Читать книгу Algorithms in Bioinformatics - Paul A. Gagniuc - Страница 50

A second hypothesis for the origin of multicellularity proposes that unicellular organisms may aggregate to form unitary colonies that can achieve multicellularity and cell specialization over time. According to this theory, multicellularity emerged from cooperation between unicellular organisms. Examples of cooperation among organisms have been observed in nature at different scales and in various forms. One of the simplest integrated multicellular organisms is Tetrabaena socialis in which four identical cells constitute the individual [144]. The nuclear genome of T. socialis dictates the number of cells in the colony [145]. Another example is the choanoflagellate (Greek and Latin – khoánē, “funnel”; flagellate, “flagellum”) Salpingoeca rosetta, which can exist as a unicellular organism or it can switch to form multicellular spherical colonies called rosettes (form bridges between cells by incomplete cytokinesis), showing a primitive level of cell differentiation and specialization. Formation of multicellular colonies is induced by different signal molecules. The source of such signal molecules can originate from individuals of the same species (i.e. slime molds) or from individuals of different species (i.e. bacterium species) [146]. In the case of S. rosetta, the signal molecules for colony formation originates from the food source, namely the Algoriphagus machipongonensis bacterium (phylum Bacteroidetes) [147, 148]. Choanoflagellates, sponges and algae of the genus Volvox are more complex examples of first evolutionary stages that indicate the border between colonial organisms and multicellular organisms. Choanoflagellates are the closest relative of metazoans (all animals composed of cells differentiated into tissues and organs) [149, 150]. Some genes required for multicellularity in animals, such as genes for adhesion, genes for signaling, and genes for extracellular matrix formation, are also found in choanoflagellates [151]. This suggests that these genes may have evolved in a common ancestor before the transition to multicellularity in animals [152]. Sponges are one of the oldest primitive multicellular organisms in the fossil record. Choanoflagellates are small single-celled protists, partially similar in shape and function with some of the sponges cells (choanocytes) [153]. Many associations have been made in the past between choanocytes and choanoflagellates. However, the transcriptome of sponge choanocytes is the least similar to the transcriptomes of choanoflagellates and is significantly enriched in genes unique to either animals or sponges alone [154]. Slime molds are also interesting examples, which can indicate how some multicellular organisms formed. Slime molds are unrelated eukaryotic organisms that can live as single cells. In certain conditions (i.e. starvation), single cells of the same species can aggregate to form multicellular reproductive structures [155]. For instance, the multicellular aggregate (a slug-like mass of a few thousand cells called a grex) of amoebae Dictyostelium discoideum can show cellular adhesion, cellular specialization, tissue organization, and coordination that allows for mechanical movement [156, 157]. Although the behavior of D. discoideum is not necessarily a close example of the process that led to multicellular organisms, it can certainly serve as a clue for detailed research on the emergence of multicellularity.