Caloplaca coralloides can reproduce both
sexually and asexually.
Sexual Reproduction:
An ascomycete with apothecia:
Caloplaca coralloides is an ascomycete, which means that its
fruiting bodies contain sacs, called asci, within which sexual fusion of nuclei
takes place and spores develop. Some ascomycetes have fruiting bodies called
apothecia while others have perithecia. Apothecia are disc or cup-shaped fruiting
bodies in which the fertile tissue is exposed to the environment, whereas perithecia
are flask shaped structures built into the thallus of the lichen in which the
fertile tissue is less exposed. The fruiting bodies of C. coralloides are
apothecia. The apothecia of C. coralloides are bright orange-ish yellow
and are 0.4-2 mm wide. The picture to the left shows apothecia of C.
Coralloides. The brighter orange circle in the center of the discs
is the fertile layer, called the hymenium. Surrounding the hymenium is
the exciple, the wall of the apothecia. Outside of the exciple is a layer
of the thallus, which is more yellowy orange.
Where does the apothecium harbor its spores?
The structure of the fruiting body of the lichen is determined
by the fungal component. The upper tissue layer is the hymenium, made up of
thread-like hyphae (paraphyses) and small sacs called asci. The hymenium is
generally 65-90 um thick and the paraphyses are simple or occasionally branched. It
is within the asci that sexual fusion of fungal cell nuclei takes place, followed
by meiosis and mitosis, and resulting in the development of 8 ascospores (each
10-16 x 5-7um). When the spores are mature and environmental conditions are
good for spore germination (when it is wet), the walls of the asci soak up
water and swell. This structural pressure is used to shoot the spores out of
the apothecium into the environment. Therefore, lichen need water to reproduce
sexually.
*Cross
section of C. Coralloides apothecium
taken
with scanning electron microscope with the help of
Chris Patton, Hopkins Marine Station, Monterey,
California.
Spores:
The sexually produced lichen spores in the apothecia are spores of the
fungal component of the lichen. Only the fungal component of the lichen reproduces
sexually. The lichenized algae reproduce by mitosis and are not packaged with
the sexually produced fungal spores. Therefore, the lichen spores do not have
everything that they need to grow into new lichens. Once the spores are released
from the apothecium, land on a suitable substratum and germinate, they need
to encounter an appropriate algal cell. Only then can the symbiosis be recreated.
This is a picture of the sexually produced lichen spores of C. coralloides.
They are colorless, two-celled and ellipsoid.
Pycnidia:
Another group
of structures that are important in sexual reproduction of lichen are the pycnidia.
Pycnidia occur in all lichen groups. They are flask shaped structures embedded
in the thallus of the lichen (similar in shape and size to perithecia) that
produce hundreds of little flecks of fungal hyphae called conidia. Conidia
can function as male spermatia, fusing with the “female” nucleus
in the asci. However, conidia can also function as asexual fungal spores by
falling onto the substratum, germinating, encountering a suitable alga and
forming another lichen.
