Clues to how animals switched from laying eggs to live birth may be found in the well-studied fruit fly’s ecology and genes.The fly is one of a dozen species of Drosophila to have recently had their genomes sequenced, information that should provide abundant opportunities for identifying genetic changes that cause females of this species, and not others, to retain their fertilized eggs until they are ready to hatch.
Even those Seychelles fly eggs that emerged unhatched were at an advanced state of development, the team reports in forthcoming issue of the Journal of Evolutionary Biology. Most larvae emerged within two hours compared to an average of nearly 23 hours for the other 10 species in the study.
Live birth could result from changes to the male reproductive strategy as well. Proteins found in the semen of the well-known lab fruit fly, Drosophila melanogaster, stimulate egg laying in the female. A modification of these signals could be responsible for the switch.
Early hatching offers advantages, the authors say. Mobile larvae can burrow into the ground to avoid becoming inadvertent hosts to the eggs of parasitic insects or a predator’s meal. But harboring offspring for a longer period of time costs the female.
One other fly in the study, Drosophila yakuba, also occasionally laid larvae instead of eggs, and their eggs also hatched fairly quickly, most in under 14 hours. It too specializes in a particular fruit, that of the Pandanus tree.