PARASITOID SEX RATIOS

Introduction

The fact that many animals produce equal numbers of sons and daughters has fascinated biologists for centuries. R.A. Fisher (1930) provided an evolutionary explanation for equal sex ratios that was based on the fact that all diploid sexual organisms have one mother and one father--i.e., the average mating success of males and females must be equal. This means that parents will realize the greatest fitness if they invest equally in sons and daughters. More specifically, fitness is maximized if the cumulative effort at the end of the period of parental care is equal for both sexes. When male and female offspring cost the same to produce, sex ratios will approximate unity. When one sex is more expensive to produce, equal investment will result in a biased sex ratio favoring the cheaper sex.

Fisher's arguments apply only to panmictic populations. When breeding is structured, either through differential mating successes or spatial organization, equal ratios may not lead to maximal fitness. Several authors have demonstrated that optimal sex ratios may depend on factors as diverse as the physiological status of a female, the social position of a female or her mates, and the population sex ratio that offspring will experience.

Populations that are spatially structured provide an interesting case in which optimal sex ratios may not be 1:1. Consider, for example, a large population that is subdivided into many mating groups composed of the offspring of one or more females. If sons typically mate with their sisters, who then disperse to found new groups, selection will favor parents who produce an excess of daughters. The actual ratio of daughters to sons will depend on the number of females who found each population. When single females always start populations, females should produce only enough sons to inseminate their daughters. As the number of additional foundresses increases, the optimal sex ratio approaches unity.

The timing of reproduction also affects optimal ratios. If females contribute their broods to the population sequentially, and do not know how many additional females will follow them in the sequences, then the optimal sex ratio for the first female will be strongly female biased, that for the next female will be less biased, that for the third even less. Finally, the relative contribution of each female to the total population size will be a determinant of optimal sex ratios. If a first foundress leaves a large brood with many daughters and few sons, and the second foundress leaves a very small brood, her optimal ratio will be 100% sons. These relationships make up the theory of local mate competition, first proposed by Hamilton (1967) and subsequently expanded and tested by Werren (1980, 1983). Various parasitic wasps make especially good example cases, since one or more females oviposit on a host, females are inseminated by their brothers, and fecund females disperse to start new populations. Under these conditions, the optimal sex ratio for an individual female depends on the size of her brood, and the probability that her sons will compete with the sons of other females. Furthermore, local mate competition theory predicts that females should be sensitive to the density of competing foundresses within a habitat. If wasp densities are very low, then the probability of joint founding is small, and initial foundresses may produce very few (1 or 2) sons per brood. As wasp densities increase, brood sex ratios should become more equitable.

The jewel wasp Nasonia vitripennis is an organism with which predictions of local mate competition theory can be tested. In this lab we will examine sex ratios in the broods of this wasp and evaluate the ability of individual females to optimize their brood sex ratios.

The Study Organisms

Female jewel wasps (see figure) oviposit in the pupae of various cycloraphous flies. Wasp development proceeds inside the fly puparium, eventually resulting in the death of the fly. At 29o C, average development time is about l4 days: 2 days as an egg, 6 days as a larva, and 6 days as a pupa. Adults emerge from the fly puparium through an exit hoLe. Males eclose first and compete for positions near the exit hole. Females mate as they emerge from the puparium. Males mate many times and, since they have vestigial wings, do not disperse. Females mate once and disperse to find new fly pupae. As in most hymenopterans, males are haploid (resulting from unfertilized eggs) and females are diploid. Internal structures appear to allow females to oviposit fertilized or unfertilized eggs, giving females control over the sex ratio of their broods. Superparasitism of fly pupae does occur in wild populations. The second female attacking a fly pupa can apparently detect the fact that it has already been parasitized and can vary her brood sex ratio in response. When only one female attacks a fly pupae she produces 5-25% sons. Second foundresses may produce up to 100% sons, depending on their brood size. When many foundresses attack single flies, total population sex ratios approach 50%.

Anaesthetized jewel wasps, Nasonia vitripennis. The female has a larger body, wider abdomen. and wings that are about as long as her total body. The male is much smaller, has a narrower abdomen, and has wings that are greatly reduced.

Methods

Choose one of the following experiments, and after discussion with other members of your group, decide on how many replicates to perform.

Experiment #1: Responses of second foundresses

For this analysis we will parallel the methods of Werren (1980), and take advantage of the fact that genetically homogeneous strains of jewel wasps are available. Jack Werren at the University of Rochester kindly sent us wild-type (LBII) and scarlet eye (ST) mutants to use for this lab. It is safe to assume that all adult females in a culture are mated. Females should be isolated from fly pupae and fed honey for 24 hours prior to use. Place a single wild type female in a petri dish with four large fly pupae (e.g., Sarcophaga). After 24 hours, remove the female and introduce a female scarlet eye. Allow her to superparasitize the pupae for 24 hours and remove her. Place the pupae in individual vials, and incubate at room temperature. Count the numbers and phenotypes of males and females emerging at eclosion. Repeat this procedure so that at least 50 pupae are parasitized. For completeness, replication should involve an equal number of trials in which wild-type females follow scarlet eyes.

Experiment #2: Consequences of population density

For this analysis we will parallel the methods of Werren (1983). Place 4 fly pupae in a petri dish and add 1, 2, 3, 4, 8, or 12 female wasps to the dish. Genotype is unimportant here. Replicate each wasp density at least 4 times. Incubate at room temperature and count the numbers of males and females that emerge at eclosion.

Analyses

In experiment #1 we are interested in comparing the responses of wasps with the predictions made by local mate competition theory. It is actually possible to predict the expected sex ratio using calculations outlined by Werren (1980). However, you can also test several hypotheses using chi-square tests. For example, do the sex ratios of the first and second female deviate from 1:1? Do they differ from each other?

In experiment #2 we are interested in the relationship between wasp density and sex ratio. Again, this can be analyzed graphically by plotting sex ratio as function of foundress density. You may want to examine the significance of this relationships using Spearman (nonparametric) correlation.

Interpretation

This set of experiments is designed to stimulate your thoughts about the factors that affect sex ratios. Think about the following questions when interpreting your analyses: Why don't jewel wasps produce equal numbers of sons and daughters? What makes jewel wasps different from those organisms, such as ourselves, that do produce equal numbers of each sex? What other organisms might be expected to show the same kinds of responses found in wasps (include plants)? How would you expect a wasp to respond to an array of oviposition sites ranging from small (like a housefly) to large (like a fleshfly)? How might a female decide whether or not another female has already used a fly pupa? How could you test this hypothesis? Would the first female to use a pupa want to hide the fact from other females? Was there variability between strains of wasps in their sex ratio? Why?

Suggested References