Phylum CILIOPHORA Doflein, 1901

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Introduction

Ciliated protozoa (phylum Ciliophora) are unique amongst the unicellular eukaryotes because they are the only group to exhibit nuclear dualism. Individual cells possess two different types of nuclei; vegetative macronuclei and reproductive micronuclei. Asexual reproduction occurs by transverse binary fission across rows of cilia (homothetogenic fission) whereas some species exhibit sexual reproduction by the phenomenon of conjugation (temporary fusion of two conjugates which exchange micronuclei).

As their common name implies, ciliates are also characterised by the possession of simple cilia, or compound ciliary organelles, in at least one stage of their life cycles (compound subpellicular infraciliature is universally present even when cilia are absent). Cilia are elongate hair-like extensions of the cell membrane with an internal microtubular core (universal 2+9 configuration = 2 single central microtubules surrounded by 9 peripheral doublets). They are organelles of motility used for locomotion and/or feeding. Cilia (singular, cilium) are similar in ultrastructure to flagella (singular, flagellum), and they are collectively often called undulipodia (singular, undulipodium) because both use cross-linked proteins (dynein-walking mechanism) to undulate about their basal kinetosome (unlike the rotary motion unique to flagella in bacteria).

Ciliates, together with dinoflagellates and apicomplexans, have subpellicular alveoli which are membrane-bound sacs beneath the plasma membrane. Alveoli are thought to serve many varied functions: ranging from support (helping maintain body shape, act as fulcrum for undulipodia); metabolism (storage); osmoregulation (mucocysts); excretion (extrusomes); protection (toxicysts, trichocysts); and even hunting (haptocysts).

Most ciliate species are free-living in aquatic or terrestrial habitats but many are commensals in vertebrate or invertebrate hosts and some are parasitic. Early classification systems recognised three main classes of ciliates mainly on the basis of their patterns of somatic (body) and buccal (oral) ciliation. The ‘lower holotrichs’ have simple body and oral ciliature; most are free-living species but some are highly specialized symbionts aiding cellulose digestion in herbivores. The ‘higher holotrichs’ have simple body ciliature but more specialized oral ciliature forming membranelles; most occur as free-living organisms but some live as commensals or parasites in a range of animals. The ‘spirotrichs’ have reduced body ciliation but well-developed oral ciliature forming an adoral zone of membranelles; most are bactivores living in aquatic and terrestrial habitats.

More recently, 10 major monophyletic lineages were recognised on the basis of their infraciliature, i.e. the ultrastructural organization of their kinetids (comprising basal bodies (= kinetosomes) and associated microtubular ribbons and fibrils). These lineages (ranked as classes) are well supported by modern molecular biological studies using several gene sequences. The classification scheme used in this document follows that of Lynn & Small (2000).

The subphylum Postciliodesmatophora contains ciliates that have somatic dikinetids with postciliodesmata or overlapping postciliary microtubular ribbons. Two classes are recognised: the Heterotrichea (‘different hair’) in which the left oral polykinetid does not encircle the body and the macronuclei do not divide; and the Karyorelictea (‘surviving nucleus’) which exhibit simple nuclear dualism and when the macronucleus divides, microtubules occur outside the macronuclear envelope. The subphylum Intramacronucleata is a diverse group, whose members are united by the presence of microtubules inside the macronuclear envelope during division. Eight classes are recognised: the Spirotrichea (‘coiled hair’) with conspicuous oral membranelles (previously known as polyhymenophoreans); the Litostomatea (‘simple mouths’) with a noncurved tubular cytopharngeal apparatus (rhabdos); the Phyllopharyngea (‘leaf throated’) with cytopharyngeal phyllae; the Colpodea (‘breast shaped’) with reniform body profiles; the Prostomatea (‘before mouth’) with simple apical mouths; the Nassophorea (‘pot bearer’) with curved tubular cytopharngeal apparatus (cyrtos or nasse); the Plagiopylea (‘misshapen marker’) with twisted oral tubes; and the Oligohymenophorea (‘few membrane-bearer’) with an adoral zone of three membranelles.

Diagnosis

Ciliates; with nuclear dualism, conjugation, alveoli, cilia.

General References

Lynn, D.H. & Small, E.B. 2000. Phylum Ciliophora Doflein, 1901. pp. 371-656 in Lee, J.J., Leedale, G.F. & Bradbury, P. (eds). An Illustrated Guide to the Protozoa. Lawrence, Kansas : Society of Protozoologists, Allen Press Inc. Vol. I.

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