Figure 1.

Breeding scheme followed to introgress S. commersonii (Cmm) into S. tuberosum Group Tuberosum (Tbr-4x) gene pool. A tetraploid clone of Cmm PI 243503 (Cmm-4x) was produced by in vitro tissue culture of leaf explants (16). Phureja-Tuberosum haploid hybrids (Phu-Tbr) came from the Wisconsin Breeding Program. Cmm-4x was crossed with Phu-Tbr, and resulting 3x F₁ hybrids were backcrossed to Tbr-4x (cultivars Blondie, Carmine, Tollocan and clone Wis 482) to obtain BC₁ 5x hybrids. These were used as both female and as male parents in further backcrosses with Tbr-4x to produce PTH BC₂ hybrids. 3x hybrids were coded as A1, B3, B10, B16, and C1; 5x hybrids were coded as P3 (B3 × Tbr-4x) and P5 (B10 × Tbr-4x).