Sex-specific transcriptome dynamics of Anopheles gambiae during embryonic development

Malaria-transmitting mosquitoes are extremely sexually dimorphic in their anatomy and behaviour. Sex-specific gene expression in Anopheles gambiae is well-studied in adult stages, but its onset during embryogenesis, apart from sex-determination factors like Yob, remains largely unknown. Here, we report a comprehensive single-embryo transcriptome atlas of A. gambiae males and females to understand the earliest stages of establishing the sex-specific expression networks. Our dataset reveals embryonic RNA isoform diversity including a global shift towards distal alternative polyadenylation (APA) sites during the maternal-to-zygotic genome transition. Sex-biased gene expression and alternative splicing are limited during embryogenesis, with most sex-specific patterns emerging post- embryonically. X chromosome dosage compensation is established shortly after zygotic genome activation concomitant with direct binding of the master regulator protein SOA to X- linked promoters. Unlike the dosage compensation regulators in Drosophila or mammals, SOA operates in a one-step fashion, directly binding CA-rich promoter motifs without prioritizing certain gene groups over others. We propose that the Anopheles dosage compensation system represents an evolutionary endpoint of a gene-by-gene regulatory mechanism that evolved to a chromosome-wide scale.