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Use of orphan genes and protein evolution are shaped by Drosophila development

A substantial fraction of coding regions in any genome sequenced consist of genes that have no recognizable homolog in distantly related species. The role of these so called “ orphan genes” in the evolutionary process is still enigmatic despite recent efforts to characterize them. Previous study in Drosophila pointed to the possibly biased use of orphans during development. Here I analyzed expression patterns of orphan genes in more details. Using publicly available EST and microarray data I detected that expression of orphans, in general, is more often stage restricted, compared to non-orphans. Further analysis of stage specific transcripts shows increased frequency of orphan genes in adults, compared to embryos. Moreover, in the same type of comparison, analysis of evolutionary rates of all stage restricted genes uncovers faster evolution of transcripts restricted to adults. However, separate treatment of orphan and non-orphan genes reveals that overall faster evolution of the adult specific transcripts is mainly due to the higher evolutionary rates of orphans expressed in the adult stage. These results suggest that distinct selective pressures are acting on the developmental stages which footprint could be detected on the molecular level. It seems that orphan genes as a group are the most sensitive part of the genome to the differences in selection posed by the development. I discuss potential causes of different selective regimes in Drosophila embryo and adults.

orphan genes; evolution; Drosophila

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