Protistology 4 (1) 5-55 (2005)
Kuidong Xu 1, 2 and Wilhelm Foissner 1
1 Universität Salzburg, FB Organismische Biologie, A-5020 Salzburg, Austria
Summary
We studied and reviewed the morphology, ontogenesis and encystment of two closely related spathidiids,
Arcuospathidium cultriforme (Penard, 1922) Foissner, 1984 and A. scalpriforme (Kahl, 1930) Foissner, 2003, using live
observation, protargol impregnation, morphometry, scanning electron microscopy, populations from different geographic regions, and
literature data. Both species are biogeographic flagships. They are 200-300 µm long and have a long, steep oral bulge. Our
investigations show that they are very similar, differing mainly in the length of the oral bulge and the arrangement of the oral bulge
extrusomes. Thus, we classify them as subspecies within the Arcuospathidium cultriforme complex, which includes the following
taxa: A. cultriforme cultriforme (Penard, 1922), A. cultriforme scalpriforme (Kahl, 1930), A. cultriforme megastoma
Foissner et al., 2002, and A. lorjeae Foissner et al., 2002. The latter taxa are likely to have a restricted geographic
distribution, and there is evidence that the South American and Rwandan populations of A. cultriforme cultriforme could represent
further distinct (sub)species. During encystment of A. cultriforme, the macronucleus becomes strongly shortened and the
infraciliature appears to be resorbed. The mature cyst is unique in having a very thick, faceted wall. A detailed scenario of the
ontogenetic processes is provided, showing that the ontogenesis of A. cultriforme matches those of haptorids in general and
other spathidiids in particular. However, there are several characteristic features, for some of which we could find reasonable
explanations which, in turn, provided models for the spathidiid ontogenesis and evolution: (i) the “outgrowth model” suggests that the
slope of the long spathidiid oral bulge is obtained mainly by faster growth of the dorsal than the ventral side; (ii) the “row
detachment model” proposes that the Arcuospathidium and Epispathidium ciliary patterns are variations of the Spathidium
pattern which evolved from the Protospathidium pattern; and (iii) a model is proposed for a Dileptus-like ancestor of
the spathidiids.
Key words: Hawaii, outgrowth model, phylogeny, row detachment model, South Africa, Spathidium cultriforme, Spathidium
lionotiforme, Spathidium scalpriforme
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