Evolution of mating preference and sexual dimorphism

A quantitative genetic model of the joint evolution of female mating preferences and sexual dimorphism in homologous characters of the sexes is described for polygamous species with no male parental effort, such that mating preferences are selectively neutral and evolve only by indirect selection on genetically correlated characters. The male character and the homologous female character are each under stabilizing natural selection toward an optimum phenotype. At an evolutionary equilibrium the female character under natural selection is at its optimum, whereas there is a line of possible equilibria between female mating preferences and the male character. The line of equilibria may be stable or unstable, depending on the intensity of natural selection, the type of mating preferences, and the inheritance of the characters. Various mechanisms for maladaptive evolution of mating preferences and sexual dimorphism are discussed.     

昆虫体型及性体型二型性的地理变异

体型是昆虫基本的形态特性,它会影响到昆虫几乎所有的生理和生活史特性。同种昆虫不同地理种群在体型上常表现出明显的渐变,导致这些渐变的环境因素包括温度、湿度、光照、寄主植物、种群密度等,并且多种环境因素也会对昆虫种群内个体体型产生影响。雌雄个体的体型存在差异,称性体型二型性。性体型二型性也显示了地理差异。这些差异形成的途径已经得到详细的分析,其形成机制导致多个假说的提出,这些假说又在多种昆虫中得到验证。本文从同一种昆虫不同种群间、同一种群内、雌雄虫个体间3个水平,对种内昆虫体型变异的方式,影响昆虫种群间体型变异和种群内昆虫体型的变异的环境因素,以及昆虫性体型二型性及其地理变异的现象等方面的研究进行了综述,并对未来的相关研究提供了建议。     

Achieving high sexual size dimorphism in insects: females add instars

Abstract.  1. In arthropods, the evolution of sexual size dimorphism (SSD) may be constrained by a physiological limit on growth within each particular larval instar. A high SSD could, however, be attained if the larvae of the larger sex pass through a higher number of larval instars.
2. Based on a survey of published case studies, the present review shows that sex-related difference in the number of instars is a widespread phenomenon among insects. In the great majority of species with a sexually dimorphic instar number, females develop through a higher number of instars than males.
3. Female-biased sexual dimorphism in final sizes in species with sexually dimorphic instar number was found to considerably exceed a previously estimated median value of SSD for insects in general. This suggests a causal connection between high female-biased SSD, and additional instars in females. Adding an extra instar to larval development allows an insect to increase its adult size at the expense of prolonged larval development.
4. As in the case of additional instars, SSD is fully formed late in ontogeny, larval growth schedules and imaginal sizes can be optimised independently. No conflict between selective pressures operating in juvenile and adult stages is therefore expected.
5. In most species considered, the number of instars also varied within the sexes. Phenotypic plasticity in instar number may thus be a precondition for a sexual difference in instar number to evolve.     

Evolution of sexual dimorphism in phenotypic covariance structure in Phymata

Sexual dimorphism is a consequence of both sex‐specific selection and potential constraints imposed by a shared genetic architecture underlying sexually homologous traits. However, genetic architecture is expected to evolve to mitigate these constraints, allowing the sexes to approach their respective optimal mean phenotype. In addition, sex‐specific selection is expected to generate sexual dimorphism of trait covariance structure (e.g., the phenotypic covariance matrix, P ), but previous empirical work has not fully addressed this prediction. We compared patterns of phenotypic divergence, for three traits in seven taxa in the insect genus Phymata (Reduviidae), to ask whether sexual dimorphism in P is common and whether its magnitude relates to the extent of sexual dimorphism in trait means. We found that sexual dimorphism in both mean and covariance structure was pervasive but also that the multivariate distance between sex‐specific means was correlated with sex differences in the leading eigenvector of P , while accounting for uncertainty in phylogenetic relationships. Collectively, our findings suggest that sexual dimorphism in covariance structure may be a common but underappreciated feature of dioecious populations.     

