Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 308

Bridge crossing is a technique of indirectly crossing two parents that differ in ploidy levels through a transitional or an intermediate cross (Figure 6.4). For example, R. C. Buckner and his colleagues succeeded in crossing the diploid Italian ryegrass (Lolium multiflorum, 2n = 2x = 14) with the hexaploid tall fescue (Festuca arundinacea, 2n = 6x = 42) via the bridge cross technique. The intermediate cross was between L. multiflorum and diploid meadow fescue (Festuca pratensis, 2n = 2x = 14). The resulting embryo was rescued and the chromosome number doubled to produce a fertile but genetically unstable tetraploid hybrid (ryegrass‐meadow fescue). Using tall fescue as recipient, the L. multiflorum x F. pratensis product was backcrossed to tall fescue, resulting in the transfer of genes from L. multiflorum to F. arundinacea. A 42‐chromosome cultivar of tall fescue with certain Italian ryegrass traits was eventually recovered and stabilized. Another example of a successful bridge cross is Allium cepa receiving genes from A. fistulosum through the A. roylei bridge.

Figure 6.4 An example of a bridge cross. In order to hybridize Italian ryegrass and tall fescue, the breeder may first make an intermediary cross with meadowgrass, followed by chromosome doubling.