The Elements of Evolution (42-3) Ciliate Protozoa

 Ciliate Protozoa

Paramecia are typical ciliate protozoa. These single-celled eukaryotes are widespread in freshwater, and fond of forming scums. Brackish and saltwater paramecia also exist.

Many ciliates stay grounded by living in soil. Others toil as symbionts in the guts of their host, from termites to ungulates, digesting cellulose.

Cilia are tiny hair-like protuberances on the external membrane that cells use to move or sense their environment. Paramecia propel themselves by waving their cilia in coordinated unison.

Ciliates can be as large as 2 mm. They are among the most complex protozoans.

Ciliates are conspicuous for their odd genome arrangement. Each cell of a ciliate protozoan has 2 nuclei: a micronucleus and a macronucleus. The small micronucleus carries the germline chromosomes but is transcriptionally silent. (Germline is the gene set employed for reproducing offspring.) The larger macronucleus is the working genome. Transcriptionally active, the macronucleus produces the functional RNAs for cell growth and living.

When stressed (e.g., starved), ciliates can respond by constructing a germline genome for their offspring with a radically different organization. This creative construction is an adaptive response to changed conditions.