A maior rede de estudos do Brasil

12 pág.
Cap 13

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
 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.

Crie agora seu perfil grátis para visualizar sem restrições.