Worm Breeder's Gazette 10(3): 143

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Some Observations on Meiosis

Donna G. Albertson

The meiotic chromosomes of C.  elegans, in common with those of 
other animals and plants with holocentric chromosomes, appear to lack 
any recognizable kinetochore or centromere region.  In organisms with 
monocentric chromosomes the centromere serves two functions in meiosis;
to hold the half bivalents together until anaphase II and to attach 
the chromosome to the meiotic spindle for disjunction.  In certain 
insects where extensive cytological studies of meiosis have been made, 
the chromatids of the meiotic bivalent are held together in an end-to-
end association.  They are oriented parallel to the equatorial plate (
equatorial orientation) and microtubules appear to project into the 
chromatin.  Chiasmata which have terminalized by diplotene-diakinesis 
hold the half bivalent together until anaphase II.
Some years ago Nichol Thomson and I investigated the behavior of C.  
elegans chromosomes in meiosis by reconstructing meiotic spindles from 
serially sectioned male and hermaphrodite gonads.  We observed that in 
C.  elegans each half bivalent lies just above or below the equatorial 
plate and that the half bivalents are held together by a chromatin 
thread.  Microtubules appeared to project into the chromatin.  The 
small size of the chromosomes made it impossible to determine whether 
the homologues were associated end-to-end in diakinesis or whether the 
chromosomes were aligned in an equatorial or axial (long axis of the 
chromosome perpendicular to the plate) orientation on the plate.  
However, it appears from cytological observations that the chromosomes 
probably are associated end-to-end in diakinesis.  Translocation of 
part of the X chromosome to the right arm of LGI in mnDp10 and mnDp25 
chromosomes resulted in cytologically visible knobs at either end of 
the bivalent in diakinesis.  Furthermore in heterozygotes the knobs 
were seen on only one end of the bivalent (Herman et al., 1979).  
Similarly, in mnT2(X;II) hermaphrodites the bivalent consisting of 
LGII and mnT11(X;II) is asymmetric, one half bivalent being longer 
than the other at diakinesis.
Recently, I have looked at the orientation of the meiotic bivalents 
again, but this time have used in situ hybridization to label the ends 
of LGI and LGII.  A biotin labeled ribosomal DNA probe was hybridized 
with 'whole mounts' of N2 or let(e2000);eDp20(I;II) gonads and embryos 
and the site of hybridization, detected by immunofluorescence was 
visualized by two-color confocal microscopy so that the fluorescently 
stained chromatin could be precisely aligned with the hybridization 
signal.  At diakinesis the ribosomal probe was observed on the outside 
ends of both LGI and LGII (eDp20) bivalents.  It has also been 
possible to make a few observations on the orientation of the 
bivalents at meiosis I and in these cases the ribosomal probe was seen 
on the ends of the chromosome pointing towards the spindle poles.  
Therefore in C.  elegans the bivalents orient axially at meiosis I and 
for LGI and LGII the right end of the chromosome leads to the spindle 
pole, while the bivalent is held together until anaphase I by the end-
to-end association of the left ends of these homologues.  It is 
possible that the chromatin threads seen extending across the 
equatorial plate in the electron microscope are terminalized chiasmata 
that serve to hold the half bivalents together at meiosis I.  These 
observations suggest that the two functions of the meiotic centromere 
are divided between the two ends of the chromosome in C.  elegans.  
One end takes on the function of holding the bivalent together until 
anaphase I (rather than anaphase II) possibly by terminalized 
chiasmata, while the other end may be involved in spindle attachment.  
Since the bivalents are oriented axially, the first meiotic division 
is reductional, causing associations through terminalized chiasmata to 
be broken at anaphase I.  Therefore another mechanism is required to 
hold the half bivalents together at metaphase II.