Genetic-gonadal-genitals sex (3G-sex) and the misconception of brain and gender,or, why 3G-males and 3G-females have intersex brain and intersex gender

The categorization of individuals as “male” or “female” is based on chromosome complement and gonadal and genital phenotype. This combined genetic-gonadal-genitals sex, here referred to as 3G-sex, is internally consistent in ~99% of humans (i.e., one has either the “female” form at all levels, or the “male” form at all levels). About 1% of the human population is identified as “intersex” because of either having an intermediate form at one or more levels, or having the “male” form at some levels and the “female” form at other levels. These two types of “intersex” reflect the facts, respectively, that the different levels of 3G-sex are not completely dimorphic nor perfectly consistent. Using 3G-sex as a model to understand sex differences in other domains (e.g., brain, behavior) leads to the erroneous assumption that sex differences in these other domains are also highly dimorphic and highly consistent. But parallel lines of research have led to the conclusion that sex differences in the brain and in behavior, cognition, personality, and other gender characteristics are for the most part not dimorphic and not internally consistent (i.e., having one brain/gender characteristic with the “male” form is not a reliable predictor for the form of other brain/gender characteristics). Therefore although only ~1% percent of humans are 3G-“intersex”, when it comes to brain and gender, we all have an intersex gender (i.e., an array of masculine and feminine traits) and an intersex brain (a mosaic of “male” and “female” brain characteristics).     

Male mate choice,male quality,and the potential for sexual selection on female traits under polygyny

Observations of male mate choice are increasingly common, even in species with traditional sex roles. In addition, female traits that bear the hallmarks of secondary sexual characters are increasingly reported. These concurrent empirical trends have led to the repeated inference that, even under polygyny, male mate choice is a mechanism of sexual selection on female traits. It is often either assumed or argued that in these cases females are competing for males of superior “quality”; females might experience sexual selection under polygyny if they compete for mates that provide either direct or indirect benefits. However, the theoretical foundation of this testable hypothesis remains largely uninvestigated. We develop a population genetic model to probe the logic of this hypothesis and demonstrate that, contrary to common inferences, male mate choice, variation in male quality (in the form of a direct fecundity benefit to females), and female ornamentation can coexist in a population without any sexual selection on female ornamentation taking place at all. Furthermore, even in a “best case scenario” where high quality males with a preference for ornamented females are able to mate disproportionately more often with them, the evolution of female traits by sexual selection may be relatively weak. We discuss the implication of these findings for ongoing empirical and theoretical research on the evolution of sexual‐signaling in females.     

Signal trait sexual dimorphism and mutual sexual selection in Drosophila serrata

Abstract The evolution of sexual dimorphism may occur when natural and sexual selection result in different optimum trait values for males and females. Perhaps the most prominent examples of sexual dimorphism occur in sexually selected traits, for which males usually display exaggerated trait levels, while females may show reduced expression of the trait. In some species, females also exhibit secondary sexual traits that may either be a consequence of a correlated response to sexual selection on males or direct sexual selection for female secondary sexual traits. In this experiment, we simultaneously measure the intersex genetic correlations and the relative strength of sexual selection on males and females for a set of cuticular hydrocarbons in Drosophila serrata . There was significant directional sexual selection on both male and female cuticular hydrocarbons: the strength of sexual selection did not differ among the sexes but males and females preferred different cuticular hydrocarbons. In contrast with many previous studies of sexual dimorphism, intersex genetic correlations were low. The evolution of sexual dimorphism in D. serrata appears to have been achieved by sex-limited expression of traits controlled by genes on the X chromosome and is likely to be in its final stages.     

