On the origin of differentiation.

Following the origin of multicellularity in many groups of primitive organisms there evolved more than one cell type. It has been assumed that this early differentiation is related to size the larger the organism the more cell types. Here two very different kinds of organisms are considered: the volvocine algae that become multicellular by growth, and the cellular slime moulds that become multicellular by aggregation. In both cases there are species that have only one cell type and others that have two. It has been possible to show that there is a perfect correlation with size: the forms with two cell types are significantly larger than those with one. Also in both groups there are forms of intermediate size that will vary from one to two cell types depending on the size of the individuals, suggesting a form of quorum sensing. These observations reinforce the view that size plays a critical role in influencing the degree of differentiation.

Oxygen and differentiation in Dictyostelium discoideum.

We showed previously that if Dictyostelium discoideum cells are sucked up into a small glass capillary with air at one end and plugged with mineral oil at the other, a sharp band of fast moving cells with pres talk characteristics formed within a minute at the air end of the cell mass. We now demonstrate that oxygen inside the capillary is responsible for the initiation and positioning of the sharp division line between prestalk-like and pres pore-like cells, and that the length of the prestalk zone is regulated by the oxygen concentration. Our results are compared to a quantitative theory, showing good agreement with the experiments. We also discuss the relevance of these observations to the differentiation of pres talk and prespore zones in normal slugs and the origins of polarity in this organism.

Why does slug length correlate with speed during migration in Dictyostelium discoideum.

Taking advantage of the fact that static electricity in plastic Petri dishes will produce very long, thin migrating slugs of Dictyostelium discoideum, it was shown that these slugs moved particularly rapidly. This is consistent with the demonstration of Inouye and Takeuchi that speed varies with length for slugs migrating on agar. Based on these observations it is suggested that slug speed is controlled by both the resistance at the tip and some factor that correlates With slug size, such as the concentration of endogenously produced ammonia.

Social insects and social amoebae.

The evolution of social groupings in insects, especially wasps, is compared to that of social amoebae (cellular slime moulds). They both show a gamut of colony sizes, from solitary forms to complex colonies with a division of labour. The various ideas as to how there might have been an evolution of complexity within insect societies, such as the role of genetic relatedness, the role of mutualism, the origin of sterility, the manipulation and exploitation of some individuals by others within a colony, are discussed, and then applied to social amoebae. The result is both interesting and instructive: despite some differences, there are many striking parallels, which suggests that there are some common denominators in the formation and evolution of a social existence among organisms.