Effects of worker manipulation on the sex ratio of a Japanese ant species, Myrmecina nipponica

In order to test the effects of colony size and nutritional condition on the survivorship and sex ratio of ants, Myrmecina nipponica colonies were housed in a laboratory in colony sizes of 10 or 30 individuals and fed either daily or weekly. Under all conditions, most of the larvae successfully grew into adults, which suggests that survivorship was not significantly affected by either colony size or nutritional condition. However, the number of new queens was significantly higher in colonies that were fed daily. These results indicate that workers do not control the proportion of diploid and haploid broods by eliminating some larvae and that nutritional condition exerts a significant effect on sex ratio.     

PSR (paternal sex ratio) chromosomes: the ultimate selfish genetic elements

PSR (paternal sex ratio) chromosomes are a type of supernumerary (or B) chromosomes that occur in haplodiploid arthropods. They are transmitted through sperm but then cause loss of the paternal chromosomes (except themselves) early in development. As a result, PSR chromosomes convert diploid fertilized eggs (which would normally develop into females) into haploid males that carry a PSR chromosome. Because they act by completely eliminating the haploid genome of their hosts, PSR chromosomes are the most extreme form of selfish or parasitic DNA known. PSR was originally described in the parasitic wasp Nasonia vitripennis (Pteromalidae). A second PSR chromosome has been found in Trichogramma kaykai, an egg parasitoid from a different family of Hymenoptera (Trichogrammatidae). We argue that PSR chromosomes are likely to be widespread in haplodiploid organisms, but have so far gone under reported due to a paucity of population genetic studies in haplodiploids. We describe the two known PSR systems and related phenomena, and models indicating the conditions conducive to increase of PSR like chromosomes in haplodiploids.     

Brood sex ratio variation in a cooperatively breeding bird

In cooperatively breeding species, the fitness consequences of producing sons or daughters depend upon the fitness impacts of positive (repayment hypothesis) and negative (local competition hypothesis) social interactions among relatives. In this study, we examine brood sex allocation in relation to the predictions of both the repayment and the local competition hypotheses in the cooperatively breeding long-tailed tit Aegithalos caudatus. At the population level, we found that annual brood sex ratio was negatively related to the number of male survivors across years, as predicted by the local competition hypothesis. At an individual level, in contrast to predictions of the repayment hypothesis, there was no evidence for facultative control of brood sex ratio. However, immigrant females produced a greater proportion of sons than resident females, a result consistent with both hypotheses. We conclude that female long-tailed tits make adaptive decisions about brood sex allocation.     

Seasonal sex ratio and unbalanced investment sex ratio in the Banksia bee Hylaeus alcyoneus

Abstract 1. Hylaeus alcyoneus is an endemic solitary bee common on coastal heaths of Western Australia. The bee is unusual in that males are larger than females. This size dimorphism presents an opportunity to test the theory of resource-dependent sex allocation, in which theory predicts that when resources are low the sex ratio should be biased towards the smaller sex. In most bees, females are larger than males and, in line with theoretical prediction, sex ratios are male biased when resources are scarce.
2. The emerging sex ratio and brood mass from a natural population of H. alcyoneus using trap nests was studied over two seasons (1999, 2000). A switch from a male- to a female-biased sex ratio through the season was found, which was related to a reduced floral resource.
3. Fisherian sex ratio theory predicts that total investment in each sex throughout a season should be equal and that the sex ratio should be biased towards the smaller sex. By measuring the mass of the emerging progeny, the total investment was found to favour males. Possible explanations for this bias in investment are discussed.     

