Worm Breeder's Gazette 8(3): 33

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.

mab-5 Levels Can Determine the Size of the Ray Domain

C. Kenyon

In lin-22(n372) males (B.  Fixsen and R.  Horvitz, 1), V2-V5 and 
sometimes V1 generate rays instead of body seam cells.  (The cells 
undergo a V5-like lineage, in which Vn.pppa as well as Vn.pppp usually 
produces a ray.) In the wild type, the generation of V rays requires 
mab-5 activity.  Examining the requirement for mab-5 activity in ray 
production in lin-22(n372) males has led to some surprising and 
significant conclusions.
Body seam cells generate alae, which are easy to score using 
Nomarski optics.  I have measured the extent and position of alae in 
lin-22(n372) males carrying different numbers of mab-5+ genes copies.  
The animals were grown at 15 C.
1.  In lin-22(n372) males with three (or four) copies of mab-5+, 
alae are generally absent, or present only in the head.  The mab-5 
dosage was increased by using sDp3 III, a free duplication very kindly 
provided by R.  Rosenbluth and D.  Baillie.
2.  In lin-22 males (two copies of mab-5 ), alae overlie the head 
and often the V1 region.  Occasionally patches of alae are present 
posterior to V1.
3.  In mab-5(e1239)/+,lin-22(n372) males (one copy of mab-5+), alae 
usually extend into the V2 and sometimes the V3 region.  In addition, 
patches of alae are more often present posteriorly.
4.  In mab-5,lin-22 males (no mab-5+ copies) alae extend along most 
of the body, with some gaps in the posterior.
V lineages have been determined in several mab-5;lin-22 males.  In 
these animals, as in lin-22 males, the V cells usually divided to 
produce two seam cells following L2 lethargus.  Cells located in the 
anterior (and sometimes posteriorly), generated body seam cell 
descendants, which produced alae.  Posterior seam cells often 
generated rays, with no overlying alae.  In addition, cells that could 
not easily be classified as body or tail seam cells sometimes were 
produced.  These cells generally produced faint (or no) alae.
A preliminary but possibly important observation is that Vn.pppp 
cells have shown a greater tendency to produce rays than Vn.pppa cells.

These results are significant for two reasons.  First, they 
demonstrate that in lin-22 mutants, the decision to produce rays 
instead of body seam cells is influenced by the level of mab-5 
activity.  The probability of any V descendant adopting a ray instead 
of a body seam fate can be increased by raising or decreased by 
lowering the level of mab-5 activity.  Thus, mab-5 can act as a 
developmental switch in lin-22 animals, and probably in the wild type 
as well.
Second, in each of the four mutant strains described above, 
posterior cells are more likely to produce rays than anterior cells.  
Each increase in mab-5 dosage increases the anterior extent of ray 
production.  This suggests 1) that in the wild type the level of mab-5 
activity defines the size of the posterior ray domain, and 2) that at 
least one component of the mab-5 regulatory circuit is present in a 
graded distribution among the seam cells.  This graded signal need not 
be mab-5 itself, and in principle could be generated within the V 
lineages or externally.