Recessive suppressors in Aspergillus nidulans closely linked to an auxotrophic mutant which they suppress

Reversion to prototrophy by a further mutation which' suppresses' an auxotrophic mutant is well-known.The suppressor can be dominant or recessive, site-specific or not, and linked or not to the suppressible site.A recessive suppressor can be used (e.g.Pontecorvo & Kafer, 1956) for mapping by means of mitotic crossing-over since, starting from diploids heterozygous for the suppressor and homozygous for the auxotrophy, segregants homozygous for the suppressor can be selected.Using a recessive suppressor is this way the section of chromosome which can be mapped is that between the centromere and the suppressor.As suppressors are distributed all over the map only few happen to be conveniently distal.A deliberate search was therefore started for such distal suppressors, with the additional working hypothesis that recessive suppressors might exist which are either very closely linked to the locus which they 'suppress' or even within that locus, i.e. intra-cistron.The present note reports: (1) that suppressors of this kind-i.e.recessive and very closely linked to the system which they suppress-do exist; (2) that by choosing an auxotrophic locus distally located, the isolation of suppressors of the type just mentioned offers an easy way of providing distal selectors for genetic analysis via mitotic crossing-over.