Dispersal and mating structure of a parasitoid with a female-biased sex ratio: Implications for theory

Summary Species of parasitic Hymenoptera that manifest female-biased sex ratios and whose offspring mate only with the offspring of the natal patch are assumed to have evolved biased sex ratios because of Local Mate Competition (LMC). Off-patch matings, i.e. outcrossing, are inconsistent with the conditions favouring biased sex ratios because they foster a mating structure approaching panmixia. Such a mating structure favours parents who invest equally in daughters and sons, assuming the production of each sex is of equal cost.Pachycrepoideus vindemiae (Rondani) is a solitary pupal parasitoid of patchily distributed frugivorousDrosophila, whose offspring manifest a female-biased sex ratio. Thus this species appears to manifest a population structure and progeny sex ratio consistent with LMC. However, preliminary observations and subsequent greenhouse experiments suggest that the males participate in off-patch matings and that this propensity is unlikely to be an experimental artefact. FemaleP. vindemiae dispersed from patches in which either the males were lacking (12% of the emigrant females), both resident (sibling) and immigrant males were present (23% of the females), only immigrant males were present (14% of the females), or their opportunity to mate could not be determined (14% of the females). Of the 12% that emigrated from a patch lacking males, an estimated 7% mated at an oviposition site and 5% remained unmated, presumably because they arrived at an oviposition site that lacked males before they were dissected to determine whether they were inseminated. Thus the degree of bias in the sex ratios of the progeny (18% males), coupled with the suggested outcrossing potential from the experiments (26–37%), is inconsistent with the assumptions of LMC or variants of it, i.e. asynchronous brood maturation. Thus the explanation for a biased sex ratio in the offspring ofP. vindemiae remains a conundrum. More importantly,P. vindemiae does not appear to be an isolated example.     

Adaptive biases in offspring sex ratios established before birth in a marsupial, the common brushtail possum Trichosurus vulpecula

Offspring sex ratios in the common brushtail possum are malebiased in many populations, and there is evidence that inter-populationdifferences in sex ratios represent adaptive responses to localconditions. However, how these biases are produced is not known.Using comparisons between populations with and without biasedoffspring sex ratios, we show that biases in this species arenot produced by sex-differential mortality between birth andweaning or sex-selective termination of pregnancy. Rather,adjustment in the sex ratio of offspring are evidently dueto shifts in the probability of conceiving male and femaleoffspring.     

Litter size and fetal sex ratio adjustment in a highly polytocous species: the wild boar

For species in which reproductive success is more variable inone sex than the other, the Trivers and Willard model (TWM)predicts that females are able to adjust their offspring sexratio. High-quality mothers should provide greater investmentto one sex than the other. Previous tests of the TWM have beeninconsistent, and whether the TWM applies to species with severaloffspring per litter is unclear due to possible trade-offs betweensize, number, and sex of the offspring. Williams' model (WM)accounts for confounding effects of these trade-offs on sexratio variation. Lastly, the "extrinsic modification hypothesis"predicts changes in offspring sex ratio in relation to climaticconditions and population density. Using wild boar as a model,we tested 1) whether the WM fitted observed sex ratio variationand 2) whether sex ratio variations were related to maternalattributes (test of the TWM) and/or to resource availability(test of the extrinsic modification hypothesis). Females adjustedtheir litter size rather than their litter composition, so thatthe WM was not supported. Likewise, changes in resource availabilitydid not influence the fetal sex ratio, so that the extrinsicmodification hypothesis was not supported. The fetal sex ratiowas negatively related to increasing litter size, providingsome support for the TWM. Sex ratio was male biased for littersizes up to 6 and then became female biased in larger litters.Our results provide the first case study showing marked changesin sex ratio in relation to litter size in a large mammal.     

