Polymorphism in the immunoglobulin-like domains of the receptor tyrosine kinase from the sponge Geodia cydonium

Sponges [Porifera] are the phylogenetically oldest phylum of the Metazoa. They are provided with both cellular and humoral allorecognition systems. The underlying molecules are not yet known. To study allorecognition in sponges we first determined the frequency of graft rejection in a natural population of the marine sponge Geodia cydonium. We then determined, for the first time at the molecular level, the degree of sequence polymorphism in segments of one molecule which may be related to sponge allorecognition and host defense: the Ig-like domains from the receptor tyrosine kinase [RTK]. Thirty six pairs of auto- and allografts were assayed, either by parabiotic attachment or insertion of grafts. All of the autografts fused, while only two allografts fused and 34 pairs were incompatible. Rejection among the parabiotic allografts was characterized by the formation of a collagenous barrier, while the allografts that were inserted into the host underwent destruction. At the molecular level we first cloned to completion the 5'-end of sponge RTK, which displays a Pro-Ser-Thr-rich sequence; this is thought to act as a module of cell adhesion proteins. Then we analyzed RT-PCR products of amplification across the two Ig-like domains of RTK (about 500 bp), from two pairs of fusing sponges and one pair of rejecting sponges. High levels of polymorphism were recorded, including 18 nucleotide-substitution positions and a tri-nucleotide deletion, which translate into 13 polymorphic amino acid positions. Two of the six sponges were scored as heterozygotes. Among 9 informative polymorphic sites that were tested for linkage disequilibrium, 11 pairwise comparisons were found to be significant, implying the possibility of distinguishable alleles in this locus. To the best of our knowledge this is the first report of polymorphism in Ig-like domains of a receptor from invertebrates that may be associated with allorecognition. This data attests also that fusion in sponges is not confined to genetically identical individuals.