title: On the evolution of the karyorelict ciliate life cycle: heterophasic ciliates and the origin of ciliate binary fission.
abstract: Karyorelict ciliates have near diploid somatic nuclei (macronuclei) incapable of division. If selective pressure favors nuclear division, how could such macronuclei have evolved? I propose that they initially evolved in the context of a diplophase stage that consisted entirely of a non-dividing trophont that was terminated by the induction of meiosis. The diploid macronucleus then differentiated, functioned and was destroyed in the absence of cell division. Such a life cycle would necessarily be heterophasic, i.e. with alternating haploid and diploid generations. I call these ancestors heterophasic ciliates. I further propose that the ability of this diploid trophont to undergo binary fission arose de novo. Ciliate binary fission would then be a derived characteristic, which possibly evolved indepedently in more than one heterophasic ciliate lineage. A progression of steps, leading to the reduction of the haplophase and the generation of the karyorelict life cycle, is proposed. The shared possession of nuclear dimorphism with non-dividing macronuclei, conjugation, and a putative heterophasic ancestry invites further investigation of the phylogenetic relationship between heterokaryotic foraminifera and karyorelict ciliates.
authors.affiliation: Department of Biological Sciences, University of California, Santa Barbara 93106.
authors.forename: E
authors.initials: E
authors.lastname: Orias
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002467:Cell Nucleus; D016798:Ciliophora / Q000166:cytology / Q000502:physiology*; D008954:Models, Biological