Studies Of Sex Allocation Research Articles

SEX-RATIO ADJUSTMENT WHEN RELATIVES INTERACT: A TEST OF CONSTRAINTS ON ADAPTATION

Studies of sex allocation offer excellent opportunities for examining the constraints and limits on adaptation. A major topic of debate within this field concerns the extent to which the ability of individuals to adaptively manipulate their offspring sex ratio is determined by constraints such as the method of sex determination. We address this problem by comparing the extent of sex-ratio adjustment across taxa with different methods of sex determination, under the common selective scenario of interactions between relatives. These interactions comprise the following: local resource competition (LRC), local mate competition (LMC), and local resource enhancement (LRE). We found that: (1) species with supposedly constraining methods of sex determination showed consistent sex-ratio adjustment in the predicted direction; (2) vertebrates with chromosomal sex determination (CSD) showed less adjustment then haplodiploid invertebrates; (3) invertebrates with possibly constraining sex-determination mechanisms (CSD and pseudo-arrhenotoky) did not show less adjustment then haplodiploid invertebrates; (4) greater sex-ratio adjustment was seen in response to LRC and LMC than LRE; (5) greater sex-ratio adjustment was seen in response to interactions between relatives (LRC, LMC, and LRE) compared to responses to other environmental factors. Our results also illustrate the problem that sex-determination mechanism and selective pressure are confounded across taxa because vertebrates with CSD are influenced primarily by LRE whereas invertebrates are influenced by LRC and LMC. Overall, our analyses suggest that sex-allocation theory needs to consider simultaneously the influence of variable selection pressures and variable constraints when applying general theory to specific cases.

Adaptive sex allocation in relation to life‐history in the common brushtail possum, Trichosurus vulpecula

Summary Maternal control of offspring sex remains a contentious topic in the evolutionary and behavioural ecology of vertebrates. Two models of adaptive sex allocation − the Trivers–Willard (TW) and local resource competition (LRC) hypotheses − have been supported in some studies, but in many other cases the forces shaping offspring sex ratios are unclear. One reason that past studies may not have provided clear support for either model is that the two processes interact, producing complex patterns of sex allocation in relation to the life histories of individual mothers. The TW hypothesis predicts that in dimorphic, polygynous mammals, females in good condition should be more likely to produce male offspring. The LRC hypothesis predicts an independent effect of age on the offspring sex ratio; females should be more likely to produce sons early in their reproductive lives and daughters later, in order to minimize the duration and reproductive cost of mother–daughter competition. We tested these two predictions in a study of sex allocation in the common brushtail possum, Trichosurus vulpecula. Offspring sex was strongly dependent on maternal age: females breeding for the first time were likely to produce sons, and at subsequent breeding attempts the sex ratio of offspring was slightly biased towards females. After the first breeding season, sex of offspring was influenced more strongly by body condition than by age, and females in good condition were more likely to have sons. Individual females that showed a between‐year gain in condition were also more likely to produce a male in the subsequent year. Body mass of male offspring at 5 months was positively correlated with maternal condition. No relationship was found between maternal condition and mass of female offspring. We conclude that patterns of change in sex allocation in relation to individual life histories of female common brushtail possums can be explained by the interaction of TW and LRC effects.

SEX-RATIO ADJUSTMENT WHEN RELATIVES INTERACT: A TEST OF CONSTRAINTS ON ADAPTATION

Studies of sex allocation offer excellent opportunities for examining the constraints and limits on adaptation. A major topic of debate within this field concerns the extent to which the ability of individuals to adaptively manipulate their offspring sex ratio is determined by constraints such as the method of sex determination. We address this problem by comparing the extent of sex-ratio adjustment across taxa with different methods of sex determination, under the common selective scenario of interactions between relatives. These interactions comprise the following: local resource competition (LRC), local mate competition (LMC), and local resource enhancement (LRE). We found that: (1) species with supposedly constraining methods of sex determination showed consistent sex-ratio adjustment in the predicted direction; (2) vertebrates with chromosomal sex determination (CSD) showed less adjustment then haplodiploid invertebrates; (3) invertebrates with possibly constraining sex-determination mechanisms (CSD and pseudo-arrhenotoky) did not show less adjustment then haplodiploid invertebrates; (4) greater sex-ratio adjustment was seen in response to LRC and LMC than LRE; (5) greater sex-ratio adjustment was seen in response to interactions between relatives (LRC, LMC, and LRE) compared to responses to other environmental factors. Our results also illustrate the problem that sex-determination mechanism and selective pressure are confounded across taxa because vertebrates with CSD are influenced primarily by LRE whereas invertebrates are influenced by LRC and LMC. Overall, our analyses suggest that sex-allocation theory needs to consider simultaneously the influence of variable selection pressures and variable constraints when applying general theory to specific cases.