Testing some hypotheses on Human Evolution and sexual dimorphism

Research on human evolution and sexual dimorphism motivates an interesting test problem. In studying hominid phylogeny it is of interest to test whether parallel evolution plays a role. With regard to sexual dimorphism it is of interest to know whether the directions of sexual dimorphism in the populations being compared are the same. We show that testing these two problems gives rise to the same type of hypothesis testing, viz. the problem of testing the hypothesis that the means of independent, normally distributed random vectors with unit covariance matrices are situated on a straight line through the origin. A test is proposed and applied to study the sexual dimorphism of 20 recent skull populations. In this example the hypothesis of equal directions of sexual dimorphism is rejected. The classical theory of constructing multiple discriminant functions (canonical variates) is adapted to the problem of comparing sexual dimorphisms.     

昆虫的雌雄二型现象

对发生雌雄二型现象的昆虫类群、生态因子及进化进行了概括总结;还特别介绍了长足虻科昆虫雌雄二型的相关方面;并简要讨论了雌雄二型与性选择的关系。     

Ossification sequence polymorphism and sexual dimorphism in skeletal development

Ossification sequence polymorphism and sexual dimorphism are prevalent in the postnatal skeletal development of the hand, foot, elbow, knee, shoulder and pelvis. For some ossification polymorphisms the sex-discriminatory efficiency is greater than 70%. Current evidence, including population comparisons, and children with kwashiorkor and marasmus, favors a genetical explanation for common sequence polymorphisms. However, ossification sequence polymorphism is more clearly defined in later-developing children, where the appearance of ossification centers is distributed among a larger number of radiographic class intervals. This observation may explain the apparent relationship between ossification sequence polymophism and developmental delay or retardation.     

Evolution of sexual dimorphism in the olfactory brain of Hawaiian Drosophila

In the fruitfly, Drosophila melanogaster, mate choice during courtship depends on detecting olfactory cues, sex pheromones, which are initially processed in the antennal lobe (AL), a primary olfactory centre of the brain. However, no sexual differences in the structure of the AL have been found in Drosophila. We compared the central brain anatomy of 37 species of Drosophilidae from the islands of the Hawaiian archipelago, uncovering an extreme sexual dimorphism within the AL in which two out of the 51 identifiable glomeruli were markedly enlarged in males. A phylogeny indicated that the sexual dimorphism of the homologous glomeruli arose 0.4-1.9 Myr ago independently in two species groups of Hawaiian endemic Drosophilidae. The corresponding glomeruli in D. melanogaster were also found to be sexually dimorphic. The formation of glomeruli of male size is prevented by the ectopic expression of female-type transformer (tra) cDNA in males, indicating that the glomerular sexual dimorphism is under the control of the sex-determination cascade of genes. It is suggested that a defined set of glomeruli in Drosophila can enlarge in response to sex-determination genetic signals, the mutations of which may result in species differences in sexual dimorphism of the brain.     

Evolution of sexual dimorphism of wing shape in the Drosophila melanogaster subgroup

Background  

Sexual dimorphism of body size has been the subject of numerous studies, but few have examined sexual shape dimorphism (SShD) and its evolution. Allometry, the shape change associated with size variation, has been suggested to be a main component of SShD. Yet little is known about the relative importance of the allometric and non-allometric components for the evolution of SShD.     

Proximate sources of sexual size dimorphism in insects: locating constraints on larval growth schedules

