ABSTRACT
We have chosen immunoglobulin, the key molecule of the immune system, to look for specific features accounting for adaptive evolution of Notothenioidei, the most abundant teleost species living in the Antarctic Ocean. Trematomus bernacchii immunoglobulin light chain was investigated and three isotypes were identified. Sequencing genomic DNA and transcripts encoding both the secreted and membrane-bound forms of the immunoglobulin heavy chain from 18 notothenioid species disclosed several unusual features. A limited diversity of the immunoglobulin heavy chain variable domain of the notothenioid teleost T. bernacchii was found, with only two VH gene families being defined. Analysis of deduced amino acid sequences of the secreted heavy chain showed an unexpected long hinge peptide, located at the CH2-CH3 boundary. To search for polymorphism, ten T. bernacchii individuals were analyzed: the largest number of polymorphic positions was found to fall within the hinge peptide suggesting that this feature may have some biological significance. In several species, alternative mRNA splicing that is responsible for the membrane-bound form synthesis generates heavy chains consisting of only two constant region domains. This domain-truncated form has a long extracellular spacer that has arisen from the insertion of multiple 39-nt repeats. Interestingly, each repeat was found to be the reverse complement of a sequence in the CH3 exon.