Brood sex ratio determination by flow cytometry in ants

AbstractSex ratio variations during brood development have important implications for the study of sex allocation in haplodiploid insects. So far, few studies have addressed this question because of the difficulty to determine the sex of the brood. We used flow cytometry to differentiate haploid males from diploid females in the ant Linepithema humile. Our data show that flow cytometry can be used successfully to distinguish between male and female brood on the basis of their DNA content, from the very first larval stage. Moreover, we show that flow cytometry allows sex brood determination in other ant species, as well as in nonsocial Hymenoptera.

Adaptive sex allocation in birds: the complexities of linking theory and practice.

We review some recent theoretical and empirical developments in the study of sex allocation in birds. The advent of reliable molecular sexing techniques has led to a sharp increase in the number of studies that report biased offspring sex ratios in birds. However, compelling evidence for adaptive sex allocation in birds is still very scant. We argue that there are two reasons for this: (i) standard sex allocation models, very helpful in understanding sex allocation of invertebrates, do not sufficiently take the complexities of bird life histories and physiology into account. Recent theoretical work might bring us a step closer to more realistic models; (ii) experimental field and laboratory studies on sex allocation in birds are scarce. Recent experimental work both in the laboratory and in the field shows that this is a promising approach.

Resource abundance and sex allocation by queen and workers in the harvester ant, Messor pergandei

The degree to which queens and workers control how limiting resources are split between reproductive males and females is central to the study of sex allocation in the eusocial Hymenoptera. We investigated the effect of resource availability on sex allocation decisions by both queens and workers in the ant Messor pergandei. We conducted the following field manipulations of resource availability: food supplementation while queens were laying reproductive brood (early-fed treatment), removal of workers while queens were laying reproductive brood (worker-removal treatment), food supplementation of colonies while workers were tending reproductive brood (late-fed treatment), and unmanipulated colonies (control). Early-fed colonies produced more alates and exhibited more strongly female-biased sex ratios than other treatments. Worker-removal colonies produced the fewest alates and the least female-biased sex ratios. Late-fed colonies yielded individual alates with the heaviest fresh masses (males and females) and dry masses (only females). Aside from worker-removal colonies, the sex and investment ratios of colonies in this study were significantly more female-biased than the relatedness asymmetries hypothesis under worker control would predict. Consistent with the multifaceted parental investment hypothesis, the timing of food supplementation relative to the reproductive cycle of the colony plays a prominent role in influencing sex allocation by both queens and workers. Early food supplements resulted in increased number of females, whereas late food supplements resulted in heavier individual females. Temporal dynamics of food availability may explain part of the tremendous inter-population variation in colony sex ratios seen in this and other ant species. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00265-002-0462-6.

Size matters: competition between male and female great tit offspring.

