Cap 6
11 pág.

Cap 6


DisciplinaZoologia2.948 materiais19.040 seguidores
Pré-visualização5 páginas
oral apparatus is 
dedifferentiated as the proter becomes the dispersal 
\u201c swarmer \u201d stage, which has a very reduced spiral 
of membranelles and does not form food vacuoles. 
Differentiation of the proter oral apparatus occurs 
upon settling and shows a similar pattern to the 
development of the opisthe oral apparatus prior to 
cell division (Mulisch & Hausmann, 1988). 
 Heterotrichs, like the karyorelicteans, have 
remarkable regenerative abilities that have long 
been exploited to probe how the development of 
cell pattern is regulated at the morphological (e.g., 
De Terra, 1985; Fahrni, 1985; Tartar, 1961; Uhlig, 
1960) and biochemical levels (Bohatier, 1981, 
1995), a subject area that is well beyond the scope 
of our treatment here, but see Frankel (1989) for a 
comprehensive review. 
 6.6 Nuclei, Sexuality 
and Life Cycle 
 The macronuclei of heterotrichs range from 
compact ellipsoid to ribbon-like and finally to 
moniliform or beaded. Correlated with their 
large cell size, heterotrich macronuclei are large 
and highly amplified. Nucleoli are often promi-
nent. In Stentor , the nuclear envelopes of both 
 macronucleus and micronuclei are surrounded 
by an additional membrane system that integrates 
them structurally to the endoplasmic reticulum 
(Mulisch, 1988). Since Mulisch (1988) used 
special fixation techniques to obtain this result, 
it may be a more general property of heterotrichs 
than is presently realized. Heterotrichs typi-
cally have many micronuclei distributed along 
the length of filiform macronuclei or associated 
singly or in groups with each bead of moniliform 
macronuclei. 
 During cell division, filiform and moniliform 
macronuclei become compact and nucleoli dedif-
ferentiate. Macronuclear division is accomplished 
primarily by extramacronuclear microtubules 
(Diener et al., 1983; Jenkins, 1973). These extrama-
cronuclear microtubules may be intimately associ-
ated with the cortex since De Terra (1983) has 
demonstrated cortical control over the direction 
of macronuclear elongation in Stentor . Since the 
majority of ciliates use intramacronuclear microtu-
bules in macronuclear karyokinesis , Orias (1991a) 
has argued that extramacronuclear microtubules 
represent an independent evolution of the capacity 
to divide the macronucleus (see Chapter 4 ). 
 Relatively little research has been done on the 
molecular biology of heterotrich nuclei. The macro-
nuclear DNA molecules or \u201cchromosomes\u201d are typically 
long, some being up to 20 µm (Hufschmid, 1983; 
Pelvat & De Haller, 1976). Thus, very little frag-
mentation of micronuclear chromosomes appears to 
occur, in contrast to other classes (Riley & Katz, 2001; 
Steinbrück, 1990). The heterotrich Blepharisma at 
least shows a deviation from the use of the universal 
stop codons \u2013 UAA, UGA, and UAG. Of the three, it 
uses at least UAA, like the spirotrich Euplotes (Liang 
& Heckmann, 1993) (see Chapter 7 ). 
 As noted above (see Life History and Ecology ), 
 conjugation may be initially stimulated by starvation 
conditions or changes in temperature. These condi-
tions stimulate transcription of a gamone gene 
(Sugiura, Kawahara, Iio, & Harumoto, 2005), 
which is followed by excretion of diffusible mating 
type substances, called gamones (Miyake, 1996) 
or mating pheromones (Luporini & Miceli, 1986). 
All species of Blepharisma excrete two gamones: 
 blepharismone , a tryptophan derivative resembling 
serotonin (Kubota, Tokoroyama,Tsukuda, Koyama, 
& Miyake, 1973; Miyake & Bleyman, 1976) com-
mon to all species; and blepharmone , a 20-kDa 
glycoprotein that is species specific (Braun & 
Miyake, 1975). Typically, the diffusion of these 
substances is sufficient to stimulate conjugation 
in receptive individuals of fresh-water species of 
Blepharisma but is apparently not sufficient in 
marine species (Ricci & Esposito, 1981). 
 There is considerable debate about the precise 
mechanisms that stimulate conjugation in hetero-
trichs at the cellular and molecular levels. Miyake 
(1996) favors his gamone-receptor hypothesis while 
Luporini and Miceli (1986) reinterpret the results 
from Blepharisma in the context of their self-recogni-
tion hypothesis . Whichever interpretation is true, the 
heterotrichs have not evolved a stable mating type 
system, as stable lines are quite rare (Demar-Gervais, 
1971; Miyake & Harumoto, 1990). Luporini and 
Miceli (1986) argued that Blepharisma (and therefore 
 heterotrichs in general?) has not yet evolved a mating 
type system and that it is only the formation of blep-
harmone-blepharismone complexes that stimulate 
 conjugation . 
 Once conjugation is stimulated and micronuclear 
meiosis has occurred, a variable number of micro-
nuclei enter the third or pregametic division: one 
in Spirostomum and Blepharisma americanum , but 
two or three in Fabrea and Blepharisma japonicum
(Raikov, 1972). Ultimately, the products of only 
one of these micronuclei form the stationary and 
migratory gametic nuclei, which fuse to form the 
 synkaryon . Thus, the exconjugants are genetically 
identical to each other although different from 
their parents. The synkaryon typically divides three 
times to produce eight products, several of which 
develop as macronuclear anlagen that may fuse to 
form the macronucleus, following separation of the 
 conjugants (Raikov, 1972). 
 6.7 Other Features 
 Given the widespread distribution of ciliates, and 
 heterotrichs in particular, and their large cell size 
and ease of cultivation, several laboratories have 
developed low-cost bioassays or microbiotests 
using Spirostomum species. Spirostomum turns out 
to be quite a sensitive indicator species, especially 
to some heavy metal contaminants (Madoni, 2000; 
Nalecz-Jawecki, 2004; Nalecz-Jawecki & Sawicki, 
2002; Twagilimana, Bohatier, Grolière, Bonnemoy, 
& Sargos, 1998). 
6.7 Other Features 139