Pelvic skeleton reduction and Pitx1 expression in threespine stickleback populations

Area of Science:

• Evolutionary biology
• Developmental genetics

Background:

• The pelvic skeleton of threespine stickleback fish (Gasterosteus aculeatus) provides defense against predators.
• Rare populations exhibit vestigial pelvic phenotypes, associated with low ionic strength water and absence of predators.
• Lack of Pitx1 gene expression is the genetic basis for pelvic reduction in many populations.

Purpose of the Study:

• To investigate Pitx1 gene expression patterns in relation to pelvic skeleton phenotypes across various threespine stickleback populations.
• To understand the genetic mechanisms underlying pelvic reduction and its variations.

Main Methods:

• Whole-mount in situ hybridization was used to examine Pitx1 expression.
• 19 populations of Gasterosteus aculeatus from Cook Inlet, Alaska, were studied.
• Pelvic structures and Pitx1 expression were correlated.

Main Results:

• Populations with full pelvic structures typically expressed Pitx1 in the limb bud.
• Populations with left-biased pelvic reduction generally lacked Pitx1 expression.
• Varied Pitx1 expression was observed in populations with right-biased, unbiased, or absent pelvic spines.

Conclusions:

• Pitx1 is a key gene involved in threespine stickleback pelvic reduction, acting as a primary target for natural selection.
• While Pitx1 is the main factor, other genes with smaller effects also contribute to pelvic phenotype variation.
• The study supports the 'tinkering' metaphor of evolution, where existing genetic components are modified for new functions.