PvM posted Entry 2006 on February 13, 2006 10:00 AM.
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A while ago on ASA, Glenn Morton referenced the work by Boraas. I have always been fascinated by this reference but unable to find much relevant literature. Until recently, when I accidentally ran across more recent reearch in this area. I would like to share what I learned and how these findings may help understand evolution of multicellularity.
The original references was to a paper published in EOS called “Predator-mediated algal evolution in Chemostat culture”. In 1998, Boraas published another paper titled “Phagotrophy by a flagellate selects for colonial prey: A possible origin of multicellularity” in Evolutionary Ecology 1998, 12, 153-164
According to the original abstract, an unknown predator had invaded a chemostat containing algae. While many of the algae fell prey to the predator, a news species arose consisting of clusters of multiple cells. This seems the first logical step towards full multi-cellularity where cells take on specific roles.
The original abstract reads
An unidentified microflagellate specie (4-12 [mico]m) and Chlorella Prenodosa (2-5 [micro]m) were grown at 25 C in mixed-species chemostats with constant light and sterile, inorganic medium flow. The flagellate readily consumed the algae and grew rapidly (doubling time ca. 6 h). Size distributions of both species are shown in the Figure (area = biovolume). After an initial oscillation (curves 1,2), the system apparently stabilized with both species coexisting. The algal population now consisted of clusters of 4 to tens of cells that were immune to predation by the flagellate (curve 3). The mean cluster size then steadily decreased (curve 4) and stabilized at 4-8 cells (curve 5). These, and other, observations support the hypothesis: (1) a multicellular algal form was selected as a response to predation pressures, (2) a minimum cluster size was selected due to nutrient competition (large clusters have a smaller surface area per unite biomass) and (3) genetic, morphological, and structural diversity of the system increased as a response to predation. Flagellate predation influences both the genetics and the dynamics of microalgal population.”
the 1998 paper reports on experiments with predation
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.
The article showed how when a common uni-celllular alga “Chlorella vulgaris” was exposed to a predator “Ochromonas vellesiaca”, a phagotrophic flagellate, within 100 generations or so, a multicellular (colonial) specie arose .
In other words, the innovsative step from single to multi-cellular may well have taken place under selective pressures of predation. The work by Boraas shows that even accidents such as allowing predators iside chemostats can lead through hard work to fascinating new scientific findings and insights.
The authors mention that other than in rare instances, the Chlorella culture had always exhibitied its normal unicellular morphology over a timeframe of 2 decades.
When the predator was introduced, predictably the prey density declined and the predator density increased. When the predators started to run out of food, they started to decline and the reduction in predation led to a recovery of the Chlorella population (this is a classical pre-predator interaction). During the recovery phase, it was noticed that in addition to unicellular forms, there now existed colonial forms with the numbers of cells ranging from four to hundreds. Eventually the system entered a steady state with the Chlorella population consisting of colonies of 8 cells. These colonies were not only stable but also self-replicating
The authors conclude that the muli-cellular form is a rare mutation which was selected by predation and thus ‘amplified’. The authors also discuss the issue of induction, namely that the flagellates released a substance that caused colony formation. Given that it took almost 20 generations before colonies became apparant, the authors reject this alternative. Additionally, multicellular colonies were maintained even in low density cultures and finally, when the colonies are allowed to reproduce by themselves, they reproduce as colonies not single cells. The authors finally show how these experiments support a thesis by Stanley that multicellular life arose late into the pre-Cambrian under selective pressure of predation.
Was the Cambrian explosion in other words, an arms race between prey and predator?
That under selective pressure, mulicellular colonies arose, shows how the simple processes of variation and selection can surely explain innovation and increase in complexity.
Stanley, S.M. (1973) An ecological theory for the sudden origin of multicellular life in the Late Precambrian PNAS 70, 1486-1489.
According to modern ecological theory, high diversity at any trophic level of a community is possible only under the influence of cropping. Until herbivores evolved, single-celled algae of the Precambrian were resource-limited, and a small number of species saturated aquatic environments. In the near-absence of vacant niches, life diversified slowly. Because the changes required to produce the first algae-eating heterotrophs were therefore delayed, the entire system was self-limiting. When the “heterotroph barrier” was finally crossed in the late Precambrian, herbivorous and carnivorous protists arose almost simultaneously, for no major biological differences separate the two groups. These events automatically triggered the formation of a series of self-propagating feedback systems of diversification between adjacent trophic levels. Comparable systems arose among multicellular groups, which radiated rapidly from the newly diversifying protist taxa. The sudden proliferation of complex food webs formed by taxa invading previously vacant adaptive zones produced an explosive diversification of life over a period of a few tens of millions of years. The rapid appearance of skeletons in various groups, though of special geological importance, was no more dramatic than other aspects of the radiation. The overall rate of diversification was comparable to rates for less-extensive adaptive radiations of the Phanerozoic.
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