The Effect of Gonadotropin Releasing Hormone on Opsin Gene Expression and Spectral Sensitivity in Zebra Cichlid Fish (Metriaclima zebra).
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Sexual selection and the maintenance of species diversity in Lake Malawi cichlid fishes are greatly dependent on optical communication, which is influenced by environmental, physiological and endocrinological factors. The diversity in spectral sensitivity of cichlids has been partially attributed to differences in opsin gene expression, with each species preferentially expressing a subset of seven possible genes. Hormones such as gonadotropin releasing hormone (GnRH) can mediate changes in gene expression and the presence of GnRH immunoreactive fibers and GnRH receptors throughout the retinal layers make it an excellent candidate for mediating changes in visual processes. Effects of exogenous GnRH administration on the visual system of zebra cichlids (Metriaclima zebra) via prolonged release cholesterol implants and intubation was investigated using electroretinogram (ERG) recordings, quantitative real-time RT-PCR and in situ hybridization. Three week and ten week sampling periods were used in the intubation study. No obvious differences in spectral sensitivity were evident when looking at a-wave, b-wave and d-wave components of the ERG waveform in any of the treatment groups. A multiple mechanism model was used to describe the cone mechanisms mediating spectral sensitivity and this analysis showed that the activity of cones was shaped by opponent and non opponent cone interactions based on subsets of five opsin genes previously described in cichlids (SWS1, SWS2b, RH2b, RH2aβ, and RH2aα). Although differences in the spectral sensitivity between control and GnRH-treated fish were not evident on a functional level, there were changes in the gonadosomatic index in the intubation group. Quantitative real-time RT-PCR (qRT-PCR) and in situ hybridization demonstrated that treatment with a synthetic GnRH3 analogue using the oral intubation delivery system resulted in statistically significant changes in opsin gene expression in both three week and ten week treatment groups, specifically the upregulation of RH2b and the downregulation of RH2a opsin genes. Moreover, in situ hybridization analysis showed that the pattern of labeling for the RH2a and RH2b riboprobes corroborated the changes in opsin gene expression found in the qRT-PCR data. In contrast, GnRH treatment using the cholesterol implant delivery system did not result in significant changes in spectral sensitivity or opsin gene expression.