Cap 7
33 pág.

Cap 7

DisciplinaZoologia3.765 materiais31.375 seguidores
Pré-visualização17 páginas
completely, also similar to many stichotrichs , 
from primordia that develop beside or in between 
the parental infraciliature (Fig. 7.8) (Song, 1993; 
Petz & Foissner, 1992). 
 Finally, brief mention should be made of division 
morphogenesis in the Family Reichenowellidae . 
This has been described for Balantidioides
(= Transitella ?) (Iftode et al., 1983) and character-
ized as pleurotelokinetal by Foissner (1996b) since 
the oral primordium arises by kinetosomal replica-
tion in several right lateral kineties. Replication in 
somatic kineties apparently occurs throughout the 
length of the kineties and without replacement of 
the parental kinetids. These features are more akin 
to the heterotrichs and some colpodeans (Foissner, 
1996b; Iftode et al., 1983), thus justifying our cur-
rent placement of this family as incertae sedis in 
the Subclass Hypotrichia . 
 Hypotrichs and stichotrichs have been model 
organisms for developmental biologists who are 
interested in probing the underlying mechanisms 
of pattern formation (e.g., see Frankel, 1989, 1991; 
Nanney, 1980). The pattern formed in each spe-
cies is undoubtedly under genic control, as has 
been demonstrated for the ventral ciliature of both 
groups (Génermont et al., 1992; Jerka-Dziadosz 
& Czupryn, 1995; Jerka-Dziadosz & Dubielecka, 
1985) and for the dorsal ciliature of hypotrichs 
(Heckmann & Frankel, 1968). These genetic traits 
in patterns of the infraciliature can be conserved 
across groups of species, as has been carefully 
demonstrated for Euplotes species (Gates, 1977, 
1978b, 1988; Machelon & Génermont, 1984), 
suggesting that the positional information systems 
determining pattern can be conserved while other 
genetic traits, such as enzyme polymorphisms 
evolve. Nevertheless, there can be considerable 
phenotypic variation in the numbers of cirri on 
the ventral surface of hypotrichs (Walker & Grim, 
1973). As occurs in the heterotrichs , this varia-
tion can be related to the cell size : smaller cells 
have proportionately fewer oral polykinetids in the 
adoral zone (Jerka-Dziadosz, 1976) and propor-
tionately fewer kinetosomes in each polykinetid 
(Bakowska & Jerka-Dziadosz, 1980). These are 
clearly issues that practising taxonomists need to 
consider as they decide whether to establish a new 
 morphospecies based on quantitative differences 
in cirral patterns. Qualitative differences occurring 
within hypotrich species further confound the tax-
onomist\u2019s job: Génermont et al. (1992) reported on 
a mutant of Euplotes that had disturbed positioning 
of the ventral cirral pattern compared to that of the 
wild type! 
 In a now classical series of papers on the 
ultrastructure of morphogenesis in spirotrichs , 
Jerka-Dziadosz (1980, 1981a, 1981b, 1982) pro-
vided not only a model for future studies but also 
revealed the basic details of the assembly of the 
somatic and oral infraciliature during the division 
morphogenesis of the stichotrich Paraurostyla (Fig. 
7.9). The model described below is consistent with 
the light microscopic observations on the division 
morphogenesis within the spirotrich subclasses. 
Specifically, the anarchically arranged dikinetids on 
the left side of the oral primordium are assembled 
into rows of dikinetids, and subsequent kinetosomal 
replication adjacent to the anterior kinetosome of 
each dikinetid adds additional kinetosomal rows 
(Fig. 7.9a) (Jerka-Dziadosz, 1981a). Dikinetids on 
the right side of the oral primordium rotate so that 
the dikinetid axis is transverse to the cell anterior-
posterior axis, forming an anterior-posterior file 
of kinetosomes as the primordium for the paroral 
structures, the paroral and endoral \u201cmembranes\u201d 
develop (Fig. 7.9b) (Jerka-Dziadosz, 1981b). Within 
the somatic ventral streak primordia, dikinetids 
align in an anterior-posterior file. Subsequently, 
these become partitioned in groups by intrastreak 
microtubules while kinetosomal replication at the 
structurally \u201canterior\u201d end of each dikinetid adds 
kinetosomes to complete cirral development (Fig. 
