Double-bridge hybrids of Solanum bulbocastanum and cultivars of Solanum tuberosum

Summary Solanum bulbocastanum (2n=2x=24) has valuable characters for potato breeding, but cannot be hybridized directly with S. tuberosum cultivars. Both S. acaule (2n=4x) and S. phureja (2n=2x) were used as bridging species. Triploid S. acaule × S. bulbocastanum were doubled with colchicine and the resulting fertile hexaploid F₁'s crossed with S. phureja. The triple hybrids obtained were tetraploid or nearly so. The two genomes of S. acaule in these triple hybrids probably pair preferentially, which may provoke pairing and possibly crossing over between the chromosomes of S. bulbocastanum and S. phureja.More than 20000 pollinations of the triple hybrids with four potato cultivars had to be made to produce 40 quadruple hybrids. These highly vigorous hybrids varied greatly in many morphological characters, resistance to Phytophthora infestans, fertility and crossability. The chromosome numbers are 48 (24 hybrids), 49 and 46, but some higher ploidy levels (65, 66, 72 chromosomes) were found as well. Their origin is to be sought in the fusion of an unreduced egg cell from triple hybrids (either euploid or hypoploid) and a reduced male gamete from the cultivars. This view is corroborated by their extreme resistance to Phytophthora. Also some 48-chromosome hybrids are highly resistant, which may indicate introgression from S. bulbocastanum.Most quadruple hybrids are readily inter-crossable and crossable as females with cultivars; several also as males. Two could be hybridized with S. bulbocastanum, but the few seeds dit not germinate.Studies of pachytene stage of meiosis revealed the presence of a S. bulbocastanum chromosome in at least one tetraploid hybrid, which is highly resistant to Phytophthora. At metaphase I of meiosis chromosome associations higher than quadrivalents were not found. Except in one hybrid, the frequency of quadrivalents did not exceed one per cell and the average proportion of chromosomes associated as bivalents amounted to 90%.The quadruple hybrids (double-bridge hybrids) appear good starting material for breeding programmes aimed at introducing genes from S. bulbocastanum into S. tuberosum cultivars.