The mechanism of sex ratio adjustment in a pollinating fig wasp

Sex ratio strategies in species subject to local mate competition (LMC), and in particular their fit to quantitative theoretical predictions, provide insight into constraints upon adaptation. Pollinating fig wasps are widely used in such studies because their ecology resembles theory assumptions, but the cues used by foundresses to assess potential LMC have not previously been determined. We show that Liporrhopalum tentacularis females (foundresses) use their clutch size as a cue. First, we make use of species ecology (foundresses lay multiple clutches, with second clutches smaller than first) to show that increases in sex ratio in multi-foundress figs occur only when foundresses are oviposition site limited, i.e. that there is no direct response to foundress density. Second, we introduce a novel technique to quantify foundress oviposition sequences and show, consistent with the theoretical predictions concerning clutch size-only strategies, that they produce mainly male offspring at the start of bouts, followed by mostly females interspersed by a few males. We then discuss the implications of our findings for our understanding of the limits of the ability of natural selection to produce 'perfect' organisms, and for our understanding of when different cue use patterns evolve.     

Obligatory female philopatry affects genetic population structure in the ant Proformica longiseta

Evolution of sociality has instigated many changes in the biology of social insects. Particularly, evolution towards complex social systems in ants affects how individuals move in space, usually by making females philopatric. Proformica longisetais well-suited for studying the effects of female philopatry, because female sexuals are wingless and do not actively disperse. We studied genetic population structure in P. longiseta in local scale both as genetic viscosity within one subpopulation, and as differentiation between closely (0.1–1.5 km) located subpopulations, by using nuclear (microsatellites) and mitochondrial (SSCP) markers. Dependent colony founding by splitting old nests is the only known nest founding strategy in P. longiseta. However, no genetic viscosity was detected at the nuclear markers within the subpopulation studied, possibly due to the dynamic nature of P. longiseta populations. The extreme female philopatry showed as strong structure between closely located subpopulations in the mitochondrial genome, but there was no isolation by distance showing that the differentiation pattern was random. Genetic structure in the nuclear genome was much weaker, and there was an indication of isolation by distance. This suggests that male dispersal is strong but not totally free across the area. Finally, non-dispersing P. longiseta females necessarily mate locally raising the possibility of inbreeding, but inbreeding coefficients showed that mating is random. Received 10 January 2006; revised 13 April 2006; accepted 20 April 2006.     

Size structure, sex ratio and onset of sexual maturity of two species of Hypostomus

Length-frequency analyses showed that male Hypostomus affinis are bigger than females, but H. luetkeni showed no such differences. The sex ratio was estimated at 1:1 for both species. The temporal variation of sex ratio indicated a decrease in the male population when GSI was at its maximum value, suggesting that males are less prone to capture during the spawning season. According to the decrease of males during the reproductive period, paternal behaviour is proposed for both species. H. affinis reached sexual maturity at a smaller size than H. luetkeni. Considering the length weight relationship and the mortality rate of the reproductive females in both species, the physiological consequences of delay or precocity of the first maturation are discussed.     

Colony genetic structure in the mono- and polygynous sibling species of the antsCamponotus nawai andCamponotus yamaokai: DNA fingerprint analysis

Using the ant-derived probe (pMY7), we performed DNA fingerprinting in monogynous and polygynous sibling ant speciesCamponotus nawai andCamponotus yamaokai. In monogynousC. nawai, band-sharing probabilities were low between unrelated individuals (mean 0.09), but those and relatedness estimates were consistently high between workers of the same nest (mean 0.85 and 0.74–0.83, respectively), suggesting that the queen mated once and nestmate workers are super-sisters. It also suggested monoandry: that is, that all nestmate workers shared most of the bands which were considered to have derived from a male. In polygynousC. yamaokai, band-sharing probabilities were low between queens of different populations (mean 0.13), moderate between queens of different nests in the same population (mean 0.25), but very high between queens of the same nest (within-nest means were 0.84–0.96). These results suggest that nestmate queens are genetically closely related with each other. Relatedness estimates between colony members sometimes reached 1. This might result from successive intranidal mating (inbreeding or large Wahlund effect) and adoption of new queens into the natal nests.     

