Heterosis Estimates for Several Characteristics in a Cross Between a Gynoecious Inbred of Cucumis Sativus L. and C. hardwickii R.

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Cucurbit Genetics Cooperative Report 3:20-21 (article 11) 1980

James Nienhuis and R. L. Lower
University of Wisconsin, Madison, WI 53706; R. R. Horton, North Carolina State University, Raleigh, NC 27650

Characteristics such as multiple laterals and prolific fruiting make Cucumis hardwickii a potentially useful source of germplasm in a breeding program to improve yield of pickling cucumbers (1). The use of C. hardwickii also represents an opportunity to broaden the genetic base of common cucumber cultivars (3). Information about the nature of differences in fruit setting and other morphological characteristics between Cucumis sativus and C. hardwickii would be very beneficial to plant improvement programs. The object of this study was to estimate heterosis for yield components and several vegetative characteristics in a cross between a gynoecious pickling cucumber inbred line, Gy 14, and a C. hardwickiiline, ‘LJ 90430’.

The two parental lines, Gy 14 and ‘LJ 90430’, (designated as P1 and P2 respectively), and their F1 were utilized in this study. The parents had been maintained by selfing for several generations and were assumed to be homozygous. The three generations were grown in a randomized complete block design with ten blocks at the Horticulture Crops Research Station in Clinton, NC, in 1978. A replication consisted of five plant single row plots for each generation; the plants were spaced on 1.5 m centers.

Analysis of variance was used to test mean differences among generations. Generation by block interaction was used as an estimate of experimental error. Heterosis above the midparent was defined as the superiority of the F1 over the respective high parent for the trait under consideration. A t-test was used to determine the significance of differences between generations. Because of the short day nature of the C. hardwickii line, flowering was reduced and data on fruiting parameters were estimated from an unweighted least squares analysis of generation means (40). The estimate of the fruiting parameters obtained were gathered on individual plants at maturity as follows: number, weight and length/diameter ratios of mature fruit, lateral number and main stem vine length. Lateral number was counted as the number of lateral branches (primary, secondary, etc.) between the cotyledonary node and the first 1m of main stem.

Heterosis above both mid and high parent was observed for fruit weight per plant and main stem vine length (Table 1). Heterosis below the midpoint was observed for lateral number (Table 1). The length/diameter ration and number of fruit showed no deviation of the F1 from the midparent. When estimates of heterosis fall above the high parent, it is tempting to conclude that this is overdominant type gene action; however, Moll and Stuber (2) point out that other non-additive types of gene action and linkage disequilibrium might result in effects which mimic those of overdominance.

From a plant breeder’s point of view, heterosis estimates indicate whether homozygotes or heterozygotes represent the more ideal genotype. With no heterozygote advantage, breeding efforts should be directed towards homozygous inbred populations; alternatively, if there is a heterozygote advantage, then breeding methods such as reciprocal recurrent selection, which capitalize on heterosis, should be explored.

Table 1. Generation means and heterosis estimates for five characteristics in a C. sativus x C. hardwickii cross.z

Generations

Fruit no. per plant

Fruit weight per plant (kg)

Lateral no.

Main stem vine length

Length/diameter ratio of fruit
P1 Gy 14C. sativus 4.03 2.21 4.10 121.31 2.24
P2 ‘LJ 90430’, C. hardwickii 93.91x 2.20x 39.81 137.15 1.47x
F1 53.12 6.70 16.50 290.32 1.99
Heterosis above midparentz 4.15 NS 4.49 ** -5.46* 161.09** 0.14 NS
Heterosis above high parenty – 4.49** – 153.17** –

z F1 – (P1+P2)/2.
y F1 – high parent.
x Least square estimate from generation means, which included F2, BC1, and BC2.
**Significant at .01 level ; *significant at .05 level.

Literature Cited

  1. Horst, E. K. and R. L. Lower. 1977. Cucumis hardwickii: A Source of Germplasm for the Cucumber Breeder. Cucurbit Genetics Coop. Rpt. 1:5.
  2. Moll, R. H. and C.W. Stuber. 1974. Adv. Agr. 26:277-315.
  3. National Academy of Sciences. 1972. Genetic Vulnerability of Major Crop Plants.
  4. Nienhuis, J. and R. L. Lower. Unpublished data.