Temperature and length-dependent modulation of the MH class IIβ gene expression in brook charr ( Salvelinus fontinalis) by a cis-acting minisatellite

Abstract

It is widely recognized that the variation in gene regulation is an important factor from which evolutionary changes in diverse aspects of phenotype can be observed in all organisms. Distinctive elements with functional roles on gene regulation have been identified within the non-coding part of the genome, including repeated elements. Major histocompatibility complex (MHC) genes have been the subject of an abundant literature which made them unique candidates for studies of adaptation in natural populations. Yet, the vast majority of studies on MHC genes have dealt with patterns of polymorphism in sequence variation while very few paid attention to the possible implication of differential expression in adaptive responses. In this paper, we report the identification of a polymorphic minisatellite formed of a 32 nucleotides motif (38% G + C) involved in regulation of the major histocompatibility class IIβ gene (MHIIβ) of brook charr ( Salvelinus fontinalis). Our main objectives were: to analyze the variability of this minisatellite found in the second intron of the MHIIβ gene and to document its effect to the variation of expression level of this gene under different environmental conditions. Distinctive number of the minisatellite repeats were associated with each different MHIIβ alleles identified from exon 2 sequences. Relative expression levels of specific alleles in heterozygous individuals were determined from fish lymphocytes in different genotypes. We found that alleles carrying the longest minisatellite showed a significant 1.67–2.56-fold reduction in the transcript expression relatively to the shortest one. Results obtained in three different genotypes also indicated that the repressive activity associated to the longest minisatellite was more effective at 18 °C compared to 6 °C. In contrast, no significant difference was observed in transcript levels between alleles with comparable minisatellite length at both temperatures. We also depicted a significant up-regulation of the total MHIIβ transcript at 6 °C relative to 18 °C. These results reveal for the first time that a temperature-sensitive minisatellite could potentially play an important role in the gene regulation of the adaptive immune response in fishes.