A maior rede de estudos do Brasil

11 pág.
Cap 6

Pré-visualização | Página 3 de 5

reversal when the intensity of 
incident light suddenly increases. The mechanism is 
likely due to a release of H + by the pigment. These 
ions are then translocated to the cytoplasm, causing 
electrical potential changes in the plasma membrane 
and subsequent ciliary reversal and reorientation of the 
cell (Fabczak, Fabczak, & Song, 1993a; Fabczak et 
al., 1993b; Matsuoka & Kotsuki, 2001; Menzies, Das, 
& Wood, 2004; Sobierajska, Fabczak, & Fabczak, 
2006), perhaps involving a G-protein-mediated sig-
nalling pathway (Fabczak, Sobierajska, & Fabczak, 
2004). Certainly, a photophobic response might be a 
selective advantage, keeping the ciliate hidden from 
potential predators. However, one cannot help but 
wonder which trait is under selection: the photopho-
bic response mediated by the pigment chemicals or 
the toxic nature of the pigment chemicals themselves 
(Lobban, Hallam, Mukherjee, & Petrich, 2007). 
 Heterotrichs can survive several weeks without food 
(Jackson & Berger, 1985a, 1985b). Nevertheless, 
 encystment is a common feature of this class, stimu-
lated by a variety of factors, such as absence of food 
or excess metabolites (Giese, 1973; Repak, 1968). 
The cyst wall is formed of several layers, which 
may contain chitin (Mulisch & Hausmann, 1989). 
Excystment occurs through a cyst pore plug or 
 micropyle . It may be induced by freshly bacterized 
medium (Giese, 1973; Repak, 1968), and is perhaps 
promoted by a substance liberated from excysting 
conspecifics (Demar-Gervais & Génermont, 1971). 
 Conjugating Stentor have been observed rarely in 
nature (Burchill, George, Lindberg, & Sims, 1974; 
Tartar, 1961). Stentor coeruleus mates with some 
frequency in the laboratory, perhaps induced by 
elevated temperatures (Rapport, Rapport, Berger, 
& Kupers, 1976), while Blepharisma has served 
as a model for understanding heterotrich sexual 
processes (Miyake, 1996). The marine heterotrich 
Fabrea requires a complex organic medium to 
complete conjugation (Demar−Gervais, 1971). 
 6.3 Somatic Structures 
 The heterotrich cell body is quite variable in shape 
(Fig. 6.1) depending upon whether the ciliate is 
a benthic or substrate oriented species or a more 
planktonic species. The body is covered by numer-
ous bipolar somatic kineties composed of dikinetids 
(Tuffrau, 1968). There is a fine glycocalyx on top 
of the plasma membrane, which is underlain by 
an alveolar layer that is often not well-developed 
and appears to be discontinuous. The epiplasm is 
very thin and inconspicuous. Mucocysts and pig-
mentocysts are found beneath the alveolar layer, 
giving the range of “living colors” in this group 
– black, blue, blue-green, brown, rose, and yellow. 
Chlorella symbionts may impart a grass-green 
color to those species harboring them. 
6.3 Somatic Structure 133
134 6. Subphylum 1. POSTCILIODESMATOPHORA: Class 2. HETEROTRICHEA – Once Close to the Top
 The somatic kinetids are typically dikinetids that 
lie 20–40° to the long axis of the kinety (Grain, 
1984; Lynn, 1981, 1991; Fabrea – Da Silva Neto & 
Grolière, 1993; Condylostomides – Da Silva Neto, 
1994b). The anterior kinetosome is often the only 
one ciliated and it bears a tangential transverse 
ribbon of about 6 microtubules near triplets 3, 4, 
and 5. This ribbon is usually doubled by a single 
microtubule on the right inside edge (Fig. 6.2). 
The posterior kinetosome is less often ciliated 
and may have a transverse ribbon associated with 
it, oriented in a variety of ways. There is a kineto-
desmal fibril homologue originating near triplets 
5, 6, which extends laterally, usually associating 
with the postciliary ribbon originating from the 
next anterior dikinetid. It is called a homologue 
because it usually does not have the obvious 
periodic striation found in other ciliates, although 
it arises from the same triplet region. The post-
ciliary ribbon of this kinetosome is divergent and 
extremely well-developed, numbering 12 or more 
