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from research on conjugation in Dileptus , and so 
any generalizations from its features must be taken 
with caution. Three mating types , which show serial 
dominance , have been identified in this species 
(Miyake, 1996). Each mating type excretes a gamone 
that can attract cells of complementary mating type 
(Afon’kin & Yudin, 1987). The gamone is thought 
to be a small polypeptide (Parfenova, Afon’kin, 
Yudin, & Etingof, 1989). Homotypic pairs (i.e., 
pairs of cells of the same mating type) may form, but 
are usually not stable, possibly due to the cessation 
of the expression of adhesive molecules (Afon’kin, 
1991). Intraclonal conjugation or selfing has been 
reported in Spathidium (Williams, 1980). 
 Litostomes are generally characterized as under-
going a preconjugation cell division , although this 
is not a universal trait (Xu & Foissner, 2004). The 
 preconjugation division can be equal, but then 
yields cells that are smaller than trophic cells since 
no growth takes place prior to conjugation (Raikov, 
1972). In entodiniomorphids , the preconjugation 
division can be unequal (e.g., Opisthotrichum ) and 
so produce macroconjugants and microconjugants 
(Raikov). Tavrovskaja (1974 in Miyake, 1996) has 
demonstrated that the gamones of Dileptus stimu-
late the preconjugation cell division . 
 Cell fusion usually occurs in the oral region, 
often involving some dedifferentiation of oral 
structures in haptorians (Xu & Foissner, 2004). 
Cell membranes of the two partners fuse, often over 
a considerable area (e.g., Didinium – Karadzhan, 
1979; Dileptus – Golińska & Afon’kin, 1993; 
Homalozoon – Leipe & Hausmann, 1993). Thus, 
cytoplasmic organelles, as well as the migratory 
 pronuclei , might be exchanged during the process, 
which may take several days. 
 There are typically three maturation divisions 
during micronuclear meiosis in haptorians and 
 trichostomes but only two in entodiniomorphids 
(Raikov, 1972; Xu & Foissner, 2004). In the vesti-
buliferids , the micronuclear mitotic spindle is much 
enlarged and the division products remain swollen 
and spindle-like during conjugation . The migratory 
 gametic nucleus of entodiniomorphids has portions 
of the telophase spindle attached so that it appears 
“spermatozoon-like” (Raikov, 1972). Following 
fertilization, the synkaryon of haptorians typically 
undergoes two or three divisions (Raikov, 1972; 
Serrano, Martín-González, & Fernández-Galiano, 
1990; Xu & Foissner, 2004). Trichostomes typi-
cally undergo one synkaryon division although one 
of these products may undergo a second division 
in Paraisotricha and Balantidium species, which 
is followed by fusion of these two division prod-
ucts (Raikov, 1972). We do not know whether this 
feature is a homologous or convergent one in these 
two genera. 
 Williams (1980) has observed clonal aging in 
the haptorian Spathidium . This aging process is 
observed as a reduction in daily fission rate. The 
time of greatest reduction is species specific and 
is reversed in part by intraclonal conjugation or 
 encystment . 
 9.7 Other Features 
 The free-living litostomes have been recorded in 
a variety of habitats, including anaerobic ones 
(Foissner, 1988a; Madoni & Sartore, 2003), from 
which they have been collected by an electromigra-
tion apparatus (Wagener, Stumm, & Vogels, 1986).
Some of the anaerobic species (e.g., Lacrymaria ) 
may harbor endosymbiotic methanogens , while 
9.7 Other Features 207
other genera (e.g., Lagynophrya ) may harbor 
Chlorella species (Finlay & Maberly, 2000). 
Didinium and Spathidium have been recorded in 
 waste treatment facilities where their prey is also 
abundant (e.g., Rivera et al., 1988). Myrionecta
rubra appears to be sensitive to oil pollution in 
the marine environment when compared to other 
ciliates (Dale, 1988). Didinium appears to be more 
sensitive to copper stress than its prey Paramecium , 
a feature that has some impact on the stability of 
this predator-prey interaction (Doucet & Maly, 
1990). If haptorians are generally more sensitive to 
toxicants than their prey, this may have significant 
impacts on the dynamics of microbial food webs . 
However, we know nothing yet of the generality of 
this susceptibility among haptorians or its impact 
on the stability of planktonic food webs, especially 
those dominated by protistan predators.