quote:From one cell to many: How did multicellularity evolve? Date: January 25, 2014 Source: American Journal of Botany
Summary: In the beginning there were single cells. Today, many millions of years later, most plants, animals, fungi, and algae are composed of multiple cells that work collaboratively as a single being. Despite the various ways these organisms achieved multicellularity, their conglomeration of cells operate cooperatively to consume energy, survive, and reproduce. But how did multicellularity evolve?
Representative diverse origins of multicellularity are shown on a highly redacted and unrooted phylogenetic diagram of the major eukaryotic clades (modified from a variety of sources). Although some lineages or clades are entirely unicellular or multicellular (e.g., lobose amoeba and the land plants, respectively), most contain a mixture of body plans such as the unicellular and colonial body plans (e.g., choanoflagellates) or a mixture of the unicellular, colonial, and multicellular body plans (e.g., ciliates and stramenopiles). In general, early-divergent persistent (EDP) lineages are dominated by unicellular species (e.g., prasinophytes in the chlorobiontic clade), whereas later-divergent lineages contain a mixture of body plans (e.g., chlorophycean and charophycean algae). Species-rich, late-divergent persistent (LDP) lineages tend to be exclusively multicellular (e.g., the land plants and metazoans).
Karl Niklas (Cornell University, Ithaca, NY), a plant evolutionary biologist, is interested in how plants have changed over the past few million years, in particular their size, shape, structure, and reproduction. As the first article in a series of Centennial Review papers celebrating 100 years of the American Journal of Botany, Niklas reviews the history of multicellularity and the changes that cells must have had to go through -- such as aspects of their shape, function, structure, and development -- in order to be able to functionally combine with other cells. He also explores the underlying driving forces and constraints (from natural selection to genetics and physical laws) that influence the evolution of multicellularity.
mutant forms can lead to increased morphological complexity
Abstract: Predation was a powerful selective force promoting increased morphological complexity in a unicellular prey held in constant environmental conditions. The green alga,Chlorella vulgaris, is a well-studied eukaryote, which has retained its normal unicellular form in cultures in our laboratories for thousands of generations. For the experiments reported here, steady-state unicellular C. vulgaris continuous cultures were inoculated with the predator Ochromonas vallescia, a phagotrophic flagellated protist (’flagellate‘). Within less than 100 generations of the prey, a multicellular Chlorella growth form became dominant in the culture (subsequently repeated in other cultures). The prey Chlorella first formed globose clusters of tens to hundreds of cells. After about 10–20 generations in the presence of the phagotroph, eight-celled colonies predominated. These colonies retained the eight-celled form indefinitely in continuous culture and when plated onto agar. These self-replicating, stable colonies were virtually immune to predation by the flagellate, but small enough that each Chlorella cell was exposed directly to the nutrient medium.
M.E. Boraas, D.B. Seale, and J.E. Boxhorn. (1998) "Phagotrophy by a flagellate selects for colonial prey: A possible origin of multicellularity." Evolutionary Ecology. 12(2): 153-164.
quote:Is the change the result of mutations? This question also has been raised about the classical example of evolutionary change - the peppered moth. In the Chlorella example, clustering of algal cells was extremely rare prior to the experiment, occurring about two or three times per year over two decades. This is easily explained as the result of rare mutations but very difficult to explain as the persistence of genetic variants from the original wild population, given the large number of generations involved and the extreme rarity of the observation of clustering.
In short, without evidence to the contrary, it is reasonable to conclude that this experiment demonstrates that natural selection acting on rare mutant forms can lead to increased morphological complexity during time scales observable by man. Natural selection and mutation thus remain viable mechanisms for historical, adaptive evolutionary change.