Logo Passei Direto

A maior rede de estudos do Brasil

10 pág.
Cap 11

Pré-visualização | Página 4 de 5

These lamellae develop from the marginal micro-
tubules of the nematodesmata, forming a ribbon 
and eventually gaining dynein-like arms. The cyrtos
of nassulids has both Y or subcytostomal lamellae 
and, as noted above, Z or cytostomal lamellae , 
neither of which are found in the microthoracid 
 cyrtos (Eisler, 1988). 
 The structure and function of the cytopharyngeal 
basket of nassophoreans has been described in detail 
for Nassula (Tucker, 1968) and Pseudomicrothorax
(Hausmann & Peck, 1978). Microfilaments bind 
the nematodesmata at the oral or distal end and 
may extend along much of the length of the cyrtos 
while a denser annulus binds the nematodesmata 
of Nassula at a more proximal level. Displacement 
of the nematodesmata, possibly by contraction of 
the microfilamentous systems facilitates ingestion 
of the cyanobacterial filaments . The arm-bearing 
microtubules of the X or nematodesmal lamel-
lae have been implicated in endocytosis of these 
filaments. Tucker (1978) argued that the arms 
in Nassula act indirectly on a highly gelated 
cytoplasm that is strongly associated with the 
 food vacuole membrane. Hausmann and Peck 
(1979) argued that the arms in Pseudomicrothorax
Fig. 11.4. Somatic cortex of a typical nassophorean interpreted based on the somatic cortex of Pseudomicrothorax . 
(Modified after Peck, 1977b.)
are associated with microfilaments that interact 
directly with the food vacuole membrane, trans-
porting it inwards at up to 15 µm sec −1 . Subsequent 
research on Pseudomicrothorax has confirmed the 
presence of actin , α-actinin , and ATPase in the 
basket, implicating an actin-based motility system 
in feeding (Hauser & Hausmann, 1982; Hauser, 
Hausmann, & Jockusch, 1980). 
 Hundreds of square micrometers of food vacu-
ole membrane must be formed in minutes during 
the ingestion of cyanobacterial filaments in these 
ciliates. Both Tucker (1978) and Hausmann and 
Peck (1979) have observed cytoplasm and vesicles 
entering the cyrtos between the nematodesmata at 
its oral or distal end. Many of these vesicles are 
probably primary lysosomes that serve a double 
function of providing membrane for the expanding 
food vacuole and hydrolases to begin the very rapid 
digestion of their food (Peck & Hausmann, 1980). 
Subsequent folding of the food vacuole mem-
branes and vesiculation of the food vacuole may 
facilitate resorption of nutrients (Hausmann, 1980; 
Hausmann & Rüskens, 1984). Thus, we have now 
detailed knowledge of how oral structures function 
in both nassulids and microthoracids . How similar 
is the process in synhymeniids? 
 11.5 Division and Morphogenesis 
 Nassophoreans typically divide while swimming 
freely. The parental oral structures are almost com-
pletely dedifferentiated and then redifferentiated in 
synchrony with those of the opisthe (e.g., Eisler & 
Bardele, 1986; Tucker, 1970a). Foissner (1996b) 
established mixokinetal stomatogenesis to character-
ize division morphogenesis in these ciliates: both 
the parental oral apparatus and the somatic ciliature 
simultaneously participate in stomato genesis – a mix -
ture of origins. Broadly, the parental paroral gives 
rise to the opisthe paroral while the synhymenium or 
 hypostomial fringe is derived from somatic kineties. 
 Eisler (1989) and Eisler and Bardele (1986) have 
provided the most detailed comparative analysis of 
 stomatogenesis in the nassophoreans (Fig. 11.5). In 
 nassulids , the parental paroral splits longitudinally 
to form a new Kinety 1' from its right kinetosomes 
and a new paroral from the left kinetosomes. The 
kinetosomes of the paroral serve as nucleation sites 
for the development of the oral nematodesmata, 
which subsequently close to form the circular pali-
sade of the differentiated cyrtos (Eisler & Bardele, 
1986; Tucker, 1970a). The microtubule nucleating 
