A crozier is an anatomical feature of many fungi in the phylum Ascomycota that form at the base of asci and look like hook-topped shepherd’s staffs or stylized religious crosiers. The term is also used in describing the unfurled frond of certain fern species.
During the ascus initial formation the crozier helps to maintain a dikaryotic state in the ascus initial and its side branch that will continue the spreading growth of the ascogenous hyphae in Ascomycota fruitbodies. The tips of developing asci on these ascogenous hyphae curl over. One haploid nucleus migrates into the curved tip while the other compatible haploid nucleus remains in the penultimate space below the hook. The ascus initial itself forms as a radiating spur branch at the top of the hook. Each nucleus divides resulting in the formation of a pair of compatible nuclei, i.e. a dikaryon, in the ascus. Two sister nuclei remain, one in the basal cell and the other in the crozier. The tip of the crozier fuses with the penultimate cell while walling itself off from the ascus by the formation of a septum. The nucleus from the crozier migrates into the penultimate cell joining the other nucleus, thus maintaining a dikaryotic state. These nuclei migrate into a side branch growing from the base of the ascus that repeats the ascus-crozier formation innumerable times. Mature croziers are detectable through microscopic examination of mature asci as small curved bridges at the basal septa. A minority of Ascomycota lack crosiers, hence the presence or absence of croziers is an important taxonomic character. Croziers resemble and function similarly to clamp connections on the dikaryotic hyphae of Basidiomycota.
- C.Michael Hogan. 2010. Fern. Encyclopedia of Earth. National council for Science and the Environment. Washington, DC.