HYP LOCI CONTROL CELL PATTERN-FORMATION IN THE VEGETATIVE MYCELIUM OFASPERGILLUS-NIDULANS

Aspergillus nidulans grows by apical extension of multinucleate cells called hyphae that are subdivided by the insertion of crosswalls calle d septa. Apical cells vary in length and number of nuclei, whereas sub apical cells are typically 40 mu m long with three to four nude Apical cells hare active mitotic cycles, whereas subapical cells are arreste d for growth and mitosis until branch formation reinitiates tip growth and nuclear divisions. This multicellular growth pattern requires coo rdination between localized growth, nuclear division, and septation. W e searched a temperature-sensitive mutant collection for strains with conditional defects in growth patterning and identified six mutants (d esignated hyp for hypercellular). The identified hyp mutations are non lethal, recessive defects in five unlinked genes (hypA-hypE). Phenotyp ic analyses showed that these hyp mutants have aberrant patterns of se ptation and show defects in polarity establishment and tip growth, but they have normal nuclear division cycles and can complete the asexual growth cycle at restrictive temperature. Temperature shift analysis r evealed that hypD and hypE play general roles in hyphal morphogenesis, since inactivation of these genes resulted in a general widening of a pical and subapical cells. Interestingly, loss of hypA or hypB functio n lead to a cessation of apical cell growth but activated isotropic gr owth and mitosis in subapical cells. The inferred functions of hypA an d hypB suggest a mechanism for coordinating apical growth, subapical c ell arrest, and mitosis in A. nidulans.