Lichens can reproduce sexually and asexually. Sexual reproduction in lichens is not comprehensively understood.
This involves the fusion of two haploid cells. In the case of lichens, these are spore cells. The sexual reproductive structures are pycnidia and ascomata.
The pycnidia produce germ cells termed conidia. These are very special cells in that they can act as ‘male’ cells in fertilization, or they can germinate without fusion to produce hyphae. In other words, they can reproduce asexually.
The ascomata reproductive structures come in two types: apothecia and perithecia. Both produce haploid germ spores.
Apothecia are jam tart-like structures in many lichens. Generally they are composed of a rim that is either the same colour as the thallus, or a contrasting colour. The rim surrounds a disc. The disc can be flat, concave or convex. The surface of the disc represents the tips of the asci or sacs that hold the spores. Apothecia may be immersed in the thallus, or sit on top of it (sessile) or sit on a small stalk. All these characteristics of apothecia are useful in lichen identification. In the Cladonia group, the apothecia sit on top of the podetia either along the rim or just inside it.
Like the apothecia they produce spores in asci. They are either immersed in the thallus with a pore or ostiole opening to the exterior, or they protrude a little from the thallus.
Fusion of haploid germ cells
Spore fusion in lichens has been observed is situations of self fertilization. Fusion of spores between separate lichen individuals has not yet been observed, but it is reasonable to assume that it does happen. What is little understood is how the zygote (essentially a fungal zygote) or the early developmental cells ‘capture’ a photobiont to initiate the symbiotic relationship that constitutes a lichen.
Asexual or vegetative reproduction occurs at its most fundamental through the process of fragmentation; a piece of lichen breaks off and grows at a new location into a new and complete lichen. This is common in many lichen groups, but some groups seem to benefit from it substantially. For example, Cladonia species fragment when dry and are dispersed by animals. Usnea species fragment and disperse in a similar way.
There are two asexual reproductive structures produced by lichens: soredia and isidia.
Soredia are spherical bundles of fungal filaments and algal cells that are produced in the algal region (usually the medulla) and rupture through the thallus to the outside. They give the thallus a powdery appearance. The soredia blow away and ‘germinate’ to produce a new lichen. Aggregates of soredia on the lichen surface are called soralia. Soredia and soralia may be laminal or marginal.
Isidia are tiny peg-like or finger-like structures containing fungal filaments and some photobiont cells. They are found on the surface of lichens and are nearly always laminal. Isidia break off and colonise new areas in a similar manner to soredia.
An interesting feature of the vegetative reproductive methods is they specifically account for the fungal and photobiont elements of the lichen.