330 Eukaryotes, Origin of No matter the theories concerning the origin of eukaryotes, there are hypotheses that can account for their chimeric nature Woese has proposed that early in the evolution of eukaryotes horizontal gene flow was much more common, and as genomes became more complex and integrated these rates decreased Doolittle posits that the advent of the cytoskeleton and ability to phagocytoze gave eukaryotes the potential to incorporate DNA from ingested organisms Over time, numerous organisms contributed to the genome of early eukaryotes While it is difficult to test either of these hypotheses, if future comprehensive phylogenetic analyses across a great many organisms point to ‘few’ donors for the origin of eukaryotes, then these ideas would be disproved Origin of Eukaryotes An important feature of eukaryotes when discussing their evolution is that they are believed to have originally lacked cell walls or a rigid membrane as found in most prokaryotes These early eukaryotes would have needed some kind of cytoskeleton to maintain integrity and shape Cytoskeletal elements are also involved in eukaryotic reproduction, mobility, and the ability to endocytoze Curiously, no solid evidence exists for the evolution of the cytoskeleton from some prokaryotic precursor (though actin and tubulin have weak similarity to archeon proteins) Thus, it seems likely that the cytoskeleton evolved largely de novo within the early eukaryote and also that many of the structural and functional characteristics of modern eukaryotes were possibly present in some of the earliest eukaryotes Reasons for the loss of a cell wall are unclear though it may have been in response to bacteriasecreted antibiotics that often target cell walls Probably the simplest theory for the origin of eukaryotes is that the proto-eukaryote diverged from an archeon and that it later derived its bacterial characteristics from its protomitochondrial symbiote; however, another popular theory holds that a bacteria and archea genomes fused prior to the acquisition of the mitochondrion ancestor In these scenarios it is unclear whether the early eukaryote possessed a nucleus before the mitochondrial event or whether it developed afterwards Some hypothesize that the nucleus must have been in place prior to prevent harmful genetic interactions with the proto-mitochondria genome, others that the nucleus evolved in response to the acquisition Other theories hold that the nucleus is the result of a host bacteria fusing with an archeon or virus (something resembling mimiviruses which have rather large genomes and are less dependent on their hosts cellular machinery; referred to as viral eukaryogenesis) In some versions the bacterial host is RNA-based and the genes controlling the manipulation of genetic code were replaced by those of the DNA-based archeon or virus, leaving the other cellular machinery bacterial Figure shows the four major competing theories for the origin of early eukaryotes resulting from ambiguity of two major events First, have they diverged from an archeon or are they the result of a fusion between an archeon and a bacterium, and second, did eukaryotes exist in an amitochondriate phase during their evolution? The ‘hydrogen hypothesis’ proposed by Martin and Muller holds that a methanogen archeon (which metabolizes hydrogen and carbon dioxide and releases methane) endocytozed the future mitochondria and that it provided the host cell with hydrogen and carbon dioxide as by-products of anaerobic respiration This idea was inspired by hydrogenosomes which are simplified mitochondria often found in anaerobic eukaryotes and are believed to have evolved independently multiple times Lynn Margulis has proposed that eukaryotes originated when an archeon developed an ectosymbiotic relationship with a spirochete (a type of bacteria) and eventually their genomes fused; this places an exogenous origin for the flagellum and occurs before acquiring a bacterial endosymbiont This idea was inspired when observing that some eukaryotes rely on symbiotic spirochetes for their motility (parabasalids within the termite gut) This theory has many bacteria-like genes originating from the spirochete and not the later mitochondria However given that there is little similarity between spirochetes and eukaryotic flagella, this theory is currently rather unpopular Endosymbionts Two of the more observable organelles in eukaryotes are mitochondria and plastids (plastids which contain chlorophyll a and b are chloroplasts) Mitochondria are often referred to as cellular ‘power plants’ and are responsible for oxidative phosphorylation; this enables aerobic respiration (which yields 15 times as much ATP from glucose than does anaerobic respiration alone), while chloroplasts allow for photosynthesis The acquisition of these organelles would clearly have been an energetic boon to the eukaryotes possessing them Unlike theories on the origins of the eukaryotes, the ‘endosymbiotic theory’ has remained relatively unchanged for decades, has a host of strong evidence, and is almost universally accepted The endosymbiotic theory dates back to 1883 when Andreas Schimper observed that chloroplasts divided similar to cyanobacteria (blue-green algae which are bacteria despite their common name) In 1905 Konstantin Mereschkowsky suggested that plastids were originally endosymbionts (beneficial organisms living within another organism), and in the 1920s, Ivan Wallin suggested the same for mitochondria These ideas were based on visual similarity when viewed under light microscopy and were largely ridiculed until the later half of the 1960s, after which Lynn Margulis helped popularize the ‘endosymbiotic theory’ in her book Origin of Eukaryotic Cells (1970) The ‘endosymbiotic theory’ states that mitochondria and plastids evolved from an engulfed bacteria and cyanobacteria, respectively It is believed that a host cell engulfed an anaerobic bacterium (perhaps as prey or a parasite) and that over time a symbiosis arose, eventually becoming an obligate symbiosis (a mutually beneficial interaction required by both participants) before evolving into mitochondria Later a mitochondria-harboring host engulfed a cyanobacterium, which similarly evolved into plastids The mitochondrial event is believed to have occurred near the origin of eukaryotes