were significantly higher than in dioecious populations (n = 6, mean = 0.11). Moreover, marker-based estimates of inbreeding depression (delta) indicated strong selection against inbred offspring in both monoecious (mean delta = 0.83) and dioecious (mean delta = 0.84) populations. However, the difference in selfing rate between the sexual systems was not reflected in contrasting levels of genetic variation. Our surveys of 12 loci in 15 monoecious and 11 dioecious populations revealed no significant differences in the proportion of polymorphic loci (P), number of alleles per locus (A), or observed and expected heterozygosity (H(o) and H(e), respectively). Strong inbreeding depression favoring survival of outcrossed offspring may act to maintain similar levels of diversity between monoecious and dioecious populations. Despite geographical overlap between the two sexual systems in southern Ontario, a dendrogram of genetic relationships indicated two distinct clusters of populations largely corresponding to monoecious and dioecious populations. Reproductive isolation between monoecious and dioecious populations appears to be governed, in part, by observed differences in habitat and life history. We suggest that selfing and inbreeding depression in monoecious populations are important in the transition from monoecy to dioecy and that the maintenance of distinct sexual systems in S. latifolia is governed by interactions between ecology, life history, and mating.     

Long-term experimental evolution in Escherichia coli. V. Effects of recombination with immigrant genotypes on the rate of bacterial evolution

This study builds upon an earlier experiment that examined the dynamics of mean fitness in evolving populations of Escherichia coli in which mutations were the sole source of genetic variation. During thousands of generations in a constant environment, the rate of improvement in mean fitness of these asexual populations slowed considerably from an initially rapid pace. In this study, we sought to determine whether sexual recombination with novel genotypes would reaccelerate the rate of adaption in these populations. To that end, treatment populations were propagated for an additional 1000 generations in the same environment as their ancestors, but they were periodically allowed to mate with an immigrant pool of genetically distinct Hfr (high frequency recombination) donors. These donors could transfer genes to the resident populations by conjugation, but the donors themselves could not grow in the experimental environment. Control populations were propagated under identical conditions, but in the absence of sexual recombination with the donors. All twelve control populations retained the ancestral alleles at every locus that was scored. In contrast, the sexual recombination treatment yielded dramatic increases in genetic variation. Thus, there was a profound effect of recombination on the rate of genetic change. However, the increased genetic variation in the treatment populations had no significant effect on the rate of adaptive evolution, as measured by changes in mean fitness relative to a common competitor. We then considered three hypotheses that might reconcile these two outcomes: recombination pressure, hitchhiking of recombinant genotypes in association with beneficial mutations, and complex selection dynamics whereby certain genotypes may have a selective advantage only within a particular milieu of competitors. The estimated recombination rate was too low to explain the observed rate of genetic change, either alone or in combination with hitchhiking effects. However, we documented comple x ecological interactions among some recombinant genotypes, suggesting that our method for estimating fitness relative to a common competitor might have underestimated the rate of adaptive evolution in the treatment populations.     

Offspring viability benefits but no apparent costs of mating with high quality males

Traditional models of sexual selection posit that male courtship signals evolve as indicators of underlying male genetic quality. An alternative hypothesis is that sexual conflict over mating generates antagonistic coevolution between male courtship persistence and female resistance. In the scarabaeine dung beetle Onthophagus taurus, females are more likely to mate with males that have high courtship rates. Here, we examine the effects of exposing females to males with either high or low courtship rates on female lifetime productivity and offspring viability. Females exposed to males with high courtship rates mated more often and produced offspring with greater egg-adult viability. Female productivity and lifespan were unaffected by exposure to males with high courtship rates. The data are consistent with models of sexual selection based on indirect genetic benefits, and provide little evidence for sexual conflict in this system.     

DOES GENETIC DIVERSITY REDUCE SIBLING COMPETITION?

An enduring hypothesis for the proximal benefits of sex is that recombination increases the genetic variation among offspring and that this genetic variation increases offspring performance. A corollary of this hypothesis is that mothers that mate multiply increase genetic variation within a clutch and gain benefits due to genetic diversity alone. Many studies have demonstrated that multiple mating can increase offspring performance, but most attribute this increase to sexual selection and the role of genetic diversity has received less attention. Here, we used a breeding design to generate populations of full-siblings, half-siblings, and unrelated individuals of the solitary ascidian Ciona intestinalis. Importantly, we preclude the potentially confounding influences of maternal effects and sexual selection. We found that individuals in populations with greater genetic diversity had greater performance (metamorphic success, postmetamorphic survival, and postmetamorphic size) than individuals in populations with lower genetic diversity. Furthermore, we show that by mating with multiple males and thereby increasing genetic variation within a single clutch of offspring, females gain indirect fitness benefits in the absence of mate-choice. Our results show that when siblings are likely to interact, genetic variation among individuals can decrease competition for resources and generate substantial fitness benefits within a single generation.     

