Application of a Gene-Based Population Dynamics Model to the Optimal Egg Size Problem: Why Do Bivalve Planktotrophic Eggs Vary in Size?

Abstract

The presumption is that egg quality influences larval survival and that egg size influences egg quality. Thus, larger eggs should be favored by selection. Counterweighing the tendency for egg size to increase is the number of eggs that can be produced if egg size remains small. We examine how egg size and egg number counterbalance in Crassostrea oysters, resulting in an average egg size near 50 mm. Simulations imposing a diversity of ranges in larval survivorship-from little advantage for large eggs relative to small eggs to a great advantage-yield some anticipated outcomes in which genotypes generating larger eggs are favored. In other simulations, however, genotypes generating smaller eggs became increasingly common. In these cases, egg size declines, as does the likelihood of survival of individual larvae: the antithesis of expectation. Few simulations identify preferred egg sizes near the size typically observed, suggesting that, under most field conditions, a selective advantage exists for smaller or larger eggs than those typically spawned. However, the extremes in egg size are rarely advantageous. Most simulations resolve an optimal intermediate egg size. Thus, observed egg size is a balance between the chanciness of larval survival enhanced by the production of a larger number of eggs and the genetically predisposed, but environmentally modulated, individual probability of larval survival that is a function of egg size, with environment determining the optimal size. The 50-mm size observed likely represents the median outcome of a range of larval survivorship probabilities, each selecting for relatively larger or smaller eggs, imposed stochastically over multiple generations. In this scenario, each year the population is pulled toward smaller or larger egg sizes, but in the next year the impetus is independent of the previous year. Reduced generation time, by disease or fishing, modifies the extent, but not the direction of trend. Thus, environmental stochasticity retains preeminence in stabilizing a balance between the probabilities of survival modulated by egg number and by egg size. The influence of shortened generation time-by disease, for example-is unlikely to be manifest in a modification in egg size and hence egg number.

A modelling study of the role of marine protected areas in metapopulation genetic connectivity in Delaware Bay oysters

“…The Dynamic Population Genetics Engine (DyPoGEn) (Powell et al ., , ; Munroe et al ., ) is a numerical model that simulates the genetic structure and population dynamics for a metapopulation. The model was parameterized to simulate a metapopulation containing four connected populations of eastern oysters ( Crassostrea virginica ) in Delaware Bay (Figure ) for the decade of the 1970s and the decade of the 2000s.…”

“…Additional details of the single population model structure and formulation, on which the metapopulation model is based, are given in Powell et al . (, ). The model processes relevant to the neutral allele behaviour used in this study, encompassing specifications for the processes of growth, reproduction, and mortality, are described below (see also Munroe et al ., ) and shown schematically in Figure .…”

Meiotic and Early Zygotic Development in Crassostrea virginica Observed through Confocal Microscopy

“…On average between the cohorts, the diameter of eggs from diploid Crassostrea virginica was 48.2 mm. The egg size differing significantly from each other (female from first cohort versus female from second) was surprising because the egg size of C. virginica is highly regulated, undergoing constant balancing selection between fecundity and nourishment (Powell et al 2011). At the same time, eggs from the two cohorts came from two different highly selected lines and the effect of domestication on the egg size is unknown.…”

Ontogeny of bivalve immunity: assessing the potential of next‐generation sequencing techniques

“…Dynamic equilibrium is essential for organism maintenance (growth and survival; Pettersen et al 2010;Powell et al 2011), assimilation of food resources, allocation of energy reserves (metabolic mechanisms, dynamic energy budgets), stress tolerance and disease resistance (defence mechanisms and local adaptation; Genard et al 2011;Samain 2011). To make advancements in the field of bivalve immunology, we need to improve our understanding of the factors that influence and stimulate stress responses (Fitzgerald-Dehoog et al 2012;Guerra et al 2012).…”

“…Success of the immune defence in early larvae and post-larvae is highly dependent on energy sources (nutrients; Marshall et al 2010), adequate energy allocation for growth (development; Bayne 2004;Wang et al 2012a), the capability of dynamic adaptation (selective pressures; Powell et al 2011;Barros et al 2013), and the ability to initiate defence and annihilate specific aggressors (cell management; Yue et al 2013b; Fig. 1).…”