Collaborative Research: Evolution of Physical and Chemical Attributes of Gametes in Free Spawning Marine Invertebrates: A Functional and Phylogenetic Approach

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The goal of this project is to examine the evolutionary significance of gamete traits that influence fertilization success in free-spawning marine organisms. Basic research in this field is crucial for understanding the reproductive ecology of harvested invertebrates, for aquaculture and for conservation biological reasons. Recent work on the fertilization ecology of free-spawners has raised diverse questions regarding character evolution at the gamete stage of the life cycle. Theory and empirical studies suggest that the size of eggs and egg coats, the density of sperm receptor sites, the speed and longevity of sperm, and the action of sperm-activating compounds released by eggs can each influence fertilization success by affecting rates of sperm-egg contact. These studies, however, have not resolved whether particular traits are a product of selection on fertilization success, and whether such traits appear to co-evolve as a result of their functional interactions. This project will involve a set of at least 9 sympatric species of marine brittlestars in the genus Macrophiothrix (Echinodermata: Ophiuroidea) from coral reef ecosystems. This group includes at least 40-fold variation in egg volume and provides an ideal comparative system for assessing the importance of size and other gamete attributes in sperm-egg collision. Interest in this genus is especially motivated by our discovery of an unusual degree of species-specificity in sperm chemotaxis, which offers a unique opportunity to examine the role of sperm chemotaxis in promoting reproductive isolation. The research will integrate phylogenetic information, tests of fertilization performance, and measures of gamete attributes to address fundamental questions about the evolution and co-evolution of gamete traits. In order to test hypotheses about gamete function in a phylogenetic context, this study has the following four objectives: OBJECTIVE 1: To construct a phylogeny for the study species using molecular markers. Phylogenetic information is needed to establish historical patterns of character change, as well as to control for correlation among traits due to common ancestry. OBJECTIVE 2: To examine the importance of egg size and other physical gamete attributes in fertilization. In vitro fertilization assays and variation in gamete traits will be used to test the hypothesis that egg target size and sperm swimming speed contribute to interspecific variation in fertilization success. OBJECTIVE 3: To examine the role of spermegg chemical attraction in promoting reproductive isolation. OBJECTIVE 4: To test hypotheses concerning patterns of covariance among gamete traits. Phylogenetic information will be used to test hypotheses of coevolution where theory predicts functional interactions between pairs of traits: ovum size and jelly coat size; egg target size and sperm speed; sperm speed and longevity; egg target size and the strength of sperm chemotaxis. This research is designed to fill critical gaps in our understanding of fertilization ecology and the evolution of gamete characters. This will be a comprehensive study in a phylogenetic context of both physical and chemical traits that can influence rates of gamete contact . By using phylogenetic information for a closely-related group of ecologically similar organisms, this research explicitly tests evolutionary hypotheses about functional relationships among such traits. Furthermore, the unique pattern of sperm chemotaxis in this system provides an opportunity to examine the role of pre-contact chemical specificity in promoting the reproductive isolation of synchronously-spawning species. Because free-spawning is the dominant and likely ancestral mode of fertilization, results from this project will have widespread relevance to the evolution of reproductive systems in marine environments.