Worm Breeder's Gazette 13(5): 16 (February 1, 1995)
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.
*Departments of Biology, New York University and **Molecular Genetics, A.
Einstein College of Medicine
As a first step toward understanding the mechanism by which
morphological diversity has arisen in rhabditid male tails, we have
compared ray pattern, morphology and development in ten species of
Rhabditidae, the family of nematodes that includes C. elegans.
In these species, 8 (Rhabditis blumi), 9 (Caenorhabditis spp.,
Rhabditis sp. "br" [EM435] and Rhabditella axei) or 10 (Teratorhabditis
palmarum and Pelodera strongyloides dermatitica) pairs of genital
papillae (rays) extend outward from the body within the folded cuticle of
the fan (bottom sketch in panels B-H, Fig. 1). In the two species with
10 pairs of papillae, one pair is actually the phasmids, which, relative
to those in C. elegans, are displaced anteriorly into the domain of fan
formation. Ray placement also appears to differ among the species with
regard to both anteroposterior and dorsoventral axes. For example, the
entire arrangement of rays in the Caenorhabditis species is shifted
posteriorly relative to that in the other species. Also, some species
(e.g., R. blumi and R. sp.) show relatively uniform spacing of rays along
the anteroposterior axis whereas other species (e.g., Rh. axei and T.
palmarum) have arrangements with some rays close together separated by
large gaps from other rays. Along the dorsoventral axis, two rays
generally open onto the dorsal fan surface and the other rays generally
open at or very near the fan margin; in Caenorhabditis, however, there is
more extensive dorsoventral differentiation such that 3 rays (the 1st,
5th and 7th, counting from the anterior) open dorsally and 2 (the 2nd and
4th) open ventrally.
The fan and rays form--after the ray tips have been anchored in the
L4 cuticle--in a morphogenetic process that is superficially similar in
all of the species: the cells in the tail "retract" anteriorly, causing
the newly formed adult cuticle to collapse and form the fan around the
ray processes. The 2-D pattern of papillae in the L4 thus prefigures the
species-specific 3-D pattern of rays in the adult.
We investigated the developmental changes in ray and hypodermal cell
shape and position by immunofluorescent staining of L3, L4 and adult
stages of the various species with the anti-zonula adherens antibody,
MH27. Because 4-cell clusters and an Rn.p hypodermal cell accompany the
generation of each ray, we propose that the ray (Rn) sublineages in each
species are identical to those in C. elegans. Furthermore, these 9
clusters are produced in the lateral hypodermis of these species in an
arrangement that is nearly identical to that in C. elegans (Fig. 2). The
only differences are that the R8 sublineage does not appear in R. blumi,
and the phasmids of T. palmarum and P. strongyloides are positioned far
anterior of the tail tip cells near R5.p (unlike the rays, the phasmids
in adults of these species take up FITC dye, as in C. elegans, suggesting
phasmid function may remain unchanged). We have thus proposed a system
of ray homologies based on the relative positions at which these ray
cells are born in the lateral hypodermis (Fig. 2). If the rays were
anchored at the positions in which the ray cells were born, the
arrangements of rays in adults of different species would be essentially
identical (a hypothetical pattern that we have called the "rhabditid
ground plan"--panel A, Fig. 1). The differences in adult ray patterns
must therefore be due to morphogenetic differences that take effect only
after the ray cells are born.
After they are born, the 3 cells that will make each ray aggregate
at specific and reproducible junctions between hypodermal cells. The 2
neuronal cells then sink below the epidermal surface, leaving the ring-
like structural cells at these positions (the numbered circles in the
central sketches of each panel in Fig. 1); the ray tips (visible as
papillae at the surface) become anchored in the epidermis. Species
differences in ray patterns are readily traced at this stage by
comparison to the ground plan (the top sketch in each panel of Fig. 1,
with species-specific "shifts" from this pattern indicated by arrows).
For example, ray 2 in Rh. axei is shifted posteriorly (relative to the
ground plan) to a position near ray 3, resulting in a large gap between
ray 1 and ray 2 in this species; ray 4 is shifted posterior of ray 5 and
clusters with ray 6. These shifts are always accompanied by differences
in the associations between the ray cells and other ray or hypodermal
cells; e.g., the posterior shift of ray 4 in Rh. axei is accompanied by
an association between the processes of rays 4 and 6 with the junction of
R6.p and R7.p. One conserved feature is that the dorsally positioned
rays 5 and 7 always open on the dorsal fan surface. In each case, the
cell-cell associations correlate with and predict the arrangement of rays
in the adult. Hence, these data support the model (Baird et al., 1991,
Development 113:515-526) that these cellular associations determine ray
arrangement.
Several genes are already known to be involved in determining ray
identities (and thus the cell-cell associations made by the ray cells
during L4 morphogenesis); variation in such genes might be involved in
the evolution of male tail morphology. For example, increased dosage of
mab-5 or gain-of-function mab-5 mutations cause anterior-to-posterior
transformations of ray identities (Chow & Emmons, 1994, Development
120:2579-2593). Similar changes may have occurred in the evolution of
ray patterns such as those of Rh. axei.
Fig. 1. Schematic comparisons of L4 development (represented by the
stage just after the neuronal processes sink, leaving the structural
cells at the surface, visualized by MH27 staining as small circles which
are numbered according to our ray homology system). Comparisons of
development in the different species (panels B-H) are made relative to
the hypothetical ground plan (A) described in the text: arrows in the
top sketches of panels B-H represent differences in cell arrangements
between the rhabditid ground plan and the various species.
Fig. 2. Schematic of the relative positions at which ray cells (numbered
according to our ray homology scheme) and their
associated Rn.p hypodermal cells are born in the ten rhabditid species
studied. In C. elegans, one cell of each 4-celled ray cluster undergoes
a programmed cell death, 2 cells go on to form neuronal processes, and
one cell forms the ring-like structural cell. se: body seam. Ph:
phasmid.