A maior rede de estudos do Brasil

33 pág.
Cap 7

Pré-visualização | Página 16 de 17

stichotrichs (e.g., Urostyla , 
Pseudourostyla , Heckmann, 1965; Raikov, 1972, 
Takahashi, 1974). Cells are typically of similar size 
although cells of different size have been shown to 
mate preferentially under some conditions (Gold 
& Pollingher, 1971). Laval-Peuto (1983) observed 
cultured forms of Favella to mate even when the 
loricae of cells were of different types. Fusion of 
cells typically occurs near the oral regions often 
accompanied by disassembly of cortical alveoli 
and some oral structures and assembly of microtu-
bules and microfilaments (Dallai & Luporini, 1989; 
Geyer & Kloetzel, 1987a, 1987b; Laval-Peuto, 
1983; Rosati, Verni, & Dini, 1998). As in the Class 
 Oligohymenophorea (see Chapter 15 ), microtu-
bules and microfilaments are presumably involved 
in the positioning and movement of the gametic 
nuclei . Completion of macronuclear maturation 
and post-conjugation morphogenesis (see below) 
in hypotrichs and stichotrichs may take several 
days during which feeding does not occur. During 
this time, energy may be supplied by programmed 
 autophagocytosis of cell constituents (Sapra & 
Kloetzel, 1975). As in other ciliates, spirotrichs are 
induced to conjugate by a variety of factors, with 
reduction in food resources being a dominant one 
(e.g., Adl & Berger, 2000; Gates & Ramphal, 1991). 
Their sexual life cycle is characterized by a period of 
 immaturity in which conjugation cannot effectively 
occur, a period of maturity or conjugation compe-
tence , and a period of senescence (Miyake, 1996; 
Smith-Sonneborn, 1981). The development to matu-
rity is not an instantaneous switch in all cells (Dini 
& Nyberg, 1994), while the length of the immaturity 
period can be determined by cytoplasmic factors 
(Dini, Bleyman, & Giubbilin, 1990). The intensity 
of mating reactivity may show daily rhythms in 
some Euplotes species (Gates & Ramphal, 1991; 
Miyake & Nobili, 1974). 
 The breeding systems of spirotrichs are gener-
ally multipolar , that is with many more than two 
mating types: many more than 100 mating types 
have been recorded in the stichotrich Stylonychia
(Ammermann, 1982; Ammermann & Schlegel, 
1983); and up to 38 have been recorded in Euplotes
species (Dini & Luporini, 1979, 1985; Heckmann, 
1964; Kimball, 1942; Nobili, 1966). On the other 
hand, bipolar breeding systems have been observed 
in other Euplotes species (Katashima, 1959) 
and its hypotrich relative Aspidisca (Dini et al., 
1987). Strains isolated from geographically distant 
localities may show varying intensities of mating 
reactivity , but this has not resulted in genetic par-
titioning as there is still clear evidence of gene 
flow (Ammermann et al., 1989; Kusch, Welter, 
Stremmel, & Schmidt, 2000; Mollenbeck, 1999; 
Valbonesi, Ortenzi, & Luporini, 1992). 
 Preparation for conjugation in stichotrichs and 
 hypotrichs may be mediated by chemical signals, 
called pheromones or gamones , which diffuse 
through the medium and induce cells of com-
plementary mating type to prepare for conjuga-
tion (Esposito, Ricci, & Nobili, 1976; Heckmann 
& Kuhlmann, 1986; Luporini & Miceli, 1986; 
Luporini, Vallesi, Miceli, & Bradshaw, 1995). 
Secretion of multiple pheromones appears to 
occur sequentially as the cells mature sexually 
(Kuhlmann & Heckmann, 1989). The pheromones 
are proteins whose genes have been sequenced 
(Miceli, La Terza, Bradshaw, & Luporini, 1992; 
Raffioni, Luporini, & Bradshaw, 1989). Several 
models now exist to explain how pheromones from 
different mating types may interact with each other 
to induce mating reactivity in cells of complemen-
tary mating types (Luporini & Miceli; Miyake, 
1996; Ortenzi et al., 2000). Pheromones may also 
function in an autocrine fashion, stimulating mito-
sis in some Euplotes species (Luporini, Alimenti, 
Ortenzi, & Vallesi, 2005). There is suggestive 
