Logo Passei Direto

A maior rede de estudos do Brasil

23 pág.
Cap 10

Pré-visualização | Página 1 de 11

Abstract Phyllopharyngeans are divided into four 
subclasses, only one of which is dominated by free-
living forms. The Subclass Cyrtophoria includes 
common members of biofi lm communities, from sea 
ice in Antarctica to waste water treatment plants. 
The Chonotrichia, Rhynchodia, and Suctoria are 
typically found as symbionts: chonotrichs are ses-
sile ectocommensals on the appendages of crusta-
ceans; rhynchodians are ectoparasites on the gills of 
bivalves; and suctorians are epibionts on metazoans, 
from crustaceans to turtles. The cyrtophorians and 
chonotrichs have heteromerous macronuclei, which 
suggests that they might be united into a larger tax-
on, and this is supported by small subunit rRNA 
gene sequences. The somatic kinetid is a monoki-
netid with a distinctly shaped and laterally directed 
kinetodesmal fi bril, and is underlain by sub kinetal 
microtubules. The cytopharyngeal apparatus is 
lined by microtubular ribbons, called phyllae, from 
which the class derives its name. Cyrtophorians 
and chonotrichs feed on bacteria and small algae. 
Rhynchodians use a single sucker to ingest cyto-
plasm of host cells, while suctorians typically have 
multiple sucking tentacles to catch primarily other 
ciliates. Division in cyrtophorians is characterized 
as merotelokinetal, but in chonotrichs and suctori-
ans is characterized by specialized forms of bud-
ding. The macronuclear DNA is apparently highly 
fragmented, and as in the spirotrichs, differentiates 
after the formation of polytene chromosomes. 
Keywords Internally eliminated sequences, poly-
tene chromosome 
 The ciliates included in this class are very diverse 
in body form, influenced by the fact that three of 
the four subclasses are symbionts of metazoans. 
Members of the only free-living group, the 
 cyrtophorians , range from small to medium-sized. 
Their oral region has a prominent cytopharyn-
geal basket or cyrtos , which gave rise to their 
name (Corliss, 1979). Typically dorsoventrally 
flattened, they are conspicuous members of the 
 Aufwuchs or biofilm communities of marine and 
freshwater habitats to which they attach with 
their ventral ciliature and attachment organelles 
(Deroux, 1976a, 1976b, 1977). The chonotrichs are 
typically found on the appendages of a variety of 
 crustaceans , and range from 40–350 µm in length 
(Fernández-Leborans, 2001; Jankowski, 1973b; 
Mohr, Matsudo, & Leung, 1970). As the name 
suggests, their body is cone-like, particularly at 
its apex where the ciliature is enclosed in a cone-
like structure. The rhynchodids are very small to 
small (i.e., 20–50 µm in length), typically found on 
the gills and within the mantle cavity of molluscs 
and other invertebrates (Chatton & Lwoff, 1949, 
1950; Raabe, 1967, 1970a, 1970b) where they 
attach to the tissues with a single tentacle-like oral 
structure. The suctorians , some, like Dendrosoma 
radians , as large as 5 mm, are epibionts on a wide 
range of metazoans from crustaceans (Fernández-
Leborans & Tato-Porto, 2000a) to turtles (Matthes, 
1988). Called tentaculiferans because of their 
feeding tentacles and “ acinétiens ” because they 
apparently lacked ciliated kineties, they have 
been related to heliozoans and hydrozoans at 
various times. Careful study of their complete life 
 Chapter 10 
Diverse in Form, Related in Structure 
cycle showed that they were indeed heterokaryotic
unicells or ciliates, had ciliated kineties in the 
dispersal swarmer stage, and underwent conjugation 
(Collin, 1912; Guilcher, 1951; Kormos & Kormos, 
1957a, 1957b, 1958). Phyllopharyngeans are typi-
cally not dominant members in microbial com-
munities. However, cyrtophorians , particularly in 
the genus Chilodonella , are important indicators 
of water quality (Foissner, 1988a) and can be 
troublesome ectoparasites on the skin and gills of 
