Sex determination in hydra: control by a subpopulation of interstitial cells in Hydra oligactis males

The stability of sexual phenotype was examined in a single clone of Hydra oligactis males maintained at two culture temperatures, 18 and 22 degrees C. At these temperatures animals of this species do not reproduce sexually, but reproduce asexually by budding, and males and females are morphologically indistinguishable. When the temperature is lowered to 10 degrees C gametogenesis is induced and sexual phenotype can be assayed. Males cultured for several years at 18 degrees C expressed a stable sexual phenotype when induced to undergo gametogenesis; males remained male. Those cultured at 22 degrees C for 1 year, however, expressed a low frequency of sex reversal from male to female; males ceased sperm differentiation and began producing eggs. Male sex reversal in cultures maintained at the higher temperature was correlated with the loss of a specific subpopulation of interstitial cells, those that bind the monoclonal antibody, AC2, which labels cells specific to the spermatogenic pathway in H. oligactis males. When interstitial cells restricted to this pathway were reintroduced into sex-reversed males (phenotypic females), the male phenotype was reestablished and animals reverted to sperm production. To further investigate the role of AC2+ cells in the masculinization of females, normal males (containing AC2+ cells) and sex-reversed males (lacking AC2+ cells) were grafted to females. In grafts between normal males and females, egg production ceased and sperm differentiation ensued, whereas those between sex-reversed males and females continued to produce eggs. Thus, the presence of AC2+ interstitial cells is strictly correlated with male sexual phenotypes and it is only in their absence that the female phenotype is expressed.     

Female remating in butterflies: interaction between female genotype and nonfertile sperm

Female mating rate is fundamental to evolutionary biology as it determines the pattern of sexual selection and sexual conflict. Despite its importance, the genetic basis for female remating rate is largely unknown and has only been demonstrated in one species. In paternally investing species there is often a conflict between the sexes over female mating rate, as females remate to obtain male nutrient donations and males try to prevent female remating to ensure high fertilization success. Butterflies produce two types of sperm: fertilizing, eupyrene sperm, and large numbers of nonfertile, apyrene sperm. The function of apyrene sperm in the polyandrous, paternally investing green‐veined white butterfly, Pieris napi, is to fill the female’s sperm storage organ thereby reducing her receptivity. However, there is large variation in number of apyrene sperm stored. Here, I examine the genetic basis to this variation, and if variation in number of apyrene sperm stored is related to females’ remating rate. The number of apyrene sperm stored at the time of remating has a genetic component and is correlated with female remating tendency, whereas no such relationship is found for fertilizing sperm. The duration of the nonreceptivity period in P. napi also has a genetic component and is inversely related to the degree of polyandry. Sexual conflict over female remating rate appears to be present in this species, with males using their apyrene sperm to exploit a female system designed to monitor sperm in storage. Ejaculates with a high proportion of nonfertile sperm may have evolved to induce females to store more of these sperm, thereby reducing remating. As a counter‐adaptation, females have evolved a better detection system to regain control over their remating rate. Sexually antagonistic co‐evolution of apyrene sperm number and female sperm storage may be responsible for ejaculates with predominantly nonfertile sperm in this butterfly.     

Determination of the sensitive stages for gonadal sex-reversal in Xenopus laevis tadpoles

The response of developing gonads of the clawed toad Xenopus laevis tadpoles to estradiol benzoate (EB) was studied between stages 44 and 67 using high resolution techniques. In presumptive genetic males the following results were obtained: 1) 100% sex reversal was induced when EB was administered before translocation of primordial germ cells (PGCs) from the gonadal epithelium into the medullary region (stages 44-50). 2) Ambiguous gonads were formed when EB treatment was initiated at stages 51-54, when PGCs were migrating into the medullary region. 3) Finally, normal testes differentiated when EB treatment began after the primordial germ cells had completed their translocation into the medulla (stages 55-56). These results suggest that EB might induce sex-reversal in genetic males by disruption of early somatic-germ cell interactions in the medullary region of the gonad. Consequently, later morphogenetic events might be deranged, preventing differentiation of testis. We propose a hypothesis in which precocious production of estradiol (E2) by genotypic females is the mechanism for primary sex differentiation.     

