Pattern Formation in the Blue-Green Alga Anabaena

Abstract

We present further evidence for an interactive mechanism in the formation of the spaced pattern of heterocysts in Anabaena. The evidence comes from experiments which are an extension of those described earlier, in which filaments of the alga are broken near to a proheterocyst. We argue that a proheterocyst depends upon neighbouring vegetative cells for the removal of an inhibitory substance: when the proheterocyst is deprived of these supporting vegetative cells it will be forced to regress. We showed earlier that such regressions do occur in early proheterocysts when a filament is broken on one side only. We now find that advanced pro-heterocysts can be made to regress when double breakages are performed to leave small fragments containing the proheterocysts. The probability of a proheterocyst regressing is correlated with its stage of development and with the size of the fragment: the smaller the fragment, the more advanced is the stage at which regression will occur. To formulate this we have defined developmental stages in terms of ultrastructure and compiled the results of a diversity of breakage operations with the cells at these specified stages. Certain compounds affect the spacing of the heterocyst pattern, causing it to become wider or narrower. These compounds have the predicted effect upon regression frequencies, up-holding our assumption that regressions express an underlying competitive mechanism.