My research
theoretical
I am developing a life-history / energy allocation model based on the life cycle of a unicellular volvocine alga, Chlamydomonas reinhardtii. The model provides a framework for understanding the physiological mechanisms underlying the cost of reproduction and for understanding morphological changes in terms of life-history and energy allocation strategies. It predicts how colony morphology will change (e.g. cell size, cell number, volume of extracellular matrix) in response to selection on colony size. Measurements of the sizes and growth rates of colonies will provide parameter estimates for the model and test whether it accurately simulates colony development.
empirical
Two trends are apparent in the
relationship between
size and level of
specialization in volvocine algae: larger colonies have higher levels
of specialization, and,
within the germ-soma specialized forms, larger species have higher
proportions of
somatic cells. Trends among
species suggest that trade-offs between motility and
fecundity have been involved in the evolution of terminally
differentiated somatic cells. However, trends among species can differ
substantially from those within
species. If the trends observed among volvocine species reflect
selective
pressures within species imposed by trade-offs between motility and
fecundity, these trade-offs should also be measurable within species. I am using artificial selection experiments
to
address the following questions: (1)
Does reproduction impose a cost on volvocine colonies in the form of
reduced
motility? (2) Does the cost of
reproduction increase with increasing colony size? (3)
Do colonies change their investment in soma in response to
changes in size? (4) Does the
response to change in size differ in environments in which motility
differs in
importance? (5) What
constraints restrict the response to selective pressures on size and
motility? To increase the
genetic variability in colony size to a point sufficient to measure
changes in the cost of reproduction, colonies of Pleodorina starrii are being selected for increasing and decreasing colony 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.
comparative
The transition from unicellular to differentiated multicellular organisms constitutes an increase in the level complexity, because previously existing individuals are combined to form a new, higher-level individual. The volvocine algae represent a unique opportunity to study this transition because they diverged relatively recently from unicellular relatives and because extant species display a range of intermediate grades between unicellular and multicellular, with functional specialization of cells. Following the approach Darwin used to understand “organs of extreme perfection” such as the vertebrate eye, this jump in complexity can be reduced to a series of small steps that cumulatively describe a gradual transition between the two levels. I used phylogenetic reconstructions of ancestral character states to trace the evolution of steps involved in this transition in volvocine algae. The history of these characters includes several well-supported instances of multiple origins and reversals. The inferred changes can be understood as components of cooperation-conflict-conflict mediation cycles as predicted by multilevel selection theory. One such cycle may have taken place early in volvocine evolution, leading to the highly integrated colonies seen in extant volvocine algae. A second cycle, in which the defection of somatic cells must be prevented, may still be in progress.
old stuff
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