Quantity matters: male sex pheromone signals mate quality in the parasitic wasp Nasonia vitripennis

Sexual selection theory asserts that females are well adapted to sense signals indicating the quality of potential mates. One crucial male quality parameter is functional fertility (i.e. the success of ejaculates in fertilizing eggs). The phenotype-linked fertility hypothesis (PLFH) predicts that functional fertility of males is reflected by phenotypic traits that influence female mate choice. Here, we show for Nasonia vitripennis, a parasitic wasp with haplodiploid sex determination and female-biased sex ratios, that females use olfactory cues to discriminate against sperm-limited males. We found sperm limitation in newly emerged and multiply mated males (seven or more previous matings) as indicated by a higher proportion of sons in the offspring fathered by these males. Sperm limitation correlated with clearly reduced pheromone titres. In behavioural bioassays, females oriented towards higher doses of the synthetic pheromone and were attracted more often to scent marks of males with a full sperm load than to those of sperm-limited males. Our data support the PLFH and suggest that N. vitripennis females are able to decrease the risk of getting constrained to produce suboptimal offspring sex ratios by orienting towards gradients of the male sex pheromone.     

Sex ratio distorter reduces sperm competitive ability in an insect

Selfish genetic elements (SGEs) are ubiquitous in animals and often associated with low male fertility due to reduced sperm number in male carriers. In the fruit fly Drosophila pseudoobscura , the meiotic driving X chromosome "sex ratio" kills Y-bearing sperm in carrier males (SR males), resulting in female only broods. We competed SR males against the ejaculates of noncarrying standard males (ST males), and quantified the number of sperm transferred by SR and ST males to females. We show that SR males are very poor sperm competitors, which is partly related to transfer of fewer sperm during mating. However, sperm numbers alone cannot explain the observed paternity reduction, indicating SR males' sperm may be of reduced quality, possibly due to damage during the killing of the noncarrying Y-sperm. The reduction in sperm competitive ability due to SR is large enough to potentially stabilize the spread of sex ratio drive through populations. The poor sperm competitive ability of SR males coupled with their low fitness as mates could favor increased remating by females to reduce paternity by SR males. Given the generally poor performance of SGE-carrying males in sperm competition, this may generate strong selective pressure favoring polyandry in many species.     

Social polyandry, parental investment, sexual selection, and evolution of reduced female gamete size

Abstract Sexual selection in the form of sperm competition is a major explanation for small size of male gametes. Can sexual selection in polyandrous species with reversed sex roles also lead to reduced female gamete size? Comparative studies show that egg size in birds tends to decrease as a lineage evolves social polyandry. Here, a quantitative genetic model predicts that female scrambles over mates lead to evolution of reduced female gamete size. Increased female mating success drives the evolution of smaller eggs, which take less time to produce, until balanced by lowered offspring survival. Mean egg size is usually reduced and polyandry increased by increasing sex ratio (male bias) and maximum possible number of mates. Polyandry also increases with the asynchrony (variance) in female breeding start. Opportunity for sexual selection increases with the maximum number of mates but decreases with increasing sex ratio. It is well known that parental investment can affect sexual selection. The model suggests that the influence is mutual: owing to a coevolutionary feedback loop, sexual selection in females also shapes initial parental investment by reducing egg size. Feedback between sexual selection and parental investment may be common.     