Evolution of increased competitiveness in cows trades off with reduced milk yield,fertility and more masculine morphology

In some species females compete for food, foraging territories, mating, and nesting sites. Competing females can exhibit morphological, physiological, and behavioral adaptations typical of males, which are commonly considered as secondary sexual traits. Competition and the development of traits increasing competitiveness require much energy and may exert adverse effects on fecundity and survival. From an evolutionary perspective, positive selection for increased competitiveness would then result in evolution of reduced values for traits related to fitness such as fecundity and survival. There is recent evidence for such evolutionary trade‐offs involving male competition, but no study has considered competing females so far. Using data from competitions for dominance in cows (Bos taurus), we found negative genetic correlations between traits providing success in competition, that is, fighting ability and fitness traits related to milk production and with fertility (the inverse of parity‐conception interval). Fighting ability also showed low but positive genetic correlations with “masculine” morphological traits, and negative correlations with “feminine” traits. A genetic change in traits over time has occurred due to selection on competitiveness, corresponding to an evolutionary process of “masculinization” counteracting the official selection for milk yield. Similar evolutionary trade‐off between success in competition and fitness components may be present in various species experiencing female competition.     

Functional morphology of amplexus (clasping) in spinicaudatan clam shrimps (Crustacea,Branchiopoda) and its evolution in bivalved branchiopods: A video‐based analysis

Male clam shrimps (Crustacea: Branchiopoda: Laevicaudata, Spinicaudata, and Cyclestherida) have their first one or two trunk limb pairs modified as “claspers,” which are used to hold the female during mating and mate guarding. Clasper morphology has traditionally been important for clam shrimp taxonomy and classification, but little is known about how the males actually use the claspers during amplexus (clasping). Homologies of the various clasper parts (“movable finger,” “large palp,” “palm,” “gripping area,” and “small palp”) have long been discussed between the three clam shrimp taxa, and studies have shown that only some structures are homologous while others are convergent (“partial homology”). We studied the clasper functionality in four spinicaudatan species using video recordings and scanning electron microscopy, and compared our results with other clam shrimp groups. General mating behavior and carapace morphology was also studied. Generally, spinicaudatan and laevicaudatan claspers function similarly despite some parts being nonhomologous. We mapped clasper morphology and functionality aspects on a branchiopod phylogeny. We suggest that the claspers of the three groups were adapted from an original, simpler clasper, each for a “stronger” grip on the female's carapace margin: 1) Spinicaudata have two clasper pairs bearing an elongated apical club/gripping area with one setal type; 2); Cyclestherida have one clasper pair with clusters of molariform setae on the gripping area and at the movable finger apex; and 3) Laevicaudata have one clasper pair, but have incorporated an additional limb portion into the clasper palm and bear a diverse set of setae. J. Morphol. 278:523–546, 2017. © 2017 Wiley Periodicals, Inc.     

Different effects of mating group size as male and as female on sex allocation in a simultaneous hermaphrodite

Sex allocation theory predicts that the optimal sexual resource allocation of simultaneous hermaphrodites is affected by mating group size (MGS). Although the original concept assumes that the MGS does not differ between male and female functions, the MGS in the male function (MGSm; i.e., the number of sperm recipients the focal individual can deliver its sperm to plus one) and that in the female function (MGSf; the number of sperm donors plus one) do not always coincide and may differently affect the optimal sex allocation. Moreover, reproductive costs can be split into “variable” (e.g., sperm and eggs) and “fixed” (e.g., genitalia) costs, but these have been seldom distinguished in empirical studies. We examined the effects of MGSm and MGSf on the fixed and variable reproductive investments in the sessilian barnacle Balanus rostratus. The results showed that MGSm had a positive effect on sex allocation, whereas MGSf had a nearly significant negative effect. Moreover, the “fixed” cost varied with body size and both aspects of MGS. We argue that the two aspects of MGS should be distinguished for organisms with unilateral mating.     

