Formation of cytoplasmic synaptonemal-like polycomplexes at leptotene and normal synaptonemal complexes at zygotene in Ascaris suum male meiosis

A thread-like (more than 70 cm long) testis of Ascaris suum, when examined under the light and electron microscope, reveals the linear succession of meiotic stages. Beginning from, at least, late leptotene, the spermatocytes are synchronous in their development. Thus within each transverse section of the testis all the spermatocytes are in the same stage. The spermatocytes at each stage of prophase I occupies several (4 to 10) cm of the whole testis length. — At leptotene, synaptonemal-like polycomplexes of lateral and central stacked elements are formed in the cytoplasm of spermatocytes. At late leptotene, the polycomplexes are attached to the external nuclear membrane. The polycomplexes disappear at zygotene. Slightly discernable axial cores are observed in the late leptotene chromosomes. The synaptonemal complexes (SCs) are formed at the zygotene stage, their structure being characteristically tripartite. The SCs disappear from the nuclei at the diffuse stage of prophase I. In other organisms completely developed polycomplexes of stacked lateral and central elements were never found during the presynaptic period of meiosis, although single or two parallel layers of aggregated central regions of SC were found in Neottiella meiocytes at the stage prior to chromosome pairing (Westergaard and von Wettstein, 1970, 1972). — First appearance of the polycomplexes in the cytoplasm insetead of the nucleus is also a novel fact. It is concluded that the polycomplexes at leptotene are formed by a self-assembly of the SC molecular material precociously synthesized in the cytoplasm. Two hypotheses regarding possible function and the further fate for leptotene polycomplexes are discussed.