Different levels of sexual size dimorphism (SSD) have usually been explained by selective forces operating in the adult stage. Developmental mechanisms leading to SSD during the juvenile development have received less attention. In particular, it is often not clear if the individuals of the ultimately larger sex are larger already at hatching/birth, do they grow faster, or do they grow for a longer time. In the case of insects, the question about sexually dimorphic growth rates is still open because most previous studies fail to adequately consider the complexity of larval growth curve, the existence of distinct larval instars in particular. Applying an instar-specific approach, we analysed ontogenetic determination of female-biased SSD in a number of distantly related species of Lepidoptera. The species studied showed a remarkable degree of similarity: SSD appeared invariably earlier than in the final instar, and tended to accumulate during development. The higher weight of the females was shown to be primarily a consequence of longer development within several larval instars. There was some evidence of higher instantaneous growth rates of females in the penultimate instar but not in the final instar. Egg size, studied in one species, was found not to be sexually dimorphic. The high across-species similarity may be seen as an indication of constraints on the set of possible mechanisms of size divergence between the two sexes. The results are discussed from the perspective of the evolution of insect body size in general. In particular, this study confirms the idea about limited evolvability of within-instar growth increments. An evolutionary change towards larger adult size appears always to be realised via moderate changes in relative increments of several larval instars, whereas a considerable change in just one instar may not be feasible.     

Growth and the development of sexual size dimorphism in lorises and galagos

Three fundamental ontogenetic pathways lead to the development of size differences between males and females. Males and females may grow at the same rate for different durations (bimaturism), grow for the same duration at different rates, or grow at a mix of rate and duration differences. While patterns of growth and the development of adult body size are well established for many haplorhines, the extent to which rate and duration differences affect strepsirrhine growth trajectories remains unclear. Here, we present iterative piecewise regression models that describe the ontogeny of adult body mass for males and females of five lorisoid species (i.e., lorises and galagos) from the Duke Lemur Center. We test the hypotheses that, like most haplorhines, sexual size dimorphism (SSD) is a result of bimaturism, and males and females of monomorphic species grow at the same rate for a similar duration. We confirm that the galagos in this sample (Galago moholi and Otolemur garnettii) show significant SSD that is achieved through bimaturism. Unlike monomorphic lemurids, the lorises in this sample show a diversity of ontogenetic patterns. Loris tardigradus does follow a lemur-like trajectory to monomorphism but Nycticebuscoucang and Nycticebus pygmaeus achieve larger adult female body sizes through a mixture of rate and duration differences. We show that contrary to previous assumptions, there are patterns of both similarity and difference in growth trajectories of comparably sized lorises and galagos. Furthermore, when ontogenetic profiles of lorisoid and lemurid growth are compared, it is evident that lorisoids grow faster for a shorter period of time.     

Why do males emerge before females? Sexual size dimorphism drives sexual bimaturism in insects

Conspecific females and males often follow different development trajectories which leads to sex differences in age at maturity (sexual bimaturism, SBM). Whether SBM is typically selected for per se (direct selection hypothesis) or merely represents a side-effect of other sex-related adaptations (indirect selection hypothesis) is, however, still an open question. Substantial interspecific variation in the direction and degree of SBM, both in invertebrates and vertebrates, calls for multi-species studies to understand the relative importance of its evolutionary drivers. Here we use two complementary approaches to evaluate the evolutionary basis of SBM in insects. For this purpose, we assembled an extensive literature-derived data set of sex-specific development times and body sizes for a taxonomically and ecologically wide range of species. We use these data in a meta-analytic framework to evaluate support for the direct and indirect selection hypotheses. Our results confirm that protandry – males emerging as adults before females – is the prevailing form of SBM in insects. Nevertheless, protandry is not as ubiquitous as often presumed: females emerged before males (= protogyny) in about 36% of the 192 species for which we had data. Moreover, in a considerable proportion of species, the sex difference in the timing of adult emergence was negligible. In search for the evolutionary basis of SBM, we found stronger support for the hypothesis that explains SBM by indirect selection. First, across species, the direction and degree of SBM appeared to be positively associated with the direction and degree of sexual size dimorphism (SSD). This is consistent with the view that SBM is a correlative by-product of evolution towards sexually dimorphic body sizes. Second, within protandrous species, the degree of protandry typically increased with plastic increase in development time, with females prolonging their development more than males in unfavourable conditions. This pattern is in conflict with the direct selection hypothesis, which predicts the degree of protandry to be insensitive to the quality of the juvenile environment. These converging lines of evidence support the idea that, in insects, SBM is generally a by-product of SSD rather than a result of selection on the two sexes to mature at different times. It appears plausible that selective pressures on maturation time per se generally cannot compete with viability- and fecundity-mediated selection on insect body sizes. Nevertheless, exceptions certainly exist: there are undeniable cases of SBM where this trait has evolved in response to direct selection. In such cases, either the advantage of sex difference in maturation time must have been particularly large, or fitness effects of body size have been unusually weak.     

