Chromosome-associated paracrystalline nuclear inclusions in the spermatocytes of a pulmonate snail, Planorbarius corneus L.

Chromosome-associated paracrystalloids are regularly found in the spermatocytes of snails which were reared in the laboratory. They seem to be largely specific for the male gametocytes as they have been observed only in few cases in the oocytes. It is likely that paracrystalloids are formed during pachytene at the site of large heterochromatic knobs which originate by fusion of heterochromatic terminal segments of some bivalents. During diplotene they are always connected with the telomeres of three or four bivalents, thus forming a large trefoil-like structure. During metaphase I paracrystalloids are shed off from the chromosomes and transferred to the cytoplasm. In early spermatids they are found again in the nuclei, where they "fade away" during spermiogenesis. Histochemically they consist of basic proteins, which are probably crystallized in the cubic system. Radioactive labeling of the structure could not be achieved, neither by 3H-uridine or thymidine, nor by amino acids. The functional significance of this peculiar structure in unknown. Certain features justify a comparison with synaptonemal polycomplexes.