Aggregative egg distributions may promote species co-existence - but why do they exist?

Although the effects of aggregated egg distributions for the co-existence of Drosophila species are fairly well understood, the driving forces that select for such egg distributions remain unclear. This study investigated the fitness consequences of Drosophila oviposition behaviour on natural breeding substrates under controlled conditions. We used fruits of Sorbus aucuparia as an oviposition substrate and recorded the clutch sizes that Drosophila subobscura females produce and the way in which individuals distribute their eggs across fruits. We found that D. subobscura females significantly aggregated their eggs. We tested the two most obvious hypotheses that might explain the distribution pattern of the eggs. Neither the hypothesis that larvae benefit from resource-dependent facilitation when feeding in groups, nor the hypothesis that Sorbus fruits might provide a negatively density-dependent refuge against parasitoid attack, is supported by our data. In contrast, our results suggest that flies should distribute their eggs regularly across fruits if the maximization of offspring survival is the selecting force. We discuss under which circumstances the observed distribution pattern might be adaptive.