evidence that pheromones may also attract cells of 
the complementary mating type (Kosaka, 1991a) 
and influence the locomotory behavior of cells 
7.6 Nuclei, Sexuality and Life Cycle 171
by eliminating rhythmic, spontaneous membrane 
depolarizations (Stock, Kruppel, Key, & Lueken, 
1999). While diffusible pheromones appear to be 
the rule among hypotrichs and stichotrichs , we do 
not know how widespread they are within the Class 
 SPIROTRICHEA . Indeed, there are some species 
of Euplotes in which the signal substances appear 
to be firmly bound to the cell surface (Heckmann 
& Siegel, 1964). 
 There are typically three prezygotic divisions of 
the micronucleus of spirotrichs , but as few as two 
and as many as four have been observed (Raikov, 
1972). In some Euplotes species, the gametic nuclei 
may not be sister nuclei. Thus, the two exconju-
gants will not be isogenic (Baird & Klobutcher, 
1988; Katashima, 1960; Kuhlmann & Heckmann, 
1991) nor will autogamous forms be homozygous 
(Dini, Raikov, & Bracchi, 1999). 
 Once fertilization has occurred, cells separate 
when the fusion zone is resorbed and/or contrac-
tile processes operate (Geyer & Kloetzel, 1987a). 
During conjugation and the long postconjugation 
period, hypotrichs typically undergo two rounds of 
cortical reorganization while stichotrichs undergo 
three rounds of cortical reorganization (Ng, 1990; 
Tuffrau, Fryd-Versavel, & Tuffrau, 1981; Tuffrau, 
Tuffrau, & Genermont, 1976). The first reorganization 
is correlated with separation of the conjugants . 
Since the cytostome is non-functional, this neces-
sitates additional rounds of reorganization. Ng 
(1990) has analyzed these processes in terms of 
 developmental heterochrony in which he believes 
the sexual cycle overlaps the preceding asexual 
cycle . The first reorganization is similar to asexual 
development in that it bears similarities to asexual 
 cortical development and the micronucleus is dis-
pensable to this process (Ng, 1990; Zou & Ng, 
1991). In the hypotrich Euplotes , the second corti-
cal reorganization requires the presence of mater-
nal macronuclear fragments (Fidler, Jayaraman, 
& Kloetzel, 1985), while in stichotrichs a macro-
nuclear anlage and micronuclei are necessary to 
proceed through both the second and third cortical 
reorganizations (Lu, Shi, & Ng, 1991; Ng, 1990). 
In Paraurostyla weissei , which undergoes total 
conjugation , an exconjugant zygocyst is formed 
as the entire infraciliatures of both donor and 
recipient cells are resorbed. Nevertheless, there 
are still three rounds of cortical reorganization 
(Frontczak-Baniewicz & Jerka-Dziadosz, 1992). 
Retention of the interphase or vegetative pattern of 
superficial cortical microtubules indicates that the 
“cell pattern” is retained throughout this complex 
 conjugation process (Fleury & Laurent, 1994). 
Whether these differences in numbers of cortical 
reorganizations during and following conjugation 
are phylogenetically significant is an open ques-
tion. Ng (1990) noted that the stichotrich Kahliella
apparently undergoes only two cortical reorganiza-
tions (Fleury & Fryd-Versavel, 1982). Could this 
reflect it’s presumed basal position in the adaptive 
radiation of this subclass? And we do not yet know 
how many cortical reorganizations oligotrichs and 
 choreotrichs might undergo. 
 Senescence follows the maturity period of the 
life cycle, and is characterized by reduced growth 
rate and increased mortality (Smith-Sonneborn, 
1981). Clones can be rejuvenated by undergoing 
 conjugation . Hypotrichs and stichotrichs typically 
have long immaturity and maturity periods , which 
means that they are typically outbreeding organ-
isms: the probability of encountering cells of 
like type being decreased the longer the cell has 
a chance to disperse (Nanney, 1980; Sonneborn, 
1957). In circumstances where complementary 
mating types are not present, some hypotrichs may 
undergo intraclonal