ABSTRACT PI: ZUCKER and WEEKS PROPOSAL NUMBERS: 9631042/9614226 Reproductive tactics in a mixed mating system. The widespread presence of males in most populations has been intriguing evolutionary biologists for decades, since there are many potential disadvantages in requiring two individuals to come together in order to reproduce. First there is the time and energy needed to locate and identify an individual of the same species but opposite sex. Also, natural selection favors any activity that results in more of an individual s genes getting into the next generation. Thus, if an individual develops a means of producing offspring without having to share half of its genes with a partner, that individual immediately adds twice as many of its own genes to the next generation. Some plants and animals have, in fact, evolved means to reproduce with a single parent, either by parthenogenesis (direct development of offspring from an unfertilized egg) or by producing both eggs and sperm within a single individual (called a hermaphrodite) and fertilizing their eggs with their own sperm. Why, evolutionary biologists have asked, do the overwhelming number of organisms still reproduce with two parents? Several possible mathematical solutions (called models ) have been suggested by theoreticians, but few of them have been experimentally tested, especially in animal systems. We will test one of the proposed models by utilizing an animal species with a recently discovered unusual mating system in which some individuals are hermaphrodites and others are males. No females are found. Hermaphrodites can fertilize their eggs with their own sperm or have their eggs fertilized by males. That is, they can reproduce with or without a partner. Hermaphrodites can not, however, fertilize the eggs of other hermaphrodites. This unusual animal is the clam shrimp, Eulimnadia texana , a small crustacean found in te mporary pools throughout the southwestern U.S. Males are found in lower proportions than hermaphrodites but appear to be present in almost all populations. This is called a mixed mating system since reproduction can occur by the typical means of two individuals mating (male with a hermaphrodite) or by a single individual (a hermaphrodite) fertilizing its own eggs. Early models of mating system evolution, however, predicted that only one or the other type of mating system would evolve and be maintained over long periods of time, not both. Nevertheless, more recent models have suggested that a mixed system, in which both forms of reproduction are present, could, under appropriate conditions, be stable over long periods of time. It is our goal to experimentally test these more recent models to document whether these conditions hold in these shrimp. By studying an animal with a mixed mating system we can learn what the advantages and the disadvantages are for individuals that reproduce by themselves and for those that reproduce with a partner. Recently, Otto, et al. (1993) developed a mathematical model aimed at explaining why the mixed mating system of E. texana appears to be stable over a long period of time. In order to test the validity of the model, it is necessary to learn more about the natural history and genetics of this species. In particular we need to collect data on four aspects of the biology of clam shrimp that are required by the model. We have thus decided to begin our exploration of the stable mixed mating system of Eulimnadia texana by quantifying those four aspects of the biology of clam shrimp. A combination of behavioral observations/manipulations and genetic crossings will be used to learn about these four aspects of the biology of clam shrimp under a range of semi- natural and controlled environmental conditions. These behavioral and genetic data can then be used to determine how accurate the Otto, et al.(1993) model is and/o r how it will be necessary to modify the model to produce a truer picture of the evolution of this mixed mating system. By studying the life history, genetics and reproductive biology of an animal with a mixed mating system, we will gain a better understanding of how a few species have managed to persist in the absence of males and, more importantly, why most species have maintained a reproductive system requiring two parents, in spite of the costs of doing so.
