The loss of pelvic spines in freshwater populations of stickleback is due to?

Question

BlackTom AI · Accepted Answer

Deletion of a specific enhancer.

Relevant concepts: developmental genes like Pitx1 control body-part formation; enhancers are cis-regulatory DNA elements that drive tissue-specific expression; cis-regulatory mutations can change when and where a gene is expressed without altering the coding sequence. Pelvic spine formation in sticklebacks is controlled by Pitx1 expression in the developing pelvic region, driven by a pelvic-specific enhancer.

Step-by-step reasoning:
1. Phenotype observation: many freshwater stickleback populations lack pelvic spines while marine populations have them.
2. Genetic mapping located the trait to the Pitx1 locus. This implicated Pitx1 in pelvic development.
3. Researchers compared coding sequence versus regulatory regions. The Pitx1 coding region was intact and produced functional protein in other tissues (for example head structures).
4. Detailed molecular analysis found a deletion of a pelvic-specific enhancer (often called the Pel enhancer) upstream of Pitx1 in pelvic-reduced populations. This deletion abolishes Pitx1 expression specifically in the pelvic region.
5. Transgenic experiments: introducing the marine pelvic enhancer (or restoring enhancer function) into pelvic-reduced fish restores pelvic Pitx1 expression and rescues spine formation. This demonstrates causation: loss of the enhancer -> loss of pelvic Pitx1 expression -> loss of pelvic spines.

Why the correct answer is correct: The causal change is regulatory (loss of a tissue-specific enhancer) rather than a change in the Pitx1 protein. This explains why Pitx1 functions normally in other tissues while pelvic expression is lost.

Why other options are incorrect:
- Deletion of the Pitx1 gene would remove Pitx1 function broadly, causing pleiotropic defects; this is not observed.
- Missense mutations in Pitx1 would alter protein function in all tissues and produce broader phenotypes, which is not supported by experiments.
- Altered RNA splicing has not been observed; the molecular defect is a cis-regulatory deletion, not splicing change.

Conclusion: The loss of pelvic spines in freshwater sticklebacks is due to deletion of a specific enhancer (the correct answer).

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