对叶榕传粉小蜂性比率的调节和稳定

传粉榕小蜂呈现偏雌的性比率,单双倍体性别决定系统、局域配偶竞争和近交效应被认为是调节偏雌性比率的3个主要机制。通过研究影响对叶榕传粉小蜂性比率的因素,结果表明传粉榕小蜂的偏雌性比率随局域配偶竞争强度的降低而增加;受母代雌蜂交配次数的影响,随着母代雌蜂交配次数的增加,子代的偏雌性比率逐渐降低,这一结果首次揭示了传粉榕小蜂的交配制次数对性比率的影响,并在个体水平上定量了性比率变异与雌蜂交配频次的关系。传粉小蜂的性比率与共生的非传粉小蜂的关系,非传粉小蜂的介入直接减少了传粉小蜂的数量,甚至对传粉小蜂的种群有显著影响,结果发现非传粉小蜂对传粉小蜂雌雄性的分配比率没有显著影响,传粉榕小蜂仍能正常地进行繁殖。传粉与非传粉者小蜂之间作用关系的确定,可为进一步理解两者的稳定共生的机制提供科学证据。     

A genetic perspective on mating systems and sex ratios of parasitoid wasps

Parasitoid sex ratios are influenced by mating systems, whether complete inbreeding, partial inbreeding, complete inbreeding avoidance, or production of all-male broods by unmated females. Population genetic theory demonstrates that inbreeding is possible in haplodiploids because the purging of deleterious and lethal mutations through haploid males reduces inbreeding depression. However, this purging does not act quickly for deleterious mutations or female-limited traits (e.g., fecundity, host searching, sex ratio). The relationship between sex ratio, inbreeding, and inbreeding depression has not been explored in depth in parasitoids. The gregarious egg parasitoid, Trichogramma pretiosum Riley, collected from Riverside, CA (USA) produced a female-biased sex ratio of 0.24 (proportion of males). Six generations of sibling mating in the laboratory uncovered considerable inbreeding depression (∼ 20%) in fecundity and sex ratio. A population genetic study (based upon allozymes) showed the population was inbred (F _it = 0.246), which corresponds to 56.6% sib-mating. However, average relatedness among females emerging from the same host egg was only 0.646, which is less than expected (0.75) if ovipositing females mate randomly. This lower relatedness could arise from inbreeding avoidance, multiple mating by females, or superparasitism. A review of the literature in general shows relatively low inbreeding depression in haplodiploid species, but indicates that inbreeding depression can be as high as that found in Drosophila. Finally, mating systems and inbreeding depression are thought to evolve in concert (in plants), but similar dynamic models of the joint evolution of sex ratio, mating systems, and inbreeding depression have not been developed for parasitoid wasps. Received: November 13, 1998 /Accepted: January 8, 1999     

SEX‐SPECIFIC INBREEDING DEPRESSION DEPENDS ON THE STRENGTH OF MALE–MALE COMPETITION

Inbreeding depression has become a central theme in evolutionary biology and is considered to be a driving force for the evolution of reproductive morphology, physiology, behavior, and mating systems. Despite the overwhelming body of empirical work on the reproductive consequences of inbreeding, relatively little is known on whether inbreeding depresses male and female fitness to the same extent. However, sex‐specific inbreeding depression has been argued to affect the evolution of selfing rates in simultaneous hermaphrodites and provides a powerful approach to test whether selection is stronger in males than in females, which is predicted to be the consequence of sexual selection. We tested for sex‐specific inbreeding depression in the simultaneously hermaphroditic freshwater snail Physa acuta by comparing the reproductive performance of both sex functions between selfed and outcrossed focal individuals under different levels of male–male competition. We found that inbreeding impaired both male and female reproductive success and that the magnitude of male inbreeding depression exceeded female inbreeding depression when the opportunity for sperm competition was highest. Our study provides the first evidence for sex‐specific inbreeding depression in a hermaphroditic animal and highlights the importance of considering the level of male–male competition when assessing sex differences in inbreeding depression.     

