7. Meeûs Square

The reproduction of lichens 


Below you will find the audio and text of the walk !
Have a good adventure!

Note: In the audio, I talk about the presence of the lichen Melanohalea exasperatula. I misspell it – always complicated these Latin names ! You now have the name if you ever want to do more research !


How do lichens disperse and reproduce? 

Like fungi, lichens have quite complex and diverse reproductive strategies. The strategies that lichens use depend on the environmental conditions.

To begin with, only the fungal partner can reproduce sexually. The photobiont (the photosynthetic partner) and the mycobiont (the fungal partner) together can reproduce asexually.

But what is the difference between sexual and asexual reproduction?

Asexual reproduction means that the offspring is created by only one parent without the fertilisation and the production of reproductive cells – called the gametes.

In sexual reproduction, offsprings are produced by two partners of different sexes. These partners produce gametes that are morphologically and genetically different. The union of these gametes produces a zygote that will become the offspring. 


ASEXUAL REPRODUCTION IN LICHENS

This is done by fragmentation, by the breakage of specialised cells called soredia and isidia, which are dispersed by the force of the wind or by the trampling of animals. For example, when a bird walks on the branches of a tree and fly to another tree, it may disperse the isidia on this other tree. 

Isidia are small structures formed on the surface of the thallus that can detach from it. Both partners (fungus and algae) are present inside the isidia (see diagram). The forms of this structure are varied and constitute an important element in taxonomy. The second picture in the slideshow is a picture of the isidia of Parmelia saxatilis (Credit: Ed Ubel on Wikimedia CC BY-SA 3.0). All schematics of the reproductive apparatuses were made by the author (inspired by the course organised by the British Lichen Society, thanks very much to them) and are under a CC BY-SA 4.0 license.

Soredia, (soradium in singular) produced by soralia (soralium in singular), are small mealy or granular masses consisting of small clusters comprising an algal cell surrounded by fungal hyphae. These soralia are created as a result of interruptions in the cortex (the upper surface of the lichen thallus). Soralia can be diffuse and found all over the thallus or they can be well delimited. They are liminal when they develop on the thallus, marginal when they are formed at the margin of the thallus and terminal when they are located at the end of the lobes. The second picture on the diaporama was taken by the author (2020) and is under a CC BY-SA 4.0 license.

Finally, the mycobiont (the fungus) can produce spores that are not involved in the sexual process. These spores are called conidia (or pycnospores). They are produced at the end of hyphae and vary in shape and size. The organs that contain them are called pycnidia. The pycnidia containing the conidia are dispersed without the photobiont and therefore must find an algal partner  in order to reconstitute a lichenised thallus.  

SEXUAL REPRODUCTION IN LICHENS

There are two main forms of sexual reproduction: apothecia and perithecia. These forms produce spores in asci (plural, ascus in singular) (see diagram). When the spores are dispersed, they must find an algal partner in order to form a lichen. 

How does the fungus overcome the problem of finding a photosynthetic partner to “lichenise” ?

Different species of lichen will find different strategies to overcome this problem. Some species will have their spores agglutinated with some cells of the alga or cyanobacteria so as to have a better chance of developing. Others may survive by forming a symbiosis with other algae (which are not normally their partners) or may insert their hyphae into a neighbouring lichen to steal some of its algae (see Science Infuse file).

Note: Today, one in five of all known fungi species can form a lichen. Some fungi formed lichens earlier in their evolution but have lost this ability. Other fungi have had different photosynthetic partners during their evolution. Finally, some fungus species can choose to live in symbiosis with algae or live independently (Sheldrake, 2020).

The apothecia are cups that can be concave, convex or flat (depending on the species). There are two types of apothecia, those called lecanorine which contain a layer of cells of the photobiont – the algae or cyanobacteria – and lecidine which do not. The second picture on the diaporama was taken by the author (2020) and is under a CC BY-SA 4.0 license.

Perithecia are small wombs embedded in the thallus. Contact with the outside environment is through a small punctiform cover at the top of the perithecia called an ostiole. The lichen in the slide show below is Bagliettoa calciseda (photo shared at the British Lichen Society meeting). 

The process by which the genes of the algal partner and the fungus are combined is still poorly understood despite the fact that we know the structures well.

Hopefully, this introduction will help you identify lichens as the reproductive apparatus is an important criterion. In the next step, we will discuss identification and these steps.

Activity

Can you identify the lichens we already know ? There are some specimens of Physcia adscendens and Xanthoria parietina

We will introduce two new species: Physconia grisea and Melanohalea exasperatula.

Physconia grisea has a white, grey or brown thallus when dry and green when wet. On the lobes – the leaf-like tips of the thallus – you will see small white dots, these are called pruina. They are small, whitish, shiny crystals (made of calcium oxalate). The origin and the function of the pruina is still unknown. Credit: The first two photos are by Christian Thirion, used with permission under a CC BY-SA 4.0 license. The other pictures were taken by the author and are under a CC BY-SA 4.0 license.

Melanohalea exasperatula has a dark green thallus. The lower cortex is pale, white (see photo below). This lichen has isidia on its thallus. These are the asexual reproductive structures that disperse by fragmentation (see above). This lichen can be found on tree branches, sometimes on rocks and walls and regularly on nutrient rich surfaces (as is the case here as we are in the city). This lichen is quite difficult to spot as it camouflages itself with its colour. It is found on the side of the tree closer to the road. 


Check out the identification guide I have created for urban lichens. It contains the description of 28 urban lichens found on the tree bark. For each species, there is a description of its sensitivity to pollutants in the city !


References

For more detailed information on lichens, as well as to have access to a determination key, I recommend you check this PDF link out (on google scholar, only accessible in French), here are the references: 

Sérusiaux, E., Diederich, P., & Lambinon, J. (2004). Les macrolichens de Belgique, du Luxembourg et du nord de la France. Clés de détermination.

Sheldrake, M. (2020). Entangled life: how fungi make our worlds, change our minds & shape our futures. Random House.

All diagrams were created by the author and are under a CC BY-SA 4.0 license.

Let’s meet at the Parc Royal! As always, more details on the location on the map 👇🏽

If you don’t continue the walk, can you give us feedback on your experience here ? It will help us improve !

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