The evolutionary significance of ultrastructural variations in the micronuclear spindle apparatus in the genus Paramecium.

Ultrastructural variations of micronuclear mitosis in three species of Paramecium have been reviewed for the purpose of hypothesizing possible evolutionary relationships. P. bursaria is considered to be the most primitive species because it has complex kinetochore-equivalents termed microlamellae which attach to microfilaments. P. multimicronucleatum is considered more advanced since it has typical kinetochores which intercalate between the kinetochores and polar microfilaments. P. aurelia is considered the most advanced member of the genus, and has typical diffuse or well defined kinetochores and chromosomal microtubules while it lacks obvious meshworks of microfilaments. A hypothetical model of chromosome movement in the genus involves the interaction of microtubules and microfilaments.