Worm Breeder's Gazette 13(4): 54 (October 1, 1994)

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.

Defects in Cleavage and Partitioning Lead to Cell-Fate Transformations in mes-1 Mutant Embryos.

Susan Strome1, Paula Martin2, Einhard Schierenberg3

1 Biology, Indiana University, Bloomington, IN 47405.
2 current address: Molecular Biology, University of Oregon, Eugene, OR.
3 Zoologisches Institut der Universitat Koln, Kerpener Str. 15, 50923 Koln, Germany

Figure 1

  The mes-1 gene was identified in screens for maternal-effect sterile mutations,
which cause homozygous mutant mothers to produce sterile progeny (Capowski et al.
Genetics 129, 1061-1072, 1991). mes-1 mutant progeny are sterile because they fail to
form primordial germ cells during embryogenesis. The primary defect in mes-1
embryos appears to be in the process of polarity reversal in P2 .Defects in polarity
reversal lead to altered patterns of cleavage and cytoplasmic partitioning and to
transformations in cell fate, primarily a transformation of the germ-line founder cell,
P4 ,into a muscle precursor, like its sister, D. Our results are summarized below.
(see figure)
  All 10 mes-1 alleles are both temperature sensitive and incompletely expressed.
Homozygous mutant mothers generally produce 10-20% sterile progeny at the
permissive temperature (16260C) and 60-80% sterile progeny at the restrictive
temperature (25260C) (Capowski et al., 1991). This is probably the null phenotype, since
mes-1 mutations behave similarly to a deficiency of the region. Thus, even in the
absence of wild-type mes-1 product, the process in which the gene participates can be
executed normally, especially at low temperature.
  The earliest and probably primary defect in mes-1 embryos is in polarity reversal
in P2 .In wild-type embryos, a series of unequal P-cell divisions set the germ line apart
from the somatic lineages. In wild-type embryos released from the constraints of the
eggshell (i.e. "partial embryos"), P1 and P2 are generated to the posterior and P3 and
P4 to the anterior of their somatic sisters. None of the 23 partial mes-1 embryos
examined at 25260C displayed normal polarity reversal. Instead, the divisions of P2 and
P3 displayed a variety of deviations from the normal pattern; for example, some
divisions were oriented along the A-P axis but were relatively symmetric and others
were asymmetric but oriented transverse to the A-P axis. These division patterns
reflect mis-positioning of the spindle in the P2 and P3 cells of mes-1 embryos.
  mes-1 embryos display defects in the divisions of P2 , P3 ,and P4 .In intact mes-1
embryos, the timing of divisions and sizes of blastomeres are normal until the 7-cell
stage, after which they differ from wild-type embryos in the following ways: 1) The
divisions of P2 and P3 do not display the pronounced asymmetry seen in wild-type
embryos. 2) P4 often divides precociously and undergoes extra rounds of division, at
about the same time as its somatic sister, D. Embryos that display precocious and
supernumerary divisions of P4 hatch into L1 sthat lack the germ-line cells Z2 and Z3
and develop into sterile adults. Thus, the sterility of mes-1 animals is due to defects in
the divisions that normally generate the primordial germ cell, P4 .Fertile mes-1 adults
arise from embryos that undergo a normal lineage.
  mes-1 embryos display defects in cytoplasmic partitioning. P granules are
correctly partitioned to P1 and to P2 in mes-1 embryos, but are mis-partitioned during
the divisions of P2 and P3 .For example, in the 16- to 46-cell embryos examined, 18%
contained P granules in only P4 (as seen in wild type), 80% contained P granules in
both P4 and D, and 2% contained P granules in D alone. The cause of the
mis-partitioning appears to be that P granules are often segregated to the side of the
spindle in P2 and P3 ,instead of to the pole of the spindle destined for the P-cell
daughter (as in wild type). On consequence of mis-partitioning is that P granules are
often located in ectopic cells (probably muscle) in mes-1 larvae.
  In mes-1 embryos P4 is transformed into a muscle precursor. Our lineage
analysis suggested that in at least some mes-1 embryos P4 follows the fate of its sister,
D, and generates body muscle instead of germ line. Two results support this germ line
to muscle transformation hypothesis. First, sterile mes-1 adults contain up to 20 extra
body muscle cells, the number normally generated by D. Using a transgenic muscle
reporter construct from Andy Fire, sterile mes-1 worms contain 103 261 7.8 muscle
nuclei compared to 94.7 261 1.5 in wild-type worms and 94.6 261 1.9 in fertile mes-1 worms.
Second, "isolated" P4 blastomeres in mes-1 embryos generate muscle cells. In
laser-operated wildtype embryos (see Bowerman et al. Cell 74, 443-452, 1992 for
procedure), isolated D blastomeres generate muscle and isolated P4 blastomeres do not,
as judged by the ability of the cells to contract and by staining with anti-myosin
antibody (Ab from David Miller). In contrast, in mes-1 embryos almost all (18/19)
isolated P4 blastomeres produced muscle cells; D blastomeres also produced muscle.
Thus, in mes-1 embryos, P4 cells contain the potential to generate muscle and are most
likely responsible for generating the extra body muscle cells observed in sterile mes- 1
  Summary and model. We hypothesize that either the program or the machinery
underlying polarity reversal is inherently sensitive to temperature, and that wild-type
mes-1 product guarantees the successful execution of polarity reversal at both high
and low temperature. This insures that the progeny of P2 (C, D, and P4 )have the
correct sizes, contents, and neighbors, and that they follow their normal fates. In the
absence of mes-1 product, there is loss of coordinate regulation of spindle positioning
and cytoplasmic partitioning in P2 and P3 (see below), leading to progeny cells with
altered sizes, contents, and developmental programs. The main developmental defect
is a transformation of the germ-line cell, P4 ,into a muscle precursor, like its sister.

Figure 1