Seasonal variations in gonad morphology and hypothalamic GnRH-I and GnIH in Eurasian Tree Sparrow, a multi-brooded passerine

The hypothalamic-pituitary-gonadal (HPG) axis ubiquitously regulates seasonal reproduction, following the rhythmicity of a suite of environmental cues. Birds display prominent seasonal variations in gonad size regulated by two hypothalamic peptides, gonadotropin-releasing hormone-1 (GnRH-I) and gonadotropin-inhibiting hormone (GnIH). How multi-brooded avian species adjust GnRH-I and GnIH to regulate gonadal morphology seasonally remains largely unknown. Here, we studied the variations in the hypothalamic immunoreactivity (ir) of GnRH-I and GnIH, gonadal proliferation, and apoptosis in a typical multi-brooded species, the Eurasian Tree Sparrow (Passer montanus), across the pre-breeding (PB), first breeding (FB), second breeding (SB), pre-basic molt (PM), and wintering stages (WS). Our results showed that both sexes had higher preoptic area (POA)-GnRH-I-ir but lower paraventricular nucleus (PVN)-GnIH-ir neurons during the breeding stages (FB and SB) relative to other life-history stages, with no significant differences between two broods. The testes and ovaries were significantly larger during the breeding stages. Testicular volume increased during the two broods due to anincreased diameter of seminiferous tubules. Furthermore, there were more testicular apoptotic cells in PB and WS stages than in breeding stages, and in PB stage than in PM stage. Males had higher POA-GnRH-I expression than females during the breeding stages, but both sexes had comparable PVN-GnIH expression throughout the annual cycle. Both sexes of the sparrows may undergo a similar pattern of life-history stage-dependent variation in the hypothalamic GnRH-I, GnIH, and gonadal morphology, except that during breeding stages, males may display higher expression of POA-GnRH-I relative to females. The higher expression of POA-GnRH-I-ir in breeding male sparrows may be critical for male-dependent parental care.