Petite colonie mutation

Petite mutants spontaneous mutants, chiefly yeasts, with chemical or physical defects in the respiratory chain. P. m. grow very slowly and form small ( petite ) colonies on nutrient agar. The same phenotype can be produced by a chromosomal mutation (segregational petite), or a mutation in the mitochondrial DNA (vegetative or neutral petite). In the latter case, mitochondrial structure is considerably altered, largely due to changes in the amino acid composition of the structural proteins of the inner mitochondrial membrane. Since these structural proteins are important for the correct arrangement and conformation of the respiratory chain enzymes, the effect of petite mutation on the respiratory chain is probably secondary. 

In conclusion, it must be emphasized that the induction of the RD or petite phenotype (petite colony phenotype due to slower growth) can be brought about by mutagens as well as by definitely nonmutagenic regimens and, moreover, that induction of this alteration is possible in only a few yeast species. Consequently, results obtained using this mutation system cannot be considered as of general relevance. 

---