A maior rede de estudos do Brasil

10 pág.
Cap 11

Pré-visualização | Página 1 de 5

Abstract Ciliates in the Class NASSOPHOREA 
have played a pivotal role in phylogenetic schemes 
of the evolution of diversity of ciliates. Their 
 simplified oral structures were thought to represent 
the ancestral condition of the more well-developed 
oral polykinetids of oligohymenophoreans, hetero-
trichs, and spirotrichs. They are united by two 
ultrastructural features: alveolocysts are a presumed 
synapomorphy of all representatives, although they 
have not been observed yet in synhymeniids; and 
the nematodesmata of the nasse bear nematodesmal 
or X-lamellae, which are not found in the phyllo-
pharyngean cytopharyngeal basket. The highly 
developed nasse is used to ingest various “algae”, 
typically cyanobacteria such as Anabaena and 
Oscillatoria , whose natural populations in rare 
instances nassophoreans may control. The somatic 
cortex has a highly developed epiplasm. In addition 
to the nasse, there is a set of “oral” polykinetids 
that extends often around the body circumference 
as a linear assemblage called a frange or synhy-
menium. This is why stomatogenesis in these forms 
is considered mixokinetal because both somatic 
and oral kinetal elements are involved. The genetics 
of these ciliates is virtually unexplored so details 
of conjugation, mating type system, and nuclear 
 development remain to be discovered. 
Keywords Cyrtos, articulins, B-cartwheel, pavés, 
blue-green algae 
 The ancestors of Pseudomicrothorax , a ciliate now 
assigned to the Class NASSOPHOREA , were 
argued to have played a pivotal role in the evolu-
tion of the oligohymenophoreans (Corliss, 1958a, 
1958b; Thompson & Corliss, 1958). This was based 
on both the revelation by silver staining of three 
 adoral polykinetids , similar in position to those of 
the Class OLIGOHYMENOPHOREA , and in the 
mode of stomatogenesis. The “oral” ciliature of 
 nassophoreans is typically arranged as a hyposto-
mial “frange” , an extensive ventral band of more 
complex kinetids that courses slightly posterior 
to the cytostome and may extend onto the dorsal 
surface (Fig. 11.1). Fauré-Fremiet (1967a, 1967b) 
analyzed this ciliary “frange” and the adoral struc-
tures of other nassulid -like ciliates, Chilodontopsis , 
Nassulopsis , Nassula , Cyclogramma , Paranassula , 
and Pseudomicrothorax , and argued that, despite 
their diversity, these oral structures could all be 
considered homologues, justifying the recognition 
of a clade of nassulid ciliates. De Puytorac, Grain, 
Legendre, and Devaux (1984) demonstrated that 
cortical ultrastructural features related peniculines 
(e.g., Paramecium , Frontonia ) and nassulids , 
separating them from the hymenostomes (e.g., 
Glaucoma , Tetrahymena ). This analysis expanded 
on the previous, more restricted analysis of Lynn 
(1979a) who had shown that nassulids , peniculines , 
and hymenostomes were all related using phyl-
lopharyngeans as the outgroup taxon: nassulids 
were the basal clade of the three (Lynn, 1979a). 
Sequence analyses of the large and small subunit 
rRNA genes have confirmed a close relationship 
between nassulids , peniculines , and hymenostomes 
(Baroin-Tourancheau, Villalobo, Tsao, Torres, 
& Pearlman, 1998; Bernhard, Leipe, Sogin, & 
Schlegel, 1995; Strüder-Kypke, Wright, Fokin, & 
Lynn, 2000b). Histone gene sequence similarities 
 Chapter 11 
 Subphylum 2. 
NASSOPHOREA – Diverse, Yet Still 
Possibly Pivotal 
Fig. 11.1. Stylized drawings of representative genera from the orders in the Class NASSOPHOREA . The synhyme-
niids Nassulopsis , Chilodontopsis , and Scaphidiodon . The nassulid Obertrumia
related nassulids and hymenostomes (Bernhard 
& Schlegel, 1998) although the α-tubulin gene 
sequence of Zosterodasys does not support this rela-
tionship (Baroin-Tourancheau et al., 1998). Overall, 
the earlier conception that nassulid -like ciliates 
were ancestors for the oligohymenophoreans still 