Selection for costly sexual traits results in a vacant mating niche and male dimorphism

The expected strong directional selection for traits that increase a male's mating ability conflicts with the frequent observation that within species, males may show extreme variation in sexual traits. These male reproductive polymorphisms are usually attributed to direct male–male competition. It is currently unclear, however, how directional selection for sexually selected traits may convert into disruptive selection, and if female preference for elaborate traits may be an alternative mechanism driving the evolution of male polymorphism. Here, we explore this mechanism using the polyandric dwarf spider Oedothorax gibbosus as a model. We first show that males characterized by conspicuous cephalic structures serving as a nuptial feeding device (“gibbosus males”) significantly outperform other males in siring offspring of previously fertilized females. However, significant costs in terms of development time of gibbosus males open a mating niche for an alternative male type lacking expensive secondary sexual traits. These “tuberosus males” obtain virtually all fertilizations early in the breeding season. Individual‐based simulations demonstrate a hitherto unknown general principle, by which males selected for high investment to attract females suffer constrained mating opportunities. This creates a vacant mating niche of unmated females for noninvesting males and, consequently, disruptive selection on male secondary sexual traits.     

Intersex,a temperature-sensitive mutant of the nematode Caenorhabditis elegans

A temperature-sensitive mutation, isx-1(hc17), is reported in the nematode Caenorhabditis elegans which alters the sexual phenotypes of both genotypic sexes. At the restrictive temperature, XX animals are functionally female rather than hermaphroditic due to the absence of spermatogenesis, and XO animals develop as intersexes. These intersexes have normal male head and tail structures and exhibit some mating behavior, but possess hermaphrodite-like gonads which produce no sperm and usually contain a few oocytes. An abortive vulva is usually present and evidence is presented which suggests that the formation of the vulva by the hypodermis is induced by the underlying gonad. The direct effects of the mutation are confined to the descendants of four primordial gonad cells. Gametogenesis and gonad sheath development do not seem to be tightly coupled and are shown to differ in their responses to X-chromosome dosage. The interaction of the intersex mutation with mutant alleles of two transformer genes tra-1 and tra-2 is discussed and a model for the action of these genes in gonad development and sex determination is proposed.     

Intersexuality in the Pig

In 3 relatively large samples intersexuality was found in 0.2 % of the pigs. Litters with intersexes had a deficit of male piglets. The total number of piglets in such litters was correspondingly reduced. Pig losses until the age of 5 weeks were reduced in intersex litters. Other malformations, especially scrotal hernia, were more common in these litters than in normal ones. Forty-five intersexes showed big variation in the development of sexual organs. All had testicles, however. Thirteen (29 %) were so-called true hermaphrodites whereas 32 were male pseudohermaphrodites. All 42 cases being karyotyped were genetical females. There are arguments for genetical predisposition for the defect. The hereditary transmission is probably complicated. In any case it is not monogenic recessive. Practical consequences of the defect are: almost complete sterility, slaughter losses due to bad smell and taste of the meat and increased risk of cassation (local or general inflammations), increased frequency of other malformations in litters holding intersexes.     

Sexual size dimorphism in a Drosophila clade, the D. obscura group

The Drosophila obscura clade consists of about 41 species, of which 20 were used for analyses of wing and thorax length. Our primary goal was to investigate the magnitude of sexual size dimorphism (SSD) of these traits within this clade and to test Rensch's Rule [when females are larger than males, SSD (e.g., female/male ratio) should decrease with body size]. Our secondary goal was methodological and involved evaluating for these flies alternative measures of SSD (female/male ratio, female/male absolute difference, female/male relative difference), developing a bootstrap method to estimate the magnitude of intraspecific variation in SSD, and applying a new method of estimating allometric relationships that is phylogenetically based and incorporates error variance in both traits. All indices of SSD were strongly correlated for both size traits. Nevertheless, female/male ratio is the best index here: it is easily interpretable and essentially independent of size. For both traits, SSD (F/M) varied interspecifically, showed a strong phylogenetic signal, but did not differ for the main phylogenetic subgroups or correlate with latitude. Factors underlying variation in SSD in this clade are elusive and might include genetic drift. SSD (wing) tended to decrease with increasing size, as predicted by Rensch's Rule, though not consistently so. SSD (thorax) was unrelated to size. However, analysis of published data for thorax length of Drosophila spp. (N=42) with a larger size range showed that SSD decreased significantly with increasing size (consistent with Rensch's Rule), suggesting our ability to detect SSD-size relations in the D. obscura data may be limited by low statistical power.     

