Submitted to: Molecular Breeding
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/15/2007
Publication Date: 5/20/2008
Citation: Robbins, M., Casler, M.D., Staub, J.E. 2008. Pyramiding QTL for multiple lateral branching in cucumber using inbred backcross lines. Molecular Breeding. 22(1):131-139. Interpretive Summary: Cucumber is the fifth most widely grown vegetable crop worldwide (2,427,436 hectares harvested in 2004) and ranks seventh in the U.S. in area harvested (68,660 hectares). Although the yield of U.S. processing cucumber has reached a plateau in the last 20 years, evidence from several studies indicates that selection for multiple lateral branching (MLB) types (i.e., plants with several lateral branches) may increase cucumber yield. In addition, highly-branched plant types are desirable for once-over machine harvest of U.S. processing cucumber. In order to increase yield in cucumber by adding more lateral branches, it is necessary to understand the interaction of genes (units of inheritance on chromosomes) which control their development. This phenomenon can be understood by evaluating genes as the are expressed in unique, novel plants. Such plants were developed through intercrossing and selection to produce an array of twin plants that differ only in the number of lateral branches they produce. The genes controlling MLB were known for earch plant by DNA analysis. By comparing these plants under the same growing conditions in two field environments in Wisconsin, it was determined that some genes contribute more to the development of lateral branches than other genes. This information will allow public and private plant breeders interested in increasing yield in cucumber to be more effective in developing cultivars with more MLB. Such novel MLB plant types will allow the U.S. grower to improve profit margin and thus become more competitive in the global market.
Technical Abstract: Multiple lateral branching (MLB) is a quantitatively inherited trait associated with yield in cucumber (Cucumis sativus L.; 2n=2x=14). Although quantitative trait loci (QTL) have been identified for MLB and QTL-marker associations have been verified by marker-assisted selection, the individual effects of these QTL have not been characterized. To test the effects of pyramiding QTL for MLB, molecular genotyping was utilized to create two sets (standard- and little-leaf types) of inbred backcross (IBC) lines possessing various numbers of QTL that promote branching. These IBC lines were evaluated for lateral branch number in two Wisconsin environments at three plant densities. Highly significant differences in the number of primary lateral branches were detected between spacings, leaf types, and lines, but not between locations. Lateral branch number decreased at higher plant densities in all genotypes, while genotype by environment and QTL by environment interactions were marginally non-significant (P = 0.065). As the number of QTL increased among IBC lines, the number of branches did not generally change in the little-leaf lines, but decreased in the standard-leaf lines, demonstrating an epistatic effect related to genetic background during lateral branch development. The genomic location with the greatest effect on MLB was confirmed as the QTL that was previously mapped near the little-leaf locus (ll), while the addition of one specific QTL consistently decreased the number of lateral branches in standard-leaf lines. Although pyramiding QTL for MLB did not uniformly increase the number of lateral branches (increase was QTL dependent), pyramiding QTL in IBC lines allowed further characterization of individual QTL involved in MLB. Our results, coupled with those of previous studies indicate that lateral branch development in cucumber is determined by growing environment (i.e., plant spacing), genetic background, and QTL composition.