Diallel crossing among doubled haploids of cucumber reveals significant reciprocal-cross differences

Cytoplasmic effects on plant performance have been documented, but are not well understood. Cucumber (Cucumis sativus) is a useful plant for studying organellar effects on phenotypes because chloroplasts show maternal transmission and mitochondria paternal transmission. We produced doubled haploids (DH) from divergent cucumber populations, generated reciprocal hybrids in a diallel crossing scheme, measured fresh and dry weights of plants 22-30 days after planting seed, estimated combining abilities and heterosis for early plant growth, and assessed performance differences between reciprocal hybrids with identical nuclear genotypes. Across experiments, general and specific combining abilities and reciprocal effects, as well as their interactions with replicated experiments, were all highly significant (P < 0.001). Hybrids consistently out-performed parental lines with average heterosis over midparent values between 14% and 30%. A mitochondrial mutant (MSC3) showed negative effects when used as the male due to paternal transmission of mitochondria, but not as the female parent. Reciprocal hybrids among wild-type DH parents were identified that differed significantly (P = 0.032 to 0.001) for dry and fresh weights across experiments, indicating that cucumber breeders should evaluate both directions of crosses when producing hybrid cultivars. Reciprocal hybrids from DH cucumbers offer a unique opportunity to study biological factors contributing to significantly better performances, due to specific nuclear-cytoplasmic combinations and/or parent-of-origin effects in identical nuclear backgrounds