Evolutionary Dynamics of the Pericentromeric Heterochromatin in Drosophila virilis and Related Species.

Alexander P Rezvykh,Lyubov N Chuvakova,Sergei Yu Funikov,Justin P Blumenstiel,Elena S Zelentsova,Michael B Evgen’Ev,Dina A Kulikova,Lyudmila A Protsenko

doi:10.3390/genes12020175

Abstract

Pericentromeric heterochromatin in Drosophila generally consists of repetitive DNA, forming the environment associated with gene silencing. Despite the expanding knowledge of the impact of transposable elements (TEs) on the host genome, little is known about the evolution of pericentromeric heterochromatin, its structural composition, and age. During the evolution of the Drosophilidae, hundreds of genes have become embedded within pericentromeric regions yet retained activity. We investigated a pericentromeric heterochromatin fragment found in D. virilis and related species, describing the evolution of genes in this region and the age of TE invasion. Regardless of the heterochromatic environment, the amino acid composition of the genes is under purifying selection. However, the selective pressure affects parts of genes in varying degrees, resulting in expansion of gene introns due to TEs invasion. According to the divergence of TEs, the pericentromeric heterochromatin of the species of virilis group began to form more than 20 million years ago by invasions of retroelements, miniature inverted repeat transposable elements (MITEs), and Helitrons. Importantly, invasions into the heterochromatin continue to occur by TEs that fall under the scope of piRNA silencing. Thus, the pericentromeric heterochromatin, in spite of its ability to induce silencing, has the means for being dynamic, incorporating the regions of active transcription.

Highlights

Introduction published maps and institutional affilLarge segments of the eukaryotic genome are packed into two basic forms of chromatin termed euchromatin and heterochromatin [1]
In order to obtain insight into the structural composition of pericentromeric heterochromatin of D. virilis, we searched for assembled contigs in the genome of D. virilis sequenced by the 12 Drosophila genomes consortium that would be highly enriched with the heterochromatic mark H3K9me3 [43]
A distinctive well-known feature of heterochromatin is the ability to silence euchromatic genes placed within the heterochromatic environment, a phenomenon called position effect variegation (PEV) [2,44]

Summary

Introduction

Large segments of the eukaryotic genome are packed into two basic forms of chromatin termed euchromatin and heterochromatin [1]. Euchromatic regions comprise most of the length of the chromosomes and are highly enriched with actively-transcribed loci, including protein-coding genes [2]. Heterochromatin is largely concentrated at pericentromeric and telomeric chromosomal domains termed constitutive heterochromatin [1,2]. Constitutive heterochromatin tends to be late replicating and shows low frequency of meiotic recombination [3]. These genomic segments are dominated by repetitive DNA sequences (up to 80%), including tandem repeats or satellite DNA as well as remnants of diverse transposable elements (TEs) [1,2]. A distinctive property of heterochromatin in Drosophila is its enrichment with di- or trimethylated H3K9 and heterochromatin protein

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Results

Conclusion