1. Studies of sex allocation in birds have traditionally centred on Fisher's (1930) theory of equal parental investment in male and female offspring. They concentrate particularly on sexually dimorphic species, where costs of rearing offspring are assumed to vary between male and female young because of body size differences. 2. Higher mortality of the larger sex (males), particularly in poorer conditions, is expected to result in female-biased sex ratios in the great tit, Parus major. Several studies have found the contrary, reporting male-biased fledging and recruiting sex ratios when conditions are poor. One reason why this may be the case is that males can gain more food resources than their sisters because of the competitive advantage afforded by their larger size. They may thus suffer less mortality in the nest or fledge in better condition, thereby enhancing their survival prospects. 3. This study investigates the importance of size in competitive interactions between nestlings of different sexes. A cross-fostering design was employed to create broods of mixed size through swapping half a brood of 2-day-old 'small' and half a brood of 4-day-old 'large' nestlings. Nestling morphometrics and mortality were measured during the nestling period. Nestlings were sexed by PCR amplification of sex-linked genes. To test for a male advantage in competitive environments, size and mortality measures were compared between 'small' males and females, and 'large' males and females (i.e. the interaction term 'size treatment' and 'sex'). 4. There was greater sexual dimorphism between small nestlings than large nestlings at fledging. This is interpreted as revealing enhanced competitive ability of male offspring under stressful conditions. Offspring from the 'large' group suffered lower mortality, but there was no difference in mortality according to sex, and no significant size*sex interaction. Similarly, no difference in recruitment was found, although this may be due to the small overall proportion of birds recruiting (3·1%). 5. The study suggests that male biased fledging/recruitment sex ratios in great tits may be explained by enhanced competitive ability of male nestlings in poor rearing environments.

Sex ratio variation as a function of host size in Pseudacteon flies (Diptera: Phoridae), parasitoids of Solenopsis fire ants (Hymenoptera: Formicidae)

Some Pseudacteon (Diptera: Phoridae) flies are parasitoids of Solenopsis (Hymenoptera: Formicidae) ant workers in North and South America. Laboratory studies of sex allocation revealed a pattern of sex ratio variation as a function of host size, with more females arising from larger hosts. Environmental sex determination is a possible mechanism for the observed pattern, and examination of Pseudacteon life history reveals several traits assumed to be important in models predicting conditions under which environmental sex determination is favoured. Sex allocation patterns of Pseudacteon are compared with theoretical predictions and empirical data from better-studied hymenoptcran parasitoids that have haplodiploid sex determination. The pattern of sex ratio variation observed has important implications for biocontrol efforts of imported Solenopsis fire ants by the introduction of Pseudacteon parasitoids.

Sex ratio variation as a function of host size inPseudacteonflies (Diptera: Phoridae), parasitoids ofSolenopsisfire ants (Hymenoptera: Formicidae)

SomePseudacteon(Diptera: Phoridae) flies are parasitoids ofSolenopsis(Hymenoptera: Formicidae) ant workers in North and South America. Laboratory studies of sex allocation revealed a pattern of sex ratio variation as a function of host size, with more females arising from larger hosts. Environmental sex determination is a possible mechanism for the observed pattern, and examination ofPseudacteonlife history reveals several traits assumed to be important in models predicting conditions under which environmental sex determination is favoured. Sex allocation patterns ofPseudacteonare compared with theoretical predictions and empirical data from better-studied hymenopteran parasitoids that have haplodiploid sex determination. The pattern of sex ratio variation observed has important implications for biocontrol efforts of importedSolenopsisfire ants by the introduction ofPseudacteonparasitoids.

Studies of Sex Allocation in Birds and Molecular Tools for Sex Identification

The recent development of molecular methods for sex identification makes large-scale studies of sex allocation in birds possible. This review identifies three types of sex allocation studies of birds. These relate either to the population primary sex ratio, the facultative manipulation of the sex ratio, or to sex-skewed provisioning. Many recent studies have suggested that facultative sex ratio manipulation may be more widespread among birds than has previously been considered. In contrast, only a few studies have been made of the population primary sex ratio, and of sexskewed provisioning. Despite all these studies, the mechanism for adjusting the sex ratio remains a mystery. In particular, estimating the cost of making sex ratio adjustments is indispensable for quantitative predictions of sex allocation. The relationships between the three aspects outlined above have not yet been sufficiently clarified. In both Budgerigars Melopsittacus undulatus and Great Reed Warblers Acrocephalus arundinaceus, fathers provide extra-feeding for a certain sex of offspring, while mothers manipulate the primary sex ratio. I suggest that the distinction between the sexes in terms of providing extra-feeding or manipulating the sex ratio, may in fact affect the population primary sex ratio and cause a bias towards the sex for which fathers provide extra-feeding.

