Egg incubation temperature differently affects female and male hatching dynamics and larval fitness in a leafhopper

Temperature effects on ectotherms are widely studied particularly in insects. However, the life-history effects of temperature experienced during a window of embryonic development, that is egg stage, have rarely been considered. We simulated fluctuating temperatures and examined how this affects the operational sex ratio (OSR) of hatching as well as nymph and adult fitness in a leafhopper, Scaphoideus titanus. Specifically, after a warm or cold incubation we compared males and females hatching dynamics with their consequences on the sex ratio in the course of time, body size, weight, and developmental rate of the two populations, all reared on the same posthatching temperature. Males and females eggs respond differently, with females more sensitive to variation in incubation temperature. The different responses of both sexes have consequences on the sex ratio dynamic of hatchings with a weaker protandry after warm incubation. Temperatures experienced by eggs have more complex consequences on posthatching development. Later nymphal instars that hatched from eggs exposed to warm temperature were larger and bigger but developmental rate of the two populations was not affected. Our study demonstrates how incubation temperature could affect operational sex ratio and posthatching development in an insect and how this may be critical for population growth.     

Reproductive limits and heterogeneity of male twospotted spider mites

This paper determines reproductive limits and variation in performance of male twospotted spider mites Tetranychus urticae Koch when virgin females are provided ad libitum over the first eight days of adult life. Theree phases of male reproduction were studied: copulations, insemination and reproduction. An average one day old male copulated 15 times, inseminated 15 females, and contributed to 336 daughters. All parameters declined with age. The average male inseminated 70 females and contributed to 1145 daughters in the first days. Variability between males was small for all parameters considered. These data suggest that the number of sperm transferred at each insemination, rather than the number of inseminations, is the limiting step to higher male reproductive output.

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Résumé La reproduction est un processus qui nécessite généralement un mâle et une femelle. Cependant la biologie des populations ne s'est traditionnellement intéressée qu'à la contribution des femelles dans ce processus. Un effet de cet accent sur les femelles est que l'activité reproductrice des mâles a été très ignorée. Cet article examine, dans un contexte démographique, l'activité reproductrice des mâles de T. urticae, acarien arrhénotoque et phytophage. Trois séquences de l'activité reproductrice des mâles ont été examinées: 1) la copulation, 2) l'insémination, 3) la reproduction.En moyenne un mâle de 1 jour a copulé 15 fois, inséminé 15 femelles et a eu 336 filles. Pendant les 8 premiers jours, le mâle moyen a inséminé 70 femelles et a eu 1145 filles. Ces résultats suggèrent que le nombre de spermatozoïdes transférés à chaque insémination, plus que le nombre d'inséminations, est le facteur limitant d'une forte contribution du mâle à la reproduction.

     

Nestling sex ratio is associated with both male and female attractiveness in rock sparrows

According to theory, in species in which male variance in reproductive success exceeds that of the females, sons are more costly to produce; females mated with high quality males or those in better condition should produce more sons. In monogamous species, however, the variance in the reproductive success of the two sexes is often similar and mate choice is often mutual, making predictions regarding sex allocation more difficult. In the rock sparrow Petronia petronia, both males and females have a sexually selected yellow patch on the breast, whose size correlates with individual body condition. We investigated whether the brood sex ratio co‐varies with the size of the yellow patch of the father and the mother in a sample of 173 broods (818 chicks) over 8 breeding seasons. While the size of the yellow patch of the mother and the father did not predict per se a deviation from the expected 1:1 sex ratio, brood sex ratios were predicted by the interaction of male and female yellow patch size. This result is surprising, as the ornament is sexually selected by both males and females as an indicator of quality in both sexes and should therefore be inherited by all offspring irrespective of their sex. It indirectly suggests that other sex‐specific traits associated with patch size (e.g. polygyny in males and fecundity in females) may explain the sex allocation bias observed in rock sparrows. Thus, female individual quality alone, as expressed through the size of the yellow patch, was not associated with the biases in sex ratios reported in this study. Our results rather suggest that sex allocation occurs in response to male attractiveness in interaction with female attractiveness. In other words, females tend to preferentially allocate towards the sex of the parent with more developed ornament within the pair.     

