In this article we will discuss about the modes of reproduction in lichens.
A. Reproduction of Lichen Thallus:
It multiplies by vegetative methods of fragmentation and also by the formation of highly differentiated propagules as follows:
(i) Fragmentation:
It consists in the breaking up of the established thalli into segments which are distributed to start new growths. Fragmentation is accomplished by ageing and accidental severing. In the first case, the older cells in the posterior or basal parts of the thallus die and separate the branches or lobes.
Each of these continues to grow independently by apical growth. In this way a number of separate plants, all derived from the parent thallus, are formed. Sometimes larger portions of the established thalli are accidentally broken off the parent plant.
Each of these, provided it contains both the symbionts, develops into a new lichen individual. In some fruticose lichens, these detached portions are carried by the wind to other trees. There they develop into new lichen plants.
Vegetative reproduction by fragmentation thus is an efficient means of propagation. It leads to increase in the number of individuals of the species as well as constant invasion of new territories.
(ii) Propagules:
Lichens also reproduce by vegetative structures peculiar to themselves. These are the soredia and isidia. Both these reproductive bodies or propagules contain elements of the lichen thallus (fungus and alga).
(a) Soredia (Fig. 20.7):
These are small, rounded granules or bud-like outgrowths which develop in the form of a greyish white or greyish green powder in extensive patches usually over the upper surface or edges of the thalli of many species of lichens.
Each soredium contains one or a few algal cells closely surrounded, by a little weft of fungal hyphae produced by branching of a hypha from the algal region (Fig. 20.7 A). Both the fungus and the alga are the same as in the parent thallus.
Sometimes soredia are developed in a more organised manner and occur in localised, pustule-like areas of thallus known as soralia (Fig. 20.7 D). In Parmelia and Physcia, the soralia are often seen as small, white pustules.
Soredia are produced from the algal layer of the lichen thallus. As the soredium is formed it is pushed outward by the elongation of the supporting hyphae. The soredia are exposed by the rupture of the overlying cortex.
They are readily rubbed off the thallus and are dispersed by the wind. At times the soredia forms an extensive coating of greyish powder called the soredial dust on the trees. Each soredium on falling on a suitable substratum germinates. It grows into a new lichen thallus with all the characteristics of the parent.
(b) Isidia (Fig. 20.6):
These are small, conical warts developed on the thallus of many lichens. Isidia are usually constricted at the base and thus can easily be broken off. Under favourable conditions each isidium grows into a new lichen thallus. The isidium differs from a soredium in being covered by cortex.
B. Reproduction of Algal Component: (Phycobiont):
The green algal component of the lichen thallus multiplies by cell division. Ahmadjean (1967) reported the formation of aplanospores. Slocum et al. (1980) observed zoospores in the thallus of Parmelia with Trebauxia as the algal component.
The blue-green algal components in the algal thallus have been reported to multiply by cell division, hormogonia, akinetes and heterocysts.
C. Reproduction of Fungal Component (Mycobiont):
The fungal component of the lichen thallus reproduces by spore formation (Sporulation) and sexual reproduction.
1. Sporulation (Asexual Reproduction) (Fig. 20.8):
The fungal partner produces small, non- motile asexual spores known as the pycnidiospores. They are produced in large numbers in special, conical, flask-shaped cavities called the pycnidia (A). The pycnidia are found sunk on the upper surface of the lichen thallus in certain species.
Each pycnidium opens to the surface through a small pore called an ostiole. The wall of the pycnidium consists of sterile fungal hyphae. From the wall arise fertile hyphae. The latter abstrict asexual spores (pycnidiospores) at their tips (B).
The pycnidiospores in certain species of lichens are capable for germination. Each produces a fungal hypha which coming in contact with an appropriate alga develops further into a new lichen thallus.
2. Sexual Reproduction (Fig. 20.9):
It is entirely the concern of the fungal partner on which the sexual organs are developed. The male reproductive organ is called the spermogonium and the female carpogonium.
(a) Spermogonia:
In certain species of lichens the pycnidia-like structures (Fig. 20.8 A) are reported to function as spermogonia. Each spermogonium is a flask-shaped receptacle immersed in a small elevation on the upper surface of the thallus.
It opens by a small pore, an ostiole, at the surface. The cavity of the spermogonium is filled with the fertile and sterile hyphae. The fertile hyphae abstrict minute, rounded cells at their tips. These are the male cells and are called the spermatia.
They are non-motile and are produced in large numbers in each receptacle. Each spermatium has a cell wall around it. The spermatia are set free in a slimy mass which oozes out through the ostiole.
(b) Carpogonia (Fig. 20.9):
The carpogonium (Fig. 20.9) is a special cellular filament. It consists of two portions, the lower coiled portion and the upper straight portion. The coiled portion constitutes the ascogonium. It is multicellular. The cells are uninucleate.
In certain species they are multinucleate. The ascogonium lies deep in the medullary region of the thallus. The carpogonia either develop from the hyphae of the medullarly region or from the hyphae deep in the algal layer of the lichen thallus.
The straight upper portion of the carpogonium is called the trichogyne. It is also multicellular. The component cells are elongated, the septa between the cells have minute holes in the centre, one each.
The terminal portion of the trichogyne ends in a long cell which projects beyond the surface of the thallus and has a gelatinous cell wall.
From the account given above it is evident that the sexual apparatus of the lichens differs from that of the Red Algae in the following respects:
(i) Coiled ascogonium sunken in the medulla.
