Abstract
Simple SummaryBirds perceive light differently than mammals. Unlike mammals, birds’ retinas do not participate in the non-image forming, seasonal breeding aspects of light perception. Birds have deep brain photoreceptors that are involved with the seasonal increase in fertility. Until this study, it was unknown how these brain photoreceptors developed after hatching. Our novel finding is that at least one of the photoreceptors, OPN4, is expressed at high levels on the day of hatching, which suggests that this photoreceptor has functions beyond just the reproductive system. These findings could change how we light poultry barns to improve the growth, health and welfare of our poultry species.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.
Highlights
GnRH mRNA levels increased following the deep brain photoreceptor (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
A series of deep brain photoreceptors (DBPs) have been described in the diencephalon that are involved with maintaining fertility in response to daylength in poultry
Summary
Long daylengths are necessary in order to maintain reproduction in all poultry species, and the duck is no exception [1,2,3]. Previous studies have attempted to understand how types of lighting, intensity of light, and color spectra of light are critical to maximize the reproduction and welfare of both meat and breeder ducks [4,5,6,7]. As with all of our poultry species, ducks are seasonal breeders and as such require a minimum of 14 h of light per day in order to maintain fertility [8]. Despite being maintained on a long daylength, there are still inexplicable drops in fertility during winter months, along with increased unwanted behaviors, such as feather picking, aggression, and mislaid eggs [9,10,11]. A series of deep brain photoreceptors (DBPs) have been described in the diencephalon that are involved with maintaining fertility in response to daylength in poultry
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