Explosive Expansion of βγ-Crystallin Genes in the Ancestral Vertebrate

In jawed vertebrates, betagamma-crystallins are restricted to the eye lens and thus excellent markers of lens evolution. These betagamma-crystallins are four Greek key motifs/two domain proteins, whereas the urochordate betagamma-crystallin has a single domain. To trace the origin of the vertebrate betagamma-crystallin genes, we searched for homologues in the genomes of a jawless vertebrate (lamprey) and of a cephalochordate (lancelet). The lamprey genome contains orthologs of the gnathostome betaB1-, betaA2- and gammaN-crystallin genes and a single domain gammaN-crystallin-like gene. It contains at least two gamma-crystallin genes, but lacks the gnathostome gammaS-crystallin gene. The genome also encodes a non-lenticular protein containing betagamma-crystallin motifs, AIM1, also found in gnathostomes but not detectable in the uro- or cephalochordate genome. The four cephalochordate betagamma-crystallin genes found encode two-domain proteins. Unlike the vertebrate betagamma-crystallins but like the urochordate betagamma-crystallin, three of the predicted proteins contain calcium-binding sites. In the cephalochordate betagamma-crystallin genes, the introns are located within motif-encoding region, while in the urochordate and in the vertebrate betagamma-crystallin genes the introns are between motif- and/or domain encoding regions. Coincident with the evolution of the vertebrate lens an ancestral urochordate type betagamma-crystallin gene rapidly expanded and diverged in the ancestral vertebrate before the cyclostomes/gnathostomes split. The beta- and gammaN-crystallin genes were maintained in subsequent evolution, and, given the selection pressure imposed by accurate vision, must be essential for lens function. The gamma-crystallin genes show lineage specific expansion and contraction, presumably in adaptation to the demands on vision resulting from (changes in) lifestyle.