The “Normalization” of Intersex Bodies and “Othering” of Intersex Identities in Australia

Once described as hermaphrodites and later as intersex people, individuals born with intersex variations are routinely subject to so-called “normalizing” medical interventions, often in childhood. Opposition to such practices has been met by attempts to discredit critics and reasserted clinical authority over the bodies of women and men with “disorders of sex development.” However, claims of clinical consensus have been selectively constructed and applied and lack evidence. Limited transparency and lack of access to justice have helped to perpetuate forced interventions. At the same time, associated with the diffusion of distinct concepts of sex and gender, intersex has been constructed as a third legal sex classification, accompanied by pious hopes and unwarranted expectations of consequences. The existence of intersex has also been instrumentalized for the benefit of other, intersecting, populations. The creation of gender categories associated with intersex bodies has created profound risks: a paradoxically narrowed and normative gender binary, maintenance of medical authority over the bodies of “disordered” females and males, and claims that transgressions of social roles ascribed to a third gender are deceptive. Claims that medicalization saves intersex people from “othering,” or that legal othering saves intersex people from medicalization, are contradictory and empty rhetoric. In practice, intersex bodies remain “normalized” or eliminated by medicine, while society and the law “others” intersex identities. That is, medicine constructs intersex bodies as either female or male, while law and society construct intersex identities as neither female nor male. Australian attempts at reforms to recognize the rights of intersex people have either failed to adequately comprehend the population affected or lacked implementation. An emerging human rights consensus demands an end to social prejudice, stigma, and forced medical interventions, focusing on the right to bodily integrity and principles of self-determination.     

Female Attraction to Higher Pitched Male Enticement Calls in Barn Swallows

Current sexual selection studies demonstrate that female mate preference almost always favors exaggerated male traits, such as colorful plumage and elongated tails. However, such studies focused mainly on secondary sexual characteristics, i.e., traits that are fully developed at maturation, while ignoring others, which precludes generalization of the findings. We studied the expression of the male enticement call, a nestling‐like courtship trait, in relation to female attraction and to the expression of secondary sexual ornaments in the Japanese barn swallow Hirundo rustica gutturalis. First, we showed that the peak amplitude frequency of male enticement calls was, on average, lower than that of nestling food‐begging calls, indicating that higher pitched calls are more similar to nestling food‐begging calls than are lower pitched calls. Second, we conducted an experimental playback of male enticement calls during the courtship period with the female's mate removed, and found that females were more attracted to heightened‐pitch calls than to control calls. Lastly, the peak amplitude frequency of male enticement calls was negatively related to the ornamentation, as measured by throat coloration. Taken together, these findings suggest the following: (1) females are attracted to a more nestling‐like trait, and (2) similarity to nestling food‐begging calls increases with decreasing secondary sexual ornaments. Nestling‐like male traits that are not secondary sexual characteristics should be considered when studying the general pattern of female attraction to male traits.     

The effects of pond duration on the life history traits of an ephemeral pond crustacean,Eulimnadia texana

We examined the relationship between pond duration and life history characters of the clam shrimp Eulimnadia texana, a species inhabiting ephemeral ponds in southwestern North America. Since the shrimp live in temporary habitats, we predicted that there should be high selection pressure on life history characteristics associated with rapid development (e.g., fast growth, early maturity, etc.), rather than selection for increased longevity. Pond duration was estimated using a combination of average monthly rainfall and pond size (surface to volume ratio). Shrimp that live in smaller ponds (high surface to volume ratio)in areas with low average rainfall should, on average, experience a shorter total time available for development than those in larger ponds or in areas of higher rainfall. These shrimp should have an earlier age at maturity, reduced longevity, lower fecundity, and faster growth. Five replicate populations of clam shrimp were collected as cysts from five ponds. These shrimp were raised in a common garden experiment in the laboratory. Daily measurements of growth and egg production were taken and ages at maturity and death were recorded. Shrimp from areas with higher average rainfall had slower growth, higher fecundity, greater longevity, and an earlier age at maturity than those from areas with lower average rainfall. If average rainfall is an accurate measure of pond duration, then the first three of these life history traits differ in the directions expected. However, age at maturity varied in a manner opposite to that expected, being earlier in the ponds with longer duration. Surface to volume ratio was not helpful in further resolving differences in these life history characters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mechanisms of parasite-induced sex reversal in Gammarus duebeni

