Over the past 250 years, biologists have described about 1.8 million species. Of these, 75% or so fall within the Opisthokonta (Fig. 27.10), a group that encompasses animals, fungi, and some protists. The name for this superkingdom is derived from Greek words meaning “posterior pole,” calling attention to the fact that cell movement within this group is propelled by a single flagellum attached to the posterior end of the cell. Not all cells in this superkingdom move around. In humans, for example, the flagellum is limited to sperm. Opisthokonts are heterotrophic, although some species harbor photosynthetic symbionts. Animals are the most diverse and conspicuous opisthokonts. More than 1.3 million animal species, mostly insects and their relatives, have been described. Fungi are also diverse, with more than 75,000 described species, including the visually arresting mushrooms (Fig. 27.10a). Here, however, we focus on opisthokont protists, somewhat poorly known microorganisms that hold clues to the origins of complex multicellularity in this superkingdom (Chapter 28).

It isn’t easy to find morphological characters that unite all opisthokonts. One reason is that animals and fungi have diverged so strikingly from what must have been the ancestral condition of the group. However, as molecular sequence comparisons began to reshape our understanding of eukaryotic phylogeny, it soon became clear that fungi and animals are closely related.

Even more closely related to the animals are the choanoflagellates, a group of mostly unicellular protists characterized by a ring of microvilli, fingerlike projections that form a collar around the cell’s single flagellum (Fig. 27.10b). About 150 choanoflagellate species have been described from marine and freshwater environments, where they prey on bacteria. As early as 1841, the close similarity between choanoflagellates and the collared feeding cells of sponges suggested that these minute organisms might be our closest protistan relatives. This view gained widespread popularity with the discovery in 1880 of Proterospongia haeckeli, a choanoflagellate that lives in colonies of cells joined by adhesive proteins.

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Molecular sequence comparisons now confirm the close relationship between animals and choanoflagellates. For example, the complete genome of the choanoflagellate Monosiga brevicollis shows that a number of signaling molecules known to play a role in animal development are present in our choanoflagellate relatives, although their function in these single-celled organisms remains largely unknown. More generally, many genes once thought to be unique to animals have now been identified in choanoflagellates and other unicellular opisthokonts, underscoring the deep evolutionary roots of animal biology.

Microsporidia form another group of single-celled opisthokonts. Microsporidia are parasites that live inside animal cells. Only their spores survive in the external environment, where they await the opportunity to infect a host and complete their life cycle. Microsporidia infect all animal phyla, so the approximately 1000 known species probably represent only the tip of the iceberg. More than a dozen species have been isolated from human intestinal tissues. Microsporidia infections can be particularly devastating in AIDS patients.

Beyond their importance in human health, microsporidians have attracted the attention of biologists because of features they lack: Microsporidian cells have no aerobically respiring mitochondria, no Golgi apparatus, and no flagella. Furthermore, they have a highly reduced metabolism and among the smallest genomes of any known eukaryote. The cellular simplicity of microsporidians does not mean that they are early-evolved organisms, however. Molecular sequencing studies show that microsporidians are the descendants of more complex organisms. Their simplicity is an adaptation for life as an intracellular parasite. It is now widely accepted that microsporidians are closely related to the fungi.

Other protistan opisthokonts have been identified in recent years. These are mostly bacteria-eating unicells or parasites of aquatic animals, but like the choanoflagellates described earlier, their biology is beginning to illuminate the evolutionary path to complex multicellularity in animals and fungi.