Polski
Keeping in mind that in biology there are exceptions to almost every rule, it is simplistic to assume that a eukaryotic cell has at least two genomes. One is located in the nucleus and is organized in the form of chromosomes (nuclear genome), while the other is located in the mitochondria (mitochondrial genome) and structurally more closely resembles the bacterial genome. Photosynthesizing eukaryotes have yet a third genome, also similar to that of bacteria, located in chloroplasts (chloroplastic). The nuclear genome is the largest and controls almost all functions of the cell. The genomes of the other organelles are smaller and mainly responsible for certain functions related to oxygen metabolism or photosynthesis. In the course of evolution, some organelle genes were taken over by the nuclear genome and their autonomy diminished. As a result, cell organelles are not fully independent and partially depend on the cell nucleus.
How were the genomes obtained? – endosymbiosis
Today, an almost certain hypothesis indicates that mitochondria are heir to aerobic bacteria and chloroplasts are heir to cyanobacteria (cyanobacteria), which were once endosymbionts of primitive eukaryotes. For the simplest plants, glaucophytes, chloroplasts are still very similar to free-living cyanobacteria. In the case of other plants – kelp and green algae, which in principle should also include terrestrial plants – chloroplasts are already much more reduced and their resemblance to cyanobacteria is not so striking to – armed only with a microscope – eyes. The nuclear genome, on the other hand, seems to be the inheritance not of bacteria, but of archaeons, Therefore, in some modern taxonomic approaches, the main axis of division of organisms is not between prokaryotes and eukaryotes, but between prokaryotic bacteria and prokaryotic archaeons together with eukaryotes.
Thus, the distinct genomes of cell organelles are evidence of endosymbiotic evolution. It is likely that an archeon-like organism once engulfed an aerobic bacterium, but instead of digesting it, it adopted it as a mitochondrion. The same happened with the assimilation of cyanobacteria as later chloroplasts. Not only bacteria and plants are capable of photosynthesis. Just as first the primordial kelp and green algae became hosts to cyanobacteria, turning them into their own chloroplasts, later more protozoa assimilated single-celled kelp and green algae, becoming diatoms or euglenins.
This process is also endosymbiosis, but already of the second order, that is, secondary. Today, some protozoa (e.g., periwinkles) or animals (e.g., corals or snails) store whole algal cells (called zoochlorells or zooxanthellae, depending on their color) or their chloroplasts (called kleptoplasts, a reference to theft) in their cells or intercellular spaces. However, they tend to be more distinct from host cells than chloroplasts.
Fourth genome of cryptomonads is already a fad
Secondary chloroplasts of diatoms, euglenews or brown algae, like primary chloroplasts, result from the reduction of symbiont cell structures. In contrast, they have more membranes, because they have retained cell membranes inherited not only from cyanobacteria, but also from kelp or green algae. Nuclear genomes, however, were generally not inherited and remained at three. However, there are exceptions to this rule. For example, in the chloroplasts of the flagellate algae cryptomonads, in addition to the usual chloroplast genome with a structure resembling that of cyanobacteria, organelles resembling the highly simplified kelp nucleus, called nucleomorphs, are preserved between the membranes. So cryptomonads have as many as four genomes – nuclear, mitochondrial, chloroplast and nucleomorph genomes.
Cryptomonads are among the less well-studied algae. In textbooks, they are either omitted or a chapter is devoted to them much shorter than to green algae, diatoms or even goldilocks. Not even the discovery of the nucleomorph more than a quarter century ago helped. They are sometimes underestimated in ecological analyses. They are relatively small, so considered less important. Sometimes, however, they are included in aggregate floristic lists because they can temporarily dominate the phytoplankton. They are mentioned as having endured very light-poor conditions and therefore occurring at record depths. Their systematic position is not stable – sometimes they are combined with furrows or haptophytes, sometimes even separated as a separate kingdom Cryptista. Hydrobiologists often call them cryptophytes, but the name is also used in terrestrial botany in a completely different sense.
Third level of nesting, or more cryptomonad genomes
Having cryptomonads have four genomes, twice the usual number, is a lot. However, it turns out that this is not the end of the endosymbiotic capabilities of these algae. In cells from a strain of Cryptomonas gyropyrenoidosa, cultured in a laboratory at the University of Göttingen for more than half a century, as early as the 1980s. In the 1970s. two species of bacteria, Grellia numerosa and Megaira polyxenophila, were discovered living permanently [1]. Bacteria infecting cells is nothing new, but in this case there is no indication of their pathogenicity. The bacteria detected are apparently solid, though still quite distinct, endosymbionts, and there is no reason why their two genomes should not be added to those of the four, typical of cryptomonads.
Symbiosis, however, can have further levels. Like any bacterium, Megaira polyxenophila is also infected by viruses of the bacteriophage type. It turned out that infection with a virus named MAnkyphage by the scientists is not dangerous to M. polyxenophila, and seems to even add to its fitness, so the virus is not a pathogen, but a symbiont. Thus, the number of genomes that can be extracted in a Cryptomonas gyropyrenoidosa cell is as many as seven, and their sources include a former archaeon – now a nucleus, a former bacterium – now a mitochondrion, a former cyanobacterium – now a chloroplast, a former crayfish – now a nucleomorph, the endosymbiotic Grellia numerosa and Megaira polyxenophila, and the symbiotic bacteriophage MAnkyphage.
Are genomes separate or integral elements? Good question
Currently, the record number of distinct genomes in a single cell is considered to be seven. This may, of course, change as new discoveries are made. The number depends, for example, on the criteria adopted. Here the reference is clearly to a single cell, which avoids the dilemmas typical of the situation of colony organisms, such as lichens, or those bound by extracellular symbiosis.
Genomes organized into chromosomes is a simplification. Some genes can easily change places and are called transposons. In the genomes of organisms, it is not uncommon to discover entire sections from other organisms, such as viruses. Once they become an integral part of the genome and are passed on through reproduction, they can hardly be considered separate. However, some viruses incorporate their genes into the host genome for many years, but after some time they are released (this is the case with the chickenpox virus, which can cause hemiplegia years later).
It turns out that distinguishing between a symbiont and a parasite is not at all as easy as the tables in textbooks depict. An interesting example is the protozoan Mixotricha paradoxa , which lives in the gut of a termite. Attached to its body are spirochetes that function like cilia, and depriving the host of these symbionts (with antibiotics, for example) ultimately kills it. The question, then, is whether we should treat the protozoan and the spirochete that inhabits it as separate individuals. And if determining the boundaries of an individual is not so obvious, how can we count genomes in this situation?