九莲塘萼花臂尾轮虫种复合体结构的快速变化及其生态学机制

自2011年7月16日起,通过每周1次的轮虫采集、实验室克隆培养、DNA提取、COI基因扩增、序列测定和分析,研究了九莲塘水体中萼花臂尾轮虫种复合体结构的快速变化,并在28 ℃和32 ℃及1.0×10⁶、3.0×10⁶和5.0×10⁶ cells·mL^-1斜生栅藻密度下研究了轮虫两姊妹种的适合度特征.结果表明: 所得4批35条COI基因部分序列共定义22个单倍型,其中共享单倍型3个.基于COI基因部分序列构建的系统发生树将22个单倍型聚合为2个支系(支系Ⅰ和支系Ⅱ);支系Ⅰ和支系Ⅱ间的序列差异百分比为14.8%～15.6%,它们应属不同的姊妹种(姊妹种Ⅰ和姊妹种Ⅱ).姊妹种Ⅱ的相对丰度较低(仅占1/35),出现时间较短(仅在第2批次出现);姊妹种Ⅰ种群内,3个共享单倍型所代表的克隆均存在重叠现象,而其他克隆存在替代现象.三因素方差分析表明,温度对轮虫的净生殖率、种群内禀增长率和后代混交率有显著影响,食物密度对轮虫的平均寿命、净生殖率和种群内禀增长率有显著影响,姊妹种对轮虫的平均寿命、种群内禀增长率和后代混交率有显著影响,温度和姊妹种的交互作用对轮虫的净生殖率和种群内禀增长率有显著影响,温度和食物密度的交互作用对轮虫的后代混交率有极显著影响,食物密度和姊妹种的交互作用对轮虫种群内禀增长率有显著影响.姊妹种Ⅰ的种群内禀增长率显著高于姊妹种Ⅱ,平均寿命和后代混交率均显著短于或低于姊妹种Ⅱ.     

Elimination of altered karyotypes by sexual reproduction preserves species identity.

Resolving the persistence of sexual reproduction despite its overwhelming costs (known as the paradox of sex) is one of the most persistent challenges of evolutionary biology. In thinking about this paradox, the focus has traditionally been on the evolutionary benefits of genetic recombination in generating offspring diversity and purging deleterious mutations. The similarity of pattern between evolution of organisms and evolution among cancer cells suggests that the asexual process generates more diverse genomes owing to less controlled reproduction systems, while sexual reproduction generates more stable genomes because the sexual process can serve as a mechanism to "filter out" aberrations at the chromosome level. Our reinterpretation of data from the literature strongly supports this hypothesis. Thus, the principal consequence of sexual reproduction is the reduction of drastic genetic diversity at the genome or chromosome level, resulting in the preservation of species identity rather than the provision of evolutionary diversity for future environmental challenges. Genetic recombination does contribute to genetic diversity, but it does so secondarily and within the framework of the chromosomally defined genome.     

Centromeric genotyping and direct analysis of nondisjunction in humans: Down syndrome

In species with chiasmate meioses, alterations in genetic recombination are an important correlate of nondisjunction. In general, these alterations fall into one of two categories: either homologous chromosomes fail to pair and/or recombine at meiosis I, or they are united by chiasmata that are suboptimally positioned. Recent studies of human nondisjunction suggest that these relationships apply to our species as well. However, methodological limitations in human genetic mapping have made it difficult to determine whether the important determinant(s) in human nondisjunction is absent recombination, altered recombination, or both. In the present report, we describe somatic cell hybrid studies of chromosome 21 nondisjunction aimed at overcoming this limitation. By using hybrids to “capture” individual chromosomes 21 of the proband and parent of origin of trisomy, it is possible to identify complementary recombinant meiotic products, and thereby to uncover crossovers that cannot be detected by conventional mapping methods. In the present report, we summarize studies of 23 cases. Our results indicate that recombination in proximal 21q is infrequent in trisomy-generating meioses and that, in a proportion of the meioses, recombination does not occur anywhere on 21q. Thus, our observations indicate that failure to recombine is responsible for a proportion of trisomy 21 cases. Received: 12 January 1997; in revised form: 16 February 1998 / Accepted: 19 February 1998     

