Hypotheses for origin Multicellular organism

1 hypotheses origin

1.1 symbiotic theory
1.2 cellularization (syncytial) theory
1.3 colonial theory
1.4 synzoospore theory
1.5 gk-pid
1.6 role of viruses

hypotheses origin


tetrabaena socialis consists of 4 cells.

one hypothesis origin of multicellularity group of function-specific cells aggregated slug-like mass called grex, moved multicellular unit. slime molds do. hypothesis primitive cell underwent nucleus division, thereby becoming coenocyte. membrane form around each nucleus (and cellular space , organelles occupied in space), thereby resulting in group of connected cells in 1 organism (this mechanism observable in drosophila). third hypothesis unicellular organism divided, daughter cells failed separate, resulting in conglomeration of identical cells in 1 organism, later develop specialized tissues. plant , animal embryos colonial choanoflagellates.

because first multicellular organisms simple, soft organisms lacking bone, shell or other hard body parts, not preserved in fossil record. 1 exception may demosponge, may have left chemical signature in ancient rocks. earliest fossils of multicellular organisms include contested grypania spiralis , fossils of black shales of palaeoproterozoic francevillian group fossil b formation in gabon (gabonionta). doushantuo formation has yielded 600 million year old microfossils evidence of multicellular traits.

until recently, phylogenetic reconstruction has been through anatomical (particularly embryological) similarities. inexact, living multicellular organisms such animals , plants more 500 million years removed single-cell ancestors. such passage of time allows both divergent , convergent evolution time mimic similarities , accumulate differences between groups of modern , extinct ancestral species. modern phylogenetics uses sophisticated techniques such alloenzymes, satellite dna , other molecular markers describe traits shared between distantly related lineages.

the evolution of multicellularity have occurred in number of different ways, of described below:

the symbiotic theory

this theory suggests first multicellular organisms occurred symbiosis (cooperation) of different species of single-cell organisms, each different roles. on time these organisms become dependent on each other not able survive independently, leading incorporation of genomes 1 multicellular organism. each respective organism become separate lineage of differentiated cells within newly created species.

this kind of severely co-dependent symbiosis can seen frequently, such in relationship between clown fish , riterri sea anemones. in these cases, extremely doubtful whether either species survive long if other became extinct. however, problem theory is still not known how each organism s dna incorporated 1 single genome constitute them single species. although such symbiosis theorized have occurred (e.g. mitochondria , chloroplasts in animal , plant cells—endosymbiosis), has happened extremely and, then, genomes of endosymbionts have retained element of distinction, separately replicating dna during mitosis of host species. instance, 2 or 3 symbiotic organisms forming composite lichen, although dependent on each other survival, have separately reproduce , re-form create 1 individual organism once more.

the cellularization (syncytial) theory

this theory states single unicellular organism, multiple nuclei, have developed internal membrane partitions around each of nuclei. many protists such ciliates or slime molds can have several nuclei, lending support hypothesis. however, simple presence of multiple nuclei not enough support theory. multiple nuclei of ciliates dissimilar , have clear differentiated functions. macronucleus serves organism s needs, whereas micronucleus used sexual reproduction exchange of genetic material. slime molds syncitia form individual amoeboid cells, syncitial tissues of multicellular organisms, not other way round. deemed valid, theory needs demonstrable example , mechanism of generation of multicellular organism pre-existing syncytium.

the colonial theory

the colonial theory of haeckel, 1874, proposes symbiosis of many organisms of same species (unlike symbiotic theory, suggests symbiosis of different species) led multicellular organism. @ least some, presumed land-evolved, multicellularity occurs cells separating , rejoining (e.g. cellular slime molds) whereas majority of multicellular types (those evolved within aquatic environments), multicellularity occurs consequence of cells failing separate following division. mechanism of latter colony formation can simple incomplete cytokinesis, though multicellularity typically considered involve cellular differentiation.

the advantage of colonial theory hypothesis has been seen occur independently in 16 different protoctistan phyla. instance, during food shortages amoeba dictyostelium groups in colony moves 1 new location. of these amoeba differentiate each other. other examples of colonial organisation in protista volvocaceae, such eudorina , volvox, latter of consists of 500–50,000 cells (depending on species), fraction of reproduce. example, in 1 species 25–35 cells reproduce, 8 asexually , around 15–25 sexually. however, can hard separate colonial protists true multicellular organisms, 2 concepts not distinct; colonial protists have been dubbed pluricellular rather multicellular .

the synzoospore theory

some authors suggest origin of multicellularity, @ least in metazoa, occurred due transition temporal spatial cell differentiation, rather through gradual evolution of cell differentiation, affirmed in haeckel’s gastraea theory.

gk-pid

about 800 million years ago, minor genetic change in single molecule called guanylate kinase protein-interaction domain (gk-pid) may have allowed organisms go single cell organism 1 of many cells.

the role of viruses

genes borrowed viruses have been identified playing crucial role in differentiation of multicellular tissues , organs , in sexual reproduction, in fusion of egg cell , sperm. such fused cells involved in metazoan membranes such prevent chemicals crossing placenta , brain body separation. 2 viral components have been identified. first syncytin, came virus. second identified in 2007 called eff1, helps form skin of caenorhabditis elegans, part of whole family of ff proteins. felix rey, of pasteur institute in paris has constructed 3d structure of eff1 protein , shown work of linking 1 cell another, in viral infections. fact known cell fusion molecules viral in origin suggests have been vitally important inter-cellular communication systems enabled multicellularity. without ability of cellular fusion, colonies have formed, complex sponge not have been possible.