Competitor-to-resource ratio, a general formulation of operational sex ratio, as a predictor of competitive aggression in Japanese medaka (Pisces: Oryziidae)

Operational sex ratio (OSR), the number of potentially matingmales divided by the number of fertilizable females, playsa central role in the theory of mating systems by predictingthe intensity of intra-sexual competition and sexual selection.We introduce a general version of OSR, competitor-to-resourceratio (CRR, the number of potential competitors divided bythe number of resource units), as a potential way of predictingthe intensity of competition for any resource. We manipulatedCRR over a broad range (0.5-8) by varying both the number ofcompeting male Japanese medaka fish (Oryzias latipes) and thenumber of resources, either females or food items. We testedwhether the rate of male—male aggression differed dependingon resource type and whether it increased monotonically or followed a dome-shaped relationship with increasing CRR. The patternsof competitive aggression in relation to CRR did not differsignificantly between resource types. In addition, the percapita rate of aggression followed a dome-shaped curve; itwas low when CRR was less than one, initially increased as CRR increased, was highest at a CRR of about two, and then decreasedwhen CRR was greater than two. However, competitor number,independent of CRR, had a significant and negative effect onrate of aggression. We suggest that CRR is a valuable predictorof the rate of competitive aggression and may be a useful conceptfor synthesizing ideas about resource competition and monopolization that are currently dispersed in the separate bodies of literatureon mating systems, social foraging and territoriality.     

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.     

Host influences on sex ratio, longevity, and egg load of two Metaphycus species parasitic on soft scales: implications for insectary rearing

Metaphycus flavus (Howard) and M. stanleyi Compere (Hymenoptera: Encyrtidae) are currently being screened for use as augmentative biological control agents of citrus-infesting soft scales (Homoptera: Coccidae). Two factors were investigated, host quality-dependent sex allocation and local mate competition, which likely influence these parasitoid's sex allocation strategies and are therefore of interest for their mass-rearing. The results of these studies suggested that, under the mass-rearing protocol that is envisioned for these parasitoids, offspring sex ratios in both M. flavus and M. stanleyi are dominated by host quality (= size) influences, but not by interactions with other females. These results indicated that host size strongly influences offspring sex ratios and brood sizes; larger hosts led to more female offspring and larger broods. In contrast, increasing the number of parental females did not lead to fewer female offspring as expected under local mate competition. Additionally, within-brood sex ratios did not vary with brood size; this result is inconsistent with expected sex ratios due to local mate competition. Other results also indicated that host quality was a dominant influence on M. flavus' and M. stanleyi's sex ratios. Larger hosts led to a larger size in the emerging wasps, and larger wasps had greater egg loads and lived longer than smaller wasps. However, wasp longevity, and the influence of wasp size on longevity were mediated by a wasp's diet. Metaphycus flavus females lived the longest when they had access to hosts, honey, and water, followed by honey and water, and shortest when they had access to water alone; M. stanleyi females lived longest with honey and water, followed by hosts, honey, and water, and shortest with water alone. Greater wasp size led to greater longevity in females only when they had access to food (honey, or hosts and honey). Finally, other results suggested that both M. flavus and M. stanleyi are facultatively gregarious. Wasp size did not decrease with brood size as expected under superparasitism. Overall, the results of these studies suggested that holding newly emerged females of both M. flavus and M. stanleyi for several days in the presence of an appropriate food source before field release could enhance a female's performance as an augmentative biological control agent. It increases their initial life expectancy following release, and maximizes the females' egg load (both Metaphycus species) and resources for replacing oviposited eggs (M. flavus only).     

