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. Wherea
s 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 extre
mely variable and equal or tower than total DNA content of eusperm cysts. T
hen we focused on the spermatocytes, and we found that the cells which will
form paraspermatids undergo a peculiar Rind of nuclear fragmentation which
differ greatly from a regular cell division. During fragmentation DNA is d
istributed unevenly among the spermatids and this gives rise to a great and
variable number of parasperm with, variable DNA content. Immunocytochemica
l assays revealed that a proper meiotic spindle is never formed during frag
mentation and that actin may play an important role in the chromatin divisi
on. Electron micrographs showed that the centrioles undergo a phenomenon of
mass reproduction similar to that found in ciliated epithelia which suppli
es each of the numerous paraspermatids of its basal body. (C) 2001 Wiley-Li
ss, Inc.