7.9c) (Jerka-Dziadosz, 1980). Fleury, Le Guyader, 
Iftode, Laurent, and Bornens (1993) demonstrated 
using immunocytochemistry that a protein scaf-
7.5 Division and Morphogenesis 167
Fig. 7.9. Schematic of the development of cortical structures in the stichotrich Paraurostyla weissei . ( a ) Development 
on the left side of the oral primordium field showing the sequential formation of five oral polykinetids by assembly of 
kinetosomes and dikinetids in the anarchic region . (from Jerka-Dziadosz, 1981a.) ( b ) Development on the right side 
of the oral primordium field showing the alignment and then dissociation of dikinetids to form the endoral and paroral 
structures. (from Jerka-Dziadosz, 1981b.) ( c ) Development of the somatic polykinetids in the marginal cirral files from a 
linear file of kinetids (bottom) to separate hexagonally-packed polykinetids (top), separated during the process by groups 
of intrastreak micro tubules (oblique lines). (from Jerka-Dziadosz, 1980.)
fold appears transiently and cirral kinetosomes 
are distributed along this. Replication and devel-
opment of dorsal dikinetids begins similarly to 
that in ventral streaks, but differences exist in the 
fibrillar associates, kinetosomal connectives, and 
the ultimate arrest of development at the dikinetid 
stage (Jerka-Dziadosz, 1982). This is similar to the 
pattern already described by Grimes and Adler 
(1976) and markedly different from that of Euplotes
(Ruffolo, 1976b). 
 Following cell division , there are relatively few 
species that elaborate extracellular structures: the 
 loricae of tintinnids and the tubes of Stichotricha
being exceptional. In tintinnids , it is the proter that 
constructs the new lorica while the opisthe is left 
with the old one (Laval-Peuto, 1994). Typically, 
construction of the protolorica takes only a few 
minutes following division and is constructed from 
the posterior to the anterior. Lorica construction 
involves the somatic cilia, which manipulate the 
secreted contents of the formative vesicles. Lorica 
length can be quite variable while lorica diameter 
is less so. If the tintinnid becomes separated from 
the protolorica , it may secrete a new lorica, called 
the paralorica . The paralorica can appear quite dif-
ferent, possibly due to its formation, which takes 
place over several hours, much more slowly than 
the few minutes taken for protolorica construction 
(Laval-Peuto, 1994). 
 7.6 Nuclei, Sexuality 
and Life Cycle 
 There is a tremendous diversity in macronuclear 
shapes among the spirotrichs . Generally, macro-
nuclei are globular to ellipsoid and often in pairs, 
which are joined together by a membranous isthmus 
that may only be visible by electron microscopy 
(e.g., Walker & Goode, 1976). In some instances, 
there can be dozens of smaller macronuclei (e.g., 
Urostyla , Fig. 7.4) while in other instances, the 
 macronucleus may be an elongate \u201cC-\u201d or \u201cE-
\u201dshape as in the hypotrichs where macronuclear 
shape may even be diagnostic of a species (Fig. 
7.2) (Curds, 1975a; Tuffrau, 1960). Micronuclei 
can be globular to ellipsoid and range from one to 
many more than ten. 
 A characteristic feature of the macronuclear 
cycle of spirotrichs is the emergence during the 
time of cell division of a replication band , formerly 
called a reorganization band (Fig. 7.1). Stein (1859) 
was probably the first to observe these in Euplotes , 
whose replication bands have been the subject of 
cell biological studies (Bonifaz & Plaut, 1974;
Gall, 1959; Olins & Olins, 1994; Olins et al., 1988). 
 Replication bands in Euplotes begin at each end 
of the elongate macronucleus and proceed towards 
the middle. The bands themselves are composed 
of a forward zone in which the chromatin is