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Tuesday "Noon" Seminar: Oct. 10, 2006Matt Herron on "Evolution of soma in volvocine algae"
12:30-1:45 p.m. in Biosciences West (map of building location), Room 208 |
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Talk Abstract A universal feature of the familiar, macroscopic multicellular groups (plants, animals, fungi, red algae, brown algae, etc.) is cellular differentiation, the specialization of cells into functional types. One of the most basic distinctions between cell types is between the potentially immortal cells of the germ line and the mortal cells of the somatic tissues. Germ-soma (G-S) differentiation is an important threshold in the evolution of individuality and of fundamental importance to the process of evolution: somatic cells are an evolutionary dead end; only mutations affecting the germ cells have any chance of being passed on to future generations. The evolution of soma is a particularly challenging problem in evolutionary biology, as somatic cells (by definition) give up reproduction and, therefore, their direct fitness. The volvocine green algae are a model system for the origins of multicellularity and cellular differentiation. Members of this group display a diverse range of body sizes and degrees of differentiation, including single-celled forms and colonial forms with various degrees of cellular specialization. Species in three genera – Pleodorina, Astrephomene and Volvox – have partial or complete separation of reproductive and somatic functions. Two trends are apparent in the relationship between size and level of specialization: larger colonies have higher levels of specialization, and within the G-S specialized forms larger species have higher proportions of somatic cells. The purpose of this project is to test the hypothesis that the evolution of soma in volvocine algae was driven by trade-offs between viability and fecundity and by the increasing cost of reproduction in large colonies. Trends among species are consistent with this explanation, but trends among species can differ substantially from those within species. To measure trade-offs within a species, colonies of Pleodorina californica, a species of intermediate size and 30-50% somatic cells, will be artificially selected for increasing and decreasing body size. The goal of this selection is to create enough variability in body size to compare direct measurements of the cost of reproduction and determine whether or not this cost increases with size. By including environmental treatments in which motility is more or less important, I will test whether colonies are able to adjust their relative levels of investment in motility and fecundity. The cost of reproduction will be estimated in terms of its effect on motility, an important component of viability. The effect of colony size on this cost will be measured by comparing across colonies artificially selected for large and small size. |
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