Sympatric speciation by sexual selection

There is increasing evidence for the process of sympatric speciation, in which reproductive isolation of species occurs without physical isolation. Theoretical models have focused on disruptive natural selection as the crucial pressure for splitting a species. Here we report the theoretical finding that sympatric speciation may be caused by sexual selection even without disruptive natural selection. Specifically, we show that variation in a male secondary sexual character with two conspicuous extremes and the corresponding variance in female mating preference around no preference may jointly evolve into bimodal distributions with increasing modal divergence of the male and female traits, pulling a population apart into two prezygotically isolated populations. This mode of speciation, driven by two runaway processes in different directions, is promoted by an increase in the efficiency of females in discriminating among males or a decrease in the cost of male conspicuousness, indicating that sympatric speciation may occur more readily if barrier-free or predator-free conditions arise. Although even a slight cost of female preference would cancel the runaway process of sexual selection, it would not cancel the divergent runaway processes of sympatric speciation.     

Overcompensation and population cycles in an ungulate.

Although theoretical studies show that overcompensatory density-dependent mechanisms can potentially generate regular or chaotic fluctuations in animal numbers, the majority of realistic single-species models of invertebrate populations are not overcompensatory enough to cause sustained population cycles. The possibility that overcompensation may generate cycles or chaos in vertebrate populations has seldom been considered. Here we show that highly overcompensatng density-dependent mortality can generate recurrent population crashes consistent with those observed in a naturally limited population of Soay sheep. The observed interval of three or more years between crashes points to sharp 'focusing' of mortality over a narrow range of population density.     

Directional postcopulatory sexual selection revealed by artificial insemination

Postcopulatory sexual selection comprises both sperm competition, where the sperm from different males compete for fertilization, and cryptic female choice, where females bias sperm use in favour of particular males. Despite intense current interest in both processes as potential agents of directional sexual selection, few studies have attributed the success of attractive males to events that occur exclusively after insemination. This is because the interactions between pre- and post-insemination episodes of sexual selection can be important sources of variation in paternity. The use of artificial insemination overcomes this difficulty because it controls for variation in male fertilization success attributable to the female's perception of male quality, as well as effects due to mating order and the relative contribution of sperm from competing males. Here, we adopt this technique and show that in guppies, when equal numbers of sperm from two males compete for fertilization, relatively colourful individuals achieve greater parentage than their less ornamented counterparts. This finding indicates that precopulatory female mating preferences can be reinforced exclusively through postcopulatory processes occurring at a physiological level. Our analysis also revealed that relatively small individuals were advantaged in sperm competition, suggesting a possible trade-off between sperm competitive ability and body growth.     

Divergent sexual selection enhances reproductive isolation in sticklebacks

Sexual selection may facilitate speciation because it can cause rapid evolutionary diversification of male mating signals and female preferences. Divergence in these traits can then contribute to reproductive isolation. The sensory drive hypothesis predicts that three mechanisms underlie divergence in sexually selected traits: (1) habitat-specific transmission of male signals; (2) adaptation of female perceptual sensitivity to local ecological conditions; and (3) matching of male signals to female perceptual sensitivity. I test these mechanisms in threespine sticklebacks (Gasterosteus spp.) that live in different light environments. Here I show that female perceptual sensitivity to red light varies with the extent of redshift in the light environment, and contributes to divergent preferences. Male nuptial colour varies with environment and is tuned to female perceptual sensitivity. The extent of divergence among populations in both male signal colour and female preference for red is correlated with the extent of reproductive isolation in these recently diverged species. These results demonstrate that divergent sexual selection generated by sensory drive contributes to speciation.     

Intrasexual competition and sexual selection in cooperative mammals

In most animals, the sex that invests least in its offspring competes more intensely for access to the opposite sex and shows greater development of secondary sexual characters than the sex that invests most. However, in some mammals where females are the primary care-givers, females compete more frequently or intensely with each other than males. A possible explanation is that, in these species, the resources necessary for successful female reproduction are heavily concentrated and intrasexual competition for breeding opportunities is more intense among females than among males. Intrasexual competition between females is likely to be particularly intense in cooperative breeders where a single female monopolizes reproduction in each group. Here, we use data from a twelve-year study of wild meerkats (Suricata suricatta), where females show high levels of reproductive skew, to show that females gain greater benefits from acquiring dominant status than males and traits that increase competitive ability exert a stronger influence on their breeding success. Females that acquire dominant status also develop a suite of morphological, physiological and behavioural characteristics that help them to control other group members. Our results show that sex differences in parental investment are not the only mechanism capable of generating sex differences in reproductive competition and emphasize the extent to which competition for breeding opportunities between females can affect the evolution of sex differences and the operation of sexual selection.     

