Marker-assisted backcross selection in an interspecific Cucumis population broadens the genetic base of cucumber (Cucumis sativus L.)

Abstract

Cucumber (Cucumis sativus L.) is a major cucurbit vegetable species whose genetic base has been drastically reduced during its domestication. The crop’s narrow genetic base (3–12% DNA polymorphism) has resulted from the use of limited genetic material and intense selection during plant improvement. Recently, however, interspecific hybridization has been successful in Cucumis via mating of C. hystrix Chakr. and C. sativus, which resulted in the amphidiploid C. hytivus. We report herein a marker-assisted strategy for increasing genetic diversity in cucumber through introgression backcrossing employing C. hytivus. The comparatively late-flowering but high-yielding, indeterminate, monoecious line WI 7012A (P1; donor parent) derived from a C. hytivus × C. sativus-derived line (long-fruited Chinese C. sativus cv. Beijingjietou) was initially crossed to the determinate, gynoecious C. sativus line WI 7023A (P2; recurrent parent 1), and then advanced backcross generation progeny (BC2) were crossed with the gynoecious indeterminate line WI 9-6A (P3; recurrent parent 2). More specifically, a single F1 individual (P1 × P2) was backcrossed to P2, and then BC progeny were crossed to P2 and P3, where marker-assisted selection (MAS) for genetic diversity (8 mapped and 16 unmapped markers; designated Sel) or no selection (designated NSel) was applied to produce BC3P2 (Sel) and BC3P3 (Sel), and BC2P2 (NSel) and BC2P2S1 (NSel) progeny. Relative vegetative growth, number of lateral branches (LB), days to flowering (DF), yield (fruit number), and fruit quality [as measured by length:diameter (L:D) and endocarp:total diameter (E:T) ratios] were assessed in parents and cross-progeny. DF varied from ~20 (BC3P2Sel) to ~25 days (BC2P3Sel) among the populations examined, where progeny derived from P2 possessed the shortest DF. Differences in cumulative yield among the populations over six harvests were detected, varying from ~8 fruits per plant in BC3P2 (Sel) to ~39 fruits per plant in BC2P3 (Sel). Although the vigorous vegetative growth of line P1 was observed in its backcross progeny, highly heterozygous and polymorphic backcross progeny derived from P3 were comparatively more vigorous and bore many high-quality fruit. Response to selection was detected for LB, DF, L:D, and E:T, but the effectiveness of MAS depended upon the parental lines used. Data indicate that the genetic diversity of commercial cucumber can be increased by introgression of the C. hystrix genome through backcrossing.