Scanning electron micrographs of selected individuals arranged to illustrate the presumed sequence of the life cycle of the magnetotactic multicellular organisms. Initially a , the organism is small and spherical; as it grows b their cell size enlarges, but not the cell number.
Later c , cells synchronously divide without separating and the organism contains a larger number of smaller cells. In the next step d , the magnetotactic multicellular organisms become elliptical and then e eight-shaped, as two attached organisms. Finally f , the eight-shaped organism splits into two equal organisms.
Light microscopy sequence of a single dividing magnetotactic multicellular organism showing the final steps in the organism division. Initially, the organism is at the eight-shaped stage, and the constriction between the two halves seems to increase with time.
Finally, the organism split into two organisms that swim independently. Transmission electron microscopy showed that some magnetotactic multicellular organisms had cells containing invaginations of the cell membranes indicative of concomitant cell division in two or more cells of the same organism Fig.
Because ultra-thin sections ca. Moreover, the two peaks observed in some of the cell counts Fig. The invaginations were oriented radially and always began at the part of the cell that has direct contact with the external environment Fig. This mechanism of cell division preserves the general organization of the organism because it maintains all cells arranged radially.
Ultra-thin section of a magnetotactic multicellular organism observed by transmission electron microscopy showing invaginations arrows indicative of cell division in four of the seven cells seen in longitudinal view.
The invaginations arise in the part of the membrane that has direct contact with the environment. The magnetosomes are observed in both sides of the invaginations, showing that both daughter-cells receive the magnetosomes from the mother-cell. In unicellular magnetotactic bacteria, cell division results in partitioning of the magnetosomes, of the other intracellular inclusions such as polyhydroxyalkanoates, and possibly also flagella between the two daughter-cells [ 11 ].
Similarly, in the magnetotactic multicellular organisms the magnetosomes were disposed in both sides of the dividing cells, showing that they were distributed to the two daughter-cells during cell division and that their magnetic polarity in relation to the flagella is maintained Fig. The magnetic polarity of magnetic crystals in magnetotactic bacteria seems to be an epigenetic heritable trace [ 12 ].
Thus, these organisms should coordinate cell division and relative position of the daughter-cells to keep the magnetic polarity of the whole organism as well as generating two magnetotactic organisms with the same magnetotactic behavior as the mother-organism.
Electron micrographs Fig. Based on the helical organization of cells we hypothesize for the cell rearrangements during division to explain the maintenance of the magnetic moment after several generations. The axis of the helix would define a polar axis parallel to the direction of movement. Cell division planes would be aligned perpendicularly to the direction of the helix trace, which would maintain the general cell arrangement in the organism.
During the organism division, the cells from different turns of the helix would slide in relation to each other, causing the organism to become elliptical, then eight-shaped as seen in Figs. All cells have their magnetic moments pointing in a specific direction along the helix trace.
The projection of the magnetic moment of each cell in the plane perpendicular to the axis would cancel out with the projected magnetic moment of another cell in the opposite side of the organism.
In contrast, the components of the magnetic moments parallel to the polar axis point in the same direction. Thus, the net magnetic moment of the whole organism would be generated by the sum of the individual cell magnetic moment projection component in the direction parallel to the polar axis.
Consequently, after the separation, each new organism would present the same radial—helical distribution of cells as the mother organism, with the net magnetic moment parallel to the polar axis. Helical organization is found in early developmental stages during the cleavage in several major invertebrate animal groups, assembled under the name Spiralia.
In this case, it is determined by the division plane of blastomeres, and depends upon the positioning of centrosomes [ 13 ]. Since in magnetotactic organisms the helical organization is present probably before the cell division, and their cells have no centrosomes, this order may correspond to the best spatial accommodation of cells with finely tuned forms, or may be caused by special adhesive properties of the cells that enable each cell to find and maintain its position in the whole multicellular body.
The similarity of this organism with the previously studied magnetotactic multicelular organisms [ 2 , 3 , 4 , 5 ] is great: they are all spherical organisms composed of multiple Gram-negative prokaryotic cells containing iron sulfide magnetosomes. Interestingly, Rodgers et al. These observations are in accordance to Figs. We hypothesize that the absence of a one-cell stage in the life cycle of magnetotactic multicellular organisms is caused, at least in part, by the need to maintain the content of the internal compartment isolated from the environment.
Another possibility is to maintain the organism always too large to be preyed by most bacteria-grazing protist populations [ 14 ]. The life cycle of this prokaryotic organism is completely multicellular, generating directly two fully organized new bodies through a rather unusual morphogenetic process. Besides, this is different from most other multicellular prokaryotic or eukaryotic organisms, which present at least one part of their life cycle in a unicellular form, from which the step-by-step ontogenic processes generate the specific body-plan of the new adult organisms.
We thank R. Supplementary data associated with this article can be found, in the online version, at doi: Video Video of a dividing magnetotactic multicellular organism showing the final steps in the organism division. Initially, the organism is eight-shaped, and the constriction between the two halves seems to increase with time until the organism split into two organisms that swim independently.
Bazylinski D. Frankel R. Google Scholar. Rodgers F. Blakemore R. Blakemore N. Maratea D. Rodgers C. Keim C. Abreu F. Lins U. Lins de Barros H. Farina M. Esquivel D. Danon J. Shimkets L. Freitas F. Brosius J. Dull T. Sleeter D. Noller H. Already a subscriber? Sign in. Thanks for reading Scientific American.
Create your free account or Sign in to continue. See Subscription Options. Discover World-Changing Science. In order to be considered a multicellular creature, and organism must fulfil certain criteria: Cells must stick together! This sounds fairly obvious but it does involve mechanisms for cellular adhesion Cells must be able to communicate. In an multicellular body the cells must remain in communication, and change in response to conditions that affect the whole body Dependency.
There is a third strategy, which is to become a nitrogen fixing bacteria by night, and an aerobically respiring bacteria by day, but this requires huge amounts of energy as it means that the cell has to do a complete enzyme turnover every twelve hours The differentiated cell is called a heterocyst.
Gould on Twitter Recent Articles by S. Get smart. Sign up for our email newsletter. Sign Up. Read More Previous. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber?
Create Account See Subscription Options. Continue reading with a Scientific American subscription. Subscribe Now You may cancel at any time.
It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. From a Molecular Cell Biology introduction course web resource from the University of Illinios, which explains the general sentiment in microbiology over this question:.
There are lots of unicellular eukaryotes, including amoebas, paramecium, yeast, and so on. As to whether there are multicellular prokaryotes, the standard answer is No, but there is a lot of evidence that some bacterial species can aggregate together and divide labor so that the "colony" is working more efficiently.
This is characteristic of any traditional multicellular organism, but there's still a lot of resistance to the idea of calling these prokaryotes "multicellular. Here's a Scientific American article on the subject, which is good for the layman and will add to your understanding. Note that the most complex 'multicellular prokaryotes' known do not form comparably large or complicated structures when contrasted with eukaryotes. Even though prokaryotes have more biochemical tools available to them, their largest multicellular specimen pales in comparison to our small multicellular eukaryotes, let alone out large eukaryotes like the Armillaria ostoyae fungus in Oregon nicknamed "the largest organism on Earth" , giant sequoia trees, the dinosaurs, or blue whales.
But I digress! Total organism size is not a requisite for multicellularity, which is why some bacteria can be termed so.
Here is a paper I found in the literature on the first reported 'true' multicellular prokaryote, from Similar organisms have been observed since. See Pubmed link. The same question has also been answered extensively by specialists on Quora and on ResearchGate. Your question implies external influence. Biological evolution does not proceed teleologically , with an end goal in mind.
0コメント