The genetic base of cucumber (C. sativus var. sativus L.; 2n = 2x = 14) is extremely narrow {3% to 8% among elite and exotic germplasm and 12% between botanical varieties [C. sativus var. sativus L. and var. hardwickii (R.) Alef.]} (Dijkhuizen et al., 1996; Horejsi and Staub, 1999). In fact, the limited genetic diversity of U.S. processing cucumber (especially since 1950) may, in part, be the result of the broad use of only a few cultigens (e.g., line Gy-14) in cultivar development (Staub et al., 2008). Yield in processing cucumber has plateaued in the last 20 years, and the introgression of exotic genes into commercial cucumber may provide opportunities for germplasm enhancement. Therefore, a series of 94 C. hystrix Chakr.-derived U.S. processing market-type inbred backcross lines (IBL) were released in Jan. 2011 by the Agricultural Research Service, U.S. Department of Agriculture to provide genetic stocks for broadening the genetic base of pickling cucumber. The IBL were developed by crossing a U.S. processing cucumber (C. sativus L.; line WI 7023A) and a C. hystrix-derived (C. hystrix × C. sativus) line (WI 7012A) and is made available to U.S. cucumber breeders to supply a source from which they may develop processing market types with increased genetic diversity and yield potential suitable for field production.
Origin
The 94 IBLs were developed by crossing U.S. Department of Agriculture, Agricultural Research Service line WI 7023A (Madison, WI; U.S. processing-type recurrent parent; 2n = 14) and the C. hystix-derived line 7012A (donor parent; 2n = 14) and then selecting the most genetically diverse BC1 (30 of 392 individuals) based on molecular marker profiles (16 mapped loci; Delannay, 2009). These selected BC1 individuals were pollinated by WI 7023A and 25 BC2 progeny were recovered from 25 BC1 selections. Approximately eight seeds of each BC2 line (8*25 = 200 total) were randomly selected for self-pollination to produce the BC2S1 generation followed by single-seed descent to generate 94 BC2S3 IBLs (Tanksley et al., 1996; Wehrhahn and Allard, 1965).
The relatively high-yielding, multiple lateral branching, gynoecious, determinate line WI 7023A produces warty, light-green fruit of commercially acceptable shape and quality (Fig. 1). It was created by mating line Gy-7 (synom. G421) and line H-19, which were originally obtained from the University of Wisconsin Madison (Madison, WI) and the University of Arkansas (Fayetteville, AR), respectively. Line WI 7023A was created through selection and backcrossing with Gy-7 as the recurrent parent and H19 as the donor parent (BC4S3) to identify a small-statured genotype for once-over mechanical harvest operations. It originated from the same populations that were used to develop recombinant inbred lines for the mapping of quantitative trait loci in U.S. processing cucumber (Fazio et al., 2003; Staub et al., 2002).
Fruit of determinate U.S. processing cucumber (Cucumis sativus L.; recurrent parent) line WI 7023A, indeterminate C. hystrix Chakr.-derived [(C. hystrix × C. sativus) × C. sativus; donor parent] BC1S3 line WI 7012A, and their derived indeterminate F1 progeny used in backcrossing and as observed at Hancock, WI, during the summer of 2007.
Citation: HortScience horts 46, 10; 10.21273/HORTSCI.46.10.1428
The late-flowering, indeterminate, monoecious line WI 7012A is a BC1S3 line derived from a cross between the amphidiploid C. hystivus (2n = 2x = 24; Chen and Kirkbride, 2000) and the C. sativus long-fruited Chinese cultivar Beijingjietou (recurrent backcross parent; 2n = 2x = 14) mating (Chen et al., 2003). C. hystivus originated through the chromosome doubling of an infertile F1 (C. hystrix × C. sativus; 2n = 2x = 19) individual to produce a fertile amphidiploid (2n = 4x = 38) as identified during in vitro embryo culture (Chen et al., 1998, 2003; Chun-Tao et al., 2005). The amphidiploid was backcrossed to Beijingjietou to produce viable seed without further manipulation (i.e., in vitro culture). Then these progeny were self-pollinated for three generations (BC1S3), in which selection was practiced for reduced chromosome number in each generation. The result of this selection and selfing was WI 7012A (2n = 2x = 14) (Staub et al., 2008).
