Question

The loss of pelvic spines during adaptive evolution in sticklebacks is one of the classic examples in the field of evo-devo. What is the molecular basis of this phenotypic difference? And what important developmental biology concept is illustrated by this example?

Answer 1

Stickbacks or Gasterosteus aculeatus as the information given above suggests, serves as a classic example of the evolutionary-developmental biology or evo-devo. This means that there is a great deal of evidence implicating the evolutionary changes in the developmental biology of this fish. One such aspect is the study of the loss of pelvic spines. This loss of pelvic spine is a phenotype that has its roots in the genotype of this fish. It further means that there is a molecular aspect behind this observable phenotype in these fishes. A group lead by Michael D Shapiro in 2004 discovered that a gene termed Pitx1 has been involved in regulating the spine structure in these fishes. Their study also lead to the findings that this gene does not express a protein that can directly take a part in the spine formation, but they have implicated that it encodes for a protein that acts as a genetic switch. Yes you heard it correct, there are such things known as genetic switches, basically, these are the protein molecules that can easily bind with the regulatory regions of the DNA (you might have studied about the activators, enhancers or repressor DNA elements/sequences) and either turn it (a particular specific gene) or off for transcription (of coding RNA for protein or noncoding RNA for other purposes as per the gene). Pitx1 acts as one such gene switch whose product, a protein, binds at the regulatory regions of those protein-coding genes whose products (protein) would be actually required for the formation of the pelvic spine in the Stickbacks.

This activity of the Pitx1 to be acting as a genetic switch is an important developmental biology concept which is illustrated here. Because in conventional developmental biology, remember that an organism develops through complex changes in their organ developments and a lot of genes play important roles in organ pattern development (through their expression or supression) and any defect in their expression can further lead to abnormalities in the body. This Evo-devo relationship of the Pit1 gene regulation switch highlights that particular important concept.

Reference: Shapiro, Michael D., et al. "Genetic and developmental basis of evolutionary pelvic reduction in threespine sticklebacks." Nature 428.6984 (2004): 717-723.