Sex determination and sex ratio in the dioecious shrub Salix viminalis L.

 Various ecological factors (e.g. herbivory, difference between males and females in colonising ability) have been invoked to explain female-biased sex ratios in populations of willow species. It was implicitly assumed that genetic factors would lead to a balanced sex ratio in the absence of ecological disturbances. In an experiment carried out in a homogeneous environment and in the absence of herbivores the progeny sex ratio of 13 crosses of basket willow (Salix viminalis L.) was observed to range from extreme female bias to extreme male bias. The observed sex ratio cannot be explained by the presence of sex chromosomes without assuming that additional loci are also involved in the sex determination. Alternatively, the sex ratios in this study can be explained by a sex determination mechanism governed by multiple independent loci. Received: 1 February 1996 / Accepted: 14 June 1996     

Parental control of sex ratio in Gammarus duebeni,an organism with environmental sex determination

Parental sex ratio control was investigated in Gammarus duebeni, an amphipod with an environmentally mediated sex determining system. The effect on the F2 generation sex ratio of the photoperiodic conditions experienced by a) the P generation during and after copulation, b) the F1 generation before and after sex determination, and c) the F2 generation themselves during the period of sex determination, was tested. The photoperiodic conditions the F2 generation experienced during the period of sex determination had a significant effect on their sex ratio (more males were produced under long-day than under short-day conditions), but the photoperiodic conditions experienced by the F1 generation males and females or the P generation on the F1 male's side had no effect on the F2 sex ratio. However, the photoperiodic conditions the P generation on the F1 female's side experienced significantly affected the F2 sex ratio. When these animals experienced long-day conditions the F2 generation became female biased and when they experienced short-day conditions, male biased. It is proposed that the mechanism of control operates through the F1 generation mothers whilst in an embryonic stage of development in the P generation mother's brood pouch. The photoperiodically mediated effects of the embryonic F1 generation mother and the F2 generation on sex determination are additive. A mechanism by which both F1 generation maternal and F2 generation sex ratio control could operate in the field is proposed.     

Fitness effects of X chromosome drive in the stalk-eyed fly, Cyrtodiopsis dalmanni

Sex-ratio (SR) males produce predominantly female progeny because most Y chromosome sperm are rendered nonfunctional. The resulting transmission advantage of XSR chromosomes should eventually cause population extinction unless segregation distortion is masked by suppressors or balanced by selection. By screening male stalk-eyed flies, Cyrtodiopsis dalmanni, for brood sex ratio we found unique SR alleles at three X-linked microsatellite loci and used them to determine if SR persists as a balanced polymorphism. We found that XSR/XST females produced more offspring than other genotypes and that SR males had lower sperm precedence and exhibited lower fertility when mating eight females in 24 h. Adult survival was independent of SR genotype but positively correlated with eye span. We infer that the SR polymorphism is likely maintained by a combination of weak overdominance for female fecundity and frequency dependent selection acting on male fertility. Our discovery of two SR haplotypes in the same population in a 10-year period further suggests that this SR polymorphism may be evolving rapidly.     

Exploring the evolution of environmental sex determination, especially in reptiles

Environmental sex determination has been documented in a variety of organisms for many decades and the adaptive significance of this unusual sex-determining mechanism has been clarified empirically in most cases. In contrast, temperature-dependent sex determination (TSD) in amniote vertebrates, first noted 40 years ago in a lizard, has defied a general satisfactory evolutionary explanation despite considerable research effort. After briefly reviewing relevant theory and prior empirical work, we draw attention to recent comparative analyses that illuminate the evolutionary history of TSD in amniote vertebrates and point to clear avenues for future research on this challenging topic. To that end, we then highlight the latest empirical findings in lizards and turtles, as well as promising experimental results from a model organism, that portend an exciting future of progress in finally elucidating the evolutionary cause(s) and significance of TSD.     

