Heterochromatin is the enigmatic eukaryotic genome compartment found mostly at telomeres and centromeres. Conventional approaches to sequence assembly and genetic manipulation fail in this highly repetitive, gene-sparse, and recombinationally silent DNA. In contrast, genetic and molecular analyses of euchromatin-encoded proteins that bind, remodel, and propagate heterochromatin have revealed its vital role in numerous cellular and evolutionary processes. Utilizing the 12 sequenced Drosophila genomes, Levine et al (1) took a phylogenomic approach to discover new such protein "surrogates" of heterochromatin function and evolution. This paper reported over 20 new members of what was traditionally believed to be a small and static Heterochromatin Protein 1 (HP1) gene family. The newly identified HP1 proteins are structurally diverse, lineage-restricted, and expressed primarily in the male germline. The birth and death of HP1 genes follows a "revolving door" pattern, where new HP1s appear to replace old HP1s. Here, we address alternative evolutionary models that drive this constant innovation.
Keywords: Drosophila; HP1; Heterochromatin Protein 1; chromodomain; chromoshadow domain; gene duplication; germline; heterochromatin; phylogenomics; pseudogenization.