INBREEDING AS A STRATEGY IN SUBDIVIDED POPULATIONS

A generalized expression for coefficients of consanguinity and relationship with previous inbreeding is presented to examine various breeding strategies in subdivided populations. Conditions that would favor inbreeding are developed for: 1) nonfamilial inbreeding within a deme versus outbreeding; 2) altruistic inbreeding by females versus outbreeding; 3) sib-mating versus outbreeding; and 4) sib-mating versus nonfamilial breeding within a deme. Inbreeding behavior is advantageous under certain conditions but depends on the types of mating, the previous breeding history of the deme, the rate of accumulation of inbreeding depression, and the cost of migration. In polygynous mating systems it is genetically more advantageous for males to migrate, because female emigration may 1) leave a related male with no mate or one fewer mate, or 2) force both male and female to risk the cost of migration. Nonfamilial breeding is always a better strategy than sib-mating given previous inbreeding within the deme. Even when the cost of migration is zero, inbreeding is favored if the coefficient of relationship among relatives is greater than the ratio of the probabilities of offspring inviability to offspring viability. Although high inbreeding coefficients are probably not adaptive unless the costs of migration are great or inbreeding depression constants are small, low levels of inbreeding are advantageous in many situations. Therefore, increased genetic representation by way of inbreeding and inclusive fitness is a major component of the evolutionary process.     

Effect of inbreeding and heritability of sperm competition success in the bulb mite Rhizoglyphus robini

Sperm competition is a potent evolutionary force shaping the reproductive biology of most animal species. Here, we estimated the heritability of sperm competition success in the promiscuous bulb mite Rhizoglyphus robini. Sperm competition success was measured with the sterile male technique as the proportion of eggs fertilised by the second of three males mated with a single female. Sperm competition success responded significantly to selection. The heritability estimated from the response to five generations of selection was 0.13. We also estimated the effect of inbreeding on sperm competition success. Males produced by sib-mating (F=0.25) had a significantly lower sperm competition success than outbred males. The estimated coefficient of inbreeding depression was 0.53. Such high inbreeding depression together with moderately low heritability is consistent with the view that sperm competitive ability is under strong directional selection and strongly influences the reproductive success of males.     

寄生蜂性别分配行为

寄生蜂是性比分配行为领域的研究热点对象,其性别决定方式为单双倍型,一般情况下,未受精的单倍型卵发育成雄蜂,受精的二倍型卵发育为雌蜂。局部配偶竞争和近交等因素使得偏雌性比成为这类生物的进化稳定策略;其性比具有可调节性,产卵个体可以根据对产卵环境的判定来调控后代性比,从而获得最大适合度。在此基础上形成的局部配偶竞争理论阐述了寄生蜂性比的这种可调节性,成为进化论的优秀论据。     

Evolution of sex ratios in social hymenoptera: Consequences of finite brood size

Evolutionarily stable sex ratios are determined for social hymenoptera under local mate competition (LMC) and when the brood size is finite. LMC is modelled by the parameterd. Of the reproductive progeny from a single foundress nest, a fractiond disperses (outbreeding), while (l-d) mate amongst themselves (sibmating). When the brood size is finite,d is taken to be the probability of an offspring dispersing, and similarly,r, the proportion of male offspring, the probability of a haploid egg being laid. Under the joint influence of these two stochastic processes, there is a nonzero probability that some females remain unmated in the nest. As a result, the optimal proportion of males (corresponding to the evolutionarily stable strategy, ESS) is higher than that obtained when the brood size is infinite. When the queen controls the sex ratio, the ESS becomes more female biased under increased inbreeding (lowerd). However, the ESS under worker control shows an unexpected pattern, including anincrease in the proportion ofmales withincreased inbreeding. This effect is traced to the complex interaction between inbreeding and local mate competition.     