Description
Multivariate analyses using Rogers (Rogers, 1972) genetic distances (GD) modified by Wright (1978) were used employing 32 codominant markers to define phenotypic and genotypic relationships between the IBL and their parents (Delannay, 2009; Delannay et al., 2010). The greatest GD was between the parental lines (0.85), whereas the GD among IBLs ranged between 0.16 and 0.75. The most genetically similar IBL were lines 113 and 201 and lines 3 and 180 (GD = 0.16). In contrast, IBL with the least genetic similarity were lines 51 and 187 (GD = 0.75).
Marker-based selection for heterozygosity at BC1 did not eliminate the phenotypic diversity in BC2 progeny (Delannay et al., 2010). Based on replicated open-field trials conducted at Hancock, WI, during the summers of 2006 and 2007, IBLs differ in days to flower, sex expression, lateral branch number, number of fruits per plant, and fruit length and diameter ratio (Table 1; Fig. 1; Delannay et al., 2010). For instance, although IBL 206 developed the greatest number of fruit per plant (approximately four) and lateral branches (approximately four), IBL 3 provided the lowest yield (approximately one fruit per plant), IBL 38 the lowest length:diameter (L:D; 2.6), and IBL 188 required the longest time to flower [days to anthesis (DA) ≈50]. By comparison, IBL 119 recorded the shortest time to flower (DA ≈39) and IBL 226 developed fruit with the largest L:D (≈3.9).
Combined early and late planting means, sds, and ranges of traits of parents [WI 7023A (recurrent parent, Cucumis sativus L.) and WI 7012A (donor parent, C. hystivus derived)] and their derived cucumber (C. sativus L.) inbred backcross lines (BC2S3) as evaluated in 2008.
The genotypic and phenotypic diversity among and between IBL are not necessarily equivalent (Table 1). Although IBL 113 and 201 possess the strongest genetic similarities (GD = 0.16), they had dramatically different morphological attributes. For instance, IBL 201 produced few lateral branches and comparatively few short fruit (zero to one laterals per plant, one to two fruit per plant, and L:D = 2.7 to 2.9), and IBL 113 produced many lateral branches and many narrow fruit (approximately four laterals per plant, 3.4 fruits per plant, and L:D = 3.4).
Many IBLs are gynoecious and lack the negative attributes associated with the monoecious WI 7012A (i.e., spiny, warty, and oblong fruit). For this and other reasons (Table 1), IBLs should be considered unique germplasm that can be used directly by plant improvement programs seeking to increase genetic diversity in cucumber. Moreover, given the consistency of phenotypic differences in IBL, they should perform consistently in early and late harvest operations typical of upper-Midwestern U.S. climates (Delannay et al., 2010).
The IBLs have been phenotypically and genotypically described (Delannay et al., 2010), and thus, can be used in cucumber improvement and genetic studies. For instance, after initial evaluation of these IBLs in specific target environments, strategic crossing of these IBLs with elite lines may allow for the development of broad- and narrow-based populations for phenotypic and/or marker-assisted selection (Fan et al., 2006). Where IBL have contrasting traits, they have use for the genetic analysis of complex traits (e.g., yield and quality components; characterization of epistatic interactions) (Robbins et al., 2008; Tanksley et al., 1996), and selected IBL (e.g., high yield, early flowering, and multiple lateral branching types) could be used in mapping experiments where synteny of quantitative trait loci controlling yield components could be assessed between C. sativus and C. hystivus genomes. Additionally, these diverse IBLs will also be useful in genetic studies and/or to evaluate cross-progeny derived from matings between C. hystrix and derived germplasm [e.g., amphidiploid, diploid (2n = 14) C. hystivus × C. sativus backcross derivatives] and substitution lines (Chen et al., 2004).
Availability
Seed of C. hystrix-derived IBLs from a hand-pollinated greenhouse increase may be obtained by addressing requests to P.W. Simon (philipp.simon@ars.usda.gov), Vegetable Crops Research, U.S. Department of Agriculture, Agricultural Research Service, Department of Horticulture, University of Wisconsin, Madison, WI 53706.
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