Mechanisms of rapid sympatric speciation by sex reversal and sexual selection in cichlid fish

Mechanisms of speciation in cichlid fish were investigated by analyzing population genetic models of sexual selection on sex-determining genes associated with color polymorphisms. The models are based on a combination of laboratory experiments and field observations on the ecology, male and female mating behavior, and inheritance of sex-determination and color polymorphisms. The models explain why sex-reversal genes that change males into females tend to be X-linked and associated with novel colors, using the hypothesis of restricted recombination on the sex chromosomes, as suggested by previous theory on the evolution of recombination. The models reveal multiple pathways for rapid sympatric speciation through the origin of novel color morphs with strong assortative mating that incorporate both sex-reversal and suppressor genes. Despite the lack of geographic isolation or ecological differentiation, the new species coexists with the ancestral species either temporarily or indefinitely. These results may help to explain different patterns and rates of speciation among groups of cichlids, in particular the explosive diversification of rock-dwelling haplochromine cichlids.     

An aromatase inhibitor or high water temperature induce oocyte apoptosis and depletion of P450 aromatase activity in the gonads of genetic female zebrafish during sex-reversal

Dietary administration of a cytochrome P450 aromatase (P450arom) inhibitor (fadrozole) in genetic female juveniles of zebrafish (Danio rerio) was performed at 15-40 days post-hatching. The percentage of gonadal masculinization in the genetic all-females at 40 days post-hatching, treated with 0, 10, 100 and 1000 microg fadrozole g(-1) diet(-1) were 0, 62.5, 100 and 100%, respectively. Rearing at high water temperature in genetic all-females was performed at 15-25 days post-hatching. The percentage of gonadal masculinization in the genetic all-females at 40 days post-hatching, at water temperatures of 28.5, 35 and 37 degrees C were 0, 68.8 and 100%, respectively. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive oocytes of early diplotene and perinucleolar stages in fadrozole-treated genetic females (1000 microg g(-1) diet(-1)) were observed at 15-40 days post-hatching during sex-reversal. In contrast, apoptotic oocytes of early diplotene stage in high temperature-treated genetic females (at 35 and 37 degrees C) during sex-reversal and presumptive males of wild-type fish during sex differentiation were found at 15-27 days post-hatching. Our findings indicate that oocyte apoptosis, depletion of P450arom activity and differentiation of spermatogonia during gonadal sex-reversal are caused by treatments of aromatase inhibitor or high water temperature.     

Aspects of the reproductive biology of Gonostoma bathyphilum (Gonostomatidae)

Protandrous hermaphroditism among deep-sea fishes is known only within one family, Gonostomatidae, and therein restricted to a few species of two genera, Cyclothone (C. atraria, C. microdon) and Gonostoma (G. elongatum, G. gracile). Gonostoma bathyphilum, the deepest dwelling (700–2700 m depth) of Gonostoma spp., is also protandrous and shares with Cyclothone spp. an expression of sexual dimorphism manifested by the excessive development of olfactory structures, including the olfactory lobes, in ripe males. Sex-reversal in ripe males, therefore, involves a regression of these structures. The relationships apparent between sex, maturity and olfactory development, and those between animal size, maturity and depth of occurrence, indicate that whilst protandry is an integral component of the species' reproductive strategy some individuals develop directly as females and that among males (max. size 150 mm S.L.) sex-reversal is not obligatory. Primary females, together with those derived from prematurely reversing males, may form an early maturing female group, spawning for the first time at about 110–120 mm S.L. Most hermaphrodites were < 100 mm S.L., and within the 70–99 mm S.L. size-range a significant proportion had mature male characters. Since males of this size were all found to be immature, this suggests that these hermaphrodites are derived from a precocious element within the initial male population, and, by virtue of size-stratification by depth, are responsible for mating with the early-maturing females. The lack of a marked seasonality in the sexual cycle implies an ever presence of all sexual categories within the population. It is conjectured that sex-reversal of ripe males is a response to critical population density levels of ripe females and that the smaller, precocious males would be more prone to sex-reversal than the larger, later-maturing ones. In some respects protandry in G. bathyphilum is similar to that in G. elongatum. In the latter, however, the reproductive cycle is seasonally linked and sex-reversal in males is probably obligatory. Even so, critical density levels of ripe females can still be invoked as being causal to sex-reversal.     

