Branch initiation in Neurospora is influenced by events at the previous branch

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Growth in Neurospora proceeds by the highly polarised extension of hyphal tips and the formation of new tips (branches). While much attention has been devoted to tip growth, the mechanisms responsible for the decision to form a branch are unknown. One important issue is the degree to which the decision to branch is made independently by each tip, or is under the control of proximal regions of the colony. We address this question by comparing the lengths of branch intervals growing from one branch point. We find significant correlations between such intervals under nearly all circumstances tested. In contrast, we find no significant correlation between branch intervals adjacent in series. The strength of the correlation depends on the qualitative nature of the branches surveyed. Dichotomous/apical branching shows a high degree of correlation while lateral branching shows a weaker but still statistically significant correlation (this difference cannot be explained either by differences between the growth rates or distributions of lengths of branch intervals). We interpret the observed correlations to mean that the decision to form a branch is not independently controlled by the tip where the branch forms. Instead it is determined, at least in part, by factors at or close to the previous branch.