Sex discrimination before birth

Sex-allocation theory describes how parents should bias investment in sons and daughters when male and female offspring give different fitness returns[ 1 Charnov, E.L. (1982) The Theory of Sex Allocation, Princeton University Press Google Scholar ]. Often, studies of sex allocation are based on species where a mechanism influencing sex allocation is known to exist, for example haplodipoidy in some insects. Among birds, unusual physiological or genetic mechanisms of sex determination are not known; however, accumulating studies attribute skews to primary sex-ratio adjustment[ 2 Komdeur J et al. Extreme adaptive modification in sex ratio of the Seychelles warblers' eggs. Nature. 1997; 385: 522-525 Crossref Scopus (387) Google Scholar , 3 Ellegren H Gustafsson L Sheldon B.C Sex ratio adjustment in relation to paternal attractiveness in a wild bird population. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 11723-11728 Crossref PubMed Scopus (267) Google Scholar ].

Sequential Patterns of Sex Allocation in Simultaneous Hermaphrodites: Do We Need Models That Specifically Incorporate This Complexity?

Theoretical and empirical studies of sex allocation usually treat sequential and simultaneous hermaphroditism as distinct and disparate forms of allocation. However, the sexual patterns of numerous species have both sequential (e.g., size-based) and simultaneous components. In most cases, we have drawn from sex allocation theory developed for sequential hermaphrodites to explain ontogenetic changes in allocation and from theory developed for simultaneous hermaphrodites to explain the remaining aspects of these sexual patterns rather than develop a more integrated theory. Here I present the evolutionary stable solution (ESS) to a dynamic statevariable model that explicitly combines the effects of size and simultaneous allocation to male and female function in a dynamic game. The model structure and initial parameter values are based on the sexual pattern of the blue-banded goby, Lythrypnus dalli, a simultaneous hermaphrodite. I then compare the natural patterns of sex allocation in L. dalli with the predictions of the model and with those of a dynamic version of the size advantage model. The integrated model predicted variation in allocation, sex-specific size distributions, and seasonal sex ratio better than the sequential hermaphroditism model did. Indeed, the sequential model, using L. dalli parameter values, predicts a dioecious rather than sequentially hermaphroditic allocation pattern. The comparison of these two models illustrates the disadvantage of drawing from two bodies of theory without a formal integrated framework. Furthermore, the comparison focuses attention on the role of costs of reallocation in the evolution of mixed (or intermediate) sexual patterns.

Sex allocation in animals.

There are numerous reasons why the study of sex allocation has attracted a great deal of both theoretical and empirical attention. To begin with, sex allocation is a ubiquitous phenomenon. All sexually reproducing animals exhibit some apportionment of reproductive effort between sexual functions. Sex ratios in organisms with separate sexes (gonochores), relative investment in sexual functions in simultaneous hermaphrodites, and age or size at which individuals change from one sexual function to another in sequential hermaphrodites are all forms that apportionment may take.KeywordsSimultaneous HermaphroditeFemale BiasLocal Mate CompetitionBluehead WrasseSimultaneous HermaphroditismThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Sex allocation in functionally hermaphroditic plants: A review and critique

A number of theoretical and methodological problems plague studies of sex allocation in hermaphroditic plants. Most models assume that consumable resources limit reproduction, yet one or both sexes may be mate-limited. Most models also assume that resource limitation causes trade-offs between the allocations to male and female functions, but the sexes may be limited by different critical resources, or their resource needs may not overlap in time. Problems arise in deciding which plant parts are reproductive, when to harvest them, how to apportion them to male and female roles, and what are the appropriate measures of their cost. When energy directly limits reproduction, or other needs can be translated into energetic costs, the great variation in composition of reproductive parts makes construction costs or respiration a more potentially accurate measure of investment than energy content or especially biomass.

Sex allocation in animals.

There are numerous reasons why the study of sex allocation has attracted a great deal of both theoretical and empirical attention. To begin with, sex allocation is a ubiquitous phenomenon. All sexually reproducing animals exhibit some apportionment of reproductive effort between sexual functions. Sex ratios in organisms with separate sexes (gonochores), relative investment in sexual functions in simultaneous hermaphrodites, and age or size at which individuals change from one sexual function to another in sequential hermaphrodites are all forms that apportionment may take.