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Spores, Sexual and Asexual

Introduction

Spores are specialised cells of the fungus that can function as resting or dispersal propagules. Each spore has the potential to generate another individual of the species. In contrast to seeds, spores have no specialised components that differentiate them from other cells of the thallus. Thus any viable cell may be called a spore, when upon separation from the thallus it can form a new individual. 

Asexual Spores

The fungi form a wide range of different types of spores. Very different types of spores are formed following asexual or sexual processes.  LINK Where the spore is genetically similar to its parent, and is formed following a vegetative division, it can be called an asexual spore. In fact, two major types of asexual spore are found: those that are formed by fragmentation or conversion of an existing hypha or cell, and those that are formed and released as specialised cells following specific developmental pathways. 

Fragmentation or disarticulation of existing hyphae is a common mechanism for spore formation. The spores are called arthrospores or arthroconidia. Where the spore has been converted from an existing cell or hypha, the terms thallic spore, or thallic conidium may be used. The term chlamydospore can be seen in the older literature. The features that distinguish each type are not clear, and unfortunately the terms are used interchangeably.

The second type of asexual spore is formed following one of two processes. In the Zygomycota and Chytridiomycota, the hyphal apex or cell swells. Within the sac, called a sporangium, spores are formed following division of the cytoplasm into one or more parts. Sporangiospores may be motile or non-motile. Individual sporangispores usually are surrounded by a wall. Motile spores are called zoospores. Zoospores may lack a cell wall.

In the Ascomycota and Basidiomycota, a sporogenous region on a hypha expands like a balloon, or bud. The process is called blastic conidiogenesis, and the resultant spore a 'blastospore'. Several different types of blastic conidiogenesis have been described. Of these, the following are relatively common.

Acropetal: in a process where the hypha becomes modified, new spores are budded from the tip. Each new spore emerges from the outer wall of the most recently formed spore.

Synchronous: spores emerge from a swollen cell or vesicle at the same time.

Sympodial: the apex of the conidiogenous cell develops a blastic cell, and the conidiogenous region branches from the base of the spore. A new blastic cell emerges from the terminus of the branch, and the eventual arrangement is sympodial. 

Annellidic: The conidium emerges from the apex of a cell, and leaves behind a scar. The subsequent conidium emerges from within the scar, and the amount of remnant annular scarring increases with conidiogenesis.

Phialidic: conidia emerge from a specialised cell called a phialide, in basipetal succession. The phialide usually does not increase in size during conidiogenisis, though scars can be seen inside the phialide using EM.

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Sexual Spores

The sexual process enables a mixing of genotypes. The sexual process in fungi markedly differs from other eukaryotic organisms. While the fungi pass through karyogamy, meiosis and plasmogamy, the length of time spent in each period may be variable and characteristic of each division of the fungi. The following is a simple description that may not necessarily be relevant for all fungi.  

The sexual processes of the zoosporic fungi vary. In principle, haploid zoospores of different mating types fuse and then form a diploid sporangium. The nucleus passes through meiosis. Resultant haploid nuclei divide by mitosis before being dispersed from the sporangium as motile zoospores. LINK

The sexual spore of the Zygomycotina and several other groups is diploid. Spore formation is initiated with hormonal interaction between hyphae of opposite mating types. LINK

Plasmogamy follows elongation of laterals towards each other. The Zygospore is formed following gametangial contact. The haploid nuclei from each mating type fuse (karyogamy) to form the diploid nucleus. The zygospore is formed inside a sporangium, and usually only one spore is found in each zygosporangium. Meiosis takes place immediately prior to germination of the zygospore, and the emerging thallus is haploid.

The haploid sexual spores of the Ascomycota are formed inside an ascus. Spore formation is initiated following hormonal interaction between hyphae of different mating types. Following plasmogamy, the two haploid nuclei exist in a dikaryon for a short period. The cell multiplies using a crozier in the most complex patterns, and eventually the nuclei in apical cells fuse (karyogamy) and immediately pass into meiosis inside the ascus initial. 

Nuclei form four daughter nuclei, and then commonly divide mitotically, with the result that the ascus contains multiples of 4, commonly 8 ascospores.


Fertile tissue of a Basidiomycota showing presense of a basidium releasing spores, another collapsing after release, and immature basidia.

The fruiting bodies of Basidiomycota are commonly visible and the structure given names like rusts, smuts, puffballs, jelly fungi, club fungi, earthstars, and mushrooms. Formation of the sexual spores of the Basidiomycota differs among these different groups. In principle, plasmogamy takes place in isolation, leading to the formation of a dikaryotic hypha. Karyogamy need not proceed for some time. Diploid hyphae may coexist with haploid and dikaryotic hyphae in one thallus. During formation of the fruit body, basidial initials are formed. Nuclei pass through meiosis in the swollen probasidium. The resultant haploid nuclei pass through the outer wall of the basidium to form basidiospores often placed in short sterigma. 

The resultant fertile tissue shows spores at various stages of development. Mature spores will exist adjacent to infertile basidia, with collapsed basidia nearby. Few fungi have synchronised spore release.

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Conclusion 

Spores are formed in various ways, with quite different outcomes. The potential to increase the size of a population is entirely due to the massive proliferation of units that can function in dispersal. Some of the cells have a sexual origin, and most an asexual origin. The latter might be converted hyphae or units specifically developed in or from various structures.

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References

Kendrick B. 1992 The Fifth Kingdom. Focus Information.

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