Intensity of sexual selection along the anisogamy-isogamy continuum

Research into the evolution of giant sperm has uncovered a paradox within the foundations of sexual selection theory. Postcopulatory sexual selection on males (that is, sperm competition and cryptic female choice) can lead to decreased sperm numbers by favouring the production of larger sperm. However, a decline in sperm numbers is predicted to weaken selection on males and increase selection on females. As isogamy is approached (that is, as investment per gamete by males approaches that by females), sperm become less abundant, ova become relatively less rare, and competition between males for fertilization success is predicted to weaken. Sexual selection for longer sperm, therefore, is expected to be self limiting. Here we examine this paradox in Drosophila along the anisogamy-isogamy continuum using intraspecific experimental evolution techniques and interspecific comparative techniques. Our results confirm the big-sperm paradox by showing that the sex difference in sexual selection gradients decreases as sperm size increases. However, a resolution to the paradox is provided when this finding is interpreted in concert with the 'opportunity for selection' and the 'opportunity for sexual selection'. Furthermore, we show that most of the variation in measures of selection intensity is explained by sperm length and relative investment in sperm production.     

Mating advantage of rare males in models of sexual selection

Models of sexual selection in polygynous species of animals have been derived on the assumption that some females have preferences to mate with males with particular genotypes. The mating advantage gained by the males is always frequency-dependent because the preferred males take part in the same number of preferential matings when they are rare as when they are common; individually therefore, they mate more often when they are rare. Frequency-dependent sexual selection has been demonstrated in many experiments with Drosophila: rare males take part in a higher proportion of matings than their frequency as available mates. Ehrman and Spiess explained this phenomenon by frequency-dependence either in female preference or in male courtship. This explanation, which is difficult to interpret in behavioural terms, may not be necessary, however, because constant female preferences would entail frequency-dependent selection among the males. I show here that a simple model of constant preferences for particular phenotypes or genotypes is sufficient to explain a large body of data on frequency-dependent sexual selection in Drosophila.     

Disruptive sexual selection for plumage coloration in a passerine bird

The theory of sexual selection was developed to explain the evolution of highly exaggerated sexual ornaments. Now supported by vast empirical evidence, sexual selection is generally considered to favour individuals with the most extreme trait expression. Here we describe disruptive selection on a sexual ornament, plumage coloration, in yearling male lazuli buntings (Passerina amoena). In habitats with limited good-quality nesting cover, the dullest and the brightest yearlings were more successful in obtaining high-quality territories, pairing with females and siring offspring, than yearlings with intermediate plumage. This pattern reflects the way that territorial adult males vary levels of aggression to influence the structure of their social neighbourhood. Adult males showed less aggression towards dull yearlings than intermediate and bright ones, permitting the dull yearlings to settle on good territories nearby. Fitness comparisons based on paternity analyses showed that both the adults and dull yearlings benefited genetically from this arrangement, revealing a rare example of sexually selected male-male cooperation.     

Sexual selection and the maintenance of sexual reproduction

The maintenance of sexual reproduction is a problem in evolutionary theory because, all else being equal, asexual populations have a twofold fitness advantage over their sexual counterparts and should rapidly outnumber a sexual population because every individual has the potential to reproduce. The twofold cost of sex exists because of anisogamy or gamete dimorphism-egg-producing females make a larger contribution to the zygote compared with the small contribution made by the sperm of males, but both males and females contribute 50% of the genes. Anisogamy also generates the conditions for sexual selection, a powerful evolutionary force that does not exist in asexual populations. The continued prevalence of sexual reproduction indicates that the 'all else being equal' assumption is incorrect. Here I show that sexual selection can mitigate or even eliminate the cost of sex. If sexual selection causes deleterious mutations to be more deleterious in males than females, then deleterious mutations are maintained at lower equilibrium frequency in sexual populations relative to asexual populations. The fitness of sexual females is higher than asexuals because there is no difference in the fecundity of sexual females and asexuals of the same genotype, but the equilibrium frequency of deleterious mutations is lower in sexual populations. The results are not altered by synergistic epistasis in males.     

