Crosses of a synaptic mutant (sy-2) of 2x(1EBN) S . commersonii Dun . with 2x(2EBN) S. chacoense Bitt . resulted in the production of 3x(2EBN) hybrids . Low levels of 2n egg production in these hybrids allowed subsequent crosses to be made with 4x(4EBN) cultivars, resulting in 5x(4EBN) S . commersonii-S . chacoense-Group Tuberosum hybrids . Seed set in these crosses averaged 0 .22 seeds per fruit . Fifty-four unspotted seeds were produced in 1226 crosses of the 5x(4EBN) hybrids with 2x(2EBN) Group Phureja haploid extraction clones carrying dominant seed spot markers . These seeds yielded a total of 15 confirmed haploids having a diploid (2n=2x=24) or near diploid chromosome number . One haploid clone recovered displayed a synaptically abnormal phenotype similar to the original S . commersonii synaptic mutant . Preliminary test crosses of these haploids indicated that all haploids produced were 2EBN.
This method for the transfer of alleles across EBN levels holds promise for the recovery of both qualitative and quantitative traits from 1EBN species . In these recovered, cross-compatible forms, clones with desired characteristics may be selected and easily incorporated into breeding programs .