This electron microscopy study examined conidiophore and spore development in Aspergillus nidulans, a fungus that reproduces asexually through specialized spore-bearing structures. Using wild-type and mutant strains, Oliver characterized the ultrastructural changes during sporulation. The conidiophore develops a distinctive two-layered wall structure, with the primary wall becoming pigmented brown as spores form. A superficial lamina continuously invests the entire structure and developing spore chains. Spore walls mature through thickening and pigmentation stages, with green pigment deposition restricted to regions beneath the surface lamina. Cytoplasmic organelles, particularly microvesicles and plasmalemmasomes, distribute in organized patterns that correlate with developmental stages. Microvesicles concentrate at growing apices, while plasmalemmasomes accumulate in subapical and basal regions, particularly near developing septa. The study identifies cytoplasmic filaments associated with organelle streaming and proposes that morphogenesis involves selective placement of organelles through cytoplasmic streaming mechanisms. The bristle locus appears to control the transition between unipolar and multipolar growth patterns, affecting secondary wall formation and sterigma development.

Key findings

• Conidiophores develop a two-layered wall structure (primary and secondary layers) with distinct electron density properties; secondary walls become increasingly electron-lucent with maturation
• Pigmentation of conidiophore and spore walls involves electron-dense material deposition and is preceded by formation of a superficial lamina, with brown pigment in conidiophores and green pigment in spores
• Cytoplasmic organelles show precise spatial and temporal distribution patterns: apical microvesicles concentrate at growing tips, while plasmalemmasomes and lomasomes accumulate in subapical/basal regions and associate with septa formation
• Cytoplasmic filaments (approximately 10 nm) are associated with organelle streaming and may play a role in controlling morphogenesis through selective organelle placement
• The bristle locus mutation prevents formation of normal secondary wall and vesicle structures, suggesting this locus controls the transition from unipolar to multipolar growth patterns during conidiophore development