Submitted to: Journal of New Seeds
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/1/2002
Publication Date: 4/20/2003
Citation: Dijkhuizen, A., Staub, J.E. 2003. Effects of environment and genetic background on qtl conditioning yield and friut quality traits in cucumber (cucumis sativus l.). Journal of New Seeds. Interpretive Summary: Yields of U.S. pickling cucumber have increased dramatically since 1940 (~5% annual average increase). Improved cultural practices, the introduction of femaleness (increased number of female flowers), and the introduction of yield and disease resistance genes (groups of DNA on the chromosome) from exotic varieties (e.g., India, China, Japan) into U.S. commercial varieties has increased average yield three-fold from 4,076 (kg/ha) in 1920 to 12,550 (kg/ha) in 1980. Nevertheless, with the exception of perhaps some unique processing hybrids that are adapted to specific growing regions (e.g., specific areas in the southern U.S.), average yield has reached a plateau in the past 15 years (~13,400 kg/ha 1997). This plateau in yield is primarily due to the inability of the cucumber plant to produce more than one fruit once a fruit is produced. In order to produce another fruit on the same plant, fruit on the plant must first be harvested. A wild species of cucumber (Hardwickii) that freely mates with cucumber has been discovered in India has a growth and fruiting habit that allows for more than one fruit to develop on a branch. Breeders have been using Hardwickii in crossings with commercial cucumber to improve fruit yield. However, progress has not been remarkable partially because of a lack of knowledge of the genetics in this wild type. Thus, a study was designed to determine the genetics of yield in this wild type by examining its DNA (the basic unit of inheritance found in the cell). It was discovered that there are certain DNA in the wild type associated with yield that differs from the DNA of the commercial cucumber, and that these differences can be used by the plant breeders to increase breeding efficiency and effectiveness. The consumer of pickling cucumbers will be directly affected as new high-yielding varieties are produced more rapidly, thus lowering the cost to the consumer.
Technical Abstract: Earliness, and fruit yield and quality components of cucumber were investigated by examining cross-progeny (BC and F2S1) derived from a wide mating [gynoecious Cucumis sativus L. var. sativus line GY14 x monoeciousC. sativus var. hardwickii (R) Alef. PI 183967]. A molecular marker map constructed from F2 individuals was used to identify quantitative trait loci (QTL) for each trait examined, and to assess the consistency of QTL over years (1991 and 1992) and planting density (29,000 and 58,000 plants ha-1). QTL affecting earliness (days to anthesis and number of barren nodes), fruit yield (fruit number and weight at two harvest times) and shape [length (L), diameter (D), and L:D ratio] were identified. The traits examined were less affected by planting density than by year. While earliness and yield traits were mostly under non-additive control, components of fruit shape exhibited additive genetic variance resulting in high values for narrow sense heritability estimates. Although the number and map location of QTL was relatively consistent over environments (years and planting density), QTL for fruit shape (e.g., fruit L and D) were less consistent over years and spacing than earliness and yield components. Differences in the number and map location of QTL were found when F2S1 and BC families were compared. Some of these differences could be attributed to disparities in population size, dominance and the amount of genotypic information available (F2S1 > BC). Fruit L and D, and to a lesser extend L:D ratio, are developmentally dependent, and thus map placement of QTL was affected by the physiological stage of fruit development. QTL evaluation of the F2S1 generation revealed that earliness is determined by relatively few genes, and that the genetic control of early yield resides in the same chromosome regions as does days to anthesis. Positive genetic correlations were identified when plants of similar physiological age were compared at different harvest times in each of the environments (years) and genetic backgrounds (F2S1, BC1P1, and BC1P2 families) examined. Thus, these factors should be considered when assessing C. sativus var. hardwickii -derived germplasm and QTL profiles in cucumber.