Transcriptome-wide changes in testes reveal molecular differences in photoperiod-induced seasonal reproductive life-history states in migratory songbirds

We investigated the molecular basis of seasonal testicular states in black-headed buntings (Emberiza melanocephala), in which seasonal migrations limit reproduction to a narrow time-window during the year. We examined testicular gene expressions in buntings during short day-induced photosensitive nonreproductive state (SDSE), and during long day-induced early (LDES) and late (LDLS) gonadal maturation and regressed photorefractory (LDRF) seasonal states. Using Kallisto, we pseudoaligned the RNA-Seq transcripts and calculated the transcript abundance. We found 1,799 differentially expressed genes (DEGs) between the four photoperiod-induced seasonal states. Further pairwise comparison with SDSE revealed 1,187, 1,224, and 1,238 DEGs in LDES, LDLS, and LDRF, respectively; 852 genes were common to three comparisons. We then identified genetic pathways putatively involved in seasonal testicular cycle. DEGs that enriched calcium ion binding were highly expressed in testicular maturation states. Similarly, DEGs that enriched glycolytic pathway were highly expressed in LDES and LDRF states. More specifically, quantitative polymerase chain reaction showed significant differences between the photoperiod-induced states in testicular expression of genes (HOOK1, RGS2, PRDX4, BCL6, CYFIP2, PDCD4, P2RX4, and GABRA1) involved in gametogenesis and associated pathways. Overall, these results show significant transcriptional differences, and provide insights into the molecular basis of seasonal changes in the reproductive life-history states in a photoperiodic species.