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

USDA-ARS, Forage and Range Research Laboratory, Utah State University, Logan, UT 84322-6300, USA
Euphytica (Impact Factor: 1.39). 03/2011; 178(2):261-272. DOI: 10.1007/s10681-010-0315-8


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 parent1), and then advanced backcross generation progeny (BC2) were crossed with the gynoecious indeterminate line WI 9-6A (P3; recurrent parent2). 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~25days (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.

KeywordsGenetic diversity–
–Molecular markers–Morphological traits

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    ABSTRACT: Plant breeders constantly need to introduce desirable new alleles to refresh breeding stocks. This first requires an assessment of potential new sources of material and then identification of genotypes most able to augment existing stocks. Genetic distance analysis is widely used for both purposes, although it measures both haplotype diversity and novel allele abundance. Here, we present a more tailored approach to address these problems. Using oil palm as an exemplar, simple metrics of allelic and genetic richness, graphical genotyping and multivariate analysis were deployed to determine the overall value of Ghanaian germplasm to supplement Sumatra Bioscience (SumBio) breeding material. We next compared three methods to rank individuals. The first was based on multivariate genetic distance. However, we also developed two new systems: Global Allelic Divergence (GAD), based on novel allele abundance, and Genome Scan Allelic (GSA) divergence, which additionally considers genome context. Ghanaian material exhibited increased allelic richness, higher heterozygosity and a higher proportion of private alleles than extant SumBio breeding stocks. Graphical genotyping revealed Ghanaian material as allele-rich in genomic regions that were allele-poor in SumBio breeding stocks. Multivariate analysis showed a collective distinctness and increased variability of Ghanaian plants. Ranks of individuals varied between GSA, GAD and genetic distance. The GAD and GSA ranks correlated strongly with each other but only poorly with the genetic distance-based ranks. We conclude that GSA and GAD are superior ranking systems to identify individuals most likely to introduce valuable new alleles, whilst genetic distance analysis identifies individuals likely to require least backcrossing.
    No preview · Article · Jan 2012 · Annals of Applied Biology