template that develops in association with these 
oral kinetosomes probably controls the shape and 
pattern of the growing nematodesmata (Pearson & 
Tucker, 1977; Tucker, Dunn, & Pattisson, 1975). 
 Eisler and Bardele (1986) interpreted stomato-
genesis in the microthoracids using their model 
for nassulid stomatogenesis . They concluded that 
the paroral and kinetal segments of the opisthe in 
Pseudomicrothorax and Leptopharynx originate 
from the parental paroral and are retained as the 
so-called “residual kinetosomes” at the next cell 
division. Peck (1975) and Njiné (1980) interpreted
their origin to be from somatic Kinety 1. Regardless 
of this difference of opinion, the paroral kinetosomes 
play a key role in formation of the basket while 
the adoral polykinetids assume a highly similar 
relationship with the cytostome, strongly support-
ing the ultrastructural similarities in somatic and 
oral structures discussed above. 
 The stomatogenesis of the highly unusual micro-
thoracid Discotricha may also be mixokinetal 
(Foissner, 1996b). Wicklow and Borror (1977) ten-
tatively concluded that post-buccal Kinety 1 par-
ticipated in stomatogenesis . This kinety itself may 
ultimately be an “oral” kinety, homologous to the 
 “residual kinetosomes” of other microthoracids . 
Further study of the stomatogenesis of this highly 
unusual ciliated is warranted as is investigation of 
 stomatogenesis in the synhymeniids . 
 Cytokinesis , at least in Nassula , coincides with 
the development of a contractile ring of microfila-
ments that presumably constrict against a girdle of 
several thousand longitudinally oriented micro-
tubules, which are embedded in the epiplasm 
(Tucker, 1971b). 
 11.6 Nuclei, Sexuality 
and Life Cycle 
 There has been relatively little research on these 
aspects of the biology of nassophoreans . The 
single macronucleus is homomerous and typi-
cally globular to ellipsoid in shape (Figs. 11.1, 
11.2). Species of smaller cell-size have one 
 micronucleus while larger cells may have multi-
ple micronuclei (e.g., Nassulopsis species – Sola 
11.6 Nuclei, Sexuality and Life Cycle 241
et al., 1990a). Intranuclear microtubules are found 
during division of both the micronucleus and the 
 macronucleus , and membrane bridges link micro- 
and macronuclei during late anaphase and early 
telophase, coordinating karyokinesis of the two 
nuclei (Tucker, 1967). Raikov (1982) character-
ized the nassulid macronucleus as a polyploid 
subnuclear type because the chromatin apparently 
aggregates as diploid sub units in both Nassula and 
Nassulopsis and whole genomes are believed to 
segregate at macronuclear division . These conclu-
sions based on early work need to be verified by 
modern techniques. 
 To our knowledge, the detailed cytology of 
 conjugation has not been described for any nas-
sophorean except for four stages illustrated by 
Raikov (1972) who reported that conjugation in 
Nassula might be seasonal. The pattern of conjuga-
tion appears to be typical of the ciliates. As in the 
 cyrtophorians , the cyrtos detaches from the cortex 
and is resorbed. During meiosis in Nassula , there 
are three maturation divisions, two meiotic and one 
mitotic. The micronucleus at zygotene assumes 
a “parachute stage” , a stage homologous to the 
 “crescent stage” in other ciliates. The conjugation 
 “fusion zone” in Nassula appears as a region of 
homogeneous cytoplasm that encloses the four 
 gametic nuclei . Fertilization occurs in this cyto-
plasmic region without apparent migration of the 
 gametic nuclei (Fig. 34 in Raikov, 1972). In addi-
tion to details on the cytology, we can only assume 
that there is a life cycle and genetics of mating type 
determination as for other ciliates. But what it is 
and how it is determined remain among the many 
questions to be answered for this possibly pivotal 
group of ciliates. 
Fig. 11.5 Division morphogenesis