Towards a genetic theory for the evolution of the sex ratio. III. Parental and sibling control of brood investment ratio under partial sib-mating

Models of sex ratio evolution under partial sib-mating are investigated in haplodiploids and diploids. In the cases of parental and sibling control of the brood investment ratio between the sexes in diploids, we find that the “unbeatable” investment ratio obtained by W. D. Hamilton (Science156, 477–488) for his local mate competition model corresponds in our inbreeding models to a weak form ESS (evolutionary stable strategy) fixation state and also to the population investment ratio at certain internal equilibria of our models. For haplodiploids, “strong form ESS” values exist under inbreeding in models involving father and sister control. Under brother and mother control, however, the ESS derived from local mate competition models is unstable in our inbreeding models to the introduction of any other investment ratio. We stress important qualitative differences between models involving local mate competition and inbreeding.     

Inbreeding depression and haplodiploidy: experimental measures in a parasitoid and comparisons across diploid and haplodiploid insect taxa

Abstract. It has long been assumed that inbreeding depression in haplodiploid organisms is low due to their ability to purge genetic load in haploid males. It has been suggested that this low genetic load could facilitate the evolution of inbreeding behaviors driven by local mate competition in hymenopteran parasitoids. I have examined inbreeding depression in haplodiploids in two ways. First I show that an outbreeding haplodiploid wasp Uscana semifumipennis (Hymenoptera: Trichogrammatidae) suffers substantial inbreeding depression. Longevity was 38% shorter, fecundity was 32% lower, and sex ratio was 5% more male for experimentally inbred wasps when compared to outbred controls. There were interactions between size and both fecundity and sex ratio for inbred wasps that were not seen for outbred individuals. Second, an analysis of data from the literature suggests that when inbreeding is experimentally imposed on populations, haplodiploid insects and mites as a group do suffer less from inbreeding depression than diploid insects, although substantial inbreeding depression in haplodiploid taxa does exist. The meta-analysis revealed no difference in inbreeding depression between gregarious haplodiploid wasps, which are likely to have a history of inbreeding, and solitary haplodiploid species, which are assumed to be primarily outbred.     

Sex determination and inbreeding depression in an ant with regular sib-mating

Haplodiploidy is one of the most widespread mechanisms of sex determination in animals. In many Hymenoptera, including all hitherto investigated social species, diploid individuals, which are heterozygous at the sex locus, develop as females, whereas haploid, hemizygous individuals develop as males (single-locus complementary sex determination, sl-CSD). Inbreeding leads to homozygosity at the sex locus, resulting in the production of diploid males, which are usually sterile and constitute a considerable fitness cost. Nevertheless, regular inbreeding without diploid male production is known from several solitary wasps, suggesting that in these species sex is not determined by sl-CSD but alternative mechanisms. Here, we examine sex determination in an ant with regular inbreeding, Cardiocondyla obscurior. The almost complete absence of diploid males after 10 generations of brother-sister mating in the laboratory documents for the first time the absence of sl-CSD and CSD with two or a few unlinked sex loci in a species of social Hymenoptera. Queens, which mated with a brother, appeared to decrease the number of males in their brood, as expected from the relatedness relationships under inbreeding. In contrast, some colonies began to show signs of an inbreeding depression after several generations of sib-mating, such as shortened queen life span, higher brood mortality, and a shift to more male-biased sex ratios in some colonies, presumably due to lower insemination capability of sperm.     

