Look Through the Microscope of Evolution

Now, here, you see, it takes all the running you can do, just to keep in the same place. The Red Queen in Lewis Carroll's Alice in Wonderland. (The Origin of Species by the Means of Natural Selection, 1859)

According to Darwin, life may have started in some "warm little pond." Rather than in warm ponds, it is believed today that primitive Bacteria and Archaea arose in the vicinity of hot vents in the ancient oceans some 4 billion years ago; and microbes still rule the earth today. It was only after the first half of life's history that microbes started to share the world with eukaryotes, and at this point they successfully explored these larger organisms as ecological niches. The Cyanobacteria laid the groundwork for the development of higher (aerobic) life forms by inventing photosynthesis and the production of oxygen, and a-proteobacteria contributed to the formation of eukar-yotes by providing the ancestors of mitochondria. According to the (now widely accepted) endosymbiont hypothesis, mitochondria, plastids and hydrogenosomes originated from free-living bacteria. Without these essential endosymbiotic organelles, unicellular eukaryotes and subsequently larger multicellular organisms would probably never have seen the light of day.

Mitochondria and plastids probably represent the most intimate relationship between pro- and eukaryotic cells, and are the prime example of intracellular life. During the initial invasion step the host cell was likely just a membrane sac with a membrane compartment to concentrate the genetic information - an ancient nucleus precursor. Most likely it was still a fellow prokaryote. In 1967, Lynn Margulis (b.1938) reintroduced the endosymbiont theory to the field of evolution biology [9]. According to this hypothesis, mitochondria and plastids originate from ancient bacterial and cyanobacterial symbionts, respectively (Figure 1.2). This hypothesis was supported by the presence of two membranes surrounding these organelles,

Mixotricha Paradoxa Diagram

Figure 1.2 (a) Schematic drawing on the symbiont hypothesis how primary symbiosis between two (or three, or more) prokaryote microbes led to eukaryote cells with mitochondria, chloroplasts and flagellae. (b) Secondary symbiosis arose between heterotrophic and photosynthetic eukaryotes namely flagellate species. In both scenarios, the ultimate relationship lead to full dependency of both partners on each other and their loss of autonomy.

Figure 1.2 (a) Schematic drawing on the symbiont hypothesis how primary symbiosis between two (or three, or more) prokaryote microbes led to eukaryote cells with mitochondria, chloroplasts and flagellae. (b) Secondary symbiosis arose between heterotrophic and photosynthetic eukaryotes namely flagellate species. In both scenarios, the ultimate relationship lead to full dependency of both partners on each other and their loss of autonomy.

many structural similarities and the presence of bacterial DNA in these organelles. The theory dates back to 1883, when the German botanist Andreas Franz Wilhelm Schimper (1856-1901) postulated that chloroplasts are derived from photosynthetic bacteria, and was renewed by Konstatin Sergejewitsch Merschkowski (1855-1921) in 1905. Our current understanding is that more than 1.5 billion years ago an a-proteobacterium-like microbe invaded an ancient host cell, which was most likely another bacterium with compartmentalized chromosomes similar as found in Gemmata obscuriglobus and other 8-proteobacteria [10].

According to scenarios based on the argument that metabolic needs may have promoted formation of symbiosis between two prokaryote species, host cell and invader may have been a methanogenic Archaea and a methanotroph, respectively (reviewed in Dyall et al. [11]). In this scenario, the essential event of eukaryote evolution is set in the anoxic era, whereas others placed it in the aerobic age where an anaerobic archaeal host was protected from toxic oxygen by an aerobic symbiont.

Another version suggests that mitochondria are derived from photosynthetic bacteria due to the following arguments: (i) they release photosynthates such as glycollate for metabolic use by the heterotrophic partner (through peroxisomes as realized in higher plants); (ii) their morphological features resemble cristae of mitochondria; and (iii) 31 of the most conserved mitochondrial genes are closely related to genes in the phototrophic bacterium Rhodospirillum rubrum [12].

Whichever microbe the mitochondrial ancestor was, it is most likely that the process of mitochondrial endosymbiogenesis succeeded just once since all known current eukaryotes contain a number of original genes from the a-proteobacterial ancestor.

During a process starting some 3.5 billion years ago, atmospheric oxygen accumulated through the metabolic activity of photosynthetic bacteria. Shortly after eukaryotes with mitochondria started roaming the earth, another invasion event by cyanobacteria led to the emergence ofthe ancestors ofgreen algae and higher plants (Chlorophyta) and subsequently of red (Rhodophyta) and brown algae (Glaucophyta) (see below) [13]. Amitochondrial amoebal, trichomonad, ciliat and anaerobic fungal species still exist today. In those organisms, ATP-producing organelles, the hydro-genosomes, play a role similar to mitochondria. Although hydrogenosomes do not contain a genome, proteomic analyses suggest their relationship with mitochondria, but this is controversially discussed [11]. In some other amitochondrial organisms, such as Giardia, Entamoeba and Microsporidia, in which mitochondria-like remnants have been found, it is not clear whether their loss is a secondary event. The emergence of eukaryotes from a get-together of different bacterial species probably represents the first type of intracellular life on earth.

