Identification and analysis of retrogenes in the East Asian nematode Caenorhabditis sp. 5 genome

Identification of Retrocopies in Lepidoptera and Impact on Domestication of Silkworm

BACKGROUND

During the domestication of silkworm, an economic insect, its physiological characteristics have changed greatly. RNA-based gene duplication, known as retrocopy, plays an important role in the formation of new genes and genome evolution, but the retrocopies of lepidopteran insects have not been fully identified and analyzed, which not only severely limits researchers from exploring the effects of retrocopies on lepidopteran insects but also affects the studies on the domestication of silkworm.

METHODS

We compared the genomes and proteomes of eight lepidopteran insects and used a series of screening criteria for auxiliary screening to obtain the retrocopies in lepidopteran insects and explored their characteristics. In addition, based on the silkworm transcriptome data from the SilkDB3.0 website, we explored the functions of the retrocopies on the domestication of the silkworm.

RESULTS

A total of 1993 retrocopies and 1208 parental genes in lepidopteran insects were obtained. We revealed that the retrocopies in Lepidoptera do not conform to the "out of X" hypothesis but fit the "out of testis" hypothesis. These retrocopies were subject to strong functional constraints and performed important functions in growth and development. Transcriptome analysis revealed that the expression pattern of the retrocopies and their parental genes were irrelevant. Through the analysis of the retrocopies in silkworm generated after domestication and located in the candidate domestication regions, the possible universal connection between the retrocopies and the domestication of silkworm were found.

CONCLUSIONS

Our study pioneered the exploration of retrocopies in multiple Lepidoptera species and found the potential association between the retrocopies and the domestication of silkworm.

The Genomic Impact of Gene Retrocopies: What Have We Learned from Comparative Genomics, Population Genomics, and Transcriptomic Analyses?

Gene duplication is a major driver of organismal evolution. Gene retroposition is a mechanism of gene duplication whereby a gene's transcript is used as a template to generate retroposed gene copies, or retrocopies. Intriguingly, the formation of retrocopies depends upon the enzymatic machinery encoded by retrotransposable elements, genomic parasites occurring in the majority of eukaryotes. Most retrocopies are depleted of the regulatory regions found upstream of their parental genes; therefore, they were initially considered transcriptionally incompetent gene copies, or retropseudogenes. However, examples of functional retrocopies, or retrogenes, have accumulated since the 1980s. Here, we review what we have learned about retrocopies in animals, plants and other eukaryotic organisms, with a particular emphasis on comparative and population genomic analyses complemented with transcriptomic datasets. In addition, these data have provided information about the dynamics of the different "life cycle" stages of retrocopies (i.e., polymorphic retrocopy number variants, fixed retropseudogenes and retrogenes) and have provided key insights into the retroduplication mechanisms, the patterns and evolutionary forces at work during the fixation process and the biological function of retrogenes. Functional genomic and transcriptomic data have also revealed that many retropseudogenes are transcriptionally active and a biological role has been experimentally determined for many. Finally, we have learned that not only non-long terminal repeat retroelements but also long terminal repeat retroelements play a role in the emergence of retrocopies across eukaryotes. This body of work has shown that mRNA-mediated duplication represents a widespread phenomenon that produces an array of new genes that contribute to organismal diversity and adaptation.