EXTRALENTICULAR EXPRESSION OF XENOPUS-LAEVIS ALPHA-CRYSTALLIN, BETA-CRYSTALLIN, AND GAMMA-CRYSTALLIN GENES

Purpose. Extralenticular expression of alpha- and beta-crystallin gene s has been demonstrated in mammals and expression of gamma-crystallin genes has been shown in Xenopus laevis. To determine a possible correl ation between lens determination and crystallin gene expression, the s ite of expression of (a member of) the alpha-, beta-, and gamma-crysta llin gene families was observed before and during lens formation in X. laevis. Methods. The partial complementary DNAs (cDNAs) of alpha A- a nd beta A4-crystallin and a gamma-crystallin were cloned from an X. la evis lens cDNA library. The corresponding antisense RNAs were used to analyze the expression of these genes during X. laevis development by wholemount in situ hybridization. Results. Expression of the beta A4- and gamma-crystallin (but not alpha-crystallin) genes could first be d etected in the animal cap of the X. laevis gastrula. The beta A4- and gamma-crystallin messengers were also found in the first stage of lens development, when the ectodermal tissue overlying the optic vesicle t hickens to form the lens placode, alpha A-crystallin messenger RNAs we re only detectable when the lens epithelial cells were formed. Conclus ions. In contrast to observations in most vertebrates, expression of t he beta A4- and gamma-crystallin genes was observed to precede that of the alpha A-crystallin gene during lens development of X. laevis, ref lecting the determination that in amphibians, the (presumptive) fiber cells are formed before the epithelial cells, whereas in vertebrates, the order is reversed. Expression of beta A4- and gamma-crystallin gen es in the ectodermal tissue of the X. laevis gastrula shows that these genes are expressed when this tissue gains competence for lens format ion.