Worm Breeder's Gazette 10(2): 50

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Lineage Alterations in egl-5 Males

Andrew Chisholm

Figure 1

Mutations in the gene egl-5 III have highly pleiotropic effects.  
Hermaphrodites are Egl as a result of defective HSN migration.  A 
number of observations suggest that this is due to incorrectly 
specified HSN identity (Chand Desai, pc).  Hermaphrodites mutant for 
the candidate null allele n945am and 5 others (including the reference 
allele n486) are also Unc (spontaneous backward coiler, ventrally or 
dorsally) and are tail Mec, both from L1 onwards.  Males mutant for 
any of these 6 alleles are severely abnormal (Mab).  An exceptional 
allele, n1439, which has the lowest Egl penetrance (92%), and which is 
not itself Unc, Mec or Mab, fails to complement the others for Egl but 
complements them for Unc, Mec and Mab (Desai, pc).  To discover the 
basis for the Mab phenotype, I have examined the postembryonic 
lineages of egl-5;him-5(e1490) males.  The account below is based on 4 
n486 males and 3 n945 males lineaged from early L1 to the L3 moult or 
later, as well as several partial lineages for both genotypes.
Ventral hypodermis: In wild-type males, P(9-11).p constitute the 
preanal equivalence group, which generates the hook.  The three cells 
in the group undergo lineages designated 3 , 2 , or 1  respectively, 
based on the replacement hierarchy seen in ablation experiments; P12.p 
divides in L1 and does not participate in the equivalence group.  In 
n486 and n945 males, the whole preanal equivalence group is 
effectively shifted posteriorly: P12.p fails to divide in L1, but 
instead survives until L3 and undergoes a 1  lineage; P11.p has a 2  
fate and P.10.p is 3 .  This is seen in all animals, although there is 
some slippage: P9.p may still participate as a 3  lineage, and P10.p 
may execute a lineage intermediate between 3  and 2 .  P12.aap divides 
in late L3, implying that the transformation is at the level of the P 
cells themselves rather than their posterior daughters.  Preanal 
cytoplasmic retraction begins in L4, but no recognizable hook is made. 
A similar transformation is seen in let-23 males (Paul Sternberg, pc).
P12.p fails to divide in egl-5 hermaphrodites as well.
Lateral hypodermis: Lateral hypodermal lineages are wildtype except 
for one intriguing blip: the division of V6.pppp is unequal (
resembling the other V ray precursor parents), instead of being equal (
see Fig. 1).  V6.ppppa, instead of giving rise to R6, is hypodermal.  
A plausible tail seam is made but its 5 nuclei fail to migrate 
anteriorly in L4.  Tail cytoplasmic retraction begins normally, but 
there is no ventral retraction: a vestigial fan with 8 ray papillae on 
each side is made.  Phasmid traces and T rays are always present.
B, Y, U, F & K: The lineages of all the four male-specific blast 
cells (B, Y, U and F) are abnormal.  None of the structures formed by 
the descendants of these cells (spicules, proctodeum, etc.) is made.  
All 5 cells are distinctly smaller than normal in L1.  The first 
divisions of B and Y are on time but later divisions have abnormal 
axes and timing.  Typical B and Y lineages in n945 are shown (Fig.  2)
: the patterns are quite reproducible.  The normal symmetrical (left-
right) divisions are not observed in B, Y, U or F, although the latter 
two may divide in L3.  K never divides in either sex in n945, although 
in n486 it can execute a proliferative lineage like that of Y above, i.
e.,with a round of divisions at the end of L1, L2 and L3.  Somatic 
gonad: The male somatic gonad is grossly abnormal, as a result of 
delays and defects in the late somatic gonad lineages.  These cause 
gonad growth to grind to a halt during L3, when the reflexed arm is 
about half as long as the distal arm.  The linker cell (which appears 
normal) subsequently breaks free of the rest of the gonad (it 
sometimes takes a couple of cells with it) and ploughs onwards, ending 
up somewhere between the distal tip cell and the cloaca.  The abortive 
somatic gonad still manages to form a sac-like structure into which a 
seemingly normal germline proliferates.  Pockets of spermatogenesis 
break out at the correct time; the gonad may burst, leaking sperm into 
the body cavity.  The lineage defects are variable: representative 
ones are shown (Fig.  3).  Both vas deferens and seminal vesicle 
lineages (of both Z1 and Z4) are abnormal.  Usually the first round of 
vas divisions is OK, but thereafter chaos ensues.  Hermaphrodite 
somatic gonads look completely wild- type.
Occasionally (<5%?) QL migrates anteriorly (like QR).  Other 
embryonic and postembryonic migrations (excepting the HSN) appear 
normal.  M is wildtype at least until the divisions of the SM grand-
daughters; 10/10 n945 males had dorsal coelomocytes (SM2L.pp).  
n945am is believed to be a null allele of egl-5 because the Egl does 
not get worse in n945/nDf16 (Desai, pc).  To check this conclusion 
with respect to the Mab I have lineaged 3 n945/nDf16 males fully (and 
several partially).  The ventral hypodermal defects do not get worse 
in the deficiency het.  The lateral hypodermis is sometimes more 
disorganised.  The somatic gonad defects may be slightly worse (in 2 
animals the somatic primordium was disorganised from L1).  The 
ectoblast lineages are difficult to interpret, but don't seem much 
worse.  Therefore n945 may be null.  However, none of the extant egl-5 
alleles derives from a complementation screen in which lethal or 
sterile null alleles could have been picked up.  
The pleiotropic effects of egl-5 mutations are a puzzle.  Some of 
the defects may reflect a local role for egl-5 product in the preanal 
or rectal region (HSNs are born here, near the PLMs).  Several DA,DB 
and DD motoneurons and numerous interneurons also arise in the 
vicinity during embryogenesis, and defects in these might give rise to 
a backward coiler phenotype.  It is difficult to fit the male somatic 
gonad defects into such a picture.  
Thanks to Chand Desai for sending egl-5 strains and information.

Figure 1