Adaptation and constraint in the evolution of environmental sex determination

When environments differentially influence male and female performance, environmental sex determination (ESD) might evolve. The conclusion from several previous theoretical models was that reaction norms for sex determination should have a single, sharp threshold, with only females being produced in some environments and only males in others. These reaction norms can be disadvantageous in fluctuating environments, however, because they lead to sex-ratio fluctuations. We analysed the evolution of ESD, looking for equilibrium strategies in unconstrained as well as constrained strategy spaces. We identified situations where a single-threshold reaction norm is not evolutionarily stable. In these cases, we found stable strategies in the form of complex reaction norms, showing an oscillatory pattern of sex determination with respect to variation in an environmental variable. Considering that constraints could prevent such phenotypes from being realized, we found that certain randomized reaction norms, with probabilistic sex determination for a range of environments, would achieve nearly the same fitness. We also investigated reaction norms constrained to have a single threshold and found that genetic polymorphism in the environmental threshold value could evolve, producing a similar effect as a randomized reaction norm. We argue that the appearance of genetic variation can be regarded as an alternative outcome when constraints prevent the evolution of a more complex or a randomized strategy.     

Sex ratio selection and multi-factorial sex determination in the housefly: a dynamic model

Sex determining (SD) mechanisms are highly variable between different taxonomic groups and appear to change relatively quickly during evolution. Sex ratio selection could be a dominant force causing such changes. We investigate theoretically the effect of sex ratio selection on the dynamics of a multi-factorial SD system. The system considered resembles the naturally occurring three-locus system of the housefly, which allows for male heterogamety, female heterogamety and a variety of other mechanisms. Sex ratio selection is modelled by assuming cost differences in the production of sons and daughters, a scenario leading to a strong sex ratio bias in the absence of constraints imposed by the mechanism of sex determination. We show that, despite of the presumed flexibility of the SD system considered, equilibrium sex ratios never deviate strongly from 1 : 1. Even if daughters are very costly, a male-biased sex ratio can never evolve. If sons are more costly, sex ratio can be slightly female biased but even in case of large cost differences the bias is very small (<10% from 1 : 1). Sex ratio selection can lead to a shift in the SD mechanism, but cannot be the sole cause of complete switches from one SD system to another. In fact, more than one locus remains polymorphic at equilibrium. We discuss our results in the context of evolution of the variable SD mechanism found in natural housefly populations.     

Experimental study of temperature effects on the sex ratio of broods in Terrestrial Crustacea Armadillidium vulgare Latr. Possible implications in natural populations

The woodlouse Armadillidium vulgare is characterized by female heterogamety (ZW) and male homogamety (ZZ). However, in several populations, sex determination is influenced by cytoplasmic sex factors (endosymbiotic bacteria = F). At 20 °C these maternally transmitted bacteria reverse genetic males into functional neo-females (ZZ + F) producing highly female broods. When these neo-females were reared at 30° C, the sex ratio of their broods became male-biased. The major process involved in this heat-induced sex ratio inversion was the disappearance of bacteria in embryos in the course of their development, which allowed the young to express a phenotype that conforms with their genotype (i.e. male ZZ). No heat-sensitive stage of development was observed, but at least 35 days at 30° C seem to be necessary to induce F-degradation. The presence of F at 30° C (before its degradation) also induced mortality during vitellogenesis. Daily thermoperiods including a thermophase at 30° C had effects on F similar to that of a constant temperature of 30° C. A. vulgare can live in climates having such thermoperiods (at least during one period of the year), temperature appears to be capable of limiting the presence of F-bacteria in natural populations, and then modifying the evolution of sex-determining mechanisms in such populations.     

The evolution of sex‐determining mechanisms: lessons from temperature‐sensitive mutations in sex determination genes in Caenorhabditis elegans

Sexual reproduction is one of the most taxonomically conserved traits, yet sex‐determining mechanisms (SDMs) are quite diverse. For instance, there are numerous forms of environmental sex determination (ESD), in which an organism’s sex is determined not by genotype, but by environmental factors during development. Important questions remain regarding transitions between SDMs, in part because the organisms exhibiting unique mechanisms often make difficult study organisms. One potential solution is to utilize mutant strains in model organisms better suited to answering these questions. We have characterized two such strains of the model nematode Caenorhabditis elegans. These strains harbour temperature‐sensitive mutations in key sex‐determining genes. We show that they display a sex ratio reaction norm in response to rearing temperature similar to other organisms with ESD. Next, we show that these mutations also cause deleterious pleiotropic effects on overall fitness. Finally, we show that these mutations are fundamentally different at the genetic sequence level. These strains will be a useful complement to naturally occurring taxa with ESD in future research examining the molecular basis of and the selective forces driving evolutionary transitions between sex determination mechanisms.     

