Effects of different photoperiods on growth performance, daily rhythm of growth axis-related genes, and hormones in Nile tilapia (Oreochromis niloticus)

Abstract

The aim of this study was to explore the effects of different photoperiods on the daily rhythm of the growth axis of Nile tilapia (Oreochromis niloticus). Fish were subjected to five photoperiod treatments in this experiment: [light (L) dark (D) ratio: 0 L:24D, 8 L:16D, 12 L:12D, 16 L:8D, 24 L:0D, light intensity 1000 lx]. The growth performance and the daily rhythm of the transcript levels of growth axis-related genes (gh, ghr1, pacap1b, igf-1, igf-1ra) and the contents of growth-related hormones (GH, IGF-1 protein) were analyzed after 4 weeks of culture in the different treatments. To allow for comparisons among treatments, the start of the light period was set to 0 h Zeitberger time (0 h ZT). The results showed that the weight-specific growth rate and daily weight gain of Nile tilapia were highest under 16 L:8D. Under 12 L:12D, the transcript levels of ghr1, igf-1, and igf-1ra and the IGF-1 protein content in the liver showed a clear daily rhythm, and the peak in the daily oscillation (acrophase) was at ZT 7.83 h, ZT 23.53 h, ZT 2.54 h, and ZT 5.91 h, respectively. When the photoperiod changed, ghr1 lost its daily rhythm. Compared with the acrophase under 12 L:12D, the acrophase of igf-1 delayed to ZT 1.71 h under 8 L:16D and ZT 0.06 h under 16 L:8D; that of igf-1ra was delayed to ZT 16.19 h under 16 L:8D and ZT 16.57 h under 24 L:0D; and that of IGF-1 protein was delayed to ZT 8.23 h under 8 L:16D and ZT 8.43 h under 16 L:8D. These results indicated that 16 L:8D was most suitable for the growth of Nile tilapia, because this photoperiod increased the amplitude of IGF-1 including the transcript level and protein and advanced their acrophase. These results shed light on the daily rhythm of growth axis of Nile tilapia, and provided a theoretical basis for optimizing the industrial culture conditions and increasing the yield of this important aquaculture fish species.