The Impact of Recombination Hotspots on Genome Evolution of a Fungal Plant Pathogen

Recombination has an impact on genome evolution by maintaining chromosomal integrity, affecting the efficacy of selection, and increasing genetic variability in populations. Recombination rates are a key determinant of the coevolutionary dynamics between hosts and their pathogens. Historic recombination events created devastating new pathogens, but the impact of ongoing recombination in sexual pathogens is poorly understood. Many fungal pathogens of plants undergo regular sexual cycles, and sex is considered to be a major factor contributing to virulence. We generated a recombination map at kilobase-scale resolution for the haploid plant pathogenic fungus Zymoseptoria tritici. To account for intraspecific variation in recombination rates, we constructed genetic maps from two independent crosses. We localized a total of 10,287 crossover events in 441 progeny and found that recombination rates were highly heterogeneous within and among chromosomes. Recombination rates on large chromosomes were inversely correlated with chromosome length. Short accessory chromosomes often lacked evidence for crossovers between parental chromosomes. Recombination was concentrated in narrow hotspots that were preferentially located close to telomeres. Hotspots were only partially conserved between the two crosses, suggesting that hotspots are short-lived and may vary according to genomic background. Genes located in hotspot regions were enriched in genes encoding secreted proteins. Population resequencing showed that chromosomal regions with high recombination rates were strongly correlated with regions of low linkage disequilibrium. Hence, genes in pathogen recombination hotspots are likely to evolve faster in natural populations and may represent a greater threat to the host.     

Random genetic drift during cyclical ameiotic parthenogenesis

The classical models of population genetics assume sexual reproduction and do not apply to organisms in which parthenogenetic reproduction is alternated with sexual recombination. Under cyclic parthenogenesis, variation in rates or frequencies of parthenogenetic reproduction among clones produces selection that is independent of processes occurring in the sexual phases.In this paper I examine how selection during cyclic parthenogenesis influences random genetic drift and leads to a loss of variance among clones. To illustrate these effects, computer simulations are presented demonstrating the response of effective clone number and equilibrium clone diversity to selection and mutation.     

The ecology and genetics of fitness in Chlamydomonas. VIII. The dynamics of adaptation to novel environments after a single episode of sex

According to classical evolutionary theory, sexual recombination can generate the variation necessary to adapt to changing environments and thereby confer an evolutionary advantage of sexual over asexual reproduction. Using the green alga, Chlamydomonas reinhardtii, we investigated the effect of a single sexual episode on adaptation of heterotrophic growth on different carbon sources. In an initial mixture of isolates, sex was induced and the resulting offspring constituted the sexual populations, along with any unmated vegetative cells; the unmated mixture of isolates represented the asexual populations. Mean and variance in division rates (i.e., fitness) were measured four times during approximately 50 generations of vegetative growth in the dark on all possible combinations of four carbon sources. Consistent with effects of recombination of epistatic genes in linkage disequilibrium, sexual populations initially had a higher variance in fitness, but their mean fitness was lower than that of asexual populations, possibly due to recombinational load. Subsequently, fitness of sexual populations exceeded that of asexual ones, but finally they regained parity in both mean and variance of fitness. Although recombination was not more effective on more complex substrates, these results generally support the idea that sex can accelerate adaptation to novel environments.     

Occasional sex in an 'asexual' polyploid hermaphrodite

Asexual populations are usually considered evolutionary dead-ends because they lack the mechanisms to generate and maintain sufficient genetic diversity. Yet, some asexual forms are remarkably widespread and genetically diverse. This raises the question whether asexual systems are always truly clonal or whether they have cryptic forms of sexuality that enhance their viability. In the planarian flatworm Schmidtea polychroa parthenogens are functional hermaphrodites (as are their sexual conspecifics), copulate and exchange sperm. Sperm is required for initiation of embryogenesis but usually does not contribute genetically to the offspring (sperm-dependent parthenogenesis). Using karyology and genotyping of parents and offspring, we show that in a purely parthenogenetic population an estimated 12% of all offspring are the result of partial genetic exchange. Several processes of chromosome addition and loss are involved. Some of these result in an alternation between a common triploid and a rare tetraploid state. We conclude that genetic recombination does not necessarily require segregation and fusion within the same generation, as is the case in most sexual species. These occasional sexual processes help to explain the geographical dominance of parthenogens in our study species.     