Origin of a haplodiploid beetle lineage

The beetle family Scolytidae includes several groups having regular sib-mating and extremely female-biased sex ratios. Two such groups are known to include haplodiploid species: (i) the tribe Xyleborini and (ii) Coccotrypes and related genera within the tribe Dryocoetini. Relationships of these groups have been controversial. We analysed elongation factor 1-α (852 bp) and cytochrome oxidase 1 (1179 bp) sequences for 40 species. The most-parsimonious trees imply a single origin of haplodiploidy uniting Xyleborini (approximately 1200 species) and sib-mating Dryocoetini (approximately 160 species). The sister-group of the haplodiploid clade is the outcrossing genus Dryocoetes. The controversial genus Premnobius is outside the haplodiploid clade. Most haplodiploid scolytids exploit novel resources, ambrosia fungi or seeds, but a few have the ancestral habit of feeding on phloem. Thus, scolytids provide the clearest example of W. D. Hamilton''s scenario for the evolution of haplodiploidy (life under bark leading to inbreeding and hence to female-biased sex ratios through haplodiploidy) and now constitute a unique opportunity to study diplodiploid and haplodiploid sister-lineages in a shared ancestral habitat. There is some evidence of sex determination by maternally inherited endosymbiotic bacteria, which may explain the consistency with which female-biased sex ratios and close inbreeding have been maintained.     

More highly female-biased sex ratio in the fig wasp,Blastophaga nipponica Grandi (Agaonidae)

The sex ratio of the pollinator fig wasp,Blastophaga nipponica Grandi (Agaonidae), was examined in an experiment manipulating the number of foundresses. The sex ratio ofB. nipponica was conditional on the number of foundresses and corresponded to the qualitative prediction of the local mate competition (LMC) theory that the proportion of males increases as foundress number increases. However, the sex ratio ofB. nipponica was consistently more female-biased than predicted by extended LMC theories that incorporated effects of inbreeding, and these deviations were statistically significant. Plausible factors that would make predictions more female-biased are discussed.     

Experimental evolution of the genetic load and its implications for the genetic basis of inbreeding depression

The degree to which, and rapidity with which, inbreeding depression can be purged from a population has important implications for conservation biology, captive breeding practices, and invasive species biology. The degree and rate of purging also informs us regarding the genetic mechanisms underlying inbreeding depression. We examine the evolution of mean survival and inbreeding depression in survival following serial inbreeding in a seed-feeding beetle, Stator limbatus, which shows substantial inbreeding depression at all stages of development. We created two replicate serially inbred populations perpetuated by full-sib matings and paired with outbred controls. The genetic load for the probability that an egg produces an adult was purged at approximately 0.45-0.50 lethal equivalents/generation, a reduction of more than half after only three generations of sib-mating. After serial inbreeding we outcrossed all beetles then measured (1) larval survival of outcrossed beetles and (2) inbreeding depression. Survival of outcrossed beetles evolved to be higher in the serially inbred populations for all periods of development. Inbreeding depression and the genetic load were significantly lower in the serially inbred than control populations. Inbreeding depression affecting larval survival of S. limbatus is largely due to recessive deleterious alleles of large effect that can be rapidly purged from a population by serial sib-mating. However, the effectiveness of purging varied among the periods of egg/larval survival and likely varies among other unstudied fitness components. This study presents novel results showing rapid and extensive purging of the genetic load, specifically a reduction of as much as 72% in only three generations of sib-mating. However, the high rate of extinction of inbred lines, despite the lines being reared in a benign laboratory environment, indicates that intentional purging of the genetic load of captive endangered species will not be practical due to high rates of subpopulation extinction.     

Inbreeding depression in an insect with maternal care: influences of family interactions,life stage and offspring sex

Although inbreeding is commonly known to depress individual fitness, the severity of inbreeding depression varies considerably across species. Among the factors contributing to this variation, family interactions, life stage and sex of offspring have been proposed, but their joint influence on inbreeding depression remains poorly understood. Here, we demonstrate that these three factors jointly shape inbreeding depression in the European earwig, Forficula auricularia. Using a series of cross‐breeding, split‐clutch and brood size manipulation experiments conducted over two generations, we first showed that sib mating (leading to inbred offspring) did not influence the reproductive success of earwig parents. Second, the presence of tending mothers and the strength of sibling competition (i.e. brood size) did not influence the expression of inbreeding depression in the inbred offspring. By contrast, our results revealed that inbreeding dramatically depressed the reproductive success of inbred adult male offspring, but only had little effect on the reproductive success of inbred adult female offspring. Overall, this study demonstrates limited effects of family interactions on inbreeding depression in this species and emphasizes the importance of disentangling effects of sib mating early and late during development to better understand the evolution of mating systems and population dynamics.     

