Chromosome Behaviour during Spermatogenesis of Anopheles stephensi Sensu Stricto

Abstract

1. The testes of Anopheles stephensi sensu stricto are two pale yellowish, ellipsoidal bodies. Their wall consists of an outer, tough, pigmented envelope, and an inner squamous epithelial layer. The nuclei of various types of resting cells within the testes are characterized by a peripheral, basophilic crescent. The anterior part of the testicular cavity is divided into cysts which hold spermatogonia and primary spermatocytes. Germ cells at later stages of differentiation are held free within the posterior part of the testicular cavity.
2. The spermatogonial metaphase configuration consists of two pairs of V-shaped autosomes and a pair of unequal, subtelocentric sex chromosomes. The homologous chromosomes exhibit somatic pairing. The centromeres appear as distinct, non-staining gaps. The centromere is slightly off-median in one of the autosomal pairs.
3. The nuclei of early primary spermatocytes show long, slender, chromomeric threads and a peripheral basophilic crescent. The latter subsequently resolves into deeply staining thread-like structures representing the heterochromatic regions of the sex chromosomes and a lightly staining nucleolar body. The chromosomal threads undergo a process of uniform condensation to produce thick pachytene bivalents. The nucleolus disappears by diplotene. The sex chromosomes associate together only along short lengths of their long limbs proximal to their centromeres. Terminalization of chiasmata takes place during diakinetic condensation of the chromosomes. The sex bivalent is symmetrical. The metaphase spindle is formed within a welldefined nuclear space surrounded by a thin limiting membrane. Tiny dotlike centrioles are present. At first anaphase the autosomal univalents disjoin normally. The sex chromosomes, on the otherhand, show obligatory postreduction. The sex chromosomes lag during the first anaphase separation. In the dyads the chromatids are widely divergent except near the centromere. The telophase nuclei are oval in shape and show a peripheral concentration of chromatin.
4. A short interkinesis stage is present. At the second prophase the dyads appear as typical X or cross-shaped figures. Subsequently they constitute a tangled group at the centre of nucleus representing a prometaphase stage. A second metaphase plate is produced although the stage is of short duration only. The sex chromosomes show precession during the second anaphase. The spermatid nuclei fall into two classes on the basis of the relative dimensions of their heteropycnotic dots.