Rapid promoter evolution of male accessory gland genes is accompanied by divergent expression in closely related Drosophila species

Seminal fluid proteins (Sfps) are essential for reproductive success and evolve fast, possibly driven by post-copulatory sexual selection (PCSS) originating from sperm competition and cryptic female choice. Counterintuitively, however, the coding region only in few Sfps evolves adaptively. Hence, additional genomic and functional factors must play a role in Sfp evolution independent of the protein coding region. To shed light on drivers of Sfp evolution we focus on those Sfps predominantly expressed in male accessory glands, because this allows examination of their evolution in the tissue which produces the majority of Sfps. Moreover, accessory glands develop normally in hybrids in contrast to testis allowing to control for changes in cellular environment arising during speciation. Here, we discover that Acp promoters contain hot spots for rapid evolution from accumulation of sequence changes and insertions/deletions (indels). We further show that changes in promoter sequences are accompanied by gene expression divergence among closely related Drosophila species. We then validate these observations in Drosophila hybrids to show that species-specific expression divergence of Acps with rapidly evolving promoters are maintained in hybrids for some Acps, while others show dominance of one allele, a phenomenon termed transvection. These results indicate that cis-regulatory evolution, rather than genome background variation, drives Acp expression changes and promotes their rapid evolution.