Botany Notes – For W.B.C.S. Examination – Vegetative Reproduction In Lichens.
Many lichens will produce new individuals from fragments that break off from a thallus. Each fragment contains both mycobiont and photobiont cells and, in principle, functions in much the same way as a cutting taken by a gardener who wishes to propagate a particular plant. The gardener’s cutting is genetically identical to the parent plant and is therefore a means of vegetative reproduction and a thallus fragment also propagates an existing genetic makeup. Fragmentation involves no special structures, though it’s clear that thallus morphology plays a role since thalli with delicate growth forms are more likely to fragment than are the robust thalli. For example a number of the pendant curtain-like species that grow on trees can be torn easily by strong winds or wind-blown debris and various foliose species have thalli in which narrow, easily breakable lobes are present.Continue Reading Botany Notes – For W.B.C.S. Examination – Vegetative Reproduction In Lichens.
The most significant vegetative propagules are isidia and soredia. The former are small outgrowths of the thallus, up to a millimetre or so in length, which contain both fungal hyphae and photobiont cells. Broken off isidia may be transported by wind, water or animals (for example, caught on bird feet or in animal fur) and if deposited into a suitable habitat may generate new thalli. Soredia look like small powdery, granules, between about 20 and 100 micrometres in diameter, and each soredium consists of a few photobiont cells surrounded by fungal hyphae. Soredia are very easily dispersed by wind, water or animal.
The term symbiotic propagule is a general term for any sort of propagule in which both fungal and photobiont cells are present. As well as symbiotic propagules it is possible to have vegetative propagules where only one partner is present. Many lichens produce vegetative fungal propagules called conidia, often in tiny chambers called pycnidia that are embedded within a thallus but with an opening (or ostiole) to the outside through which the conidia can escape. In some circumstances photobiont cells may escape from a thallus and then function as vegetative propagules of the photobiont. Escape is especially likely in some species when the thalli are highly saturated with water.
From what was said in answer to the second fundamental question above it is clear that a lichen thallus could contain genetically distinct fungal or photobiont cells. If that were the case then vegetative propagules produced at different parts of a thallus could be genetically distinct with respect to mycobiont, photobiont or both. To be precise, it would be necessary to say that a vegetative propagule would produce a new thallus genetically identical to that part of the parent thallus that produced the propagule.
While on the subject of genetically diverse thalli it’s relevant to note that the generation of such thalli need not rely on fungal propagules. In both Hypogymnia and Physcia soredia have been seen to coalesce in the early stages of the formation of new thalli. Clearly, if the soredia are genetically distinct (in one or both partners) then the new thallus also will be genetically diverse.
An oddity – ascospore to conidium to mycelium
This short section has been put here simply to show you that the ascospore-to-mycelium path is not always direct. The ascospores of the species Vezdaea aestivalis have not been seen to produce a mycelium. Instead a spore produces a short hyphal outgrowth from which a conidium is formed and released. Such conidia give rise to extensive hyphal growth. If you consider fungi as a whole the production of conidia from sexual spores is not unique to Vezdaea aestivalis for there are various non-lichenized ascomycetes and basidiomycetes in which this occurs. However, in contrast to Vezdaea, the sexual spores of those other genera can germinate to produce mycelia. There are some more comments in the next reference button.
Examples of non-lichenized genera in which conidia are budded off the sexual spores include Ascocoryne, Nectria, Rustroemia (all ascomycetes) and Calocera, Dacrymyces, Exobasidium (basidiomycetes) but the phenomenon hasn’t been seen in all species of those genera. If a fungus (lichenized or not) is capable of producing conidia from both the sexual spores and from the mycelium, the conidia from the two sources need not be genetically identical. Amongst both the lichenized and non-lichenized fungi there are species capable of self-fertilization as well as those incapable of self-fertilization. In the latter the nuclei of the sexual spores must, of necessity, contain a genetic mixture from distinct parents. On the other hand, as outlined in the SEXUAL REPRODUCTION CASE STUDY, each cell of the corresponding mycelium contains nuclei from one or both parents, but with no mixing of genes from those nuclei.
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