Mating behavior of male deer ticksIxodes dammini (Acari: Ixodidae)

To analyze the sexual behavior of male black-legged deer ticks Ixodes dammini,we collected ticks infesting 202 white-tailed deer. On average, 17.7 males and 8.8 females infested each deer. Field-collected males copulated with a mean of 2.25 females, and virgin males mated with 2.4 females. On experimental hosts, males established sexual contact with feeding females and repelled other males, and about half remained paired after their mate detached. Engorged females continue to be receptive, and males mate more readily with them than with nonfed females. We conclude that male I. damminiare endowed with a repertoire of behaviors which favor an opportunistic mating before seeking a host and a preference for mating with feeding females on the host accompanied by tenacious mate guarding.     

Female production within the gall and male production on leaves by individual alates of a social aphid

The social aphid Astegopteryx spinocephala forms a banana-bunch shaped gall, consisting of several subgalls, on Styrax benzoides in northern Thailand. The aphid’s life cycle is non-host alternating. Alates (sexuparae) containing both male and female embryos appear near the end of the dry season, when many sexuals and eggs are found in subgalls guarded by sterile soldiers. Our experiments revealed that these alates give birth to almost all (99%) females within the natal subgall before flying but most (73–86%) males on leaves of the host tree after flying, and that these first-instar males intrude into live subgalls for mating. The fact that some (14–27%) males are deposited in the natal subgall indicates the occurrence of both outbreeding and inbreeding, or some level of local mate competition (LMC), in this mating system. However, the primary (investment) sex ratio was estimated to be near 0.5. This suggests that factors other than LMC, a candidate for which is local resource competition, might also affect the sex ratio in A. spinocephala. Received 19 March 2007; revised 12 July 2007; accepted 13 August 2007.     

Limited male dispersal in a social spider with extreme inbreeding

Cooperatively breeding animals commonly avoid incestuous mating through pre-mating dispersal. However, a few group-living organisms, including the social spiders, have low pre-mating dispersal, intra-colony mating, and inbreeding. This results in limited gene flow among colonies and sub-structured populations. The social spiders also exhibit female-biased sex ratios because survival benefits to large colonies favour high group productivity, which selects against 1 : 1 sex ratios. Although propagule dispersal of mated females may occasionally bring about limited gene flow, little is known about the role of male dispersal. We assessed the extent of male movement between colonies in natural populations both experimentally and by studying colony sex ratios over the mating season. We show that males frequently move to neighbouring colonies, whereas only 4% of incipient nests were visited by dispersing males. Neighbouring colonies are genetically similar and movement within colony clusters does not contribute to gene flow. Post-mating sex ratio bias was high early in the mating season due to protandry, and also in colonies at the end of the season, suggesting that males remain in the colony when mated females have dispersed. Thus, male dispersal is unlikely to facilitate gene flow between different matrilineages. This is consistent with models of non-Fisherian group-level selection for the maintenance of female biased sex ratios, which predict the elimination of male dispersal.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society 2009, 97 , 227–234.     

Points of view: Why male cannibalism won't cause a female-biased OSR -- a comment on Smith and Wootton's paper

Reviews in Fish Biology and Fisheries -     

Constrained sex allocation in a parasitoid due to variation in male quality

The theory of constrained sex allocation posits that when a fraction of females in a haplodiploid population go unmated and thus produce only male offspring, mated females will evolve to lay a female-biased sex ratio. I examined evidence for constrained sex ratio evolution in the parasitic hymenopteran Uscana semifumipennis. Mated females in the laboratory produced more female-biased sex ratios than the sex ratio of adults hatching from field-collected eggs, consistent with constrained sex allocation theory. However, the male with whom a female mated affected her offspring sex ratio, even when sperm was successfully transferred, suggesting that constrained sex ratios can occur even in populations where all females succeed in mating. A positive relationship between sex ratio and fecundity indicates that females may become sperm-limited. Variation among males occurred even at low fecundity, however, suggesting that other factors may also be involved. Further, a quantitative genetic experiment found significant additive genetic variance in the population for the sex ratio of offspring produced by females. This has only rarely been demonstrated in a natural population of parasitoids, but is a necessary condition for sex ratio evolution. Finally, matings with larger males produced more female-biased offspring sex-ratios, suggesting positive selection on male size. Because the great majority of parasitic hymenoptera are monandrous, the finding of natural variation among males in their capacity to fertilize offspring, even after mating successfully, suggests that females may often be constrained in the sex allocation by inadequate number or quality of sperm transferred.     

