Trans regulation of an odorant binding protein by a proto-Y chromosome affects male courtship in house fly

Abstract

Y chromosomes have male-limited inheritance, which favors the fixation of alleles that affect spermatogenesis, courtship, and other male-specific traits. Y-linked male-beneficial alleles can also have female-deleterious (sexually antagonistic) effects because they never experience direct selection in females. However, determining the mechanisms underlying these male-beneficial effects is challenging because it can require studying Y-linked alleles while they still segregate as polymorphism. We used a Y chromosome polymorphism in the house fly, Musca domestica, to address this challenge. Two common male-determining Y chromosomes (YM and IIIM) segregate as stable polymorphisms in natural house fly populations, and they differentially affect multiple traits, including male courtship performance. We performed a meta-analysis of RNA-seq data and identified differentially expressed genes encoding odorant binding proteins (in the Obp56h family) as candidate causal agents in the courtship differences. The Obp56h genes are not found on either the YM or IIIM chromosomes, suggesting that they must be regulated in trans by one of the house fly sex chromosomes. Using a network analysis and allele-specific expression measurements, we identified multiple genes on the house fly IIIM chromosome that could serve as trans inhibitors of Obp56h gene expression. One of those genes is homologous to D. melanogaster CG2120, which encodes a transcription factor that binds both up- and down-stream of Obp56h. We found that up-regulation of CG2120 in D. melanogaster nervous tissues reduces copulation latency, consistent with this transcription factor acting as a negative regulator of Obp56h expression. We propose the name speed date (spdt) for CG2120, with the house fly homolog named Md-spdt. The expression of spdt across D. melanogaster development and tissues suggests that evolution of higher expression in neurons may be constrained by pleiotropic or sexual antagonistic effects. We hypothesize that a cis-regulatory allele that increases expression of Md-spdt on the IIIM chromosome exists because Y-linkage of this allele releases it from those constraints. This provides evidence for a molecular mechanism by which a Y-linked gene can evolve a male-beneficial function regardless of the negative effects on females.