Inbreeding effects on progeny sex ratio and gender variation in the gynodioecious Silene vulgaris (Caryophyllaceae)

In gynodioecious species, sex expression is generally determined through cytoplasmic male sterility genes interacting with nuclear restorers of the male function. With dominant restorers, there may be an excess of females in the progeny of self-fertilized compared with cross-fertilized hermaphrodites. Moreover, the effect of inbreeding on late stages of the life cycle remains poorly explored. Here, we used hermaphrodites of the gynodioecious Silene vulgaris originating from three populations located in different valleys in the Alps to investigate the effects of two generations of self- and cross-fertilization on sex ratio and gender variation. We detected an increase in females in the progeny of selfed compared with outcrossed hermaphrodites and inbreeding depression for female and male fertility. Male fertility correlated positively with sex ratio differences between outbred and inbred progeny, suggesting that dominant restorers are likely to influence male fertility qualitatively and quantitatively in S. vulgaris. We argue that the excess of females in the progeny of selfed compared with outcrossed hermaphrodites and inbreeding depression for gamete production may contribute to the maintenance of females in gynodioecious populations of S. vulgaris because purging of the genetic load is less likely to occur.     

Sex ratio of the cyclic parthenogen Daphnia magna in a variable environment

The relationship between environmental factors, sex ratio and mating system in Daphnia magna was examined, and the adaptiveness of environmental sex determination over ametic sex determination was explored. Monthly sexual sex ratios (males over total number of males and sexual females) ranged from 0.31 to 1.0, the three-year average equalling 0.61. However, if only the samples collected during the period of frequent sexuality from August to October are included, the sexual sex ratio becomes equal, 0.51. Sexual sex ratios varied real between samples during the same period and the standard errors appeared highest in July ad August. Typical of suck times is some uncertainty in the environment, and the environmental cues can be contradictory. Sex expression in Daphnia appears to be determined by responses to complicated interactions between different environmental factors, which adaptively alter the sex ratio. The longterm sexual sex ratio of Daphnia aproaching the equilibrium 1:1, despite environmental sex %termination, gives support for Fisier's classic theory of equal parental investment in both sexes. An equal sex ratio is advantageous also during periods of small population size because it maximizes the effective population size.     

泽蛙的性腺分化及温度对性别决定的影响

通过组织学方法观察了泽蛙(Rana limnocharis)原始生殖细胞(PGCs)的迁移、原始性腺的形成和性腺分化,并且探讨在不同的培育温度条件下性腺分化的差异。泽蛙的性腺分化有其特殊性:生殖嵴形成时,其中既有体细胞,又有原生殖细胞;无论原始性腺是分化成为精巢还是卵巢,其中都出现一个初生性腔。蝌蚪孵化后的17-34d(Gosner 26-38期)为性腺分化的敏感时期。在蝌蚪孵化后的第2d(Gosner 25期),分别用不同水温18℃±1℃、30℃±1℃、32℃±1℃、34℃±1℃培育蝌蚪,直至完成变态幼蛙(Gosner 46期)形成。自然水温23℃-25℃为对照。对照组的雌、雄性比接近1∶1(1∶1.06);18℃±1℃实验组的雌、雄比例为1.83∶1,雄性率仅35.1%(P<0.01);从30℃±1℃实验组起,雄性率提高,34℃±1℃实验组的雄性率达74.0%(P<0.01)。较高的培育温度可使泽蛙蝌蚪性别分化趋向雄性,而较低的培育温度则使蝌蚪雌性化。泽蛙的性别分化属于温度依赖型性决定(TSD)。当前全球性气候变暖对两栖类性比的稳定存在着威胁。     

Mate choice and competition in the barklouseLepinotus patruelis (Psocoptera: Trogiidae): The effect of diet quality and sex ratio

