The known diversity of morphological characters in eukaryotes is simply staggering and can be attributed to the vast multitude of possible solutions to basic biological problems, such as nutrition/feeding, locomotion, defense, refuge, mate selection and reproduction. Eukaryotes are built from one or more internally differentiated cells comprised of intricate subcellular systems. Several single-celled lineages, for instance, have reached the utmost degree of morphological complexity within the confines of a single enveloping cell membrane (e.g. parabasalids, ciliates, dinoflagellates), while others have reached the lower limits of morphological complexity by becoming extremely streamlined (e.g. picophytoeukaryotes, yeasts). Moreover, some multicellular eukaryotes have struck the upper physical limits of overall body size (e.g. dinosaurs, elephants, and whales), while others are miniaturized to the point of being smaller than single-celled counterparts in the same ecosystem (e.g. gastrotrichs, tardigrades, rotifers and nematodes). Regardless of major differences in body size and morphological peculiarities, eukaryotes share many characteristics in common. Many of these characteristics are homologous for the entire group, whether comparing a blue whale to an amoeba or a human to a giant redwood tree.
Unifying Features of Eukaryotes
Below is a list of important features that are likely to have been present in the common ancestor of eukaryotes. Some of these features are still universally found in all eukaryotic diversity, while others have been lost or drastically transformed in some lineages, but are nevertheless ancestral to those groups (see Fig. 3 for examples).
- Cytoskeleton consisting of tubulin-based microtubules and actin-based microfilaments, and ancestrally including motile cell extensions called ‘flagella’ or ‘cilia’ that contain an axoneme of 9 peripheral microtubular doublets and 2 central microtubules.
- An endomembrane system that consists of endoplasmic reticulum, Golgi bodies, vacuoles, lysosomes, peroxisomes, and the nuclear envelope.
- Primary genome of each cell consisting of multiple linear chromosomes contained within a membrane-bound nucleus. Following replication of the genome the chromosomes are segregated by the process of mitosis. Cells in many species can have more than one nucleus.
- Mitochondria - organelles with diverse functions, usually including aerobic respiration, iron sulfur cluster assembly, and synthesis and breakdown of small molecules such as lipids and amino acids. Mitochondria are bounded by two membranes, and usually contain a small genome. They are the descendents of an alpha-proteobacterial endosymbiont.
- Translation machinery in the form of 80S ribosomes, each consisting of four molecules of RNA complexed with many proteins, and partitioned in a small (40S) and a large (60S) subunit.
Fig. 3. Some unifying characteristics of eukaryotes. From left to right: the 9+2 microtubule structure of eukaryotic flagella (© Patrick Keeling and Kevin Carpenter), the nucleus (highlighted in red) from the diatom endosymbiont of the dinoflagellate Durinskia baltica (© Patrick Keeling and Kevin Carpenter), the microtubules of the Arabidopsis thaliana cytoskeleton highlighted by fluorescence (© Jessica Lucas and Fred Sack), a linear chromosome from a dinoflagellate nucleus (© Patrick Keeling and Kevin Carpenter), and the Golgi body from Microjoenia fallax (© Guy Brugerolle).
Other Common Characteristics of Eukaryotes
A number of other characteristics are common to many eukaryotes and not to prokaryotes, but these are not ancestral to all eukaryotes, and many have evolved several times independently (See Fig. 4 for examples).
- Multicellularity and tissue formation (e.g. green algae, land plants, red algae, brown algae, animals and fungi).
- Secreted hard parts (e.g. mollusk shells, plant cell walls, ecdysozoan cuticles, coccoliths, vertebrate endoskeletons, chrysophyte scales, polychaete tubes, diatom frustules, brachiopod shells, cnidarian corallites, euglenophyte loricas, poriferan spicules, echinoderm ossicles, foraminiferan and radiozoan tests).
- Extrusive organelles that function in defense, prey capture or parasitic invasion (e.g. ejectisomes of cryptomonads; trichocysts of alveolates; polar tubes of microsporidian fungi, gun cells of oomycetes; nematocysts of cnidarians, myxozoans and some dinoflagellates).
- Plastids, including chloroplasts and their homologues. Referring to plastids as homoplasies is a qualified statement, since the vast majority of plastids do ultimately stem from a common primary endosymbiosis with a cyanobacterium (the one possible exception being the ‘chromophore’ of the euglyphid amoeba Paulinella), but their subsequent spread via secondary and tertiary endosymbioses has led to a complicated distribution on the tree of eukaryotes (see Symbiosis section below).
Fig. 4. Some features are common, but not necessarily ancestral characteristics of eukaryotes. For example, making hard bones, shells, or other body parts. From left to right: the shell of the cephalopod Spirula spirula (© 1996 Richard E. Young), an SEM showing the external scales of the chrysophyte Mallomonas sp. (© 2003 Brian S. Leander), a collection of human bones from the Chapela dos Ossos in Évora, Portugal (© Patrick Keeling), and Calliarthron tuberculosum from the North East Pacific (© Patrick Keeling).
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