fishes in aquaculture settings (Hoffman, 1988). 
Standardization of culture methods has meant that 
some Chilodonella species have become model 
organisms for studying the biology of this class 
(Radzikowski & Golembiewska, 1977). 
 Small and Lynn (1981) were the first to recog-
nize this group as a class within the phylum, and 
united these four groups based on the structure of 
the somatic kinetid and the presence of phyllae or 
leaf-like ribbons of microtubules surrounding the 
pharyngeal tube. This gives rise to the class name 
 PHYLLOPHARYNGEA , which is derived from 
phyllos , Greek, meaning leaf and pharynx , Greek, 
meaning mouth. Guilcher (1951) demonstrated 
the affinities of cyrtophorians and chonotrichs in 
her studies of division morphogenesis of these 
organisms. Ultrastructural data then prompted de 
Puytorac et al. (1974b) to include these two 
groups in their Superorder Phyllopharyngidea . De 
Puytorac, Grain, Legendre, and Devaux (1984) con-
firmed this using a numerical phenetic analysis and 
further supported the decision of Small and Lynn 
(1981) to unite these four groups into a class. 
 The Class PHYLLOPHARYNGEA must have 
originated at least in the Paleozoic Era . Mohr (1966) 
has argued that the chonotrichs themselves must 
have diverged not later than the Tertiary Period , 
some 225 million years ago. The class is established 
as monophyletic based on two strong synapomor-
phies (Small & Lynn, 1981). First, the structure of 
the somatic monokinetids is highly conserved with 
a distinctively shaped, laterally directed kinetodes-
mal fibril and subkinetal microtubules underlying 
the somatic kinetosomes. These monokinetids are 
observed in the somatic regions of the trophonts of 
 cyrtophorians , chonotrichs , and rhynchodians , and 
in the kineties of the dispersive swarmers or buds 
of chonotrichs and suctorians . Second, the phar-
ynx, or at least ingestatory structures in the case 
of the tentacles of suctorians , is lined by ribbons
of arm-bearing microtubules called phyllae . These 
ribbons are often disposed in a somewhat radial 
fashion when the ciliate is not feeding. The phyllae
lining the cytopharynx of cyrtophorians lie inside 
a palisade of rod-like nematodesmata, which 
together form the cyrtos or cytopharyngeal basket . 
 Cyrtophorians and chonotrichs possess heteromer-
ous macronuclei , a synapomorphy that unites them 
as a clade (Grell & Meister, 1982a), which might 
be recognized taxonomically in the future. The 
first molecular phylogeny of phyllopharyngeans 
supports these cytological affinities, strongly dem-
onstrating that the chonotrich Isochona groups well 
within the cyrtophorine clade (Snoeyenbos-West, 
Cole, Campbell, Coats, & Katz, 2004). 
 10.1 Taxonomic Structure 
 Corliss (1979) placed the four subclasses that we 
now include in this class in two orders, the Order 
 Hypostomata and Order Suctoria in the Class 
which included cyrtophorines , chonotrichs , and 
 rhynchodines , also included what we now consider 
to be unrelated forms. Small and Lynn (1981) 
emphasized the features of the somatic kinetid (i.e., 
a distinctively shaped, laterally directed kinetodes-
mal fibril and subkinetal microtubules underlying 
the somatic monokinetids) and the similarities in 
the oral structures (i.e., phyllae lining cytopharynx) 
as shared derived characters to establish the Class 
 PHYLLOPHARYNGEA , a name that had been 
proposed by de Puytorac et al. (1974b) to include 
only cyrtophorines and chonotrichs. De Puytorac 
et al. (1984) later supported the class as envisioned 
by Small and Lynn (1981). Lynn and Small (1997, 
2002) have maintained these subclass assignments. 
Leipe, Bernhard, Schlegel, and Sogin (1994) 
 demonstrated that the cyrtophorian Trithigmostoma
and the suctorian Discophrya formed a strongly 
supported clade based on sequence analyses of the 
 small subunit ribosomal