Worm Breeder's Gazette 8(3): 30

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 is Required for Regional Specification

C. Kenyon

Figure 1

The V and P ectoblasts are arranged in rows along the body axis.  
Many of the V and also the P cells undergo identical sequences of 
divisions and generate the same cell types.  However in certain 
regions, for example the hermaphrodite vulval region and the male 
preanal region, V and P cells generate more complex lineages than 
their homologs.  Immigrating sex myoblasts also produce sex muscles in 
these regions.  We wish to understand how these specialized regions 
are defined and diversified.
mab-5 may provide a clue.  Putative mab-5 null mutations 
specifically eliminate nearly all preanal specialization.  In these 
mutants, cells in this region develop like their less specialized 
homologs located elsewhere.  mab-5 mutations alter two aspects of 
regional diversification, cell lineages and cell migration.  These 
alterations are summarized in a table below.  They are listed in more 
detail here.
Changes in cell lineages.
1.  Lateral hypodermis.  In mab-5 mutants, the male V5 and V6 cells 
do not generate male-specific ray lineages.  Instead, these cells 
follow developmental sequences characteristic of their hermaphrodite 
homologs.  (The V5 postdeirid and T are unaffected.) 2.  Ventral Pn.p 
hypodermal precursors.  In mab-5 males, P(9-11).p do not divide during 
the third larval stage.  These nuclei were found within the large hyp7 
syncytium in serial section EM reconstructions of an adult mab-5 male.
Ross Francis's MH27 monoclonal antibody, which outlines cell 
boundaries at known positions of desmosomes, has proved useful in 
determining the likely time of cell fusion with hyp7.  In mab-5 males, 
P(9-11).p generally lose their antibody staining during L1 lethargus, 
suggesting that they fuse with hyp7 at this time.
The fusion of P(7,8).p is also altered.  In normal males, these 
cells lose their staining during L1 lethargus.  (Unlike P(3-6).p, 
which, surprisingly, continue to stain with MH27).  In mab-5 males P(7,
8).p behave like normal P(3-6).p.
The fact that P(7,8).p do not fuse during L1 lethargus may explain 
the phenotype of mab-5 77)dom males.  In lin-12(
n177) single mutants, P(3-6).p, but not P(7,8).p, generate 
pseudovulvae (1).  In contrast, in mab-5(e1239) 77) 
double mutants, all P(3-8).p generate pseudovulvae.
P12.p is usually normal in mab-5 animals.  However, in mab-5 animals 
of both sexes, P12.p often does not divide, but fuses with hyp7.  A 
puzzling observation is that at a lower frequency, p11.p is displaced 
posteriorly, and appears to adopt the fate of normal P12.p.
The fates of Pn.p cells that lie outside this region appear to be 
normal.  In particular, vulval morphology is normal (although cell 
lineages have not yet been determined), and the animals are egg-laying 
proficient.  3.  Pn.a neuroblasts.  P(11,12).aaap (homologous to the 
VB motoneurons) normally undergo programmed cell death in both sexes.  
In mab-5 mutants, these cells do not die, but acquire compact nuclei 
characteristic of neurons.
In wild type males, P(9-11).aap divide.  In mab-5 mutants, they do 
not divide, but, like P2.aap, remain small, compact, and assume a 
dorsolateral position within the cord.  At a low frequency these cells 
undergo programmed cell death.
Other Pn.a derivatives appear to be unaffected.  4.  Muscle.  mab-5 
mutations affect the development of the unique postembryonic myoblast 
M, located in the preanal region.  The L1 M division pattern in mab-5 
animals is variable.  In both sexes, M derivatives that would normally 
become body muscles become apparent sex myoblasts.  These cells 
increase in size and divide repeatedly during the L3.  mab-5 
hermaphrodites have never been observed to produce dorsal coelomocytes.

M produced extra sex myoblasts in wild type males when hyp7 was 
damaged with a laser microbeam, raising the possibility that mab-5 
mutations alter developmental information provided by the underlying 
Changes in migratory behaviors.  1.  QR and its descendants normally 
migrate anteriorly, while QL and its descendants migrate posteriorly.  
M.  Chalfie has shown that in mab-5 mutants, QL's descendants migrate 
anteriorly (2).  2.  The normal and ectopic mab-5 male sex myoblasts 
migrate anteriorly instead of posteriorly.  If a hermaphrodite vulva 
and gonad is provided by introducing J.  Hodgkin's tra-1(e1488) 
mutation, these cells assemble into structures that resemble vulval 
muscles under polarized light.  3.  mab-5 activity also appears to be 
necessary for the movement of V ray precursors into the tail.  In 
about half of mab-5/+ animals, the most anterior ray is not assembled 
within the fan.  The ray can often be found near its birthplace.  In 
addition, mab-5(e1239) 72) animals usually produce 
V5 and V6 rays (see following report).  Unlike wild type or lin-22 
rays, these do not move posteriorly.
These alterations probably represent the mab-5 null phenotype.  
Three alleles, e1239 (isolated by J.  Hodgkin) e1936 and e2011 are 
similar to one another and to e1239/nDf16.  e2011 was isolated in a 
complementation screen (2500 chromosomes examined) that could have 
detected more severe mab-5 alleles.  In addition, nDf16/+ and e1239/+ 
display similar ray mutant phenotypes.
These results indicate that mab-5+ functions in a process that 
distinguishes preanal cells in multiple tissues from cells elsewhere.  
This circuit appears to function as a developmental switch between two 
alternative cell fates: preanal specialization vs.  ground state.  
Presumably because the affected cells are in different regulatory 
states, individual responses to mab-5 activity differ.  The most 
striking example is that mab-5 activity leads to fusion of P(7,8).p 
but prevents fusion of P(9-11).p..
[See Figure 1]

Figure 1