Flagellate megaevolution: the basis for eukaryote diversification
ABSTRACT Flagellates are a vastly disparate spectrum of related life forms, grouped here in 111 orders and sixty classes in sixteen phyla in three kingdoms: Protozoa, Plantae, Chromista. Many (especially chromists and dinoflagellates) are photophagotrophic exceptions to classical animal/plant dichotomies; zooflagellates and phytoflagellates are both polyphyletic. Multiple evolutionary losses of mitochondria, chloroplasts and flagella confused earlier classifications. Molecular phylogeny is clarifying flagellate evolution, but conflicts between different molecular trees show widespread and misleading biases, leaving many major questions unresolved. Probably all extant eukaryotes are either flagellates or evolved directly or indirectly from them. I discuss major events in flagellate diversification and, briefly, how I think they gave rise to major non-flagellate groups. Archezoan flagellates (Metamonada; Parabasalia), which evolved by losing mitochondria or converting them to hydrogenosomes, are basal eukaryotes on rRNA trees, but protein trees and their flagellar tetrakonty suggest that they evolved from aerobic biflagellates and may be derived from other protozoa, not truly basal. Aerobic zooflagellates are very diverse: their phylogeny and classification (including some new taxa) are discussed in detail. From them all other heterotrophic groups evolved, as did the ancestral phytoflagellate, by a single symbiogenetic origin of chloroplasts from cyanobacteria; glaucophytes, green plants and red algae are derived from this ancestor. Chromistan phytoflagellates arose by symbiogenetic uptake of a red alga and may be sisters of Alveolata; in my view dinoflagellates and sporozoa got their plastids from the same symbiotic red alga. Plastids have been lost many times by chromists and alveolates. Euglenoids and chlorarachneans are chimaeras of a flagellate host and a green algal symbiont, and may have had a photosynthetic common ancestry.