Tissue-specific CLOCK isoforms modulate circadian feedback loops to govern reproductive fitness in Drosophila

Circadian rhythms, conserved across life, are governed by molecular feedback loops in Drosophila clock neurons, though how these loops adapt in peripheral tissue clocks remains unclear. In this study, we discovered that in reproductive tissues, the expression profile of Clock is notably different. It is expressed at very low levels in the ovary, while a shorter isoform of Clock is particularly abundant in the testis. Downregulation of these isoforms impaired fertility in both male and female flies. The short isoform of CLOCK inhibited the binding of the longer isoform to CYCLE, leading to a reduction in CLOCK / CYCLE co-binding in the testis. Using ChIP-seq to identify CLOCK and CYCLE binding sites in the head, ovary and testis, we found that the binding profile of CLOCK / CYCLE was distinct between these tissues, in which CLOCK / CYCLE targets to genes involve in spermatogenesis specifically in testes. Immunostaining revealed knock down of CLOCK short form results in defects in spermatogenesis. This study reveals the expression profiles and functional mechanisms of specific Clock isoforms in reproductive tissues, which highlighted the modification of clock regulatory loop and essential role of Clock in reproductive organs.