It is like following a rainbow at breakneck speed looking for the pot of gold at the end. In studying the molecular development of any organism, we are not just concerned with the signaling between cells, which is the emphasis here, but within cells as well. Some effects of oxygen on polarity in Tubularia. Moreover, we know that natural selection can and does act directly on the form and function of all organisms. Polarity As pointed out in the previous discussion, polarity is an alignment, the onset of a direction to the embryo or the cell mass. Other good examples although less elaborate ones may be found in the exchanges between parent and offspring when feeding or in responding to moments of danger.
Line lengths are arbitrary and are meant only to indicate the inferred branching order. Unfortunately, it has not been possible to exploit the sexual cycle of cellular slime molds to make crosses, but by various less direct methods the known genes can be assigned into linkage groups. Such an internally produced signal could be a morphogen, and along with it there must be a response mechanism to the signal, all of which eventually becomes incorporated into the genome. On the other hand, a forest is not an organism. When such a slug reached the dead end of a tunnel, each cell within the slug starting at the tip did a U-turn so that soon the entire slug, with its anterior red tip, was going in the opposite direction. Hypothetical First Steps in Slime Mold Development Before we think of mathematical models, it will be helpful to ask what might have been the minimal sequence of initial steps in the rise of slime mold multicellular development. Suddenly one day, the obvious solution hit me: I could use lower plants to attack the problems of embryology.
Readers acquainted with his previous books will look forward to hearing more about odd organisms that illustrate important biological principles. A unicellular stage is also favored in many asexual organisms where dispersal in the form of spores is best accomplished by the smallest bodies that are easily carried to some new feeding ground. Diatoms and ciliates have highly specialized outer coverings, which present another kind of problem. Inside there are no cross-walls, but merely a large vacuole surrounded by streaming cytoplasm containing vast numbers of nuclei. Each of the organisms forming the swarm are polar—they have heads and tails—yet there is no leader; all of them are followers. He then asks what is the most elementary way that polarity and pattern formation can be achieved.
The orientation to light and the extremely sensitive orientation to temperature gradients in the slime mold Dictyostelium discoideum. Brown Algae and Diatoms The brown algae are largely marine and are notable for having some forms, such as Macrocystis, which are over one hundred feet in length. The relatively uncomplicated and easy-to-grow cellular slime molds offer a unique opportunity to analyze development at a primitive stage and perhaps gain insight into how early multicellular development might have started. We do not know enough about dispersal mechanisms in the soil and can only ask whether this primitive clustering of cysts might somehow enhance dispersal. Slime molds, ascidians, and the utility of evolutionary theory.
Let us now examine the matter of how the polarity and the pattern are established. The relatively uncomplicated and easy-to-grow cellular slime molds offer a unique opportunity to analyze development at a primitive stage and perhaps gain insight into how early multicellular development might have started. I should add that all these polarity-inducing phenomena that I have described for Fucus and Tubularia provide textbook cases for applying the principles of reaction-diffusion models see, for example, Gierer and Meinhardt, 1972; Meinhardt, 1982; Murray, 1989. The third way to seek simplicity is through mathematical modeling. Clearly, sexuality exists among unicellular organisms; in other words, sex undoubtedly antedated multicellularity.
However, if those juveniles were fed a paste made of dead soldiers no such metamorphosis occurred; the inhibitor in the paste prevented the switch in the developmental pathway. The really basic and easy-to-grow mobile slime molds provide a different chance to research improvement at a primitive level and maybe achieve perception into how early multicellular improvement may need started. So we begin in the genome and follow the sequence through protein synthesis and the developmental activities of those proteins. Nature 161: 548— 551, 587—589. Drawn from a photograph in Bonner et al.
I shudder to think where I would be without their criticisms, constructive and otherwise. He has vacationed in Nova Scotia yearly for over 40 years. A Air in the space on the left. Through such a search it might be possible to understand the essence of development. I will pursue this possibility presently; here I am only concerned with the initial step. His proposal comes at a time when the field of evolutionary developmental biology is really taking off, and many young researchers are just beginning to formulate their conceptual and experimental approaches. The development of the small mushroom Coprinus.
It is also true for these and many other examples that the same receptors will be responsive to the amount of signaling substance: over a threshold amount, the reaction will be the reverse of that below the threshold. It is not as I once thought that all the cells on one side move faster than the other, but it is the tip region that moves, and this seems to lead all the cells behind it in a new direction. How can one simplify the mechanism of development? These transitions are especially important in their implications for natural selection because with each transition one moves from one level of selection to another. The same advantages would apply to the daughter cells, thereby giving rise to a multicellular sessile colony. Conclusion The study of the molecular development of cellular slime molds is giving us a deep insight into how development is controlled, how it is directed on the microlevel. Now we know that it is an animal, a hydroid, which is a cnidarian, and that its green color is due to a unicellular photosynthetic alga living in the cells.