ABSTRACT
Abstract 75 4.1 Introduction: What Are Chromalveolates? 76 4.2 Diversity and Unity of Chloroplast Protein Targeting in Chromalveolates 83 4.3 Alveolate Cell and Organellar Diversity 86
4.3.1 Cortical Contractility and the Degenerative Simplification of Desmate Ciliates 87
4.3.2 Why Ciliates Have Separate Germline and Somatic Nuclei: Making Giant Cells with Rapid Growth Rates 89
4.3.3 Genomic Consequences of Evolving Macronuclei: Multiploidy, Chromatin Diminution and Gene-Fragment Scrambling 90
4.3.4 Mouth Evolution and Ciliate Structural Diversity 91 4.3.5 Myzozoa and Myzocytosis: Apical Suctions and Cortical Artistry 92
4.4 Chromist Cell Diversification 96 4.4.1 Ejecting Ribbons and Cryptist Cell Diversification 96 4.4.2 Chromobiote Predation: Cell Organellar Novelty through Diverging
Modes of Prey Entrapment 98 4.4.3 Heterokont Heterogeneity 99
4.5 Biogenesis of Cortical Alveoli 101 4.6 Evolution and Biogenesis of Chromalveolate Ciliary Hairs 101 4.7 Envoi 102 References 102
Eukaryote cell architecture depends on specific topogenic proteins that control protein targeting to different genetic membranes and cell compartments or mediate mechanical attachments between them, the cytoskeleton and chromosomes. Chromalveolates are a major branch of the eukaryote tree whose cells are often marvellously more complex — and immensely more disparate — than those of animals and plants, e.g., the four-genomed cryptophytes, diatoms or dinoflagellates, but sometimes highly simplified such as the degen-
erate Blastocystis parasite of human guts. Their origin and diversification are discussed in relation to molecular and ultrastructural evidence. Chromalveolates evolved when a phagotrophic biciliate protozoan cell enslaved and merged with a unicellular red alga more than 530 million years ago. The algal plasma membrane was converted into the periplastid membrane by inserting duplicated chloroplast outer membrane translocons for importing nuclear-coded plastid proteins through it and the chloroplast envelope. Chromalveolates comprise the protozoan infrakingdom Alveolata (Myzozoa, Ciliophora) and the kingdom Chromista (Cryptista, Chromobiota, the latter comprising heterokonts and haptophytes), in which fusion of the former food vacuole membrane and nuclear envelope placed the periplastid membrane and enclosed plastid inside the rough endoplasmic reticulum. Both groups lost photosynthesis and probably plastids several times, and therefore comprise phototrophs (chromophyte algae), phagotrophs (e.g., ciliate protozoa), parasites (e.g., Plasmodium, the malaria agent) and saprotrophs. The ancestral chromalveolate probably had simple ciliary hairs, still common in the haploid biciliate Myzozoa (Dinozoa, Apicomplexa) but lost — like chloroplasts — by their diploid or multiploid multiciliate sisters the Ciliophora (ciliates, suctorians); these evolved into rigid bipartite tubular glycoprotein hairs in chromists (lost by haptophytes when predation using the haptonema evolved and by a few other chromists). Chromalveolates might be sisters of kingdom Plantae, cortical alveoli probably having evolved to create their corticate ancestor. The astonishing trophic diversity and cellular virtuosity of chromalveolates give them tremendous ecological and medical importance and unsurpassed evolutionary fascination.
