Nuclear fragmentation characterises paraspermiogenesis in Tubifex tubifex (Annelida, Oligochaeta)

It is known that tubificine oligochaetes produce two types of spermatozoa: eusperm, fertilizing sperm with regular haploid DNA content; and parasperm, with a much lower DNA content, protecting and carrying the eusperm. Whereas mature spermatozoa and spermatids of the two lines are easily recognized by their morphology and DNA content, little is known about the first steps of differentiation of the two lines. This subject is addressed here in two ways: we have measured DNA content by a new method based on confocal laser microscopy and found that the total DNA content of parasperm cysts is extremely variable and equal or lower than total DNA content of eusperm cysts. Then we focused on the spermatocytes, and we found that the cells which will form paraspermatids undergo a peculiar kind of nuclear fragmentation which differ greatly from a regular cell division. During fragmentation DNA is distributed unevenly among the spermatids and this gives rise to a great and variable number of parasperm with variable DNA content. Immunocytochemical assays revealed that a proper meiotic spindle is never formed during fragmentation and that actin may play an important role in the chromatin division. Electron micrographs showed that the centrioles undergo a phenomenon of mass reproduction similar to that found in ciliated epithelia which supplies each of the numerous paraspermatids of its basal body. Mol. Reprod. Dev. 59: 442–450, 2001. © 2001 Wiley‐Liss, Inc.