Calmodulin Gene Expression in Sarotherodon melanotheron: A Salinity-Dependent Study in Drainage Basins

Abstract

Calmodulin (CaM) is an ubiquitous Ca2+ binding protein that plays an important role in signalling events mediating the 
hormone secretion such as pituitary hormones. Previous laboratory experiments have demonstrated that this gene is 
relatively over expressed in gill tissues of Sarotherodon melanotheron acclimated to freshwater. In this study, the 
relationship between mRNA levels of the CaM in the gills and environmental salinity was investigated in wild populations 
of this species sampled during both the rainy and the dry season from six coastal, estuarine and freshwater sites in 
Senegal and Gambia. In both seasons the highest CaM mRNA levels were recorded in freshwater where the highest 
prolactin (PRL) and lowest growth hormone (GH) expression levels were previously reported. In the dry season CaM 
expression was highest at the lowest salinities (Guiers lake and Balingho) than in the hypersaline water sites of the Saloum 
estuary (Missirah, Foundiougne and Kaolack) and did not differ between these three sites. The amounts of CaM mRNA 
were not different between Balingho and Hann bay locations. The expression of CaM showed a similar pattern in rainy 
season, being highest in freshwater locations of Guiers lake and lowest in the most saline sites (Hann bay, Missirah, 
Foundiougne and Kaolack).The CaM mRNA levels were significantly negatively correlated with environmental salinity in 
both the rainy and the dry season. All together these results may indicate a role of CaM in the acclimation to hypo-osmotic 
stress possibly through the regulation of cell volume, calcium uptake and the expression of genes involved in 
osmoregulation such as PRL and GH. In the rainy season, overall mean expression of the CaM was higher than in the dry 
season, which may have reflected more variable particularly sudden fluctuations in salinity and poorer overall water 
quality.