Ontogeny of OPN4, OPN5, GnRH and GnIH mRNA Expression in the Posthatch Male and Female Pekin Duck (Anas platyrhynchos domesticus) Suggests OPN4 May Have Additional Functions beyond Reproduction

Abstract

The hypothalamic-pituitary-gonadal axis (HPG) is known to be regulated by daylength through the deep brain photoreceptor (DBP) system. The post-hatch ontogeny is not known for any of the DBPs. We set out to determine the ontogeny of OPN4 and OPN5 gene expression relative to GnRH and GnIH using qRT-PCR. Brains and serum were collected from five drakes and five hens on the day of hatching (Day 0) and again at 2, 4, 6, 10, 14, 19, 25 and 31 weeks of age and analyzed by qRT-PCR. Hen and drake serum was assayed for circulating levels of estradiol and testosterone, respectively. Data were analyzed between sexes over time using a repeated measures two-way ANOVA. Interestingly, the results show that on the day of hatching (Day 0), ducks showed adult-like levels of relative OPN4, but not OPN5, gene expression. During week 10, DBP levels increased, achieving highest relative expression levels at week 19 that maintained through week 31, typically peak fertility in ducks. GnRH mRNA levels increased following the DBP expression at the onset of puberty, and gonadal steroids increased after GnRH at week 14 while estradiol preceded testosterone. GnIH mRNA levels did not appreciably change during the time course of this experiment. These observations suggest that OPN4 may be active during the peri-hatch period and may have physiological roles beyond puberty and fertility.

Keywords: melanopsin; neonate; photoreception; puberty.

Figure 1

Ontogeny of relative mRNA expression of opsin 4 (OPN4; (A)), opsin 5 (OPN5; (B)), gonadotropin releasing hormone (GnRH; (C)), and gonadotropin inhibitory hormone (GnIH; (D)). Relative OPN4 expression was high just after hatch then dropped over the first 6 weeks of life, then once again increased prior to the increase in relative GnRH mRNA expression. OPN5 expression remained low until prior to the increase in GnRH expression. No differences were observed in GnIH relative mRNA expression at any age. Letters indicate statistically different groups at p < 0.05.

Figure 2

Serum T and E₂ concentrations in drakes and hens, respectively. (A) Serum T concentrations in drakes are at minimal concentrations from hatch through 10 weeks of age. T concentrations were significantly greater by 14 weeks of age, and again significantly increased 19 weeks and maintained into adulthood. Letters indicate statistically significant groups, a = p < 0.05, b = p < 0.01. (B) Serum E₂ concentrations were also at baseline from hatch through week 10 and were significantly increased to adult-like concentrations by week 19. a = p < 0.001. Data suggest that gonadal development is occurring by week 14 and may be more advanced in hens compared to drakes.