A recessive lethal mutation, tb, that bends the midbrain region of the neural tube in the early embryo of the medaka

A recessive lethal mutation, tb (twisted br ain), in the homozygous embryo of which the neural tube is twisted, was newly found in the medaka (Oryzias latipes). The mutation was recovered in the progeny of a male medaka which had been treated with N-ethyl-N-nitrosourea (ENU). The mutation affected morphogenic cell movements (extension and convergence) in the gastrula and neurula stages. In these stages, the embryonic body was shorter and the convergence of the neuroectodermal cells proceeded more slowly, especially in the prospective midbrain region, in the mutant embryo than in the wild-type embryo. As the neural rod formed in the mutant embryo, it curved in the midbrain region, usually protruding to the right as viewed from above. Structures lying anterior and posterior to the midbrain region were invariably present and developed fairly normally in the mutant embryo. These results show that there exists a genetically separable component in the developmental process of the formation of a straight and symmetrical neural tube.