Caracterização estrutural e de expressão do gene exuperantia (exu) em Anastrepha fraterculus (Diptera, Tephritidae) e evidências de seleção positiva em Cyclorrhapha

Abstract

The genus Anastrepha (Tephritidae) has several species of great economic importance due to their impact in fruticulture. We are particularly interested in the evolutionary history of Anastrepha fraterculus, the most relevant species in the closely related group of species fraterculus, which has undergone recent divergence and holds the majority of species of economic importance in the genus. To better understand the differentiation process and identification in the group, we need to find molecular and morphological markers that are involved with the differences between species. Reproductive genes have, in general, been very informative in this regard due to their rapid evolutionary rates, although few of them have been studied and characterized in this species so far. Therefore, the present study focuses on the exuperantia gene (exu), which participates in both oogenesis (localization of bicoid and oskar mRNAs) and spermatogenesis, since exu embryos from mutant mothers are unviable and males are sterile. Thus, the first step was to use Next generation sequencing strategies (RNA-seq) to structurally characterize and define expression patterns of this gene between sexes in the cephalic and reproductive tissues of A. fraterculus. A. fraterculus has similar structural and expression patterns to Drosophila melanogaster in reproductive tissues, in which the transcripts differ between sexes for the 5’ and 3’ untranslated regions (UTRs). We describe for the first time the expression of exu in cephalic tissues, involving a new isoform that is common to both sexes. All these alternative spliced transcripts share a common coding region, resulting in the same protein. We used the exu coding region, along with sequences from several Diptera to investigate the molecular evolution of exu in Cyclorrhapha. This group has experienced an enormous adaptive radiation, associated with molecular and morphological changes, and by comparing the dN/dS ratio between Cyclorrhapha and other Diptera we found that exu was subjected to positive selection in Cyclorrhapha for at least two sites in the coding region. One of the sites is present in the RNA exonuclease-like domain of exu. The second site is located in a not characterized region that is conserved and under purifying selection in Diptera, suggesting that it may be an important region of the protein. The adaptive changes found in exu may reflect an evolutionary gain that allowed its co-option for a new function the gene performs in Cyclorrhapha: the localization of bicoid in the anterior region of the oocyte, since bcd is a gene exclusive of Cyclorrhapha.