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conjugation or selfing (Akada, 1986; Kosaka, 1990; Machelon, 1986). This has the advantage of resetting the life cycle clock, but the disadvantage that it can only be a short term strategy as over several generations it leads to lethal inbreeding depression (Kosaka, 1982). Another route to resetting the life cycle clock is autogamy , a process of self-fertilization under- taken by a single cell. Some autogamous strains of Euplotes species are determined by a dominant allele at a single locus (Heckmann & Frankel, 1968; Dini & Luporini, 1980). Although autogamy is an extreme form of inbreeding , heterozygosity is maintained for longer periods in these species because the meiotic products of non-sister nuclei form the zygotic nucleus or synkaryon (Dini et al., 1999; Luporini & Dini, 1977). Nevertheless, auto- gamous strains are less tolerant to stresses, for example, mercury toxicity, than non-autogamous or outbreeding strains (Dini, 1981). Further, changes in body proportions of autogamous strains relative to non-autogamous strains may inhibit effective cell pairing (Gates, 1990). Sibling or cryptic species are found among spirotrichs (Valbonesi, Ortenzi, & Luporini, 1988; Valbonesi, Ortenzi, & Luporini, 1992) as they are among other groups of ciliates (Nanney & McCoy, 1976; Sonneborn, 1957, 1975). Most genetic work on the species problem in spirotrichs has focused on Euplotes species where there are competing conclusions on whether one or another \u201cspecies\u201d of Euplotes is reproductively isolated. For exam- ple, using mating tests , Valbonesi et al. (1988, 1992) claimed that Euplotes crassus is not a sib- ling species complex , but it is a species separate from Euplotes vannus . Caprette and Gates (1994) claimed that these two \u201cspecies\u201d were not repro- ductively isolated. Nevertheless, they cautioned that until the extent of interbreeding is known in nature, results of laboratory experiments must be interpreted with caution. Valbonesi et al. (1988) have also used characteristics of isoenzymes to distinguish E . crassus , E . vannus , and Euplotes minuta , all of which demonstrated discretely different patterns in five isoenzymes , differences that are as great as those used to separate species of the Tetrahymena and Paramecium sibling species com- plexes (Nanney & McCoy, 1976; Sonneborn, 1975). Isoenzyme differences clearly distinguish morpho- logically different species of the hypotrich Euplotes (Machelon & Demar, 1984; Schlegel, Kramer, & Hahn, 1988) and the stichotrich Stylonychia , even when isolated from separate continents (Ammermann et al., 1989). Schmidt, Ammermann, Schlegel, & Bernhard (2006a) have identified a single nucleotide difference in the SSUrRNA genes of Stylonychia lemnae from Eurasia and North America . This ten- tatively suggests a biogeography , a conclusion that was also tentatively reached in a study of strains of the soil stichotrich Gonostomum affine from Europe , Africa , and Asia (Foissner, Stoeck, Schmidt, & Berger, 2001). More recently, random amplified polymorphic DNA or RAPD fingerprinting has been used to demonstrate genetic diversity within Euplotes aediculatus (Kusch et al., 2000) and Euplotes octo- carinatus (Mollenbeck, 1999) and also between morphospecies of Euplotes (Chen, Song, & Warren, 2000). The intraspecific analyses concluded that there was no geographic subdivision of species despite continental separation of some strains, con- firming the results of isoenzyme studies on stichot- richs (Ammermann et al., 1989). This indicates that conjugation must be frequent enough across intercontinental geographic distances to essentially maintain a single gene pool, even though it is rarely observed in natural populations (Lucchesi & Santangelo, 2004). The rarity of conjugation in Euplotes was supported by RAPD analysis of a population of Euplotes daidaleos in Germany : the genetic diversity was very low, indicating a clonal population structure rarely undergoing conjugation (Kusch & Heckmann, 1996). 7.7 Other Features As with heterotrichs (see Chapter 6 ), the widespread distribution of hypotrichs and stichotrichs coupled with the ease of culturing them has led to their use in monitoring environmental quality. Hypotrichs and stichotrichs can be found in extremely acidic environments (Packroff & Wöfl, 2000) although some oligotrichs may be quite sensitive (Pedersen & Hansen, 2003). They are also very abundant in the biofilms of water treatment facilities (Curds, 1969; Martin-Cereceda, Serrano, & Guinea, 2001a; Perez-Uz et al., 1998), presumably playing a role by feeding upon bacteria in the biofilms (Lawrence & Snyder, 1998). Hypotrichs and stichotrichs have been used to bioassay copper, nickel, cadmium, and other organics (Albergoni et al., 2000; Madoni, 2000; Piccinni, Irato, Cavallini, & Ammermann, 1992; Stebbing, Soria, Burt, & Cleary, 1990). They showed broad variations in sensitivities to different toxicants: Euplotes species can be highly tolerant of nickel (Madoni, 2000) or highly sensitive to nickel (Madoni & Romeo, 2006) and to copper (Albergoni et al.); Halteria can be highly sensitive to cadmium (Madoni & Romeo). Resistance to heavy metals may be conferred on stichotrichs by the presence of unique metal-binding proteins, very different from metallothioneins and chelatins isolated from other protozoa (Piccinni et al., 1992). As noted above (see Life History ), spirotrichs can be important predators in microbial food webs , ingesting a variety of prey organisms from bac- teria to other ciliates and metazoa . This can have important consequences for humans. For example, Tso and Taghon (1999) demonstrated that Euplotes did not show selectivity for contaminant-degrad- ing bacteria , which may have important implica- tions for bioremediation initiatives . On the other hand, feeding by hypotrichs and stichotrichs might remove Cryptospori dium oocysts from wastewa- ters , helping to decrease the incidence of water- borne outbreaks of cryptosporidiosis (Stott, May, Matsushita, & Warren, 2001). 7.7 Other Features 173