Multiple Ig-like featuring genes divergent within and among individuals of the marine sponge Geodia cydonium.

The receptor tyrosine kinase of the marine sponge Geodia cydonium features two extracellular Ig-like domains in which we recently documented RT-PCR polymorphism among individuals. Genomic-PCR analysis presented here revealed 14 unique sequences from four sponges, differing predominantly in the sequence of an intron which splits the Ig-like domains. Nevertheless, analysis of putative coding regions in 19 distinct clones (156-159 aa) from seven sponges revealed 69 positions of nucleotide substitutions, 67.6% of them non-synonymous, translating into 43 positions of divergent residues. Excluding aa deletions, these 19 sequences share pairwise aa identities of 89-99%. In three sponges, four or five unique Ig-like coding regions were scored. PCR amplification across this intron revealed multiple bands, polymorphic among five of six sponges. Further substantiated by Southern and Northern blots, it is evident that the genome of G. cydonium harbors multicopies of moderately divergent Ig-like domains. Presently, this only appears paralleled by the human KIR multigene family of NK cells MHC class I-specific receptors, which consist of two or three moderately divergent extracellular Ig-like domains.