The relationship of a specific chromosomal region to the development of the acrosome

In early spermatids of Urodeles the chromosome segments bearing constitutive heterochromatin are localized in one half of the round nucleus; this region becomes the basal part of the long nucleus of the spermatozoon. The euchromatic chromosome segments extend toward the anterior nuclear pole in a bouquet configuration (Macgregor and Walker, 1973). In the course of spermiohistogenesis, one of the heterochromatic regions (the acrosomal chromocenter) migrates from the basal part to the anterior half of the spermatid nucleus. This heterochromatic block is identical with a species-specific, definite C-band in the karyotype. This relationship between the acrosomal chromocenter and a specific chromosomal C-band was established in Triturus cristatus, T. marmoratus, T. alpestric and Cynops pyrrhogaster. In closely related species this particular C-band lies on similar chromosomes. - While the spermatid nucleus still retains its round shape the acrosomal chromocenter despiralizes into a long heterochromatic thread (acrosomal thread). Precisely at the position of this thread the nucleus evaginates and acquires a pear-like shape. During the elongation of the nuclear protrusion the acrosomal thread remains associated with the anterior end. At termination of spermiogenesis it lies closely below the acrosome in the tip of the spermatozoon. Spontaneous aberrations which affect the acrosomal chromocenter or the thread lead to the development of spermatozoa with defective tips. - Several euchromatic segments, interspersed between the heterchromatic segments, can be recognized in the completely despiralized acrosomal thread. Genes responsible for the morphogenetic activities of both, the acrosomal chromocenter and the acrosomal thread, in the development of the spermtip, might be localized in these interspersed euchromatic segments. The existence in higher vertebrates of an acrosomal chromocenter or an equivalent chromosomal region is discussed.