Ecological opportunity and sexual selection together predict adaptive radiation

A fundamental challenge to our understanding of biodiversity is to explain why some groups of species undergo adaptive radiations, diversifying extensively into many and varied species, whereas others do not. Both extrinsic environmental factors (for example, resource availability, climate) and intrinsic lineage-specific traits (for example, behavioural or morphological traits, genetic architecture) influence diversification, but few studies have addressed how such factors interact. Radiations of cichlid fishes in the African Great Lakes provide some of the most dramatic cases of species diversification. However, most cichlid lineages in African lakes have not undergone adaptive radiations. Here we compile data on cichlid colonization and diversification in 46 African lakes, along with lake environmental features and information about the traits of colonizing cichlid lineages, to investigate why adaptive radiation does and does not occur. We find that extrinsic environmental factors related to ecological opportunity and intrinsic lineage-specific traits related to sexual selection both strongly influence whether cichlids radiate. Cichlids are more likely to radiate in deep lakes, in regions with more incident solar radiation and in lakes where there has been more time for diversification. Weak or negative associations between diversification and lake surface area indicate that cichlid speciation is not constrained by area, in contrast to diversification in many terrestrial taxa. Among the suite of intrinsic traits that we investigate, sexual dichromatism, a surrogate for the intensity of sexual selection, is consistently positively associated with diversification. Thus, for cichlids, it is the coincidence between ecological opportunity and sexual selection that best predicts whether adaptive radiation will occur. These findings suggest that adaptive radiation is predictable, but only when species traits and environmental factors are jointly considered.     

Oncogene-induced senescence as an initial barrier in lymphoma development

Acute induction of oncogenic Ras provokes cellular senescence involving the retinoblastoma (Rb) pathway, but the tumour suppressive potential of senescence in vivo remains elusive. Recently, Rb-mediated silencing of growth-promoting genes by heterochromatin formation associated with methylation of histone H3 lysine 9 (H3K9me) was identified as a critical feature of cellular senescence, which may depend on the histone methyltransferase Suv39h1. Here we show that Emicro-N-Ras transgenic mice harbouring targeted heterozygous lesions at the Suv39h1, or the p53 locus for comparison, succumb to invasive T-cell lymphomas that lack expression of Suv39h1 or p53, respectively. By contrast, most N-Ras-transgenic wild-type ('control') animals develop a non-lymphoid neoplasia significantly later. Proliferation of primary lymphocytes is directly stalled by a Suv39h1-dependent, H3K9me-related senescent growth arrest in response to oncogenic Ras, thereby cancelling lymphomagenesis at an initial step. Suv39h1-deficient lymphoma cells grow rapidly but, unlike p53-deficient cells, remain highly susceptible to adriamycin-induced apoptosis. In contrast, only control, but not Suv39h1-deficient or p53-deficient, lymphomas senesce after drug therapy when apoptosis is blocked. These results identify H3K9me-mediated senescence as a novel Suv39h1-dependent tumour suppressor mechanism whose inactivation permits the formation of aggressive but apoptosis-competent lymphomas in response to oncogenic Ras.     

Post-mating sexual selection increases lifetime fitness of polyandrous females in the wild

Females often mate with several males before producing offspring. Field studies of vertebrates suggest, and laboratory experiments on invertebrates confirm, that even when males provide no material benefits, polyandry can enhance offspring survival. This enhancement is widely attributed to genetic benefits that arise whenever paternity is biased towards males that sire more viable offspring. Field studies suggest that post-mating sexual selection biases fertilization towards genetically more compatible males and one controlled experiment has shown that, when females mate with close kin, polyandry reduces the relative number of inbred offspring. Another potential genetic benefit of polyandry is that it increases offspring survival because males with more competitive ejaculates sire more viable offspring. Surprisingly, however, there is no unequivocal evidence for this process. Here, by experimentally assigning mates to females, we show that polyandry greatly increases offspring survival in the Australian marsupial Antechinus stuartii. DNA profiling shows that males that gain high paternity under sperm competition sire offspring that are more viable. This beneficial effect occurs in both the laboratory and the wild. Crucially, there are no confounding non-genetic maternal effects that could arise if polyandry increases female investment in a particular reproductive event because A. stuartii is effectively semelparous. Our results therefore show that polyandry improves female lifetime fitness in nature. The threefold increase in offspring survival is not negated by a decline in maternal lifespan and is too large to be offset by an equivalent decline in the reproductive performance of surviving offspring.     

