It is normal for any living organism to want to live in a hospitable environment. For the fungi, which reproduce through the production of spores which are dispersed by a variety of mechanisms, there is a need for the spores to germinate only when the environmental conditions are favorable for germination. Goldstein (2011, p. 28) asserts that dormancy can be viewed as a survival mechanism for the fungi during which the fungi “remains in a state of suspended animation until the environment becomes hospitable”. Spore dormancy represents an important character influencing the competitiveness of difference species of fungi. There are two types of fungi spore dormancy namely; constitutive spore dormancy and exogenous dormancy spore dormancy.
There are distinct differences between constitutive dormancy (also known as endogenous dormancy) and exogenous dormancy which is sometime called superficial dormancy. One of the main differences between the two types of dormancies is that constitutive dormancy takes place in sexual spores while exogenous dormancy takes place in asexual spores. It is also apparent that constitutive dormancy is imposed by some endogenous mechanisms which means that the dormancy is self imposed. Exogenous dormancy on the other hand is imposed by the environment; it therefore follows that spores exhibiting this type of dormancy can only germinate when the environment presents them with conducive conditions for germination.
There are various means through which the two types of dormancies are achieved. One of the mechanisms through which constitutive is achieved is through the development of permeability barriers (Griffin, 1996). Pointedly, spores with thinner walls often germinate better and faster than spores with thick walls and spores produced by fungi that reproduce sexually make good use of this property to control the times when they germinate; increasing the thickness of the wall of a spore reduces its permeability (Griffin, 1996). The creation of metabolic blocks is yet another mechanism used by spores to achieve constitutive dormancy. Empirical research findings have shown that there are certain chemical substances known to induce constitutive dormancy. Such chemicals are always found in plenty in spores produced by sexually reproducing fungi in their dormant states even though the amount of such chemicals have been found to be overly reduced in the germinating spores of the same plants (Griffin, 1996). Constitutive dormancy can also be aided by self inhibitors.
An example of a plant that produces spores that exhibit constitutive dormancy is the Puccinia graminis that produces uredospores (commonly known as rust spore) responsible for the causation for rust disease in certain crops (Issac, 1992). These spores are usually unable to germinate when they are close to another due to the presence of a self inhibitor called methyl-cis-ferulate that is supposed to leach out or be washed away before the spore germinate (Issac, 1992). Once the spores are dispersed further apart, the inhibitor finally leaches out and the spores are able to germinate (Issac, 1992).
On the other hand, exogenous spore dormancy is imposed environmental conditions that are not favorable for germination. As seen above, constitutive spore dormancy hinders germination even when favorable germination conditions exist. However, spores that exhibit exogenous dormancy only germinate when there is favorable temperature, pH, nutrients and amount of water. For example the conidia which are produced by of Aspergillus and are at times called asexual chlamydospores are known to start germinating as soon as they are presented with favorable environmental conditions for germination.
For any biology student studying cell dormancy and has also studied evolution at some point, one cannot help wondering which mechanism is more likely to have come about first between constitutive dormancy and exogenous dormancy. In evolution, students are taught that the fennec and the arctic fox evolved from a common ancestor even though as they travelled and settled in differing environments, the fennec ended up having huge ears to aid in cooling off while the arctic fox, because it finally ended up in a snowy area ended up with small ears since larger ears would have easily frozen thereby breaking off easily. Admittedly, it is hard to tell which one of the two types of foxes evolved before the other. The same applies to constitutive dormancy and exogenous dormancy; one cannot easily tell which one is most likely to have come first. However, from a personal stand point, I can attest that it is the exogenous dormancy that is most likely to have come about first. This is because, naturally, seeds and spores are supposed to start germinating as soon as there is favorable environmental condition. Ideally, it is probably through competition and the increasing need for survival that endogenous dormancy finally came into being. As seen above, after reproduction, the spores are usually at the same place hence in the event of germination, there is a high likelihood for the spores to compete stiffly for the little available resources. Constitutive dormancy seems to be an elaborate way through which the spores can wait until they are dispersed further apart before that can finally germinate.
References
Goldstein, W. E. (2011). Sick Building Syndrome and Related Illness: Prevention and Remediation of Mold Contamination. Boca Raton, FL: CRC Press.
Griffin, D. H. (1996). Fungal Physiology. New York: John Wiley and Sons.
Issac, S. (1992). Fungal-Plant Interactions. London: Chapman & Hall.
