Distinct mechanisms decommission redundant enhancers to facilitate phenotypic evolution

Area of Science:

• Developmental biology
• Evolutionary genetics
• Molecular biology

Background:

• Morphological trait evolution often involves changes in gene regulation.
• Developmental genes frequently possess multiple redundant enhancers, ensuring robust expression.
• The mechanisms by which robust regulatory systems are dismantled for phenotypic transitions remain an area of investigation.

Purpose of the Study:

• To investigate the regulatory mechanisms underlying the evolutionary loss of larval trichomes in Drosophila sechellia.
• To understand how robust gene regulatory networks can be altered to permit significant phenotypic changes.

Main Methods:

• Analysis of the shavenbaby gene and its regulatory elements in Drosophila sechellia.
• Identification and characterization of mutations affecting embryonic enhancers.
• Comparative genomics and molecular analysis to determine inactivation mechanisms.

Main Results:

• The loss of larval trichomes in Drosophila sechellia was attributed to the independent inactivation of four embryonic enhancers of the shavenbaby gene.
• Each enhancer was inactivated through distinct mechanisms including deletion, alteration of cis-regulatory elements (loss of activators, gain of repressors), acquisition of silencers, and unmasking of preexisting repression.
• Three out of the four inactivation mechanisms involved repression, highlighting its role in rapid regulatory element loss.

Conclusions:

• Robustness in gene regulation provides multiple pathways for mutations to reduce enhancer activity.
• Repression is identified as a rapid route for the evolutionary loss of robust regulatory elements.
• Gene regulatory robustness facilitates, rather than prevents, morphological evolution by offering numerous opportunities for selection to act.