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Potential Project Description for 2008-09

 

Title: Comparing two scaly pearl oyster populations: Are differences in life history strategies evolutionary or physiological responses to environmental stress?
Faculty: Eric Cole, Biology

Life history demographics are mathematically complex, and challenging to model. In our study organism Pinctada longisquamosa, the scaly pearl oyster juveniles begin life in a planktonic form and only 0.1 mm in diameter. Within the first month of life they metamorphose into sessile bivalves, attached to the substrate with tough fibers called byssal threads. Sometime during their first year of growth, they achieve a critical mass, and differentiate sexually. Typically, all juvenile Pinctada differentiate as males. During their second year of growth (again approx-imately), these oysters achieve a second critical mass, and half re-differentiate into females. Theory tells us that this strategy allows even very small oysters to begin competing for sexual reproduction in that sperm are metabolically inexpensive and can be produced low-cost, by even very tiny individuals, whereas eggs are relatively costly to produce given their substantial stores of yolk and other macromolecules. Hence, a strategy of protandrous hermaphroditism (male-first followed by sex-change), makes physiological and adaptive sense. Curiously, we have come upon two populations of Pinctada that exhibit different life-history-trajectories. In a protected population, we see the classic protandrous life history. In a neighboring population that is uniquely exposed to repetitive annihilation by hurricane activity, the oysters have abandoned protandry for a dioecious life history (they differentiate both male female individuals immediately upon maturation). This may represent an evolutionary shift in strategy driven by annihilation of the adult (female-biased) population coupled with hurricane driven spawning events. As juveniles settle and mature following a storm, there are few females to breed with (adults having been largely exterminated). Hence, any mutation that favors precocious feminization would be favored in a male-biased population.

To properly model and test this hypothesis (the “hurricane hypothesis”), we should like to pursue quantitative approaches to both ecological and evolutionary parameters. Some possible approaches:

Ecology
1) establish growth curves, modeling if possible, the oyster’s growth dynamics. This will allow us to assign ages to oysters of given dimensions, and more accurately assess the timing of maturation and sex-change, and assess the age at which egg formation can be sustained.

2) evaluate the quality and quantity of food in their natural habitats using fluorimetry, and oxygen consumption of organisms in the water column,

3) quantify feeding rates by following the rate at which algae are cleared from a given volume of water,

4) assess the possibility that environmental estrogens may be playing a larger role than the hurricane phenomena in altering sexual differentiation within the different ponds.

Evolution

1) estimate population sizes (the effective breeding population) for each pond,

2) mathematically model the tipping point at which alleles favoring protandry should be balanced against alleles favoring dioecy, and assess the predictions against real measurements in terms of frequency of gender assignment at maturation.

3) do allele assessment to determine the amount of genetic diversity both within the populations and between populations.