A genetic interpretation of the variation in inbreeding depression

Inbreeding depression is expected to play an important but complicated role in evolution. If we are to understand the evolution of inbreeding depression (i.e., purging), we need quantitative genetic interpretations of its variation. We introduce an experimental design in which sires are mated to multiple dams, some of which are unrelated to the sire but others are genetically related owing to an arbitrary number of prior generations of selfing or sib-mating. In this way we introduce the concept of "inbreeding depression effect variance," a parameter more relevant to selection and the purging of inbreeding depression than previous measures. We develop an approach for interpreting the genetic basis of the variation in inbreeding depression by: (1) predicting the variation in inbreeding depression given arbitrary initial genetic variance and (2) estimating genetic variance components given half-sib covariances estimated by our experimental design. As quantitative predictions of selection depend upon understanding genetic variation, our approach reveals the important difference between how inbreeding depression is measured experimentally and how it is viewed by selection.     

EXPERIMENTAL EVIDENCE FOR FREQUENCY DEPENDENT SELF-FERTILIZATION IN THE GYNODIOECIOUS PLANT, SILENE VULGARIS

After over a half century of empirical and theoretical research regarding the evolution and maintenance of gynodioecy in plants, unexplored factors influencing the relative fitnesses of females and hermaphrodites remain. Theoretical studies suggest that hermaphrodite self-fertilization (selfing) rate influences the maintenance of gynodioecy and we hypothesized that population sex ratio may influence hermaphrodite selfing rate. An experimental test for frequency-dependent self-fertilization was conducted using replicated populations constructed with different sex ratios of the gynodioecious plant Silene vulgaris . We found that hermaphrodite selfing increased with decreased hermaphrodite frequency, whereas evidence for increased inbreeding depression was equivocal. We argue that incorporation of context dependent inbreeding into future models of the evolution of gynodioecy is likely to yield novel insights into sex ratio evolution.     

Single-locus sex determination in the parasitoid wasp Cotesia glomerata (Hymenoptera: Braconidae)

The parasitoid Cotesia glomerata usually produces female-biased sex ratios in the field, which are presumably caused by inbreeding and local mate competition (LMC); yet, sibling mating increases the production of males, leading to the male-biased sex ratio of broods in the laboratory. Previous studies have suggested that the sex allocation strategy of C. glomerata is based on both partial LMC in males and inbreeding avoidance in females. The current study investigated the presence of single-locus complementary sex determination (sl-CSD) as a sex-determining mechanism in this species through inbreeding experiment, cytological examination and microsatellite analysis. Cytological examination detected diploid males in nine of 17 single pairs of sibling mating, thus in agreement with the proportion of matched matings predicted by the sl-CSD model. Sex ratio shifts in these matched sibling matings were consistent with the sl-CSD model with less viable diploid males. The haploid males have a single set of maternal chromosomes (n = 10), whereas diploid males possess a double set of chromosomes (2n = 20). Microsatellite analyses confirmed that diploid males produced from the matched matings inherited segregating genetic materials from both parents. Thus, this study provides the first solid evidence for the presence of sl-CSD as a sex-determining mechanism in the braconid genus Cotesia.     

Patterns of self compatibility, inbreeding depression, outcrossing, and sex allocation in a marine bryozoan suggest the predominating influence of sperm competition

