engleski

An evolutionary analysis of orphan genes in Drosophila

Orphan genes are protein coding regions that have no recognizable homologue in distantly related species. A substantial fraction of coding regions in any genome sequenced so far consists of such orphan genes, but their evolutionary and functional significance is not understood. We present a re-analysis of the Drosophila melanogaster proteome that shows that there are still between 26 - 29% of all proteins without a signficant match with non-insect sequences and that these orphans are under-represented in genetic screens. To analyse the characteristics of orphan genes in Drosophila, we have used sequence comparisons between cDNAs retrieved from two D. yakuba libraries (embryo and adult) and their corresponding D. melanogaster orthologues. We find that the cDNA library from adults yields double as many orphan genes than the one from embryos, suggesting a higher constraint on the use of conserved genes in embryos. The orphan genes evolve on average more than three times faster than non-orphan genes, although the width of the evolutionary rate distribution is similar for both classes. In particular, some orphan genes show very low substitution rates which are comparable to otherwise highly conserved genes. We propose a general model for orphan gene evolution that takes these large rate differences into account and suggests that they are caused by episodic phases of fast and slow divergence. The general characteristics of orphan genes in Drosophila suggest that they may be involved in the evolution of adaptive traits and that slow evolving orphan genes may be particularly interesting candidate genes for identifying lineage specific adaptations.

orphan genes; evolution

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