Abstract

In the carp, nucleolar reorganization is one of the most remarkable features of the cellular adjustment that occurs upon seasonal acclimatization. To assess if the level of cellular expression of a specific rRNA is maintained between both seasonal adaptive states, we isolated a carp 5.8S rRNA gene. Sequence analyses showed its high conservation during evolution. The level of expression of 5.8S rRNA was determined by in situ hybridization assays in pituitary sections from summer- and winter- acclimatized carp. The quantification of the positive hybridization signals demonstrated a statistically significant difference between the acclimatization states. Transcription of 5.8S rRNA was considerably higher in summer- than in winter-acclimatized carp. These results indicate that the level of rRNA expression in eurythermal fish is not comparable between seasons and suggest that rRNA genes may be not constitutive. Thus, rRNA transcription should be used with caution as a reference to quantitate transcripts that are modulated as part of the acclimatization process in eurythermal fish.