Abstract
BackgroundThe Drosophila Odorant-Binding Protein (Obp) genes constitute a multigene family with moderate gene number variation across species. The OS-E and OS-F genes are the two phylogenetically closest members of this family in the D. melanogaster genome. In this species, these genes are arranged in the same genomic cluster and likely arose by tandem gene duplication, the major mechanism proposed for the origin of new members in this olfactory-system family.ResultsWe have analyzed the genomic cluster encompassing OS-E and OS-F genes (Obp83 genomic region) to determine the role of the functional divergence and molecular adaptation on the Obp family size evolution. We compared nucleotide and amino acid variation across 18 Drosophila and 4 mosquito species applying a phylogenetic-based maximum likelihood approach complemented with information of the OBP three-dimensional structure and function. We show that, in spite the OS-E and OS-F genes are currently subject to similar and strong selective constraints, they likely underwent divergent evolution. Positive selection was likely involved in the functional diversification of new copies in the early stages after the gene duplication event; moreover, it might have shaped nucleotide variation of the OS-E gene concomitantly with the loss of functionally related members. Besides, molecular adaptation likely affecting the functional OBP conformational changes was supported by the analysis of the evolution of physicochemical properties of the OS-E protein and the location of the putative positive selected amino acids on the OBP three-dimensional structure.ConclusionOur results support that positive selection was likely involved in the functional differentiation of new copies of the OBP multigene family in the early stages after their birth by gene duplication; likewise, it might shape variation of some members of the family concomitantly with the loss of functionally related genes. Thus, the stochastic gene gain/loss process coupled with the impact of natural selection would influence the observed OBP family size.
Highlights
The Drosophila Odorant-Binding Protein (Obp) genes constitute a multigene family with moderate gene number variation across species
We compared nucleotide and amino acid variation across 18 Drosophila and 4 mosquito species applying a phylogenetic-based maximum likelihood approach complemented with information of the Odorant-Binding Proteins (OBPs) three-dimensional structure and function
Positive selection was likely involved in the functional diversification of new copies in the early stages after the gene duplication event; it might have shaped nucleotide variation of the OS-E gene concomitantly with the loss of functionally related members
Summary
The Drosophila Odorant-Binding Protein (Obp) genes constitute a multigene family with moderate gene number variation across species. Likely plays an important role in the evolution of olfactory-involved genes. Olfactory-specific gene families might contribute to the host-specificity shifts occurring in the diversification of super-specialist Drosophila species [8,9]. The primary step in the olfactory perception is accomplished by the Odorant-Binding Proteins (OBPs). In spite of the similar global function of insect and vertebrate OBPs, these two protein families are evolutionarily unrelated [10]. OBPs are small globular proteins that bind odorant molecules (including pheromones) at the pores of the chemosensory sensilla, transporting them through the aqueous lymph, and delivering their ligands near the olfactory receptors (OR) [11,12]. The OBP three-dimensional (3D) structures of several insects have been determined [reviewed in [19]; these proteins share similar folds, with significant structural differences (protein length, position and conformation of α-helices, loops and C-terminus), resulting in diverse solvent access properties
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