Passage VI

Threespine sticklebacks are fish that can live in marine (saltwater) and freshwater environments. Marine sticklebacks and freshwater sticklebacks have plates (bony scales) that provide protection from predators, such as fish and birds. However, due to the low number of these predators in freshwater environments, freshwater sticklebacks typically have fewer plates than do marine sticklebacks.

The Eda gene regulates plate formation and has 2 alleles: E and e. Table 1 describes the phenotype associated with each Eda genotype.

Two studies were done to determine how moving a marine stickleback population to a freshwater environment affects the Eda genotypes of the population's offspring and how Eda genotype relates to the length and the breeding status of the offspring.

Study 1

Two hundred adult marine sticklebacks with the Eda genotype Ee were tagged and placed in an artificial freshwater pond on June 1, 2006, during their breeding season. At the beginning of each month from September 2006 to July 2007, 200 stickleback offspring were collected and their Eda genotypes were determined. Figure 1 shows the fraction of the collected offspring (FCO) with a particular genotype for each month, as well as the developmental stages of the offspring during the study.

Study 2

The length and the breeding status—sexually immature (SI) or sexually mature (SM)—of each of the offspring collected in May 2007 were determined. Figure 2 shows the results by Eda genotype.