It should, however, be mentioned that the bacterium Bdellovibrio bacteriovorus is a specialized parasite of other bacteria and invades their periplasmic space. A symbiotic a-proteobacterium, Midichloria mitochondrii, has recently been described residing in the mitochondria of tick ovary cells [14]. These facts may lead to the bold hypothesis that intrabacterial parasites/symbionts may have preceded the rise of eukaryotes, and that they were the first intracellular life forms.

The current view that interbacterial symbiosis formed the basis for the evolution of bona fide organelles is further corroborated by the identification of more recent "domestication" events of (cyano-)bacterial symbionts by eukaryotes. The filose amoeba Paulinella chromatophora harbors photosynthetic Synechococcus-type cyano-bacteria as symbionts, which have totally lost their autonomy thereby forming a primary symbiont [13] (Figure 1.2).

Subsequently to endosymbionts becoming mitochondria and chloroplasts, evolution led to further examples of endosymbioses. After green, red and brown algae emerged, secondary endosymbiosis (Figure 1.2) was born when aplastid flagellates incorporated red algae cells, thus joining the photosynthetic community. The genera Cryptophyta, Dinophyta, Heterokontaphyta and Haptophyta were the results of these joint ventures. Also, the parasite phylum Apicomplexa originated from such an endosymbiosis, which explains why parasites such as Plasmodium and Toxoplasma are affected by herbicides that target plastid enzymes [15]. Moreover, unicellular eukaryotes, probably heterotrophic flagellates, incorporated green algal cells and gave rise to Euglenophyta and Chlorarachniophyta. A very recently evolved secondary endosymbiosis is the union between the colorless flagellate "Hatena" and a green algae of the genus Nephroselmis [16]. Upon engulfment of free-living flagellated Nephroselmis cells by Hatena, the symbiont loses flagellae, cytoskeleton and endo-membranes but retains nucleus, plastides, mitochondria and eyespot. The complex feeding apparatus of the colorless host flagellates disappears after uptake of the symbiont. This event seems to coincide with the host cell's switch from heterotrophic predator to autotrophic algae. After cell division, the daughter cell lacking the symbiont becomes heterotrophic again and develops a feeding apparatus to catch a new symbiont. It has been suggested that "Hatena" could be a model for the early development of secondary symbiosis. This example shows that not only the symbiont but also the host cell may go through cellular changes upon formation of endosymbiosis. The latter notion is also corroborated by findings from the genome of the pathogenic filarial nematode Brugia malayi, which revealed adaptations, which had most probably evolved in response to the presence of Wolbachia symbionts [17].

On several occasions later in evolution, a-proteobacteria such as Wolbachia, Rickettsia and Ehrlichia species, as well as members of the Chlamydiales became settlers of eukaryotic cells as highly specialized obligate intracellular mutualists or pathogens. In free-living amoeba, more than 20 bacterial symbionts have been identified so far, belonging to the a-proteobacteria, b-proteobacteria, Bacteroidetes and Chlamydiales [18-20]. Interestingly, symbiotic Chlamydia species in amoebae have a biphasic lifecycle between metabolically active reticulate and inactive elementary bodies similar to that of pathogenic species in mammals, suggesting common ancestry between the groups. Among amoebae symbionts, differentiation between symbiosis and parasitism is difficult. In the case of Parachlamydia-related symbionts, their association with amoebal partners can also be detrimental to the host cell as they lyse their hosts at temperatures above ambient. In contrast, Neohartmanella hartmanellae is a bona fide mutualist since this bacterium promotes growth of its amoebal host [21, 22]. This suggests that there can be fine lines between mutualistic and parasitic companionships between bacteria and eukaryotes, depending on factors such as environmental conditions.

Some hypotheses of intermicrobial symbiosis go beyond metabolic mutualisms. The evolution of motility and cytoskeleton elements has been suggested to originate from a hypothetical spirochetal symbiont forming a consortium with an archea [10]. It should be noted that "living fossils" for such a scenario exist in the form of Chlorochromatium aggregatum and Mixotricha paradoxa. C. aggregatum evolved from a consortium comprising green sulfur bacterial epibionts surrounding a central motile b-proteobacterium. M. paradoxa is a flagellate in the gut of the termite Mastotermes darwiniensis and is coated with Bacteroides species and spirochetes for motility [23, 24].

Box 1.1

Definitions of interspecies relationships

Symbiosis: Partnership between two different species. Often used synonymously with mutualism, meaning that advantages usually outweigh the disadvantages for both partners.

Mutualism: Partnership between two different species with benefits for both partners.

Parasitism: Relationship between two different species, in which the smaller one, the parasite, lives in or on a larger host organism, gaining benefits and causing harm to the host.

Commensalism: Relationship between two different species benefiting one partner, the commensal, but without (known) benefits or disadvantages for the other one.

Pathogen: Microbe that induces one or more infectious diseases.

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    How eukaryotic cell originated mitochondria and chloroplast?
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