Occurrence
Multicellularity has evolved independently at least 46 times in eukaryotes,[5][6] and also in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and land plants.[7] It evolved repeatedly for Chloroplastida (green algae and land plants), once or twice for animals, once for brown algae, three times in the fungi (chytrids, ascomycetes and basidiomycetes)[8] and perhaps several times for slime molds and red algae.[9] The first evidence of multicellularity is from cyanobacteria-like organisms that lived 3–3.5 billion years ago.[5] To reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells (i.e. sperm and egg cells), an issue that is studied in evolutionary developmental biology. Animals have evolved a considerable diversity of cell types in a multicellular body (100–150 different cell types), compared with 10–20 in plants and fungi.[10]
Loss of multicellularity
Loss of multicellularity occurred in some groups.[11] Fungi are predominantly multicellular, though early diverging lineages are largely unicellular (e.g. Microsporidia) and there have been numerous reversions to unicellularity across fungi (e.g. Saccharomycotina, Cryptococcus, and other yeasts).[12][13] It may also have occurred in some red algae (e.g. Porphyridium), but it is possible that they are primitively unicellular.[14] Loss of multicellularity is also considered probable in some green algae (e.g. Chlorella vulgaris and some Ulvophyceae).[15][16] In other groups, generally parasites, a reduction of multicellularity occurred, in number or types of cells (e.g. the myxozoans, multicellular organisms, earlier thought to be unicellular, are probably extremely reduced cnidarians).[17]
Cancer
Multicellular organisms, especially long-living animals, face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development. Changes in tissue morphology can be observed during this process. Cancer in animals (metazoans) has often been described as a loss of multicellularity.[18] There is a discussion about the possibility of existence of cancer in other multicellular organisms[19][20] or even in protozoa.[21] For example, plant galls have been characterized as tumors[22] but some authors argue that plants do not develop cancer.[23]
Separation of somatic and germ cells
In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is relatively rare (e.g. vertebrates, arthropods, Volvox), as great part of species have the capacity for somatic embryogenesis (e.g. land plants, most algae, many invertebrates).
2 comments:
kmd
bin
Post a Comment