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