Retroposed new genes out of the X in Drosophila

Abstract

New genes that originated by various molecular mechanisms are an essential component in understanding the evolution of genetic systems. We investigated the pattern of origin of the genes created by retroposition in Drosophila. We surveyed the whole Drosophila melanogaster genome for such new retrogenes and experimentally analyzed their functionality and evolutionary process. These retrogenes, functional as revealed by the analysis of expression, substitution, and population genetics, show a surprisingly asymmetric pattern in their origin. There is a significant excess of retrogenes that originate from the X chromosome and retropose to autosomes; new genes retroposed from autosomes are scarce. Further, we found that most of these X-derived autosomal retrogenes had evolved a testis expression pattern. These observations may be explained by natural selection favoring those new retrogenes that moved to autosomes and avoided the spermatogenesis X inactivation, and suggest the important role of genome position for the origin of new genes.

Figure 1

RT-PCR for several genes. (A) CG10174, (B) CG13732, (C) CG17856, (D) CanB, and (E) CG9873. Lane 1 corresponds to gonadectomized male cDNA, lane 2 is testis + accessory glands cDNA; lanes 3 and 4 are the negative controls after DNA digestion for the experiments of lanes 1 and 2, respectively, and lane 5 is the negative control of the PCR. Lane 6 is the PCR experiment using testis cDNA; lane 7 is the negative control after DNA digestion, and lane 8 is the negative control of the PCR. (F) Lane 1 is CG15645 RT-PCR using cDNA from polyA selected RNA from a mixed sample of males and females; lane 2 is the PCR from this mRNA without being reverse-transcribed from the mixed sample; lanes 3 and 4 are the nested PCR experiments using the PCR products of lanes 1 and 2 as templates. The DNA marker, as shown here, is a 1-kb DNA ladder (Gibco).