CGC3-6

Cucurbit Genetics Cooperative Report 3:10-11 (article 6) 1980

An Estimate of Heritability of Fruit Number from a Cross Between a Pickling Cucumber Inbred (Cucumis sativus L.) and an Inbred of C. hardwickii R.

R.R. Horton

Department of Horticulture, North Carolina State University, Raleigh, NC 27650

R.L. Lower and J. Nienhuis

University of Wisconsin, Madison, WI 53706

Low fruit number per plant is the major factor in reducing yield of once-over mechanically harvested pickling cucumbers (4). Smith et al. (6) used a North Carolina Design I to obtain estimates of variance components associated with several characteristics of pickling cucumber, the narrow sense heritability based on full-sib families for fruit number was 0.17. The reference population was random mating and derived from 18 inbred lines obtained from several U.S. breeding programs (5). The low heritabilities and variances associated with fruit number per plant in existing populations might be increased by incorporating multiple fruiting genotypes into the germplasm pool (6). One possible source of such multiple fruiting genotypes is Cucumis hardwickii, an annual monoecious Cucurbit species, which is thought to be either a feral or progenitor species of the cultivated cucumber (Cucumis sativus L.) (1, 3). Previous studies reported that C. hardwickii averaged 80 fruit per plant under North Carolina conditions (3).

The purpose of this study was to estimate the heritability of fruit number per plant in an exotic population which incorporated germplasm from a C. hardwickii line.

Individual plants of F₂ and both backcrosses from a cross between a gynoecious inbred line 'Gy 14', and a selection from C. hardwickii were evaluated for fruit number per plant in Clinton, NC, in 1978. Half-sib offspring of 53 single plant selections based on either high fruit number (> 100) and/or high fruit weight (> 6 kg) were evaluated in a replicated trial in Hancock, WI, in 1979.

Regression of offspring on parent was significant. The heritability of fruit number per plant on an individual basis was estimated as twice the regression coefficient, and was equal to 0.88 ± 0.156 (Table 1). The 95% confidence interval of heritability ranged from 0.57 - 1.20. One of the assumptions of parent offspring regression was that environmental correlations between parent and offspring was zero (2). This was met by evaluating parents and offspring in separate environments. Additional assumptions were made:

Regular diploid inheritance
Hardy-Weinberg equilibrium
Linkage equilibrium
No epistasis
No maternal effects
No assortative mating, including selfing

Parent-offspring covariance is free of genotype x environment interaction (2).

Incorporation of C. hardwickii germplasm has resulted in increasing the variance and heritability of fruit number, suggesting that selection within this exotic population should be effective in increasing yield. Genetic correlations due to linkage or pleiotropy between high fruit number and other less desirable horticultural traits are unknown at this time.

Table 1. Analysis of variance and estimates of regression coefficients for regression of offspring on parent for fruit number/plant.

Source	df	MS
Total (Corr)	52.0	-
Regression	1.0	14469.46*
Dev. from regression	51.0	440.29
R²	0.39	-
b₀	60.94 ± 6.38	-
b₁	0.44 ± 0.078	-

*Significant at 1% level.

Literature Cited

Deakin, J.R., G.W. Bohn and T.W. Whitaker. 1971. Interspecific hybridization in Cucumis. J. Econ. Bot. 25:195-211.
Falconer, D.E. 1960. Introduction to Quantitative Genetics. Ronald Press, New York, NY.
Horst, E.R. and R.L. Lower. 1978. Cucumis hardwickii: A source of germplasm for the cucumber breeder. Cucurbit Genetics Coop. Rpt. No. 1:5.
Miller, C.H., and G.R. Hughes. 1969. Harvest indices for pickling cucumbers in once-over mechanical harvested systems. J. Amer. Soc. Hort. Sci. 94:485-487.
Smith, O.S. 1977. Estimation of heritabilities and variance components for several traits in a random-mating population of pickling cucumbers. Ph.D. Thesis. North Carolina State University, Raleigh, NC.
Smith, O.S., R.L. Lower, and R.H. Moll. 1978. Estimates of heritabilities and variance components in pickling cucumber. J. Amer. Soc. Hort. Sci. 103:222-225.