The amphipod Gammarus duebeni is host to the feminising microsporidian parasite Nosema granulosis that converts males into functional females. To test the hypothesis that the parasite acts through endocrine disruption we compared the morphology of the gonad and activity of the androgenic gland, which coordinates male sexual differentiation, in infected and uninfected animals. Male gonad consisted of testis, seminal vesicle and vas deferens that was anchored to the genital papilla on segment 7. The androgenic gland was associated with the distal end of the vas deferens. In female and intersex animals the bi-lobed ovary opened into the oviduct at segment 5, vestigial vas deferens and vestigial androgenic gland were retained. The majority of parasitised individuals (38/39) were either phenotypic females or intersexes with fully developed ovaries and an undifferentiated androgenic gland. Our data suggest that the parasite prevents differentiation of the androgenic gland. In further support of this hypothesis, mass spectrometry of a single androgenic gland from males revealed a dominant molecular ion with a mass/charge ratio of 4818.4+H, corresponding to a peptide of androgenic gland hormone from Armadillidium vulgare. In contrast the vestigial androgenic gland from parasitised and unparasitised females showed only low intensity peaks. Our observations demonstrate that the parasite manipulates host sex by preventing androgenic gland differentiation, androgenic gland hormone production and consequently male differentiation. This is in agreement with observations of A. vulgare with inherited Wolbachia infection, suggesting that phylogenetically distant feminisers manipulate hosts through a common mechanism. The high frequency of infection in intersexes (89.3%) suggests that this phenotype results from incomplete feminisation by the parasite.     

Notes on the life history of the clam shrimp,Eulimnadia texana

Several life history measures (growth rate, egg production, molt frequency, age at maturity and lifespan) were measured on several clam shrimp hermaphrodites (Eulimnadia texana Packard) grown in a laboratory setting under optimal growth conditions. Growth rates were high early in life, and then dropped dramatically when egg production began (day 5–6). Early egg production was low, and increased until approximately day 7, after which production leveled off for several days. Reproductive senescence was noted after day 17, with clutch sizes continuously dropping until death. Average molts per day was approximately 1.1, and molting seemed to be more closely associated with egg production than with growth. Growth and egg production were negatively correlated, indicating a possible trade-off between these two traits. No other trade-offs were detected. These shrimp show typical early-colonist life history traits, displaying high initial growth, early reproduction at a high rate, and then early senescence and death. This revised version was published online in August 2006 with corrections to the Cover Date.     

Origins of female genital diversity: Predation risk and lock‐and‐key explain rapid divergence during an adaptive radiation

The study of male genital diversity has long overshadowed evolutionary inquiry of female genitalia, despite its nontrivial diversity. Here, we identify four nonmutually exclusive mechanisms that could lead to genital divergence in females, and potentially generate patterns of correlated male–female genital evolution: (1) ecological variation alters the context of sexual selection (“ecology hypothesis”), (2) sexually antagonistic selection (“sexual‐conflict hypothesis”), (3) female preferences for male genitalia mediated by female genital traits (“female‐choice hypothesis”), and (4) selection against inter‐population mating (“lock‐and‐key hypothesis”). We performed an empirical investigation of all four hypotheses using the model system of Bahamas mosquitofish inhabiting blue holes that vary in predation risk. We found unequivocal support for the ecology hypothesis, with females exhibiting a smaller genital opening in blue holes containing piscivorous fish. This is consistent with stronger postmating female choice/conflict when predators are present, but greater premating female choice in their absence. Our results additionally supported the lock‐and‐key hypothesis, uncovering a pattern of reproductive character displacement for genital shape. We found no support for the sexual conflict or female choice hypotheses. Our results demonstrate a strong role for ecology in generating female genital diversity, and suggest that lock‐and‐key may provide a viable cause of female genital diversification.     

