Pré-visualização | Página 6 de 6
) (Hiller, 1993b). As noted above (see Oral Structures ), rotation of these circumoral dikinetids brings the postciliary ribbons into a position to support the oral open- ing and cytopharynx (Hiller, 1993b; Huttenlauch & Bardele). Using scanning electron microscopy, Bardele (1999) has described the formation of the circumoral in Balanion , in which a field, possibly of two rows of dikinetids, separates into an ante- rior and posterior portion. The space in the center becomes the cytostome as these two rows round up to form the circumoral. Bardele (1999) noted that this is a unique form of stomatogenesis in the pros- tomes . It should be studied by protargol staining and transmission electron microscopy to confirm his interpretation and resolve how the circumoral dikinetids rotate to form the circumoral supports. The brush kinetids derive from kineties to the left of those involved in formation of the circu- moral dikinetids (Hiller, 1992). In Coleps , the three brosse polykinetids originate one from the anterior end of each of three somatic kineties (Fig. 13.4) (Huttenlauch & Bardele, 1987). In the other proro- dontids , variable numbers of kineties are involved and each kinety is involved in the formation of all brosse rows: two somatic kineties in Urotricha , up to eight in Bursellopsis , and up to 13 in Holophrya (formerly Prorodon ) (Hiller, 1992, 1993b). This process occurs in several “waves”, each of which includes proliferation of a kinetofragment , rota- tion of that kinetofragment , and then fusion of the kinetofragments to form the mature brosse row. Allometric growth of somatic kineties following cytokinesis “pushes” the brosse into its morpho- static position (Fig. 13.4). There is some dedif- ferentiation of the proter’s oral nematodesmata during division and then regrowth as the opisthe’s nematodesmata grow. 13.6 Nuclei, Sexuality and Life Cycle Prostomes typically have a single, ellipsoid to elon- gate macronucleus of the homomerous type (Fig. 13.2) (Raikov, 1982). Cryptocaryon is a notable exception, having four linked ellipsoid to spheroid macronuclear segments in the phoront and young trophont stages. These segments fuse into one elon- gated and twisted macronucleus as cell growth pro- ceeds to the protomont stage (Colorni & Diamant, 1993). The heterochromatin in the macronucleus is Fig. 13.5. Division morphogenesis of the prorodontid Coleps . The circumoral dikinetids and brosse kinetids begin to differentiate at the equatorial ends of four somatic kineties ( a , b ). As the brosse kinetids differentiate as small polykinetids, the circumoral dikinetids then begin a clockwise migration into the fission furrow to encircle the puta- tive anterior end of the opisthe ( b – d ). (from Huttenlauch & Bardele, 1987.) 13.6 Nuclei, Sexuality and Life Cycle 267 268 13. Subphylum 2. INTRAMACRONUCLEATA: Class 7. PROSTOMATEA distributed throughout the macronucleus, but can be aggregated into one central condensation body or bodies in some Holophrya (formerly Prorodon ) species (Hiller, 1993a; de Puytorac & Savoie, 1968). There is no report of the role of microtubules in macronuclear division in prostomes. We presume that intramacronuclear microtubules are involved. There is typically a single micronucleus closely associated with the macronucleus. The micronu- cleus can be ellipsoid (e.g., Coleps , Rodrigues de Santa Rosa, 1976) or elongated and lenticu- lar (Hiller, 1993a; Sola, Guinea, Longás, & Fernández-Galiano, 1990b). There can be up to five micronuclei in the theront and phoront stages of Cryptocaryon (Colorni & Diamant, 1993). There has been no genetic research on pros- tomes to demonstrate their kind of mating type system, which undoubtedly exists as they have been reported to conjugate . Starvation appears to stimulate conjugation in Coleps (Serrano, Martín- Gonzàlez, & Fernández-Galiano, 1985) while Holophrya (formerly Prorodon ) has been reported to conjugate immediately on emerging from resting cysts (Tannreuther, 1926). Prostomes typically first encounter each other and associate using the oral and anterior ciliature (Serrano et al., 1985; Sola et al., 1990b). Cell fusion takes place near the oral region, but in Coleps the ciliates can continue to feed (Serrano et al., 1985). The micronucleus typi- cally undergoes two meiotic divisions during which the micronucleus may exhibit the “crescent stage” (Serrano et al., 1985). Three of these four products degenerate and the remaining nucleus undergoes mitosis to produce the stationary and migratory gametic nuclei (Raikov, 1972; Serrano et al., 1985). These gametic nuclei may remain connected by the telophase spindle as nuclear exchange proceeds (Serrano et al., 1985; Tannreuther, 1926). Raikov (1972) characterized the postconjugation process as involving a single division of the synkaryon to produce a new micronucleus and macronucleus. This was confirmed in Coleps by Serrano et al. (1985). However, Sola et al. (1990b) reported two or three postconjugation divisions in Lagynus . Population genetic research on prostomes has been restricted to isolates of Cryptocaryon . Variation among isolates of C. irritans has been of interest to parasitologists since it may explain the outbreaks and incidence of cryptocaryoniasis in various parts of the world’s oceans. Isolates of C. irritans show relatively low host specificity (Burgess & Matthews, 1995b; Diggles & Lester, 1996; Rigos et al., 2001). Rather, cryptocaryonia- sis typically develops in susceptible fish at tem- peratures above 19°C. However, isolates have been reported that can grow faster at lower or at higher temperatures (Diggles & Adlard, 1997; Jee, Kim, Park, & Kim, 2000). Diggles and Adlard (1997) and Yambot et al. (2003) examined the genetic variation in the rDNA gene region of isolates of C. irritans , and recognized a variety of genetic strains within this species, some global in distri- bution, and perhaps two lineages, one adapted to marine and the other to low-salinity habitats. Are these in fact cryptic species ? Wright (1999) noted that the alignment of Diggles and Adlard (1997) wrongly identified the 3'-end of the ITS-1 region. Nevertheless, there appears to be significant genetic variation among these isolates, an issue worthy of continued investigation for this commercially impor- tant parasite of fishes. 13.7 Other Features There have been a few reports of the impacts of anthropogenic pollution on prostome ciliates. Dale (1988) reported that Tiarina and other aloricate heterotrophic ciliates were more susceptible to oil pollution in the marine environment than tintin- nids . Rivera et al. (1988) reported that Urotricha farcta was the most abundant prostome in rotating biological contactor treatment plants in Mexico, along with five species of oligohymenophoreans and Didinium nasutum . Finally, Packroff (2000) noted that the community structure of acidic mining lakes in Germany appeared to favor pros- tomes over haptorians , scuticociliates , and oligot- richs . This, however, does not appear to be a global phenomenon as Beaver and Crisman (1981) noted oligotrichs dominated in acidic softwater lakes in Florida.