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.
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.