The “out of the testis” hypothesis for the emergence of new genes. This hypothesis suggests that the transcription of new gene copies/structures (green boxes) is facilitated in certain testis germ cells—meiotic spermatocytes and post-meiotic round spermatids (which are found in the seminiferous tubules, where spermatogenesis takes place)—because of the potentially overall permissive chromatin state and overexpression of key components of the transcriptional machinery in these cells. The transcriptionally active chromatin state in spermatocytes and spermatids is thought to be a result of a potentially widespread demethylation of CpG dinucleotide-enriched promoter sequences and modifications (acetylation and methylation) of histones (blue ovals), which facilitate access of the transcriptional machinery (red ovals). Once transcribed, new functional genes (transcripts shown as green wavy lines) with beneficial products may be selectively preserved and evolve more efficient promoters (a process that might be facilitated by the fact that spermatocyte/spermatid-specific expression requires only relatively simple promoters). Eventually, such new genes may also evolve more diverse expression patterns and thus also obtain functions in other (somatic) tissues.
