Marine stickleback fish have pelvic fins with long spines that provide protection from larger predatory fish. Some stickleback fish were trapped in lakes and have adapted to life in a different environment. Many lake populations of stickleback fish lack pelvic fins. Shapiro et al. (2004) (Nature 428:717.723) mapped the mutation associated with the loss of pelvic fins to the Pitx1 locus, a gene expressed in pelvic fins, the pituitary gland, and the jaw. However, the coding sequence of the Pitx1 gene is identical in marine and lake stickleback [Chan et al. (2010). Science 327:5963,302–305]. Moreover, when the Pitx1 coding region is deleted, the fish die with defects in the pituitary gland and the jaw, and they lack pelvic fins. Explain how a mutation near, but outside of, the coding region of Pitx1 may cause a loss of pelvic fins without pleiotropic effects on the pituitary gland and jaw.

Key Concepts

Gene Regulation

Gene regulation refers to the mechanisms that control the expression of genes, determining when and how much of a gene product is made. In the case of the Pitx1 gene, mutations outside the coding region can affect regulatory elements, such as enhancers or silencers, which influence the gene's expression in specific tissues. This means that while the coding sequence remains unchanged, the gene's activity can be altered, leading to specific phenotypic changes like the loss of pelvic fins.

Pleiotropy

Pleiotropy occurs when a single gene influences multiple phenotypic traits. In the context of the Pitx1 gene, it is important to understand that while the gene is involved in the development of pelvic fins, the pituitary gland, and the jaw, a mutation affecting only the regulatory regions may selectively impact pelvic fin development without disrupting the other traits. This selective effect is crucial for explaining how the loss of pelvic fins can occur without pleiotropic consequences.

Mutations and Phenotypic Variation

Mutations are changes in the DNA sequence that can lead to variations in traits among individuals. In stickleback fish, mutations near the Pitx1 gene can lead to changes in how the gene is expressed, resulting in the absence of pelvic fins. Understanding the distinction between coding and non-coding regions of DNA is essential, as mutations in non-coding regions can lead to significant phenotypic changes while preserving the function of the coding sequence, thus avoiding detrimental effects on other traits.