The 185/333 Gene Family Is a Rapidly Diversifying Host-Defense Gene Cluster in the Purple Sea Urchin Strongylocentrotus purpuratus

The genome of the purple sea urchin contains numerous large gene families with putative immunological functions. One gene family, known as 185/333, is characterized by extraordinary molecular diversity resulting from single nucleotide polymorphisms and the presence or the absence of 27 large blocks of sequences known as elements. The mosaic composition of elements, known as element patterns, that is present within the members of this gene family is encoded entirely in the second of two exons. Many of the elements correspond to one of six types of repeats that are present throughout the genes. The sequence diversity and variation in element patterns led us to investigate the evolution of the 185/333 gene family. The work presented here suggests that the element patterns are the result of both recombination and duplication and/or deletion of intragenic repeats. Each element is composed of a limited number of similar but distinct sequences, and their distribution among the 185/333 genes suggests frequent recombination within this gene family. Phylogenetic analyses of five 185/333 elements and two regions of the intron were performed using two tests: incongruence length difference and incongruence permutation. Results indicated that each pair of sequence segments was incongruent, suggesting that recombination occurs frequently along the length of the genes, including both the intron and the second exon, and that recombination is not restricted to intact elements. Paradoxically, the high level of similarity among the elements indicated that the 185/333 genes appear to be the result of a recent diversification. These results add to the growing body of evidence suggesting that invertebrate immune systems are not simple and static, but are dynamic and highly complex, and may employ group-specific mechanisms for diversification.