POLLEN MARKERS FOR GENE CENTROMERE MAPPING IN DIPLOID POTATO

The utility of two pollen genetic markers for estimating the extent of meiotic recombination between the centromere and a marker gene was te sted in 2n pollen of diploid potato clones. One of these markers was t he distal locus amylose-free (amf) on chromosome 8 and the other was t he isozyme locus alcohol dehydrogenase (Adh-1) on chromosome 4. In the case of the amf locus. the gene-centromere distance was estimated in a normal synaptic and a desynaptic genotype. In both cases the genetic analysis was confined to: (1) a direct estimation of the phenotypic ( blue vs red) segregation ratios in FDR (first-division restitution) 2n pollen and (2) a classification of the 4x progeny from 4x (nulliplex amf) x 2x (Amf/amf) crosses into duplex, simplex and nulliplex classes . The recombination frequency between the centromere and the amf locus in the normal synaptic genotype B92-7015-4 corresponded to a gene-cen tromere distance of 48.8 cM, whereas this distance amounted to 13.3 cM in the desynaptic genotype RS93-8025-1. Hence desynapsis reduced cros sing-over by 73%. The observed genetic distance of 48.8 cM in the norm al synaptic clone, B92-7015-4, is the highest gene-centromere distance reported so far in potato and this could be explained on the assumpti on of absolute chiasma interference. For the Adh-1 locus, it was found that heterozygous 2n pollen grains could be detected in pollen sample s of the diploid clones, because of the occurrence of a heterodimeric band of the isozyme. Unlike the amf locus, the genecentromere distance for the Adh-1 locus was estimated only on the basis of the duplex, si mplex and nulliplex classes in the progenies from 4x (nulliplex Adh-1( 2)) x B92-7015-4 (Adh-1(1)/Adh-1(2)) crosses and was found to be 19.4 cM. Because the accurate positions of centromeres in relation to other loci are not available in the existing genetic maps ofpotato, which a re saturated with molecular markers, half-tetrad analysis is a promisi ng additional approach to the basic genetics of this crop.