Spore Dispersal in Fungi

Introduction

There are few species of fungi relative to other organisms. However, fungi seem to be everywhere and can utilize just about any material for food. One reason that they seem to occur everywhere is that they produce large number of spores that often can be dispersed long distances. Moreover, fungal spores also have other attributes that ensure their survival. Spores are often less susceptible to adverse environmental conditions than the mycelium or yeast cells  and germination of spores oftentimes will not occur until environmental conditions are optimal for their survival.

How Many Spores do Fungi Produce?

When Carl Sagan, the noted astronomer, spoke of the numbers of stars in galaxies, he is always stereotypically thought of as saying that there are "billions and billions" of stars. This always made quite an impression on viewers listening to Sagan since it is difficult to imagine billions and billions of individuals of any species of organism.  The world's human population at approximately 6.5 billion is far greater than any other mammal in the world, but this number is insignificant when compared to the number of spores that just a single fungus may produce in a short time. Numerous studies concerned with the number of spores that fungi produce have been carried out with plant pathogenic fungi. Ustilago maydis, the Corn Smut (Figure 1a-b) may infect any part of the corn plant.  When infected, the corn plant will have black galls of various sizes. A gall that is about 1in3 may contain approximately 25 billion spores! Multiply that by all of the galls that may be present in a single corn plant and we will literally have billions and billions of spores. Ganoderma applanatum, the Artist Fungus (Figure 1c-d), produces a perennial fruiting body, which may disperse 5.4 trillion spores over a six month period, from within the pores of the fruiting body. Although the specimen in Fig 2a may seem large, The Guinness Book of World's Records recognizes a specimen that was approximately 56 inches by 37 inches and weighing 300 pounds. Imagine how many spores that one must have produced. 

Fig. 1a: Two corn smut galls of Ustilago maydis on corn cob. Fig. 1b: Numerous corn smut galls on corn cob. Fig. 1c: Ganoderma applanatum fruiting body growing on a live tree. Fig. 1d: Pores typical of Ganoderma applanatum fruiting body.

And while the number of spores produced by microscopic fungi are not of this magnitude, they are nevertheless still significant in the numbers that are produced.  Rhizopus stolonifer, the common bread mold, can be recognized by the grayish, aerial mycelium covered with numerous black "dots". These dots are the sporangia containing spores of this fungus. Each sporangium contains upwards of 50,000 spores. A single spore grown from this species, in three to four days, will produce hundreds of millions of spores. Many species of microscopic fungi are capable of producing comparable number of spores.

If even a small fraction of these spores were to survive and produce more fungi, the world would be entirely covered with fungi. Since we are not, it is safe to assume that most of those billions of spores produced will not live long enough to reproduce themselves or perhaps even reach a food source that will even allow for germination of the spores. However, there are more than enough spores that survive to make the world the moldy place that it is.

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