(ii) Long multicellular trichogyne with the terminal cell only projecting above the surface of the thallus.
(iii) Absence of supporting and auxiliary cells characteristic of red algae.
(iv) Antheridia in the form of flask-shaped cavities or receptacles sunk in the thallus, each opening through a pore on the surface of the thallus.
(v) Male cells or spermatia with cell walls.
(c) Fertilisation:
Spermatia or the male cells have been found adhering to the gelatinous wall of the projecting terminal cell of the trichogyne. From this, some Lichenologists conclude that the spermatia are functional.
At the point of contact the intervening walls between the spermatium and the trichogyne dissolve. The contents of the spermatium migrate through the spore into the trichogyne. The supporters of this view hold that only the carpogonia of which the trichogynes have been spermatised develop into ascus fruits.
The presence of empty spermatia in contact with the trichogyne supports this view. However, it is not accepted by all. The opponents hold that the actual migration of the male nuclei down the trichogyne has never been observed.
(d) Post-plasmogamy changes:
Fertilisation is followed by the following changes in the carpogonium:
(i) Gradual withering of the trichogyne.
(ii) Development of freely branched ascogenous hyphae from the ascogonium.
(iii) Formation of asci at the ends of ascogenous hyphae or their branches. In some lichens the development of ascus is direct from the terminal cell, whereas in others from the typical croziers.
(iv) Development of an envelope of sterile hyphae around the developing asci, the ascogenous hyphae and the ascogonium to form a fructification or the spore fruit which in many of the ascolichens is of apothecium type and in a few others of perithecium type.
1. Apothecia:
The apothecium is a fruit body of an ascomycete fungus borne in the lichen thallus. Generally the apothecium is a rounded, cup-shaped structure. Sometimes it may be plate-like and rarely it has an elaborate form.
The apothecia of lichens vary in colour which ranges from reddish, reddish brown, yellow to black depending upon the species. In many lichen species (Lecidea, Cladonia and Gyrophora) the apothecium consists of fungal elements only.
There is no algal component. Such apothecia are called lecideine type. Many species (Parmelia, Physcia, and Lecanora) have more highly developed apothecia in which algal component of the thallus also takes part in the formation of the apothecial margin. Such apothecia are known as lecanorine type.
(a) Structure of the Apothecium (Fig. 20.10 A):
Whatever its form, the apothecium from outside is usually seen to consist of two parts, namely, the disc and the margin.
(i) Disc of the apothecium:
It is the fertile portion of the apothecium. In many lichens it is soft and bright in colour and in others hard and black. In a vertical section of the apothecium, the disc consists of a closely packed, palisade-like layer of sac-like asci and sterile, hair-like fungal hyphae known as the paraphyses.
The stout club-shaped asci grow upwards between the paraphyses. They do not project beyond them. Each ascus contains eight ascospores. The asci and the paraphyses are closely packed to form a continuous fertile layer.
It is called the hymenial layer or hymenium. It is also known as thecium. The projecting ends of paraphyses form a smooth surface the epithecium.
Beneath the hymenium is a region consisting of a dense mass of sterile hyphae’ It is the subhymenium. It consists of closely interwoven hyphae from which arise the vertically growing paraphyses.
(ii) Margin of the apothecium:
Surrounding the disc and forming the edge of the apothecium is the proper margin. The lecideine type of apothecium has only a proper margin consisting of fungal hyphae alone.
The lecanorine type of apothecium (Xanthoria and Physcia), in addition to the proper margin has a second appreciably thickened margin surrounding the proper margin.
The second margin is an up-growth of the thallus and thus in colour and appearance it resembles the thallus itself. It is called the thalline margin. The apothecia with a thalline margin (lecanorine type) have both the fungal and the algal elements.
(b) Development of Ascospores:
The young ascus mother cell contains a single diploid nucleus formed by the union of two haploid nuclei. As it advances towards maturity, the nucleus undergoes three divisions to form eight daughter nuclei.
The first two divisions of the ascus nucleus constitute meiosis. The resultant eight nuclei are thus all haploid. By the accumulation of small amount of cytoplasm around each nucleus, eight ascospores are formed.
The residual cytoplasm in the ascus becomes gelatinous. It takes up water during wet weather and swells. Owing to the pressure from within, the mature asci burst open at their tips. The ascospores are liberated with considerable force.
Before liberation, the ascospores in many lichen species may undergo division to form two- celled to many-celled structures. The asci in the apothecium come to maturity at different times. As they mature the dehiscence takes place whenever wet weather occurs. The liberated ascospores are very light.
They remain suspended in the air after their forcible discharge and thus may be carried by wind or air over long distances. On falling on suitable soil, the ascospore germinates. It produces a fungal hypha. The latter grows into a new lichen thallus if it comes in contact with an appropriate alga.
2. Perithecia:
In the second sub-group Pyrenocarpeae, the ascus fruit is of perithecium type. It is much smaller in size than the apothecium and appears as a dot on the surface of the thallus. In a V. S., the perithecium is seen as a flask-shaped structure.
It is bounded by a dark-coloured wall. The cavity is lined with asci. The paraphyses are either few in number or lacking. The perithecium is embedded in the elevations of the thallus on its upper side. It opens at the surface through a small pore called an ostiole.
The parthenogenetic development of the ascus fruit from the ascogonium has also been reported.