Dispersal, philopatry, and genetic relatedness in a social carnivore: comparing males and females

A balance must be maintained between the proportion of individuals dispersing and the proportion remaining philopatric such that inbreeding and resource competition are minimized. Yet the relative importance of dispersal and philopatric behaviour is uncertain, especially for species with complex social systems. We examine the influence of dispersal on genetic relationships of a white-nosed coati ( Nasua narica : Procyonidae) population from Panama. Field studies of the coati indicate a social system in which all females are highly philopatric and live in bands while all adult males become solitary at maturity, but do not disperse from the home range of their natal band. Based on analyses of multilocus DNA fingerprints, we confirm that female philopatry is the rule, long-distance dispersal is rare, and that relatedness between most bands is low. However, some new bands result from fission events and these bands retain relatively high relatedness to one another for several years. Adult males inhabiting the home range of a band are closely related to band members. In contrast, males and band members whose ranges do not overlap are unrelated or only slightly related. Adult males are also more closely related to other males whose home ranges they overlap extensively than to males whose home ranges they overlap only slightly. These results indicate that males initially disperse from their natal bands to reduce resource competition and not to avoid inbreeding. Inbreeding avoidance, if it occurs, results from more extensive range movements by males during the mating season.     

Female-biased sex ratio in a wild bruchid seed-predator, Kytorhinus sharpianus. I. Larval competition and other factors

Abstract.

• 1 A wild bruchid seed-predator, Kytorhinus sharpianus, has a complex life cycle consisting of bi- and trivoltinism on a wild leguminous plant, Sophola flavescens. Observations of adults showed significant female-biased sex ratios (from 1:2 to 1:6) for nine generations over 4 years.
• 2 To investigate the potential effects of larval competition on the sex ratio, we altered the number of hatched eggs per seed and counted emergent males and females under laboratory conditions. Although only one adult could emerge per seed, the ratio of the females that emerged increased with the number of hatched eggs per seed. However, the sex ratio was not significantly different from 1:1 in the case of one hatched egg per seed.
• 3 We dissected seeds bearing two hatched eggs at regular intervals, and classified the surviving and the dead larvae according to their developmental stage. Over time, one larva within each seed always survived, while the other larva died from the second to fourth instar before the seed resource became exhausted.
• 4 In order to study the effects of the difference in the stages of two larvae in a seed on the emergence sex ratio, we manipulated intervals between the first and second ovipositions in the laboratory. As the difference in developmental stages of the two larvae increased, the closer to 1:1 the emergence sex ratio became.
• 5 Field observations, however, showed that about 60% of infested seeds were bored by only one K.sharpianus larva. This suggests that female dominance in larval competition within a seed may be relatively unimportant in causing the female-biased sex ratio in the field.

     

Local resource competition and sex ratio in the ant Cataglyphis cursor

The local resource competition (LRC) hypothesis predicts thatwherever philopatric offspring compete for resources with theirmothers, offspring sex ratios should be biased in favor of thedispersing sex. In ants, LRC is typically found in polygynous(multiple queen) species where foundation of new nests occursby budding, which results in a strong population structure anda male-biased population-wide sex ratio. However, under polygyny,the effect of LRC on sex allocation is often blurred by theeffect of lowered relatedness asymmetries among colony members.Moreover, environmental factors, such as the availability ofresources, have also been shown to deeply influence sex ratioin ants. We investigated sex allocation in the monogynous (singlequeen) ant Cataglyphis cursor, a species where colonies reproduceby budding and both male and female sexuals are produced throughparthenogenesis, so that between-colony variations in relatednessasymmetries should be reduced. Our results show that sex allocationin C. cursor is highly male biased both at the colony and populationlevels. Genetic analyses indicate a significant isolation-by-distancein the study population, consistent with limited dispersal offemales. As expected from asexual reproduction, only weak variationsin relatedness asymmetry of workers toward sexual offspringoccur across colonies, and they are not associated with colonysex ratio. Inconsistent with the predictions of the resourceavailability hypothesis, the male bias significantly increaseswith colony size, and investment in males, but not in females,is positively correlated with total investment in sexuals. Overall,our results are consistent with the predictions of the LRC hypothesisto account for sex ratio variation in this species.     

