Identification of a unique lamprey gene with tandemly repeated sequences and pharyngeal chondrocyte-specific expression.

Recent studies have revealed a common cartilage genetic regulatory network among vertebrates, cephalochordates, and arthropods. It has been proposed that this network was originally established for the dense connective tissues of ancestral invertebrates and subsequently recruited for chondrocyte differentiation in various lineages. This reasoning prompted questions about whether the evolution of cartilage from dense connective tissues occurred in the common ancestors of vertebrates. Alternatively, the evolution of cartilage may have occurred independently in agnathans and in gnathostomes, because extant agnathans (cyclostomes) are known to possess a matrix composition different from that of gnathostomes. Here, we identified the gene which is likely to encode one of the matrix proteins unique to lamprey cartilage, which we designated pharymprin. Pharymprin shows specific expression in larval pharyngeal chondrocytes. Like lamprins, which are the known matrix proteins of lamprey trabecular cartilage, pharymprin is also composed of repeated sequences. However, the repeated sequence is distinct from that of lamprins. The presence of two distinct matrix proteins in lamprey cartilage supports the hypothesis that true cartilage evolved independently in cyclostomes and gnathostomes.