Genetic analysis by mitotic recombination in Dictyostelium discoideum of growth and developmental loci on linkage group VII.

A genetic study was made of two developmental loci, stmA (mutations of which give 'streamer' mutants with large aggregation territories) and fgdB (mutations of which give a class of 'aggregateless' mutant unable to respond to exogenous cyclic AMP signals), and three growth-related loci, couA, tsgK and bsgB (sensitivity to growth with coumarin, temperature sensitivity for growth and inability to grow on Bacillus subtilis, respectively). These loci, which have previously been located on linkage group VII, were found by mitotic recombination studies to lie proximal to the recombinant selector cobAI (resistance to cobaltous chloride) with the order: centromere-couA-tsgK-(stmA-bsgB)-fgdB-cobA. Analysis of haploid segregants derived from 374 recombinant diploids allowed relative map distances to be calculated and revealed that tsgK, stmA, bsgB and fgdB lay as a loosely clustered group close to cobA. Two independently isolated mutations at another streamer locus, stmF, were found to be associated with a diploid instability phenomenon such that heterozygous (stmF/+) diploids which became homozygous at the cobA locus on linkage group VII by mitotic recombination reverted very rapidly to the haploid state. Reversion studies with the tsgK21 marker revealed that partial revertants (or suppressors) at this locus frequently introduced secondary developmental or growth-related mutations by an unexplained mechanism.