How to go extinct by mating too much: population consequences of male mate choice and efficiency in a sexual–asexual species complex

Selection acting on individuals is not predicted to maximize population persistence, yet examples that explicitly quantify conflicts between individual and population level benefits are scarce. One such conflict occurs over sexual reproduction because of the cost of sex: sexual populations that suffer the cost of producing males have only half the growth rate compared to asexuals. Male behaviour can additionally impact population dynamics in a variety of ways, and here we study an example where the impact is unusually clear: the riddle of persistence of sperm‐dependent sexual–asexual species complexes. Here, a sexually reproducing host species coexists with an ameiotically reproducing all‐female sperm parasite. Sexual–asexual coexistence should not be stable because the proportion of asexually reproducing females will rapidly increase and the relative abundance of the sexually reproducing host species will decline. A severe shortage of males will lead to sperm limitation for sexual and asexual females and the system collapses. Male mate choice could reduce the reproductive potential of the asexual species and thus potentially prevent the collapse. In the gynogenetic (sperm‐dependent parthenogenetic) Amazon molly Poecilia formosa and its host (P. latipinna or P. mexicana), males discriminate against asexual females to some extent. Using a population‐dynamical model, we examine the population dynamics of this species complex with varying strengths of male discrimination ability and efficiency with which they locate females and produce sperm. The sexual species would benefit from stronger discrimination, thus preventing being displaced by the asexual females. However, males would be required to evolve preferences that are probably too strong to be purely based upon selection acting on individuals. We conclude that male behaviour does not fully prevent but delays extinction, yet this is highly relevant because low local extinction rates strongly promote coexistence as a metapopulation.     

A species‐specific multigene family mediates differential sperm displacement in Drosophila melanogaster

Sperm competition is a postcopulatory sexual selection mechanism in species in which females mate with multiple males. Despite its evolutionary relevance in shaping male traits, the genetic mechanisms underlying sperm competition are poorly understood. A recently originated multigene family specific to Drosophila melanogaster, Sdic, is important for the outcome of sperm competition in doubly mated females, although the mechanistic nature of this phenotype remained unresolved. Here, we compared doubly mated females, second mated to either Sdic knockout or nonknockout males, and directly visualize sperm dynamics in the female reproductive tract. We found that a less effective removal of first‐to‐mate male's sperm within the female's sperm storage organs is consistent with a reduced sperm competitive ability of the Sdic knockout males. Our results highlight the role young genes can play in driving the evolution of sperm competition.     

Population structure, reproduction and sex-change in a tropical East Atlantic grouper

Spawning time and sizes at first sexual maturity and at sex-reversal are described in the tropical blue-spotted grouper Cephalopholis taeniops . All juvenile fish possess non-functional ovaries. At sexual maturity, most juvenile fish become functional females. Some of these later change sex to become males. The remaining juvenile females change immediately to males, without time as functional females. This is a new scheme for sexual differentiation of groupers.     

Variable mate-guarding time and sperm allocation by male snow crabs (Chionoecetes opilio) in response to sexual competition, and their impact on the mating success of females.