Regulators of male and female sexual development are critical for the transmission of a malaria parasite

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Internest sex-ratio variation and male brood survival in the ant Pheidole pallidula

Sex allocation in social insects has become a general modelin tests of inclusive fitness theory, sex-ratio theory, andparent-offspring conflict. Several studies have shown that colonysex ratios are often bimodally distributed, with some coloniesproducing mainly females and others mainly males. Sex specializationmay result from workers assessing their relatedness to malebrood versus female brood, relative to the average worker-relatednessasymmetry in other colonies of their population. Workers thenadjust the sex ratio in their own interest This hypothesis assumesthat workers can recognize the sex of the brood in their colonyand selectively eliminate males. We compared the primary sexratio (at the egg stage) and secondary sex ratio (reproductivepupae and adults) of colonies in the ant Pheidole pallidula.There was a strong bimodal distribution of secondary sex ratios,with most colonies producing mainly reproductives of one sex.In contrast, there was no evidence of a bimodal distributionof primary sex ratios. The proportion of haploid eggs producedby queens was 0.35 in early spring and decreased to about 0.1in summer. Male eggs also were present in virtually all fieldcolonies sampled in July, although eggs laid at this time ofyear never give rise to males. All male brood is, therefore,selectively eliminated beginning in July and continue to beeliminated through the rest of the year. Finally, the populationsex-ratio investment was female-biased. Together, these resultsare consistent with the hypothesis that workers control thesecondary sex ratio by selectively eliminating male brood inabout half the colonies, perhaps those with high relatednessasymmetry.[Behav Ecol 7: 292–298 (1996)]     

Driving a hard bargain: sex ratio and male marriage success in a historical US population

Evolutionary psychologists have documented a widespread female preference for men of high status and resources, and evidence from several populations suggests that this preference has real effects on marriage success. Here, we show that in the US population of 1910, socioeconomic status (SES) had a positive effect on men's chances of marrying. We also test a further prediction from the biological markets theory, namely that where the local sex ratio produces an oversupply of men, women will be able to drive a harder bargain. As the sex ratio of the states increases, the effect of SES on marriage success becomes stronger, indicating increased competition between men and an increased ability to choose on the part of women.     

Intersex and male development in Daphnia magna

Sex induction is environmentally stimulated in Daphnia and involves a cue-dependent response for sex determination. Somatic growth was shown to be similar in males and females during juvenile instars, but divergent due to a reduction in male somatic growth, at about the time that females produce ovaries. At this time, males appeared morphologically adult with respect to secondary sex characteristics. Intersex was rare and is unlikely to be important in natural populations. Intersex could be induced in both sexes, and observed more frequently after longer exposure to high temperature, or in the second generation following a temperature change. This indicates an impact on the ability of mothers to determine the sex of the offspring. It may be possible to use intersex characteristics for manipulative investigation of sex-determination mechanisms in Daphnia. Although sex-determination is initiated before birth, intersex occurrence suggests that development of male characters requires an additional process, probably involving hormone activity, during juvenile development and maturation.     

Facultative adjustment of the offspring sex ratio and male attractiveness: a systematic review and meta‐analysis

Females can benefit from mate choice for male traits (e.g. sexual ornaments or body condition) that reliably signal the effect that mating will have on mean offspring fitness. These male‐derived benefits can be due to material and/or genetic effects. The latter include an increase in the attractiveness, hence likely mating success, of sons. Females can potentially enhance any sex‐biased benefits of mating with certain males by adjusting the offspring sex ratio depending on their mate's phenotype. One hypothesis is that females should produce mainly sons when mating with more attractive or higher quality males. Here we perform a meta‐analysis of the empirical literature that has accumulated to test this hypothesis. The mean effect size was small (r = 0.064–0.095; i.e. explaining <1% of variation in offspring sex ratios) but statistically significant in the predicted direction. It was, however, not robust to correction for an apparent publication bias towards significantly positive results. We also examined the strength of the relationship using different indices of male attractiveness/quality that have been invoked by researchers (ornaments, behavioural displays, female preference scores, body condition, male age, body size, and whether a male is a within‐pair or extra‐pair mate). Only ornamentation and body size significantly predicted the proportion of sons produced. We obtained similar results regardless of whether we ran a standard random‐effects meta‐analysis, or a multi‐level, Bayesian model that included a correction for phylogenetic non‐independence. A moderate proportion of the variance in effect sizes (51.6–56.2%) was due to variation that was not attributable to sampling error (i.e. sample size). Much of this non‐sampling error variance was not attributable to phylogenetic effects or high repeatability of effect sizes among species. It was approximately equally attributable to differences (occurring for unknown reasons) in effect sizes among and within studies (25.3, 22.9% of the total variance). There were no significant effects of year of publication or two aspects of study design (experimental/observational or field/laboratory) on reported effect sizes. We discuss various practical reasons and theoretical arguments as to why small effect sizes should be expected, and why there might be relatively high variation among studies. Currently, there are no species where replicated, experimental studies show that mothers adjust the offspring sex ratio in response to a generally preferred male phenotype. Ultimately, we need more experimental studies that test directly whether females produce more sons when mated to relatively more attractive males, and that provide the requisite evidence that their sons have higher mean fitness than their daughters.     