Sex allocation by simultaneous hermaphrodites is theoretically influenced by selfing rate, which is in turn influenced by the benefits of enhanced genomic transmission and reproductive assurance relative to the cost of inbreeding depression. The experimental investigation of these influences in seed plants has a rich pedigree, yet although such an approach is equally relevant to colonial invertebrates, which globally dominate subtidal communities on firm substrata, such studies have been scarce. We reared self‐compatible genets of the marine bryozoan Celleporella hyalina s.l. in the presence and absence of allosperm, and used molecular genetic markers for paternity analysis of progeny to test theoretical predictions that: (1) genets from focal populations with high selfing rates show less inbreeding depression than from focal populations with low selfing rates; (2) genets whose selfed progeny show inbreeding depression prefer outcross sperm (allosperm); and (3) genets bias sex allocation toward female function when reared in reproductive isolation. Offspring survivorship and paternity analysis were used to estimate levels of inbreeding depression and preference for outcrossing or selfing. Sex allocation was assessed by counting male and female zooids. As predicted, inbreeding depression was severe in selfed progeny of genets derived from the populations with low self‐compatibility rates, but, with one exception, was not detected in selfed progeny of genets derived from the populations with higher self‐compatibility rates. Also, as predicted, genets whose selfed progeny showed inbreeding depression preferred outcrossing, and a genet whose selfed progeny did not show inbreeding depression preferred selfing. Contrary to prediction, sex allocation in the majority of genets was not influenced by reproductive isolation. Lack of economy of male function may reflect the over‐riding influence of allosperm‐competition in typically dense breeding populations offering good opportunity for outcrossing. We suggest that hermaphroditism may be a plesiomorphic character of the crown group Bryozoa, prevented by phylogenetic constraint from being replaced by gonochorism and therefore not necessarily adaptive in all extant clades. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 519–531.     

Exploring the genetic basis and proximate causes of female fertility advantage in gynodioecious Thymus vulgaris

In many gynodioecous species, females produce more viable seeds than hermaphrodites. Knowledge of the relative contribution of inbreeding depression in hermaphrodites and maternal sex effects to the female fertility advantage and the genetic basis of variation in female fertility advantage is central to our understanding of the evolution of gender specialization. In this study we examine the relative contribution of inbreeding and maternal sex to the female fertility advantage in gynodioecious Thymus vulgaris and quantify whether there is genetically based variation in female fertility advantage for plants from four populations. Following controlled self and outcross (sib, within-population, and between-population) pollination, females had a more than twofold fertility advantage (based on the number of germinating seeds per fruit), regardless of the population of origin and the type of pollination. Inbreeding depression on viable seed production by hermaphrodites occurred in two populations, where inbreeding had been previously detected. Biparental inbreeding depression on viable seed production occurred in three of four populations for females, but in only one population for hermaphrodites. Whereas the maternal sex effect may consistently enhance female fertility advantage, inbreeding effects may be limited to particular population contexts where inbreeding may occur. A significant family x maternal sex interaction effect on viable seed production was observed, illustrating that the extent of female fertility advantage varies significantly among families. This result is due to greater variation in hermaphrodite (relative to female) seed fertility between families. Despite this genetic variation in female fertility advantage and the highly female biased sex ratios in populations of T. vulgaris, gynodioecy is a stable polymorphism, suggesting that strong genetic and/or ecological constraints influence the stability of this polymorphism.     

Inbreeding and reproductive investment in the ant Formica exsecta

In social animals, inbreeding depression may manifest by compromising care or resources individuals receive from inbred group members. We studied the effects of worker inbreeding on colony productivity and investment in the ant Formica exsecta. The production of biomass decreased with increasing inbreeding, as did biomass produced per worker. Inbred colonies produced fewer gynes (unmated reproductive females), whereas the numbers of males remained unchanged. As a result, sex ratios showed increased male bias, and the fraction of workers increased among the diploid brood. Males raised in inbred colonies were smaller, whereas the weight of gynes remained unchanged. The results probably reflect a trade-off between number and quality of offspring, which is expected if the reproductive success of gynes is more dependent on their weight or condition than it is for males. As males are haploid (with the exception of abnormal diploid males produced in very low frequencies in this population), and therefore cannot be inbred themselves, the effect on their size must be mediated through the workers of the colony. We suggest the effects are caused by the inbred workers being less proficient in feeding the growing larvae. This represents a new kind of social inbreeding depression that may affect sex ratios as well as caste fate in social insects.