Fertilization selection on egg and jelly-coat size in the sand dollar Dendraster excentricus

Area of Science:

• Reproductive biology
• Evolutionary ecology
• Marine biology

Background:

• External fertilization in echinoids often faces sperm limitation.
• Larger egg targets are hypothesized to increase fertilization probability due to more frequent sperm-egg collisions.

Purpose of the Study:

• To investigate the contribution of egg cell size and jelly coat size to fertilization success in sand dollars.
• To quantify selection gradients on these two components of egg target size.

Main Methods:

• Cross-sectional analysis of correlated characters in Dendraster excentricus.
• Estimation of selection gradients on egg cell and jelly coat size in five replicate male pairs.

Main Results:

• Eggs with larger cells and jelly coats were preferentially fertilized under sperm limitation.
• Selection gradients were significantly steeper for egg cell size (average 922%) compared to jelly coat size.
• Standardized selection gradients for both components were similar, but increased egg cell volume more effectively enhanced fertilization success.

Conclusions:

• Fertilization selection acts on both egg cell and jelly coat size in echinoids.
• Egg cell volume is a more critical determinant of fertilization success than jelly coat volume.
• Replication is crucial in studies of selection on correlated traits.