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Spores and reproduction of fungi

Introduction

Spores in fungi are not the same as spores of bacteria. Fungi  use spores (asexual or sexual) to reproduce whereas the bacteria, which form spores, do so to survive adverse conditions (lack of nutrients, extreme pH, high temperature etc). Different fungi form different types of spores and, therefore, different names are used for these spores. However, also fungi can sometimes form spores to survive adverse conditions.

Asexual spores

Arthrospores are a form of primitive spores, which occur for instance in Coccidiodes immitis. They are formed by fragmentation of hyphae with septa (septated hyphae) and separation of individual hyphal cells. These hyphal cells make up the spores.

Blastospores are spores, which are formed by budding from the terminal end of the hyphae. These spores may stay attached to the hyphae for further budding, which will result in a branching chain of blastospores. Ascomyceter and basidiomyceter can produce blastospores.

Conidiospores or conidia are formed by so-called conidiophores, which grows aerially from a vegetative hyph. The conidophores branches and at the end of each branch, there is a so-called fialid cell, which produces conidia singly or in chains. Members of the phylum Ascomycota reproduce vegetatively with the help of conidiospores.

Chlamydospores can form when conditions are unfavorable for the fungus and then some hyphal cells may lose water and develop a thick cell wall. The cell, which may be at the tip of or in the middle of a hyphae, has then become a chlamydia spore. The chlamydospore can then germinate when the conditions have become favorable again. Ascomycetes and basidiomycetes (for instance Candida albicans and Histoplasma capsulatum) may develop chlamydospores.

Spores and reproduction of fungi

The figure shows different parts of a sporangium, which grows out of a hyphae (2), which is anchored to the substrate with rhizoids (1). The part closest to the hyphae is called the sporangiophor (3) and it has a septum (4). The part above the septum is called the apophysis (5) and then comes the columella (6) where the sporangiospores (8) are formed. The sporangiospores are surrounded by the wall of thesporangia (7). - Click on image to enlarge it. - Click on the image to enlarge it.

 

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Sporangiospores are produced in sac-like structures, called sporangia. Sporangia are formed at the end of a hyphae, which grow out of the substrate and are called sporangiophores. The sporangia contain a large number of haploid spores, which are attached to a bladder-like structure called columella. When the sporangia wall ruptures, spores are released into the air. Members of the genera Lichthemia, Mucor, Rhizomucor and Rhizopus produce sporangiospores. Note that the sporangia may have a slightly different structure, depending on which genus the actuall fungus belongs to.

Zoospores are unique because they are motile unlike all other types of fungal spores. Zoospores have one single flagellum, which provides mobility, and they are produced in so-called zoosporangia. Members of the phylum Chytridiomycota (chytrides) can produce zoospores.

Sexual spores

Ascospores are produced by fungi, which belong to the phylum Ascomycota, as the name implies. During sexual reproduction, there must be two different reproduction types (male and female), and one usually talks about plus (+) type and minus (-) type. The hyphal cells and the conidium have N chromosomes each. If a conidium (see above) of the minus-type encounters a plus-type of hyphae, the conidium can "fuse" by plasmogamy with the hyphal cell and form a so-called ascus, which is dikaryotic, i.e. has two cell nuclei (N + N chromosomes). Then the cell nuclei can fuse together through karyogami and then a zygote with a diploid nucleus is formed. The zygote now has two sets of chromosomes (2N) and then genetic material can be exchanged between homologous chromosomes. Then the nucleus in the zygote divides through meiosis and then first the chromosomes (4N) are duplicated and then the nucleus divides twice. This results in 4 new nuclei, each of which has one set of chromosomes. Then the chromosomes are duplicated again and the nuclei now undergo a mitosis, which results in 8 haploid ascospores.

Basidiospores are sexual spores, which are produced in a club-shaped organelle, called basidium, by fungi belonging to the phylum Basidiomycota. A typical basidium produces 4 basidiospores exogenously (from the outside) in contrast to ascospores, which are produced endogenously in an ascocarp.

Zygospores occur in fungi of the phylum, which was earlier called Zygomycota and which have (or should) be divided into two new phyla (Mucoromycota and Zoopagomycota). Zygospores are diploid and are formed by sexual conjugation between two fungi.

Resting spores

Teliospores are thick-walled diploid resting spores that occur in rust and smut fungi and are formed in their fruiting bodies (telia). When the teliospores are mature, they spread to the environment and can rest until conditions become favorable. Then the teliospores germinate resulting in a protomycelium, from which two lines of non-pathogenic, saprophytic and haploid sporidia bud. These sporidia then grow into monokaryotic hyphae, which by conjugation become dicaryotic and can invade the host plant. 

References

1. Fungal spores: Highly variable and stress-resistant vehicles for distribution and spoilage.

Updated: 2022-05-18.


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Swedish University of Agricultural Sciences