microtubules, which extend towards the cortex as 
ribbons oriented perpendicular to the cell surface 
and separated by a single microtubule (Fig. 6.2). 
These postciliary ribbons are accompanied by 
dense material called a retrodesmal fibril (Grain, 
1984) or a postciliary accessory fibre (Peck, Pelvat, 
Bolivar, & Haller, 1975). Yogosawa-Ohara, Suzaki, 
and Shigenaka (1985) suggest that this fibre may 
induce the twisting of the body during contractions 
of Spirostomum . A number of postciliary ribbons 
are integrated together to form the conspicuous 
 postciliodesma or Km fiber in the cortex of these 
 Myonemes are typically present and always so 
in contractile forms. They are predominantly longi-
tudinally arranged around the entire body, and are 
either in direct contact with the somatic kinetosomes 
or indirectly contact them via intermediate fibres. 
Transverse myonemes may integrate the longitudi-
nal bundles, either locally or throughout the cortex. 
These cortical contractile systems created consider-
able interest among cell biologists who sought to 
explain their role in changing cell shape. Huang 
and Pitelka (1973) first experimentally demonstrated 
the antagonistic relationship between the myonemes 
and postciliodesmata in Stentor : the myonemes are 
responsible for contraction of the body while micro-
tubule-on-microtubule sliding achieves the slow elon-
gation back to the “relaxed” form. This system 
has now been demonstrated in normal contractions 
of Spirostomum (Yogosawa-Ohara & Shigenaka, 
1985; Yogosawa-Ohara et al., 1985) and in the light-
induced contractions of the posterior portion of the 
body in Blepharisma (Ishida, Suzaki, & Shigenaka, 
1991a; Ishida Suzaki, Shigenaka, & Sugiyama, 
1992). Contraction and elongation involve calcium 
(Ishida et al., 1992; Legrand, 1971), which may be 
stored as hydroxyapatite in crystalline intracellular 
deposits (Takagui & Silveira, 1999) or in cortical 
alveoli (Ishida, Shigenaka, Suzaki, & Sugiyama, 
1991b). However, there are conflicting reports regard-
ing the inhibitory action of cytochalasin , an actin 
antagonist, on contraction (Ettienne & Selitsky, 1974; 
Yogosawa-Ohara & Shigenaka). Although we can-
not yet conclude that this is an actin-based system, 
a caltractin-like protein has been localized to the 
 myonemes of Stentor (Maloney, McDaniel, Locknar, 
& Torlina, 2005). 
 The contractile vacuole system is well- developed 
in heterotrichs , especially the fresh-water forms, 
which may have conspicuous collecting canals 
(Patterson, 1976, 1980). 
 Mucocysts are probably widespread among 
heterotrichs, since they often encyst . However, 
there are relatively few studies on these. Mulisch 
and Hausmann (1983) documented their role in 
 lorica construction in folliculinids . Pigmentocysts 
are considered to be a special type of mucocyst 
(Hausmann, 1978). 
 6.4 Oral Structures 
 Tuffrau (1968) presented a detailed description of 
the heterotrich oral region and its fibrillar supports, 
but this diversity has been considerably reduced 
with the removal of a number of groups from this 
class (see Taxonomic Structure above). The oral 
region is characterized by an adoral zone of polyki-
netids , which typically form a small dextral spiral 
around the cytostomal region deeper in the oral 
cavity. The oral polykinetids then extend out of 
this deeper cavity onto the cell surface of the oral 
region where variations in the pattern become more 
conspicuous: they may be organized as a linear file 
along the left border; may form a more or less com-
plete circle around the anterior end, or may extend 
out on two wing-like projections in the folliculinids 
(Fig. 6.1). The peristomial region circumscribed by 
6.4 Oral Structures 135
Fig. 6.2. Ultrastructure of the cortex of the Class HETEROTRICHEA . A Somatic dikinetids . ( a ) Blepharisma (after 
Ishida et al., 1991a). ( b ) Climacostomum (after Peck et al., 1975). ( c ) Eufolliculina

Crie agora seu perfil grátis para visualizar sem restrições.