Environmental conditions affect sex expression in monoecious, but not in male and female plants of Urtica dioica

Urtica dioica is a sub-dioecious plant species, i.e. males and females coexist with monoecious individuals. Under standard conditions, seed sex ratio (SSR, fraction of males) was found to vary significantly among seed samples collected from female plants originating from the same population (0.05–0.76). As a first step, we investigated the extent to which SSR and sex expression of male, female, and monoecious individuals is influenced by external factors. We performed experiments to analyse: (1) whether the environment of a parental plant affects the sex ratio (SR) of its offspring, (2) whether SSR can be affected by environmental conditions before flowering, and (3) whether sex expression of male, female and monoecious plants that have already flowered can be modified by environmental conditions or by application of phyto-hormones. Within the range of our experimental design, SSR was not influenced by external factors, and gender in male and female plants was stable. However, sex expression in monoecious plants was found to be labile: flower sex ratio (FSR, fraction of male flowers) differed considerably between clones from the same individual within treatments, and increased toward 100% maleness under benign conditions. These results provide strong evidence that monoecious individuals are inconstant males, which alter FSR according to environmental circumstances. In contrast, we consider sex expression in male and female individuals to be solely genetically based. The observed variation in SSR between maternal parents cannot be explained by sex-by-environment interactions.     

The importance of cytoplasmic male killing elements in natural populations of the two spot ladybird, Adalia bipunctata (Linnaeus) (Goleoptera: Goccinellidae)

Adalia bipunctata , the two spot ladybird, is polymorphic for a cytoplasmically inherited element which produces female-biased sex ratios by effecting the death of male offspring during embryogenesis. The levels of this element were assessed in Cambridge populations. Six out of 82 females tested showed both a female-biased sex ratio and low egg hatch rates consistent with the presence of this element. Population sex ratios were assessed by collecting pupae from the Cambridge area. The population sex ratio was found to be 1.15: 1, female biased, significantly different from 1:1, and consistent with predictions based on a model incorporating the observed level of the sex ratio element in the population.     

Interspecific mate choice by late-courting male western grebes

In mixed populations of western and Clark's grebes, advertisingcalls by males and females play a critical role in mate choiceand reproductive isolation. We conducted field playback experimentsthat tested whether courting western grebe males became lesschoosy in their responses to female Clark's grebe calls as themating season progressed and mating opportunities diminished.Late-courting western grebe males were much more likely to answerand approach advertising calls of Dark's grebe females thanwere males courting earlier in the season. This change in responsivenessoccurred as the operational sex ratio index of the populationapproached 3:1 male calls per female call. These and field censusdata support the hypothesis that late-season hybridization betweenthese two closely related species may be a result not of speciesmisidentification, but of active and adaptive mate choice byindividuals with limited alternatives.     

Sex ratio and male sexual characters in a population of blue tits, Parus caeruleus

Sex allocation theory proposes that parents should bias thesex ratio of their offspring if the reproductive value of onesex is greater than that of the other. In the monogamous bluetit (Parus caeruleus), males have a greater variance in reproductivesuccess than females, and high-quality males have higher reproductivesuccess than high-quality females due to extrapair paternity.Consequently, females mating with attractive males are expectedto produce broods biased toward sons, as sons benefit more thandaughters from inheriting their father's characteristics. Songand plumage color in birds are secondary sexual characters indicatingmale quality and involved in female choice. We used these malesexual traits in blue tits to investigate adaptive sex ratiomanipulation by females. We did not find any relationship betweenmale color ornamentation and brood sex ratio, contrary to previousstudies. On the other hand, the length of the strophe bout (i.e.,the mean number of strophes per strophe bout) of fathers waspositively related with the proportion of sons in their broods.The length of the strophe bout is supposed to reflect male qualityin terms of neuromuscular performance. We further showed thatsons produced in experimentally enlarged broods had shorterstrophe bouts than sons raised in reduced broods. These resultsare consistent with the hypothesis that females adjust the sexratio of their broods in response to the phenotype of theirmate.