Abstract
Drosophila teissieri and D. yakuba are two closely related species within the melanogaster species subgroup. They both live sympatrically in the Ivory Coast but in different places the ratio of their abundance is strikingly reversed. Alternative smooth or stepped cline of fecundity is observed in D. teissieri and D. yakuba respectively via an altitudinal transect in the Ivory Coast including lowland savannas of Lamto and various elevations and habitats (rainforest versus high-altitude grassland) on Mt Nimba. Crosses of parents of similar origin and, hence, altitude versus crosses of parents of different origin were conducted in order to assess the genetic basis of the differences observed. Allopatric pairs gave intermediate fecundities compared to either relevant sympatric pairs. Some cases of heterosis are observed that are dependent of the direction of cross. Fecundity uniquely and gradually increases with altitude irrespective of the habitat in D. teissieri while decreases, although with a strong stepwise inverse shift due to dramatic habitat change, in D. yakuba.
