Explosive expansion of betagamma-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.
Kappé
Guido
G
Biomolecular Chemistry 271, NCMLS, Radboud University Nijmegen, P.O. Box 9101, 6500 HB, Nijmegen, The Netherlands.
Purkiss
Andrew G
AG
van Genesen
Siebe T
ST
Slingsby
Christine
C
Lubsen
Nicolette H
NH
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D030541:Databases, Genetic; D019143:Evolution, Molecular; D005091:Exons; D007438:Introns; D008969:Molecular Sequence Data; D048428:Petromyzon / Q000235:genetics*; D010802:Phylogeny; D016415:Sequence Alignment; D014561:Urochordata / Q000235:genetics*; D038204:beta-Crystallins / Q000235:genetics*; D038222:gamma-Crystallins / Q000235:genetics*
D038204:beta-Crystallins; D038222:gamma-Crystallins