PMC

CNA1 expression and localization in Tetrahymena species. a Immunoblot analysis of Cna1p expression in a panel of Tetrahymena species and Paramecium tetraurelia. b Fixed cells from Tetrahymena species were labeled with DAPI to stain the larger macronuclei and smaller micronuclei, and anti-Cna1p antibody to visualize centromeres. Background fluorescence was reduced in close-up images of nuclei in top panels by deconvolution. Bar = 5 microns

Evolution of centromeric histones in distinct taxonomic groups. a Cladograms of Tetrahymena, Mammals, Rosids, and Drosophila show rates of synonymous substitution (dS) as a gauge of divergence from T. thermophila, H. sapiens, A. thaliana, and D. melanogaster. b Amino acid positions under positive selection (Bayesian posterior probability >0.9) are shown for Rosids (triangles) and Mammals (circles). Amino acid site numbering is according to CenH3 from A. thaliana and H. sapiens

Taxonomic diversity of meiosis in eukaryotes. Animals and plants undergo two types of meiosis: asymmetric (female) meiosis in which three of four meiotic products are eliminated (shaded), and male meiosis in which all four meiotic products have an equal likelihood of success. In contrast, some lineages (like Fungi) only undergo male meiosis, while other lineages like ciliated protozoans (including Tetrahymena) only undergo female meiosis. Each of these configurations is expected to differ in terms of their susceptibility to "centromere-drive" and its deleterious consequences (Henikoff et al. ; Malik ; Malik and Bayes ; Malik and Henikoff )

Molecular evolution of CNA1 (a) Amino acid alignment of Cna1p from a panel of Tetrahymena species. Highlighted is the border between the N-terminal tail and histone fold domain, the sequence used as an epitope for antibody production, and the highly variable loop 1 region. Sequences corresponding to primer-binding sites are indicated in lower case. b Maximum likelihood phylogeny of CNA1 nucleic acid sequences. Bootstrap values above 50 are indicated at corresponding branches and the scale of substitutions/site is shown. The arrowhead indicates where the tree would root in the context of more diverse species as shown in c. c A phylogenetic tree of Tetrahymena species based on the histone H3-H4 spacer region (adapted from Brunk et al. ). The asterisks (*) denote species that lack a germline micronucleus. ‡The identification of this strain as T. rostrata has been challenged (Segade et al. )