A teacher was explaining about constant physical contact involving almost equal physiological independence in two different thalloid forms. He was trying to explain which one of the following?
A. Mycorrhizal association
B. Establishment of heterothallism
C. Operation of heterothallism
D. The advent of lichen formation.

Hint: Lichen is the combination of two organisms which includes a fungus and cyanobacteria. Both the organisms remain in symbiotic association. Near about 17000 species of lichens are found in the environment. Lichen is a thallus shaped organism. Fungus plays a dominant role in the lichen association.

Complete answer:
1. The mycorrhizal association involves the symbiotic association between fungus and plant. The association provides benefits to both organisms. The synthesized food by the plant is transferred to fungus located in the root region. Fungus in return gives the plant essential nutrients and molecules like phosphorus to the plant from the soil.
2. In the case of some fungus, reproduction needs to involve two different organisms. Each individual provides nuclei which results in the formation of zygote. This phenomenon is known as heterothallism. Example: Aspergillus sp.
3. Lichen possesses a mutualistic relationship between the fungus and the algae. Both the fungus and algae possess different thalloid forms.

Note:
The fungus and the cyanobacteria associates to firm the lichen are known as mycobiont and phycobiont respectively. Lichens are classified into three classes. These are Foliose, fruticose, and crustose. The symbiotic relationship involves the supply of carbon to the fungi by the cyanobacteria for nutrition in return fungus protects the cyanobacteria.