Reproductive Tactics in a Mixed Mating System

PI: ZUCKER和WEEKS提案数：9631042/9614226混合交配系统中的繁殖策略。几十年来，进化生物学家一直对大多数种群中男性的普遍存在感兴趣，因为需要两个个体聚集在一起进行繁殖有许多潜在的缺点。首先，需要时间和精力来定位和识别同一物种但性别不同的个体。此外，自然选择倾向于任何能够使个体的更多基因进入下一代的活动。因此，如果一个个体发展出一种生育后代的方法，而不必与伴侣分享一半的基因，那么这个个体就会立即将自己的基因增加一倍给下一代。事实上，一些植物和动物已经进化出了单亲本繁殖的方式，要么是孤雌生殖（从未受精卵直接发育后代），要么是在一个个体内同时产生卵子和精子（称为雌雄同体），然后用自己的精子使卵子受精。进化生物学家问，为什么绝大多数生物仍然是双亲繁殖？理论家们提出了几种可能的数学解决方案（称为模型），但其中很少得到实验测试，特别是在动物系统中。我们将利用一种最近发现的不寻常的交配系统的动物物种来测试其中一个提出的模型，其中一些个体是雌雄同体，另一些是雄性。没有发现雌性。雌雄同体可以用自己的精子使卵子受精，也可以让雄性受精。也就是说，它们可以在没有伴侣的情况下繁殖。然而，雌雄同体不能使其他雌雄同体的卵受精。这种不寻常的动物是蛤虾，Eulimnadia texana，一种小型甲壳类动物，在美国西南部的临时水池中发现。雄性的比例低于雌雄同体，但似乎在几乎所有种群中都存在。这被称为混合交配系统，因为繁殖可以通过典型的两个个体交配（雄性与雌雄同体）或单个个体（雌雄同体）使自己的卵子受精来进行。然而，早期的交配系统进化模型预测，只有一种或另一种类型的交配系统会进化并维持很长一段时间，而不是两种都有。然而，最近的模型表明，在适当的条件下，存在两种繁殖形式的混合系统可以在很长一段时间内保持稳定。我们的目标是通过实验测试这些最新的模型，以证明这些条件是否适用于这些虾。通过研究具有混合交配系统的动物，我们可以了解单独繁殖的个体和与伴侣繁殖的个体的优势和劣势。最近，Otto等人（1993）开发了一个数学模型，旨在解释为什么得克萨斯河鼠的混合交配系统在很长一段时间内看起来是稳定的。为了检验该模型的有效性，有必要进一步了解该物种的自然史和遗传学。我们特别需要收集模型所需的蛤虾生物学的四个方面的数据。因此，我们决定通过对蛤虾生物学的这四个方面进行量化，来开始我们对德克萨斯蛤虾稳定的混合交配系统的探索。本文将采用行为观察/操作和遗传杂交相结合的方法来了解蛤虾在一系列半自然和受控环境条件下的这四个生物学方面。然后，这些行为和遗传数据可以用来确定Otto等人（1993）模型的准确性，以及/或如何修改模型以更真实地描述这种混合交配系统的进化。通过研究具有混合交配系统的动物的生活史、遗传学和生殖生物学，我们将更好地理解少数物种是如何在没有雄性的情况下生存下来的，更重要的是，为什么大多数物种能够维持一个需要双亲的生殖系统，尽管这样做需要付出代价。

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.