Unpredictable environments,nuptial gifts and the evolution of sexual size dimorphism in insects: an experiment

Many insects have a mating system where males transfer nutrients to females at mating, which are often referred to as ''nuptial gifts''. Among butterflies, some of the characteristic features of these species are polyandry (females mate multiple times), and relatively large male ejaculates. When males produce part of the resources used for offspring, the value of body size might then increase for males and decrease for females. The male/female size ratio is also observed to increase when the degree of polyandry and gift size increase. Butterfly species where gift-giving occurs are generally more variable in body size, suggesting that food quality/quantity fluctuates during juvenile stages. This will cause some males to have much to provide and some females to be in great need, and could be conducive to the evolution of a gift-giving mating system. In such a system, growing male and female juveniles should react differently to food shortage. Females should react by maturing at a smaller size since their own lack of reproductive resources can partly be compensated for by male contributions. Males have to pay the full cost of decreased reproduction if they mature at a small size, making it more important for males to keep on growing, even when growth is costly. An earlier experiment with the polyandrous and gift-giving butterfly, Pieris napi, supported this prediction. The pattern is expected to be absent or reversed for species with small nuptial gifts, where females do not benefit from mating repeatedly, and will thus be dependent on acquiring resources for reproduction on their own. To test this prediction, we report here on an experiment with the speckled wood butterfly, Pararge aegeria. We find that growth response correlates with mating system in the two above species, and we conclude that differences in environmental conditions between species may act as an important factor in the evolution of the mating system and sexual size dimorphism.     

Mammalian sexual dimorphism

Sexual dimorphisms (SDs) have evolved in mammals to assure greater reproductive success for individuals, usually males. Secondary sexual characteristics (SSC) developed to further this objective, tending to be more pronounced in species which are polygynous, diurnal and open-habitat dwellers. Sexual selection has underpinned many of these changes, which are not necessarily advantageous for individual survival. Domestication has affected certain characteristics, more in terms of their quantitative rather than qualitative expression. However, restrictions imposed by domestication can also affect behaviors such as isolation and post-natal bonding while artificial selection can, by focusing on certain traits, cause unforeseen effects in genetically linked traits, which, when sex-specific or sex-linked, can be reflected in SD. On a global scale, environmental changes can have important phylogenetic implications for species which rely upon environmental cues for activities as migration, hibernation and breeding, especially when SD occurs in response to such cues. Understanding the evolutionary rationale behind the development of SDs, as well as the dynamics which occur at the interface between natural and artificial selection, allows positive insights into areas as diverse as wildlife preservation and livestock management. For both, greatest "success" should be achieved when artificial selection occurs in harmony with natural selection within a supportive environment. Thus the aim of this review is to discuss current knowledge relating to the evolution, benefits and costs of mammalian sexual dimorphisms and, where possible, draw conclusions that might be beneficial for the husbandry and propagation of mammals today.     

Socio-bioenergetics and sexual dimorphism in primates

Socio-bioenergetics is presented as a practical method of estimating energy budgets of primates in a social context. Energy budgets are estimated on the basis of behavioral observations and a series of empirical formulae, which consider body weight, activity, and reproductive status. Data on a captive colony of Sykes' monkeys and baboons are incorporated as illustrations of the possible effects of group composition, body size, reproductive status, and activity patterns on energy requirements.Supported by the Wenner-Gren Foundation for Anthropological Research Incorporated and the National Science Foundation Grant GU-1598.