Agar is extract from seaweed used to solidify media. The agar used in mushrooms cultivation is usually available in powder form or premixed and ready for use.
Using the genetics of a strong mycelium to selectively grow a new generation with beneficial traits such as larger fruit, albino mutations, or any beneficial trait. This can be done in a variety of ways, common measures are to use a Grain to Agar transfer, popular with the Uncle Ben TEK, where a single grain of rice that has been colonized is placed into a agar plate to propagate a new generation. There are also Colonized Agar to Grain spawn methods that is the same process but reversed to grow a new mycelium.