Evolution of gene expression in the Drosophila olfactory system

Area of Science:

• Insect evolution
• Plant ecology
• Sensory systems biology

Background:

• Phytophagous insects' host plant shifts are crucial for evolution and ecology.
• These shifts necessitate behavioral adaptations in insect attraction/repulsion to new host plant cues.
• Changes in peripheral sensory system gene expression are hypothesized to underlie chemotactic behavior evolution.

Purpose of the Study:

• To investigate gene expression changes in the olfactory organs of Drosophila sechellia, a specialist on Morinda citrifolia.
• To compare olfactory gene expression with its generalist relatives, Drosophila simulans and Drosophila melanogaster.
• To test the hypothesis that altered gene expression in the peripheral sensory system drives chemotactic behavior evolution.

Main Methods:

• Whole-genome microarrays and quantitative polymerase chain reaction (qPCR) were employed.
• The study surveyed the expression of all Drosophila odorant receptors (ORs) and odorant-binding proteins (OBPs) in the antennae.
• Gene expression was compared between D. sechellia and its close relatives, D. simulans and D. melanogaster.

Main Results:

• Gene expression evolution for ORs and OBPs was significantly accelerated in D. sechellia compared to the genome average and other species.
• Some olfactory gene expression changes predated the divergence of D. sechellia from D. simulans.
• Several OR and OBP genes, including Or22a, showed increased expression in D. sechellia, potentially linked to M. citrifolia odorants.

Conclusions:

• Accelerated evolution of olfactory gene expression, particularly ORs and OBPs, is evident in the specialist D. sechellia.
• These expression changes likely contribute to the evolution of specialized chemotactic behavior towards specific host plants.
• Olfactory gene expression divergence is not solely explained by changes in sensilla abundance, indicating independent evolution of specific genes.