Worm Breeder's Gazette 14(4): 36 (October 1, 1996)

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.

Gutsy competition in Cephalobus - more than just a gut feeling

Oliver Wiegner, Einhard Schierenberg

Zoological Institute, University of Cologne, 50923 Koeln, Germany

To better understand the evolution of cell-specification mechanisms we
study development in other nematode species. One of them is Cephalobus
spec., an apparently more primitive representative of rhabditid
nematodes (Skiba and Schierenberg, 1992, Dev. Biol. 151:597).
Cephalobus lacks autofluorescence and birefringence of the gut
granules as found in C.elegans. In order to analyse development of
the intestine we therefore take advantage of 1.) its tissue-specific
endocytotic activity leading to accumulation of fluorescently labelled
transferrin (Bossinger et al., 1996, Roux's Arch. Dev. Biol. 205: 494)
and 2.) the mAb 1CB4 which specifically recognizes gut cells in
C.elegans  (Okamoto and Thomson, 1985, J. Neurosci. 5: 643) and
In the last WBG we reported that gut differentiation in Cephalobus
does not depend on an induction by P2 but may depend on AB. Here we
present additional data leading to a modified interpretation. When AB
was extruded at the 2-cell-stage, 87% (35/40) of the P1-derived
partial embryos displayed gut differentiation while 13% (5/40) did
not. When AB was extruded at the 3-cell stage 45% (15/33) of the
partial embryos showed gut differentiation while 55% (18/33) did not
(or very faintly). From this we conclude that in Cephalobus  gut
differentiation can take place without inductions from either P2 or AB
In another set of experiments we extruded P1 from 2-cell stages. Much
to our surprise, 62% (20/32) of the emerging AB-derived embryos
developed strong gut differentiation although lineage analysis shows
that normally the gut in Cephalobus comes exclusively from
descendants of the E- cell as in C. elegans. To exclude a potential
confusion of AB/P1 (despite their size differences), we ascertained
the typical different early lineage patterns in the non-extruded and
the extruded cell. We found that in 65% (17/26) of the embryos AB was
still able to produce gut-like cells when EMS and P2 were extruded in
late 3-cell stages. In 35% (6/17) of these we even observed an
overexpression of the gut markers in the AB lineage with 40-60 cells
being recognized by the transferrin assay and nicely outlined by the
antibody. These cells were considerably smaller than the normal 20-24
gut cells in untreated Cephalobus  embryos, suggesting one or two
additional rounds of cell division.
Our results indicate that in Cephalobus both of the first two
blastomeres carry the potential to develop gut and that an inhibitory
interaction between AB and EMS (or their descendants) is necessarty to
restrict the gut fate to the E-lineage. They also show that the
relatively few divisions that normally take place in the gut lineage
are not a prerequisite for proper differentiation. They can be
interpreted as an early example of two cells competing for a primary
fate as observed later in equivalence groups.
Preliminary observations suggest that in manipulated embryos after
the 8-AB cell stage some AB descendants acquire a slower cell-cycle
rhythm and give rise to descendants with gut characteristics. This
pattern is reminiscent of early embryogenesis in Enoplus brevis, a
marine nematode in which a visible soma/germline differentiation is
absent. In addition, other nematode species have been described in
which gut is derived from the AB blastomere (V.V. Malakhov, 1994,
Nematodes, Smithsonian Institution Press).