In the Fungus Neurospora,a Standard Lab Strain Carrying the Auxotrophic

Essay

In the fungus Neurospora,a standard lab strain carrying the auxotrophic mutation m was crossed to a prototrophic strain isolate from a burned sugar cane field.There were three types of asci as shown below,where the symbol "ab" means an aborted ascospore whose m locus constitution could not be tested.
$$\begin{array} { l l l } \text { Type I } & \text { Type 2 } & \text { Type 3 } \\m & a b & m \\m & a b & m \\m & a b & a b \\m & a b & a b \\+ & a b & a b \\+ & a b & a b \\+ & a b & + \\+ & a b & +\end{array}$$
Explain the origin of each ascus type.