Two laboratory experiments investigated mate guarding and sperm allocation patterns of adult males with virgin females of the snow crab, Chionoecetes opilio, in relation to sex ratio. Although females outnumbered males in treatments, operational sex ratios were male-biased because females mature asynchronously and have a limited period of sexual attractiveness after their maturity molt. Males guarded females significantly longer as the sex ratio increased: the mean time per female was 2.9 d in a 2 males:20 females treatment compared to 5.6 d in a 6 males:20 females treatment. Female injury and mortality scaled positively to sex ratio. Males that guarded for the greatest number of days were significantly larger, and at experiment's end had significantly smaller vasa deferentia, suggesting greater sperm expense, than males that guarded for fewer days. In both experiments, the spermathecal load (SL)--that is, the quantity of ejaculate stored in a female's spermatheca--was independent of molt date, except in the most female-biased treatment, where it was negatively related. The SL increased as the sex ratio increased, mainly because females accumulated more ejaculates. However, similarly sized males had smaller vasa deferentia and passed smaller ejaculates, such that, at a given sex ratio, the mean SL was 55% less in one experiment than in the other. Some females extruded clutches with few or no fertilized eggs, and their median SL (3-4 mg) was one order of magnitude smaller than that of females with well-fertilized clutches (31-50 mg), indicating sperm limitation. Males economized sperm: all females irrespective of sex ratio were inseminated, but to a varying extent submaximally; each ejaculate represented less than 2.5% of male sperm reserves; and no male was fully exhausted of sperm. Sperm economy is predicted by sperm competition theory for species like snow crab in which polyandry exists, mechanisms of last-male sperm precedence are effective, and the probability that one male fertilizes a female's lifetime production of eggs is small.     

Intrasexual mounting in the beetle Diaprepes abbreviatus (L.).

The weevil Diaprepes abbreviatus shows three kinds of same-sex mountings: males mount other unpaired males, males mount males already engaged in copulation and females mount other females. Four hypotheses were evaluated in order to explain same-sex matings by males: (i) female mimicry by inferior males, (ii) dominance of larger males which affects the behaviour of small males, (iii) sperm transfer in which smaller males gain some reproductive success by 'hitchhiking' their sperm with the sperm of larger males, and (iv) poor sex recognition. Data from mate choice and sperm competition experiments rejected the female mimicry, dominance and sperm transfer hypotheses and supported the poor sex recognition hypothesis. We tested three hypotheses in order to explain female mounting behaviour: (i) females mimic male behaviour in order to reduce sexual harassment by males, (ii) females mount other females in order to appear larger and thereby attract more and larger males for mating, and (iii) female mimicry of males. The results of our mate choice experiments suggested that the female mimicry of males hypothesis best explains the observed female mounting behaviour. This result is also consistent with the poor sex recognition hypothesis which is the most likely explanation for male and female intrasexual mating behaviour in many insect species.     

Sex Reversal in Zebrafish fancl Mutants Is Caused by Tp53-Mediated Germ Cell Apoptosis

The molecular genetic mechanisms of sex determination are not known for most vertebrates, including zebrafish. We identified a mutation in the zebrafish fancl gene that causes homozygous mutants to develop as fertile males due to female-to-male sex reversal. Fancl is a member of the Fanconi Anemia/BRCA DNA repair pathway. Experiments showed that zebrafish fancl was expressed in developing germ cells in bipotential gonads at the critical time of sexual fate determination. Caspase-3 immunoassays revealed increased germ cell apoptosis in fancl mutants that compromised oocyte survival. In the absence of oocytes surviving through meiosis, somatic cells of mutant gonads did not maintain expression of the ovary gene cyp19a1a and did not down-regulate expression of the early testis gene amh; consequently, gonads masculinized and became testes. Remarkably, results showed that the introduction of a tp53 (p53) mutation into fancl mutants rescued the sex-reversal phenotype by reducing germ cell apoptosis and, thus, allowed fancl mutants to become fertile females. Our results show that Fancl function is not essential for spermatogonia and oogonia to become sperm or mature oocytes, but instead suggest that Fancl function is involved in the survival of developing oocytes through meiosis. This work reveals that Tp53-mediated germ cell apoptosis induces sex reversal after the mutation of a DNA–repair pathway gene by compromising the survival of oocytes and suggests the existence of an oocyte-derived signal that biases gonad fate towards the female developmental pathway and thereby controls zebrafish sex determination.     