Increased gamma-globin expression in a nondeletion HPFH mediated by an erythroid-specific DNA-binding factor

In man, a shift from gamma- to beta-globin gene expression in erythroblasts underlies a switch from fetal to adult haemoglobin during development. In hereditary persistence of fetal haemoglobin (HPFH), inappropriately high gamma-globin expression in adult life is associated with deletions in the beta-globin cluster or with single-base changes upstream of the gamma-globin genes. To account for enhanced gamma-gene expression in HPFH of the non-deletion type, we tested the nuclear proteins of human erythroleukaemia cells that bind gamma-promoter sequences in vitro by correlating specific mutations in their binding sites with promoter activity. An erythroid-specific factor (GF-1) binds as a single molecule to the -195 to -170 region and contacts two TATCT(AGATA) motifs, but not the conserved octamer (ATGCAAAT) that separates them. We observe that a single change (at -175, T----C) found in HPFH leads to increased promoter activity only in erythroid cells. This effect is mediated by GF-1, the human counterpart of the chicken erythroid factor Eryf 1. The form of HPFH we studied here is an inherited disorder which can be ascribed to the action of a cell-specific DNA-binding factor on a mutant promoter.     

Positive selection of CD4+ T cells mediated by MHC class II-bearing stromal cell in the thymic cortex

T lymphocytes differentiate in the thymus, where functionally immature, CD4+CD8+ (double positive) thymocytes develop into functionally mature CD4+ helper cells and CD8+ cytotoxic (single positive) T cells. The thymus is the site where self-reactive T cells are negatively selected (clonally deleted) and where T cells with the capacity to recognize foreign antigens in association with self-proteins encoded by the major histocompatibility complex (MHC) are positively selected. The net result of these developmental pathways is a T-cell repertoire that is both self-tolerant and self-restricted. One unresolved issue is the identity of the thymic stromal cells that mediate the negative and positive selection of the T-cell repertoire. Previous work has pointed to a bone-marrow-derived macrophage or dendritic cell as the inducer of tolerance, whereas a radiation-resistant, deoxyguanosine-resistant thymic cell seems to mediate the positive selection of self-MHC restricted T cells. Thymic stromal cells in the cortex interact with the T-cell antigen receptor on thymocytes. Using several strains of transgenic mice that express the class II MHC molecule I-E in specific regions of the thymus, we show directly that the positive selection of T cells is mediated by an I-E-bearing cell in the thymic cortex.     

Female sticklebacks count alleles in a strategy of sexual selection explaining MHC polymorphism.

The origin and maintenance of polymorphism in major histocompatibility complex (MHC) genes in natural populations is still unresolved. Sexual selection, frequency-dependent selection by parasites and pathogens, and heterozygote advantage have been suggested to explain the maintenance of high allele diversity at MHC genes. Here we argue that there are two (non-exclusive) strategies for MHC-related sexual selection, representing solutions to two different problems: inbreeding avoidance and parasite resistance. In species prone to inadvertent inbreeding, partners should prefer dissimilar MHC genotypes to similar ones. But if the goal is to maximize the resistance of offspring towards potential infections, the choosing sex should prefer mates with a higher diversity of MHC alleles. This latter strategy should apply when there are several MHC loci, as is the case in most vertebrates. We tested the relative importance of an 'allele counting' strategy compared to a disassortative mating strategy using wild-caught three-spined sticklebacks (Gasterosteus aculeatus) from an interconnected system of lakes. Here we show that gravid female fish preferred the odour of males with a large number of MHC class-IIB alleles to that of males with fewer alleles. Females did not prefer male genotypes dissimilar to their own.     

留守儿童自尊与性虐待早期预防:孤独感的中介作用

从安徽省3市以年级分层抽样的方式选取502名留守儿童,发放罗森伯格自尊量表、预防儿童性虐待知识问卷和儿童孤独感量表进行调查。使用SPSS 22.0统计软件对数据进行相关和中介效应检验。结果:留守儿童自尊与孤独感之间均存在显著的负相关(r值分别为-0.35,P值<0.001);留守儿童孤独感与性虐待早期预防之间存在显著的负相关(r值为-0.37,P值<0.001);留守儿童自尊与性虐待早期预防之间存在显著的负相关(r值为-0.29,P值<0.01),控制孤独感检验发现自尊对性虐待早期预防的直接效应不显著(LLCI=-0.039 8,ULCI=0.134 3);孤独感的中介效应显著(LLCI=-0.188 3,ULCI=-0.110 2)。结论:留守儿童孤独感在自尊与性虐待早期预防关系中起完全中介作用。