Worm Breeder's Gazette 1(2): 13

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.

DNA Accumulation in a Mutant Blocked in Late Cell Division

D.G. Albertson, J.G. White, J.E. Sulston

Figure 1

Several mutants have been isolated that fail in post-embryonic cell 
division.  Behaviorally, the animals are uncoordinated, specially when 
going backwards.  Cytological examination revealed large 'blebs' of 
Feulgen-positive material in regions such as the ventral nerve cord, 
hypodermis and male tail where extensive post-embryonic cell division 
would normally occur.  The number and position of the blebs suggested 
that they were nuclei of precursor cells that had endoreduplicated.  
The development of the ventral nerve cord of one of these mutants, 
E1348, has been studied in detail.  E1348 is a sterile animal and is 
kept balanced over dumpy 10, E128 (II).
Ordinarily the twelve ventral cord neuroblasts divide asymmetrically 
to produce 5 daughter neurons (A,B,C,AS,D) and a hypodermal cell.  At 
the ends of the cord cell C dies (P1,P2,P9-P12) and also cell B (P11 
and P12).  When the ventral cord 'lineage' of E1348 was followed by 
Nomarski (Sulston & Horvitz, newsletter 1 p.  15) it was found that 
blebs developed in the cord through repeated failure of cells to form 
proper metaphase plates.  The precursors entered prophase, the nuclear 
membrane broke down but the metaphase plate never formed.  Eventually 
the nucleus regained the appearance of interphase until the next 
abortive metaphase.  Several animals have been followed and precursors 
P3-P8 attempt mitosis 3 times, while those at the ends of the cord may 
be blocked after 1 or 2 cycles.  From the lineage we might have 
expected to see 4 periods of mitotic activity.
The number of DNA copies made in E1348 ventral cord blebs was 
measured by microspectrofluorimetry of whole mounts using Hoechst 
33258 to stain DNA.  For blebs P3 to P8 5-6 times the diploid quantity 
of DNA was found, while at the ends of the cord precursors averaged 3-
5 times as much DNA.
Together the Nomarski observations and DNA measurements suggest that 
the asymmetric division pattern of the ventral cord neuroblasts might 
continue to operate within a single nucleus, possibly causing only 
certain sets of chromosomes to be replicated.  Furthermore these blebs 
show characteristics of the differentiated daughter cells, such as 
cell death at the ends of the cord, seen either as a bleb death or 
extruded small death, and at the ultra-structural level.
[See Figure 1]

Figure 1