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Podolsky Lab Research

Fertilization ecology and egg size evolution in broadcast-spawning organisms

Fig. 1.  Eggs of Dendraster excentricus suspended in sumi ink, showing overall target size as increased by a substantial jelly coat  surrounding a fertilized (left) and unfertilized egg (right).

A major goal of life-history theory is to understand how ecological forces shape the optimal tradeoff between offspring size and number. We are interested in how variable conditions during broadcast spawning and planktonic development, the most common mode of marine reproduction, influence the evolution of egg traits and the optimal size-number balance.

To understand forces acting on the evolution of egg size, work in the lab addresses both pre-zygotic and post-zygotic consequences of changes in egg size.  In earlier work I found that a pre-zygotic benefit of large size--an increase in the rate of collision between spawned gametes--is neither necessary nor sufficient to drive the evolution of large ova.  One common feature of free spawned eggs, however, is a large and inexpensive accessory coat. Egg coats alter both chemical and physical properties (size and density) of a spawned egg that can influence sperm-egg contact.  Using a combination of fertilization experiments and quantitative models of fertilization kinetics, I am asking what role egg coats play in fertilization ecology and how they alter size-specific tradeoffs that govern resource allocation to individual offspring.

Using sperm as an agent of selection, sand dollar eggs with larger overall target sizes (Fig. 1) were favored in
Fig. x.  Sibling pluteus larvae of the sand dollar Dendraster excentricus derived from whole egg (left) and from half egg (right).  Photos by JD Allen.
fertilization.  An analysis of selection on the correlated characters that determine target size--ovum volume and jelly coat volume--found that fertilization imposes similar direct selection pressures on the two traits.  Under a broad range of conditions, an optimization model that tests the importance of different assumptions about egg coats (on target size, resource allocation, fertilization efficiency, sinking speed, and polyspermy) predicted small variation in ovum size and large variation in jelly coat size, matching variation seen in nature. This research is demonstrating that, by decoupling physical size and energetic cost, egg coats can buffer the evolution of egg size from heterogeneity in spawning conditions.

We are also testing hypotheses of post-zygotic benefits to large egg size.  In marine invertebrates egg energy content is often greater than needed for larval development, implying that fitness benefits must offset the fecundity cost of large egg size.  Experimental manipulations of egg size offer a powerful test of this hypothesis.  Jon Allen, a former graduate student in the lab and now a faculty member at the College of William and Mary, used a classic embryological technique--blastomere separation--to ask a series of ecological and life-history questions.  By separating blastomeres at the first two cleavage stages, we are able to generate whole, half, and quarter-size siblings using several echinoid species that together span a range of egg sizes and development modes.  By growing these size classes at different food rations and with different predator types, Jon measured the interactive
Fig. 3. Clypeaster rosaceus, new  juvenile.  Photo by JD Allen.
effects of initial egg size, degree of reduction, and feeding on larval growth and development as well as susceptibility to predation.

Christina Zakas, then an undergraduate honors student in the lab working with Jon, carried the analysis to the next life-history stage by asking whether the effects of egg size and larval feeding experience carry over into effects on juvenile growth and survival.  This analysis was done with Clypeaster rosaceus, a sea urchin with an intermediate size egg and an unusual mode of development known as facultative planktotrophy. Larvae of C. rosaceus can feed but do not require food to complete larval development.  Using blastomere separation, Christina generated egg sizes characteristic of species with obligate planktotrophy, which require feeding to complete larval development.  Results of these experiments indicate a stronger effect of initial egg size than of feeding on size at metamorphosis and juvenile growth and survival.  These results indicate how the egg, larval, and juvenile life history stages are functionally connected by different energetic strategies that fuel development using maternal nutrients vs. those acquired during larval feeding.

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