Evolutionary novelties in islands: Drosophila santomea, a new melanogastersister species from Sao Tome

The finding of new melanogaster sister species may help us in understanding more about how the emergence of genetic: novelties, particularly in insula r habitats, can result in speciation. Here we report on the discovery of Dr osophila santomea, which is the first melanogaster sibling found off West-e quatorial Africa, on Sao Tome, one of the Gulf of Guinea islands, Although the eight other melanogaster sister species are remarkably conservative in their morphology except for their terminalia, the new find has a morphologi cal trait distinguishing it fr om all of these: a pure yellow body colorati on of both sexes without the normal black abdominal banding. Evidence From the terminalia, polytene and mitotic chromosomes, period gene and allozymes are provided indicating that it is nonetheless the nearest relative of Dro sophila yakuba with which it coexists on the island. The new find is a clea r-cut taxon as shown by the production of sterile male hybrids, eventually with developmental defects, in both directions of cross with yakuba and by the existence of an altitudinal divide accompanied by a hybrid zone at mid- elevation on the island, Molecular and karyotypic data further support this conclusion. In contrast to the significant divergence of their nuclear DNA s, an intriguing similarity in their cytochrome b sequences was observed in dicating a recent coalescence common to santomea, yakuba and also teissieri cytoplasms. These were shown to harbour the same Wolbachia endosymbiotic b acteria which could possibly be responsible for mitochondrial DNA hitchhiki ng across the species barrier.