Infection of Hydra by Microsporidia

A microsporidian infection in a laboratory clone of Hydra littoralis has been observed, and the parasite has been tentatively identified as a species of Plistophora . Infected hydra continue to bud and regenerate normally and show no significant physiological or morphological changes. Sexual crossing of infected and non-infected animals shows that the infection is transmitted by the ovum but not by the sperm. Continuous exposure of infected hydra to Fumidil B in solution resulted in the disappearance of all Plistophora spores after a five week period of treatment, and the clones of the treated animals have remained parasite-free for more than a year.     

Variable male potential rate of reproduction: high male mating capacity as an adaptation to parasite-induced excess of females?

Numerous animals are known to harbour intracytoplasmic symbionts that gain transmission to a new host generation via female eggs and not male sperm. Bacteria of the genus Wolbachia are a typical example. They infect a large range of arthropod species and manipulate host reproduction in several ways. In terrestrial isopods (woodlice), Wolbachia are responsible for converting males into females (feminization (F)) in some species, or for infertility in certain host crosses in other species (cytoplasmic incompatibility (CI)). Wolbachia with the F phenotype impose a strong excess of females on their host populations, while Wolbachia expressing CI do not. Here, we test the possibility that male mating capacity (MC) is correlated with Wolbachia-induced phenotype. We show that males of isopod hosts harbouring F Wolbachia possess a strong MC (i.e. are able to mate with several females in a short time), while those of species harbouring CI Wolbachia possess a weaker MC. This pattern may be explained either by the selection of high MC following the increase in female-biased sex ratios, or because the F phenotype would lead to population extinction in species where MC is not sufficiently high. This last hypotheses is nevertheless more constrained by population structure.     

Sex differences in developmental plasticity and canalization shape population divergence in mate preferences

Sexual selection of high-quality mates can conflict with species recognition if traits that govern intraspecific mate preferences also influence interspecific recognition. This conflict might be resolved by developmental plasticity and learned mate preferences, which could drive preference divergence in populations that differ in local species composition. We integrate field and laboratory experiments on two calopterygid damselfly species with population genetic data to investigate how sex differences in developmental plasticity affect population divergence in the face of gene flow. Whereas male species recognition is fixed at emergence, females instead learn to recognize heterospecifics. Females are therefore more plastic in their mate preferences than males. We suggest that this results from sex differences in the balance between sexual selection for high-quality mates and selection for species recognition. As a result of these sex differences, females develop more pronounced population divergence in their mate preferences compared with males. Local ecological community context and presence of heterospecifics in combination with sex differences in plasticity and canalization therefore shape population divergence in mate preferences. As ongoing environmental change and habitat fragmentation bring formerly allopatric species into secondary contact, developmental plasticity of mate preferences in either or both sexes might facilitate coexistence and prevent local species extinction.     

Sperm stock and mating of males in a parasitoid wasp

Hymenoptera are haplodiploid insects, consequently sex ratio depends on female's sperm management which itself arises from the reproductive capacity of neighbouring males. To study the influence of ageing on male reproductive potential, laboratory experiments were conducted on Dinarmus basalis (Hymenoptera, Pteromalidae) males, a tropical wasp in which sperm counts are known to constrain sex ratio. Two groups of virgin males were compared: 1-day and 30-days old. Parameters recorded were sperm quantity and viability in seminal vesicles, shape of testis, mating ability in both individual and competitive situations and sperm stored by females after male multiple mating. Older males had twice as much sperm as young males, but their reproductive capacities did not differ. They were able to copulate with 20 successive virgin females in a short period. Sperm stored in spermathecae decreased with female mating order. In competition, old and young males had the same access to females. The difference between old and young males was visible at the level of reproductive tract: young males have functional testis and old males have empty non-functional testis. Spermatozoa are kept viable in male seminal vesicles for long periods. In this species, the reproductive potential of males is not altered by ageing. At the population level this may represent an adaptation for maintaining continuous reserves of sperm at the disposal of females.