GENOTYPE-BY-ENVIRONMENT INTERACTIONS IN THE DETERMINATION OF THE SIZE OF A SECONDARY SEXUAL CHARACTER IN THE COLLARED FLYCATCHER (FICEDULA ALBICOLLIS)

Although genetic variation in characters closely related to fitness is expected to either become depleted by selection or masked by environmental variation, “good gene” models of sexual selection require moderate to high heritabilities of secondary sexual characters to explain the occurrence of costly female mate preferences. In this study, I investigated whether the estimated heritability of a condition-dependent secondary sexual character (i.e., the white forehead badge) in the collared flycatcher varied depending on environmental conditions experienced during offspring growth. The data were collected over a period of 14 years making it possible to exploit natural variation in natal conditions. In addition, natal conditions were experimentally altered through brood size manipulations. During unfavorable conditions caused by generally poor weather or experimentally enlarged brood size, no significant heritability based on father-sons regressions could be demonstrated (0.19 ? h² ? 0.27). In contrast, sons reared during years with favorable weather or in experimentally reduced broods significantly resembled their fathers (0.44 ? h² ? 0.65). In addition, the heritability estimates declined with increasing maternal age. The strong effect of natal environmental condition on the estimated heritability of forehead badge size suggests that the potential genetic benefit from mate choice vary according to environmental conditions (e.g., the benefit is reduced during unfavorable rearing conditions). Because sons reared during poor conditions have probably experienced a natal environment different from that experienced by their fathers, the low heritability estimates obtained under poor conditions seem to be caused by low additive genetic variation expressed in such environments and/or a low genetic correlation between the expression of the trait in the two different environments (i.e., good vs. bad). Both of these explanations imply the presence of genotype-by-environment interactions. If such interactions frequently affect the expression of secondary sexual characters, this may offer an explanation of the high heritabilites sometimes reported for such traits, despite their exposure to long-term directional selection.     

The role of juvenile hormone in immune function and pheromone production trade-offs: a test of the immunocompetence handicap principle

The immunocompetence handicap hypothesis postulates that secondary sexual traits are honest signals of mate quality because the hormones (e.g. testosterone) needed to develop secondary sexual traits have immunosuppressive effects. The best support for predictions arising from the immunocompetence handicap hypothesis so far comes from studies of insects, although they lack male-specific hormones such as testosterone. In our previous studies, we found that female mealworm beetles prefer pheromones of immunocompetent males. Here, we tested how juvenile hormone (JH) affects male investment in secondary sexual characteristics and immune functions in the mealworm beetle, Tenebrio molitor. We injected male mealworm beetles with JH (type III) and found that injection increased the attractiveness of male pheromones but simultaneously suppressed immune functions (phenoloxidase activity and encapsulation). Our results suggest that JH, which is involved in the control of reproduction and morphogenesis, also plays a central role in the regulation of a trade-off between the immune system and sexual advertisement in insects. Thus, the results reflect a general mechanism by which the immunocompetence handicap hypothesis may work in insects.     

Broad-sense sexual selection, sex gene pool evolution, and speciation.

Studies of sexual selection have traditionally focused on explaining the extreme sexual dimorphism in male secondary sexual traits and elaborate mating behaviors displayed by males during courtship. In recent years, two aspects of sexual selection have received considerable attention in the literature: an extension of the sexual selection concept to other traits (i.e., postcopulatory behaviors, external and internal genital morphology, gametes, molecules), and alternative mechanistic explanations of the sexual selection process (i.e., coevolutionary runaway, good-genes, sexual conflicts). This article focuses on the need for an extension of sexual selection as a mechanism of change for courtship and (or) mating male characters (i.e., narrow-sense sexual selection) to all components of sexuality not necessarily related to courtship or mating (i.e., broad-sense sexual selection). We bring together evidence from a wide variety of organisms to show that sex-related genes evolve at a fast rate, and discuss the potential role of broad-sense sexual selection as an alternative to models that limit speciation to strict demographic conditions or treat it simply as an epiphenomenon of adaptive evolution.