Worm Breeder's Gazette 11(4): 110

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.

Reversal of L-R Asymmetry by Micromanipulation of 6-Cell Embryos: ABa1 and ABp1 are Equivalent to ABar and ABpr, Respectively

Bill Wood

Figure 1

Both the embryo and the embryonic cell lineage are asymmetric from 
the 6-cell stage onward, particularly with regard to the ABa lineages (
1).  The general bilateral symmetry of the animal is superimposed on 
the asymmetric lineage by lineally nonhomologous cells adopting 
analogous fates at equivalent contra lateral positions during 
embryogenesis (1), but several asymmetries remain throughout larval 
development, such as the anterior-posterior positions of the left and 
right coelomocyte pairs (2) and the anterior-r, posterior-l 
orientation of the primordial and developing gonad (3).  These 
asymmetries normally have the same handedness in all individuals; in 
N2 populations grown at 16 C, no animals with reversed gonad 
handedness were found among about 2500 examined.  Handed asymmetry 
first becomes evident in the embryo between the 4- and 6-cell stages, 
when skewing of the l-r cleavages of ABa and ABp results in 
positioning of the al and pl daughters anterior to ar and pr, 
respectively, with al more ventral than ar [see Wood, WBG 11,#3(May)
:73, 1990].
Reversal of handedness was accomplished using methods similar to 
those of Priess and Thomson (4), by rolling a 4-cell embryo to ventral 
side (EMS) up with a microneedle and then pressing down and back on 
the left ventral surface of ABa during AB-cell cleavage.  When 
successful, this procedure reverses the normal skewing of both AB 
spindles and results in ar and pr being anterior to al and pl, 
respectively, at completion of the cleavage (see diagram).
The embryonic development of a reversed embryo was observed and 
recorded using the '4D Microscope' developed by J.  White.  (This 
consists of a Nomarski microscope equipped with a video camera 
connected to a computer controlled optical disc recorder and focussing 
drive motor, which automatically records complete sets of serial 
optical sections at predetermined intervals and allows subsequent 
playback of consecutive images from any desired optical section to 
facilitate lineaging.)  Cell assignments in the reversed embryo, based 
on timing of divisions, relative positions, and lineaging to confirm 
identities of AB descendants, were entirely consistent with a 
completely enantiomorphic but otherwise normal cleavage pattern.  For 
example, MS descendants were located on the left side of the early 
embryo and C descendants on the right; MSa and Ca descendants were to 
the right of MSp and Cp descendants, respectively, and so on.  In the 
L1 that hatched from this embryo, 14 normally asymmetrically placed 
nuclei scored were all l-r reversed: Z1, Z2, Z3, and Z4 in the gonad 
primordium, M, the four cc's, Q2, the exc cell, mu sph, hyp11, and PVR.
The adults that developed from other reversed embryos showed 
reversed handedness for the somatic gonad, placement of coelomocytes, 
and the positions of neuronal cell bodies relative to processes in the 
ventral nerve cord.  The reversed animals were healthy, moved normally,
and were normally fertile.
These results indicate that ABal is equivalent to ABar and ABpl to 
ABpr, and, therefore, that the left-right differences in lineage 
patterns and cell fates exhibited by descendants of ABa and ABp must 
be dictated by cell interactions, which differ on the two sides as a 
consequence of the asymmetric positioning of these cells relative to 
others in the embryo.  The respective cell contacts of the ABal-pl and 
ABar-pr pairs are still equivalent in the 8-cell embryo despite the 
asymmetry, and earlier laser ablation experiments (1) provide evidence 
for cell autonomy of many AB-cell fates subsequent to the 51-cell 
stage.  Therefore, the determinative interactions are most likely to 
occur between the 15-cell and 51-cell stages.
[See Figure 1]
Thanks to A.  Chisolm, J.  Rothman, J.  Sulston, and J.  White for 
timely coaching and sage advice.

Figure 1