Cap 8

ultrastructural studies on 
micronuclear and macronuclear division in the 
 clevelandellid Nyctotherus cordiformis . Macronuclei 
divide by intramacronuclear microtubules that are 
primarily responsible for the elongation of the 
macronucleus, which is also accompanied on its out-
side by scattered extramacronuclear microtubules 
(Eichenlaub-Ritter & Tucker, 1984; Hamelmann, 
Eichenlaub-Ritter, & Ruthmann, 1986). Micronuclear 
mitosis is an endomitosis, typical of ciliates (Raikov, 
1982). There may be three “classes” of micro tubules, 
identified by their differing responses to drugs and 
temperature, which function to accomplish micronu-
clear mitosis : (1) manchette microtubules underlying 
the nuclear envelope; (2) interpolar and kinetochore 
microtubules, which function during anaphase; and 
(3) stembody microtubules, which function during 
telophase to separate the putative micronuclei to 
each progeny cell (Eichenlaub-Ritter & Ruthmann, 
1982a, 1982b). Microtubules in the dividing nuclei 
may have more than the canonical 13-protofila-
ments (Eichenlaub-Ritter, 1985; Eichenlaub-Ritter 
& Tucker). 
 Conjugation has been studied in only a few 
examples of armophoreans since the description of 
it in Nyctotheroides (= Nyctotherus ) by Wichterman 
(1936). It is not established what factors stimulate 
conjugation in free-living forms. Wichterman (1936) 
observed it occurring only in transforming tadpoles 
of the frog Hyla versicolor . This lead to specula-
tion that gonadotropins or some other physiological 
signal derived from the host may cue these ciliates 
to begin conjugation. However, Sandon (1941a) 
observed conjugation in Paranyctotherus isolated 
from the adult clawed frog Xenopus laevis , sug-
gesting that other factors are involved. Affa’a and 
Amiet (1994) have confirmed that conjugation can 
occur in all stages of the frog life cycle – tadpoles , 
transforming individuals, and adults. Gonadotropin 
injections induced conjugation in Prosicuophora , 
even when immature stages were treated (Affa’a, 
1986b). Thus, it is unlikely that one single factor 
stimulates conjugation . 
 Fusion of the conjugants occurs in the anterior 
region, and in some Metopus species total conjuga-
tion may occur (Noland, 1927). The micronuclei 
of each partner typically undergo three maturation 
divisions – two meiotic divisions followed by a 
mitosis of one of the four haploid products (Raikov, 
1972; Martín-González et al., 1987). In the total 
 conjugation of Metopus , the cytoplasm of one con-
jugant flows into the partner carrying the gametic 
nucleus or nuclei with it. However, the old macro-
nucleus is left in the cortical shell of the disgarded 
partner (Noland, 1927). Following fusion of the 
gametic nuclei to form the synkaryon , armopho-
reans typically have one post-synkaryon division 
with one nucleus becoming the new micronucleus 
and the other becoming the new macronucleus. In 
species with more than one macronucleus, there 
may be additional post-synkaryon divisions (see 
Martín-González et al., 1987). 
 Development of the macronuclear anlage in 
 armophoreans is an extremely long process: 
Golikova (1965) recorded it taking up to 2 weeks in 
Nyctotheroides (= Nyctotherus ) while Noland (1927) 
observed a mininimum of 1 week in Metopus . In 
both these genera, it appears that polytene chromo-
somes are formed at one stage during anlage devel-
opment. Golikova (1965) concluded that one giant 
 polytene chromosome may form in Nyctotheroides
by the end-to-end joining of the individual chromo-
somes. This giant chromosome later fragments both 
transversely and longitudinally to yield the macro-
nuclear chromosomes (Vinnikova & Golikova, 
1978). Ultimately, the macronuclear chromosomes 
fragment into gene-sized pieces as happens in the 
Class SPIROTRICHEA (see Chapter 7 ), a fact 
that Riley and Katz (2001) have confirmed by 
molecular analyses of the macronuclear DNA of 
both armophorids and clevelandellids. 
 8.7 Other Features 
 The free-living armophorids have been recog-
nized for some time as strong indicators of anoxic 
aquatic environments (e.g., Bick, 1972; Foissner, 
1988a; Sládecˇek, 1973). They are commonly found 
in soils (Foissner, 1987) and have been recorded 
from a variety of municipal landfill sites in the 
United Kingdom, where they undergo an encyst-
ment - excystment cycle in response to starvation 
and water loss (Finlay & Fenchel, 1991). 
8.7 Other Features 185