Courtship varies among individuals, partly because individuals differ in quality. To explore proximate factors affecting courtship behavior, I investigated the effect of diet quality on mate choice and competition in the barklouseLepinotus patruelis Pearman (Psocoptera: Trogiidae) in the laboratory. The effect of sex ratio on mate choice was also addressed. Some males were found to exhibit active mate choice, and rejected females in both male- and female-biased sex ratio groups, although they were more likely to do so in a female-biased sex ratio group. Diet quality affected male mate choice: males on high-quality diets were significantly more likely to reject females than males on low-quality diets. Males exhibited choice significantly more often than females, who showed no overt signs of choosiness. Both males and females competed for, access to mates: both sexes attempted to interfere with mounted pairs and females grappled. The choosiness of the male may have directly affected the incidence of female competition. The results also suggest that the patterns of mate choice inL. patruelis differ from those expected by conventional sex role theory.     

Fitness effects of sex ratio response to host quality and size in the parasitoid wasp Spalangia cameroni

The parasitoid wasp Spalangia cameroni oviposited a greaterproportion of daughters in stable fly pupae than in house flypupae, even when I controlled for stable flies being smallerthan house flies. Sex ratio manipulation in response to hostquality has been modeled as being adaptive through an effectof host quality on the size and hence offspring production ofdaughters. 5. cameronis response to host species may insteadbe adaptive through an effect on larval survivorship, the developmenttime of daughters, and the size of sons. There was greater survivalof daughters than sons on stable flies. Controlling for hostsize, I found that development time of daughters was about 2%less on stable flies than on house flies. The decrease in developmenttime corresponds to a 2% increase in fitness as estimated byr, the intrinsic rate of increase, and is equivalent to abouta 9% increase in offspring production. Sons were about 2% largerfrom house flies than stable flies, which may increase offspringproduction by up to 3%. Host species had no consistent effecton size of daughters or development time of sons. In additionto the response to host species, mothers oviposited a greaterproportion of daughters in larger stable fly hosts. Whetherthis behavior is adaptive is unclear. Although offspring werelarger when they developed on larger stable flies, the rateof increase was less for daughters dian for sons. Effects ofstable fly size on offspring development time were negligible.     

The behavioral ecology of threshold evolution in a polyphenic beetle

Facultative expression of alternative male morphologies is thoughtto allow individual males to select the phenotype with the highestfitness gain given their competitive status relative to othermales with which they compete for females. Choice of, or switchingbetween, morphs commonly relies on developmental threshold responses.Evolutionary changes in developmental threshold responses arethought to provide an important avenue for phenotypic diversificationand the evolution of morphological and behavioral novelties.However, the extent to which alternative male phenotypes andtheir underlying threshold responses actually evolve in naturalpopulations is unclear. Likewise, the ecological factors thatshape the evolution of threshold responses in natural populationsare unexplored for most organisms, as are the consequences ofsuch modifications for patterns of morphological diversity.I examined the ecological basis of rapid threshold evolutionin exotic populations of the horn-polyphenic dung beetle Onthophagustaurus. Male O. taurus vary continuously in body size as a functionof larval feeding conditions. Only males that exceed a criticalthreshold body size develop a pair of long horns on their heads,whereas males below this threshold remain hornless. Populationsin two exotic ranges of this species, the eastern United Statesand western Australia, have diverged in the mean threshold bodysize, which has resulted in the evolution of highly divergentand novel horn length–body size allometries in these populations.Populations in a third and previously unstudied exotic rangeof O. taurus in eastern Australia exhibit threshold body sizesroughly intermediate between the eastern U.S. and western Australianpopulations. I tested three hypothesis to explain how differencesin ecological and demographic factors can drive allometric divergencesbetween populations, using data derived from comparative, standardizedsampling of a large number of populations in each exotic range.Results suggest that differences in the intensity of both intra-and interspecific competition have contributed to the evolutionof divergent thresholds in these populations. My results donot support the hypothesis that shifts in threshold body sizesto larger body sizes are a consequence of increases in the meanbody size of competing males. I discuss my results in the contextof Onthophagus mating systems and the evolutionary implicationsof threshold evolution.