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Reproduction And Dispersal – Botany Notes – For W.B.C.S. Examination.
প্রজনন এবং বিসরণ – উদ্ভিদ নোট – WBCS পরীক্ষা।
Reproduction and dispersal
Two forms of reproduction can be found amongst living organisms – sexual and asexual. It is by the mixing of genes from two individuals, via sexual reproduction, that genetic diversity is effected, whereas in asexual reproduction there is no such mixing of genes. Both sexual and asexual reproduction can be found amongst the lichens. When talking of plants (or lichens, which were once thought of as plants) asexual reproduction is commonly called vegetative reproduction. Though lichens, as a whole, may reproduce both sexually or vegetatively, there are species in which both types of reproduction may be common but also species where one type is rare or even unknown. In each form of reproduction propagules of some sort are produced and dispersed and there is a separate page dealing with PROPAGULE DISPERSAL. The SEXUAL VS. VEGETATIVE page gives some general comments about the two means of reproduction.Continue Reading Reproduction And Dispersal – Botany Notes – For W.B.C.S. Examination.
When talking about lichen reproduction two fundamental questions arise:
What does it mean to talk of sexual reproduction in lichens?
What is an individual lichen?
The next two sections will look at those questions.
What does it mean to talk of sexual reproduction in lichens?
After all, no lichen is an individual organism but an association between two (or more) organisms. In fact in lichens only the fungal partners may reproduce sexually. To say that lichens may reproduce sexually is really shorthand for “the fungal partners within lichens may reproduce sexually”. A number of photobiont species found in lichens can be found free-living and could then reproduce sexually but within a lichen sexual reproduction of the photobiont is suppressed.
The sexually reproducing lichens are either ascomycetes or basidiomycetes. Ascomycetes produce their sexual propagules (called ascospores) within microscopic organs called asci and basidiomycetes produce their sexual propagules (called basidiospores) on microscopic organs called basidia. Often ascospores or basidiospores are simply called spores, especially where there is no need to differentiate the two or where there is no possible confusion with vegetative propagules. Individual asci or basidia cannot be seen with the naked eye but they are produced in large numbers in easily visible structures. The most commonly seen ascospore-producing structures are the apothecia, typically disc-like to cup-like and found growing from the thallus surface. The REPRODUCTIVE STRUCTURES page gives descriptions of various structures associated with the production of ascospores and the BASIDIOLICHENpage gives examples of the structures in which basidiospores are produced.
Much is known about the processes of sexual reproduction in the non-lichenized ascomycetes or basidiomycetes since it is possible to grow many of them in the laboratory, follow all steps of their life cycles and carry out mating experiments. By these means it has been possible to see how the processes of sexual reproduction are carried out in those species. Such research has shown that the sexual reproduction in the non-lichenized ascomycetes or basidiomycetes has both similarities to and significant differences from the processes organisms such as humans or plants. It has not been possible to elucidate most of the processes in the lichen mycobionts. A number of them can be grown, to some degree, in the laboratory but not to the extent that would allow a good understanding of all the processes of sexual reproduction. In this context it is worth noting that, amongst the non-lichenized ascomycetes or basidiomycetes, it is harder to study the processes in mycorrhizal species than in saprotrophic ones. In the SEXUAL REPRODUCTION CASE STUDY you can find a brief description of some aspects of sexual reproduction in non-lichenized ascomycetes and basidiomycetes and briefer comments about the lichenized fungi.
What is an individual lichen?
You know that lichens are associations between fungi and photobionts. Let’s keep things simple for the moment and assume we have a thallus that contains just one photobiont species and one fungal species. The answer to the previous fundamental question revealed that sexual reproduction in lichens involves only the fungal partner so an important question is: Does a lichen thallus contain just one fungal individual? The answer is: Sometimes yes, sometimes no.
At this point it is useful to introduce the word mycelium, a fundamental concept for non-lichenized fungi. When a propagule of an ascomycete or basidiomycete germinates it would usually do so via a hyphal outgrowth which, by repeated extension and branching, would produce a network of hyphae and this network is called a mycelium. In a non-lichenized fungus this mycelium would spread through say soil, dung or wood, acquiring nutrients from the substrate but staying out of sight. In a lichenized fungus a mycelium associates with a species of photobiont and is contained within the lichen thallus, which is usually easily visible though the thalli of some crustose species may grow hidden in fissures within wood or rock.
Suppose a fungal propagule that has been released from a lichen comes to land near some free-living alga. It is irrelevant whether the propagule has been produced sexually or vegetatively. Suppose further that the propagule germinates to give rise to a mycelium that entraps some of the nearby alga and so develops a new lichen thallus. In this scenario, where all the hyphae within the thallus belong to a single mycelium that developed from one propagule, we can say that the thallus contains just a single fungal individual. Now suppose that a couple of genetically distinct fungal propagules of the same species have landed close together and near some free-living alga. Suppose both fungal propagules germinate, capture some algal cells, grow into mycelia and so give rise to two thalli but in this case the nearness of the two propagules leads to the two thalli merging. After the merger the naked eye sees just one thallus but within that thallus there are two fungal individuals – of the same species but genetically distinct. In the genus Xanthoria the ascospores are often released in groups of eight. Laboratory studies involving several Xanthoria species have shown that within a couple of days after ejection from the ascus the spores are held together by a mucilage that makes it impossible to separate the spores. Suppose several spores within the octet germinate and capture photobiont cells to generate a new thallus. The new thallus would be composed of genetically distinct fungal individuals if the spores themselves were genetically distinct.
The emphasis has been on the fungi since the fungal partners may reproduce sexually. Nevertheless it is worth noting that when one (or more) fungal propagules land near free-living photobionts and capture them to form a new lichen thallus, the photobionts may also have been genetically diverse. Therefore a single lichen thallus, even if it contains just one species of photobiont might still show genetic diversity amongst the photobiont cells.
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