Mating frequency and resource allocation to male and female function in the simultaneous hermaphrodite land snail Arianta arbustorum

Sex allocation theory predicts that mating frequency and long‐term sperm storage affect the relative allocation to male and female function in simultaneous hermaphrodites. We examined the effect of mating frequency on male and female reproductive output (number of sperm delivered and eggs deposited) and on the resources allocated to the male and female function (dry mass, nitrogen and carbon contents of spermatophores and eggs) in individuals of the simultaneous hermaphrodite land snail Arianta arbustorum. Similar numbers of sperm were delivered in successive copulations. Consequently, the total number of sperm transferred increased with increasing number of copulations. In contrast, the total number of eggs produced was not influenced by the number of copulations. Energy allocation to gamete production expressed as dry mass, nitrogen or carbon content was highly female‐biased (>95% in all estimates). With increasing number of copulations the relative nitrogen allocation to the male function increased from 1.7% (one copulation) to 4.7% (three copulations), but the overall reproductive allocation remained highly female‐biased. At the individual level, we did not find any trade‐off between male and female reproductive function. In contrast, there was a significant positive correlation between the resources allocated to the male and female function. Snails that delivered many sperm also produced a large number of eggs. This finding contradicts current theory of sex allocation in simultaneous hermaphrodites.     

Seed sex ratio in dioecious plants depends on relative dispersal of pollen and seeds: an example using a chessboard simulation model

Two principles are important for the optimal sex ratio strategy of plants. (1) Sib mating. Because seed dispersal is restricted, sib mating may occur which selects for a female bias in the seed sex ratio. (2) Local resource competition (LRC). If a plant produces pollen its nuclear genes are dispersed in two steps: first through the pollen and then, if the pollen is successful in fertilizing an ovule on another plant, through the seed. If the plant produces an ovule, its genes are dispersed only through the seed. By making pollen instead of ovules the offspring of a single plant is then spread out over a wider area. This reduces the chance that genetically related individuals are close together and need to compete for the same resource. The effect is the strongest if pollen is dispersed over a much wider area than seeds. Less LRC for paternally vs. maternally derived offspring selects for a male bias in sex allocation. We study the above‐mentioned opposite effects in dioecious plants (with separate male and female individuals), with maternal control over the sex ratio (fraction males) in the seeds. In a two‐dimensional spatial model female‐biased sex ratios are found when both pollen and seed dispersal are severely restricted. If pollen disperses over a wider area than seeds, which is probably the common situation in plants, the seed sex ratio becomes male‐biased. If pollen and seeds are both dispersed over a wide area, the sex ratio approaches 0.5. Our results do not change if the offspring of brother–sister matings are less fit because of inbreeding depression.     

Effect of cryopreservation on the frequency of chromosomal abnormalities and sex ratio in human sperm

The effects of cryopreservation on the frequency and type of chromosomal abnormalities in human sperm were investigated. Employing a technique that enables direct visualization of human sperm chromosomes following in vitro penetration of hamster oocytes, sperm samples from 10 normal men were examined before and after freezing in liquid nitrogen. A total of 1,960 sperm karyotypes were analyzed, 1,132 before freezing and 828 after freezing. There was no significant difference in the frequency of structural chromosomal anomalies (10.5% prefreeze vs. 8.5% postfreeze), but there was a significant decrease in the frequency of numerical abnormalities (5.2% prefreeze vs. 3.0% postfreeze). However, there was a large excess of hypohaploid complements compared with hyperhaploid complements, suggesting that the hypohaploid complements were caused by technical artefact. A conservative estimate of aneuploidy, derived by doubling the hyperhaploid frequencies, did not differ before (0.4%) and after (0.4%) freezing. There was no evidence for interdonor variability in response to sperm cryopreservation for total chromosomal abnormalities, structural abnormalities, and sex ratios. The sex ratios were also not affected by cryopreservation and did not differ significantly from the theoretical 50%. It is concluded that cryopreservation does not affect the frequencies of chromosomal abnormalities or alter the sex ratio in human sperm, provided that an adequate cryoprotective buffer and freezing system is employed.