Variation in genomic recombination rates among animal taxa and the case of social insects

Meiotic recombination is almost universal among sexually reproducing organisms. Because the process leads to the destruction of successful parental allele combinations and the creation of novel, untested genotypes for offspring, the evolutionary forces responsible for the origin and maintenance of this counter-intuitive process are still enigmatic. Here, we have used newly available genetic data to compare genome-wide recombination rates in a report on recombination rates among different taxa. In particular, we find that among the higher eukaryotes exceptionally high rates are found in social Hymenoptera. The high rates are compatible with current hypotheses suggesting that sociality in insects strongly selects for increased genotypic diversity in worker offspring to either meet the demands of a sophisticated caste system or to mitigate against the effects of parasitism. Our findings might stimulate more detailed research for the comparative study of recombination frequencies in taxa with different life histories or ecological settings and so help to understand the causes for the evolution and maintenance of this puzzling process.     

A corrected Haldane’s map function to calculate genetic distances from recombination data

The estimation of genetic distances from recombination data has no direct relationship due to the fact that multiple crossovers do not generate recombinant gametes that can be recognized in the progeny. The Haldane’s map function is the most widely used mathematical formulation able to relate the observed recombination frequency with the actual number of crossovers. Here I show that the model in which the Haldane’s correction is based on is not correct, and I present a modified map function that takes into consideration the actual number of recombinant gametes produced in cells in which different number of crossovers have occurred. My correction generates shorter genetic distances than the Haldane’s one.     

Meiotic recombination dramatically decreased in thelytokous queens of the little fire ant and their sexually produced workers

The little fire ant, Wasmannia auropunctata, displays a peculiar breeding system polymorphism. Classical haplo-diploid sexual reproduction between reproductive individuals occurs in some populations, whereas, in others, queens and males reproduce clonally. Workers are produced sexually and are sterile in both clonal and sexual populations. The evolutionary fate of the clonal lineages depends strongly on the underlying mechanisms allowing reproductive individuals to transmit their genomes to subsequent generations. We used several queen-offspring data sets to estimate the rate of transition from heterozygosity to homozygosity associated with recombination events at 33 microsatellite loci in thelytokous parthenogenetic queen lineages and compared these rates with theoretical expectations under various parthenogenesis mechanisms. We then used sexually produced worker families to define linkage groups for these 33 loci and to compare meiotic recombination rates in sexual and parthenogenetic queens. Our results demonstrate that queens from clonal populations reproduce by automictic parthenogenesis with central fusion. These same parthenogenetic queens produce normally segregating meiotic oocytes for workers, which display much lower rates of recombination (by a factor of 45) than workers produced by sexual queens. These low recombination rates also concern the parthenogenetic production of queen offspring, as indicated by the very low rates of transition from heterozygosity to homozygosity observed (from 0% to 2.8%). We suggest that the combination of automixis with central fusion and a major decrease in recombination rates allows clonal queens to benefit from thelytoky while avoiding the potential inbreeding depression resulting from the loss of heterozygosity during automixis. In sterile workers, the strong decrease of recombination rates may also facilitate the conservation over time of some coadapted allelic interactions within chromosomes that might confer an adaptive advantage in habitats disturbed by human activity, where clonal populations of W. auropunctata are mostly found.     

Influence of co-evolution with a parasite, <Emphasis Type="Italic">Nosema whitei,</Emphasis> and population size on recombination rates and fitness in the red flour beetle, <Emphasis Type="Italic">Tribolium castaneum</Emphasis>

The high prevalence of meiotic recombination—an important element of sexual reproduction—represents one of the greatest puzzles in biology. The influence of either selection by a co-evolving parasite alone or in combination with genetic drift on recombination rates was tested in the host-parasite system Tribolium castaneum and Nosema whitei. After eight generations, populations with smaller genetic drift had a lower recombination rate than those with high drift whereas parasites had no effect. Interestingly, changes in recombination rate at one site of the chromosome negatively correlated with changes at the adjacent site on the same chromosome indicating an occurrence of crossover interference. The occurrence of spontaneous or plastic changes in recombination rates could be excluded with a separate experiment.