There is widespread evidence that egg size has evolved through offspring and maternal fitness demands. Larger eggs typically bestow more nutrients, a larger birth weight and increased chance of survival on the offspring. Egg size is balanced with egg number to maximize maternal reproductive success. However, a new player has recently been added to the roster: sperm limitation, which occurs when there is an insufficient quantity of sperm to fertilize all of the available eggs. It has been suggested that in externally fertilizing animals, sperm limitation could also act as a selective force influencing egg size.
Externally fertilizing animals, such as a sea urchins or fish, release their sperm and eggs into the surrounding environment with the hope that a sperm will bump into an egg and successfully fertilize it. It has recently been shown in three species of sea urchin that larger eggs need lower sperm concentrations in order to be successfully fertilized. Christopher MacFarlane from the University of East Anglia in the UK along with colleagues from Brandon University and the University of British Columbia in Canada sought to determine whether the same could be said for sockeye salmon, a fish with substantial natural variation in egg size where males and females spawn in much closer association compared with broadcast spawning marine invertebrates like sea urchins.
In order to test whether larger sockeye salmon eggs were preferentially fertilized under conditions of sperm limitation, the authors collected sperm from 20 males and pooled eggs from 15 females. Dividing the pooled eggs into groups, the team added just enough sperm to each group of eggs to ensure fertilization success rates ranging from 20% to 80%. After allowing the eggs to incubate for 10 days, they measured the surface area of the fertilized and unfertilized eggs. Finally, the authors also included two treatments where all the eggs or none of the eggs were fertilized to account for possible changes in the size of the egg over the course of time or in response to fertilization. This also allowed the team to test whether there is a relationship between egg size and fertility when a surfeit of sperm is available.
The team found that under sperm-limitation conditions, the eggs that were fertilized successfully had a significantly larger surface area (by 7%) than the unfertilized eggs, suggesting that increased egg size could evolve under conditions of sperm limitation.
This study by MacFarlane and colleagues demonstrates that in addition to offspring and maternal fitness demands, egg size could also be influenced by sperm limitation in salmon. The next step in this line of research is to assess the importance of sperm limitation in fish in a natural setting. Though salmonids are generally considered to have high fertility in the environment, other species may not be so lucky. One thing seems to emerge clearly from this study: it's easier to hit a bigger target!
