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.

The mechanisms responsible for controlling hyphal extension and branching are still poorly understood. We have investigated these processes by studying their dependence on temperature and nutrient concentration. Tip growth is highly responsive to temperature change, increasing linearly from 4 to 37 °C. Over this range of temperatures the branching pattern shows virtually no response. Likewise, varying nutrient concentration does not affect branch distribution. Colonies subjected to rapid extreme temperature downshifts (for example from 25 to 4°) display a strong and highly predictable branching response. There are three stages to this response. First there is an initial lag phase of growth without branching. After this, the growing tips display a series of tightly spaced, dichotomous branches dubbed starbursting. Following continued growth at 4°, tips enter a recovery phase, returning to normal branching frequencies with lateral branches. The strength of the response to cold is correlated with the ratio of growth rates before and after the downshift. The combined observations point to a homeostatic set point for branch distribution that compensates for temperature and nutrient concentration.

The widespread occurrence of vegetative incompatibility in fungi and the high level of incompatibility gene polymorphisms in fungal populations means that generally parents in a cross will be vegetatively incompatible. In Neurospora crassa the mating type locus even functions as a vegetative incompatibility locus assuring vegetative incompatibility in a cross. In this paper we have tested the effect of vegetative incompatibility on regulating transmission of mitochondrial plasmids and nuclear genes in sexual interactions of N. crassa. In the absence of mating type vegetative incompatibility between the parents, transmission of plasmids from the conidial paternal parent was found to be approximately ten times higher than in normal crosses. Control experiments in which conidia of a contaminating plasmid bearing strain of similar mating type as (and fully vegetatively compatible with) the established maternal culture were added together with the presumed paternal conidia (opposite mating type) showed plasmid transmission to ascospores. Since trichogynes do not fuse with conidia of the same mating type, it may be concluded that plasmids from the contaminating strain entered the maternal tissue by somatic fusion. The fate of conidial nuclei in such sexual interactions has been investigated as well, both under conditions of vegetative compatibility and incompatibility between the strains. These experiments demonstrated that conidia that are vegetatively compatible to the established protoperithecial strain manage to get access to the resources of the maternal culture and initiate new fruiting bodies. This type of nuclear parasitism was never observed when vegetatively incompatible conidia were used. We propose that vegetative incompatibility may function in sexual crosses to protect unfertilised cultures from looting of maternal resources.