A crozier is an anatomical feature of many fungi in the phylum Ascomycota

Ascomycota

The Ascomycota are a Division/Phylum of the kingdom Fungi, and subkingdom Dikarya. Its members are commonly known as the Sac fungi. They are the largest phylum of Fungi, with over 64,000 species...

 that form at the base of asci

Ascus

An ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain eight ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number one , two, four, or multiples...

 and look like hook-topped shepherd’s staffs or stylized religious crosier

Crosier

A crosier is the stylized staff of office carried by high-ranking Catholic, Eastern Orthodox, Anglican, and some Lutheran and Pentecostal prelates...

s. The term is also used in describing the unfurled frond of certain fern species.

During the ascus

Ascus

An ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain eight ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number one , two, four, or multiples...

 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

Cell nucleus

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 divides resulting in the formation of a pair of compatible nuclei, i.e. a dikaryon

Dikaryon

Dikaryon is from Greek, di meaning 2 and karyon meaning nut, referring to the cell nucleus.The dikaryon is a nuclear feature which is unique to some fungi, in which after plasmogamy the two compatible nuclei of two cells pair off and cohabit without karyogamy within the cells of the hyphae,...

, 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

Septum

In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...

. 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 connection

Clamp connection

A clamp connection is a structure formed by growing hyphal cells of certain fungi. It is created to ensure each septum, or segment of hypha separated by crossed walls, receives a set of differing nuclei, which are obtained through mating of hyphae of differing sexual types...

s on the dikaryotic hyphae of Basidiomycota

Basidiomycota

Basidiomycota is one of two large phyla that, together with the Ascomycota, comprise the subkingdom Dikarya within the Kingdom Fungi...

.