seems a reasonable view (see below Division and 
Morphogenesis ). 
 Ciliates in this class are typically holotrichous. 
Larger nassulids , which can be >200 µm in length, 
are densely ciliated. However, some of the smaller 
 microthoracids , which may be about 10 µm in 
length, can exhibit regions of the cortex that are 
barren of cilia, including the dorsal surface in 
 discotrichids . Scaphidiodon is tentatively placed in 
this class, although it has three features that relate it 
to the cyrtophorian phyllopharyngeans : (1) a non-
ciliated dorsal surface; (2) right somatic kineties 
that arch over the anterior end onto the left ventral 
surface and terminate on the anterior suture; and 
(3) a podite -like appendage at the posterior end 
(Dragesco, 1965). The pattern of the somatic cili-
ation of other nassophoreans is also similar to that 
of cyrtophorians as the right somatic kineties may 
arch over the oral region onto the left ventral surface
(Deroux, 1994b). 
 Small and Lynn (1981) were the first to elevate 
this group to the class level, establishing the Class 
 NASSOPHOREA . The class derives its name from 
the French “nasse” meaning basket and the Greek 
phoros meaning to bear. This refers to the com-
plex cytopharyngeal basket of nematodesmata that 
are used in feeding. Original descriptions of the 
ultrastructure of the nasse (Fauré-Fremiet, 1962a) 
stimulated later research on the structure, function,
and development of this complex microtubular 
apparatus in Nassula (Tucker, 1968, 1970a, 1970b). 
Earlier demonstration of the thick epiplasm in 
Pseudomicrothorax (Fauré-Fremiet & André, 1967) 
has led to the discovery of a novel class of pro-
teins, the articulins , which are found in ciliates and 
 euglenoid flagellates (Huttenlauch & Stick, 2003; 
Huttenlauch, Peck, & Stick, 1998a). Cellular and 
biochemical research has been possible because 
these ciliates can be easily grown on filamentous 
 cyanobacteria (Peck, 1977b; Tucker, 1968). 
Members of the class are united by two synapomor-
phies: (1) the presence of alveolocysts , extensions 
of the cortical alveoli into the cytoplasm; and (2) the 
presence of nematodesmal or X lamellae , accompa-
nying the nematodesmata of the nasse (Eisler, 1989; 
Eisler & Bardele, 1983). These two features are 
presumed to be present in synhymeniids , although 
ultrastructural analysis of their nasse is needed to 
confirm this (see Taxonomic Structure ). 
 11.1 Taxonomic Structure 
 Corliss (1979) placed nassophorean ciliates 
in the Subclass Hypostomata of the Class 
 KINETOFRAGMINOPHORA based on the pres-
ence of a hypostomial “frange” that extends to 
varying degrees across the ventral surface of the 
cell and that may ultimately be restricted to the oral 
region. Small and Lynn (1981, 1985) were led by 
similarities in the somatic kinetids and extrusomes 
to include synhymeniids , nassulids , microthorac-
ids , peniculines , and hypotrichs in their newly 
conceived Class NASSOPHOREA . Gene sequence 
data have now refuted a close relationship of 
 hypotrichs with these taxa and demonstrated that 
 peniculines are a basal clade in the oligohy-
menophorean radiation (e.g., Baroin-Tourancheau, 
Delgado, Perasso, & Adoutte, 1992; Lynn & Sogin, 
1988; Strüder-Kypke et al., 2000b). 
 Fauré-Fremiet (1967a) set the conceptual perspec-
tive for phylogeny within this class by proposing a 
phylogenetic transformation series for the ciliary 
 “frange” , the French for fringe. Some synhymeniids 
are considered to represent its ancestral state: a 
transverse line of dikinetids, not well differentiated 
from the adjacent somatic monokinetids, extend-
ing completely across the ventral surface and onto 
the dorsal surface (Fig. 11.1) (e.g., Zosterodasys , 
formerly Chilodontopsis ). It is imagined that these 
dikinetids became polymerized into the “pavés” ,
French meaning paving-stone or tile, or small 
polykinetids (e.g., some Nassulopsis species). These