Heterosis in Summer Squash (Cucurbita pepo L.)

Cucurbit Genetics Cooperative Report 21:43-45 (article 15) 1998

I.T. Firpo, F. Lopez Anido, S.M. Garcia, and E. Cointry
Fac. de Cs. Agrarias, Universidad Nacional de Rosario, CC 14, Zavalla 2123, Argentina

Introduction. Cultivated cucurbita species have not followed the general pattern of inbreeding and heterosis of other cross-pollinated crops like maize and onions (9). In early reports (1,4,8) it was noted that inbreeding may not necessarily decrease vigor and yielding capacity and that selected inbreds could equal or outyield commercial checks. Nevertheless significant heterosis have been reported in crosses between certain varieties of C. pepo L. and C. maxima L. (2,5) encouraging hybrid production.

It is convenient to specify a reference population when dealing with heterosis (6). The heterosis (hI) in the cross between two inbred populations is an expression of hI+ Ho+I. The first component, ho, is the heterosis expressed in crosses between non-inbred populations (interpopulation level). The second, I, is any inbreeding effect in the populations and will increase heterosis in the same amount (6). Heterosis in this sense (intrapopulation level) has rarely been reported in C. pepo L. The present reports aim to evaluate heterosis in lines derived from a summer squash population (Cucurbita pepo L.)

the experimental material consisted of ten selected inbred lines (UR 4, 7, 18, 19, 30, 33, 34, 38, 52, and 53) derived from the highly variable summer squash population ‘Tupungato Magnif INTA’ (caserta type) and the diallel crosses including reciprocals. The experiment was established on 18 October 1996 at the Experimental Field of the Fac. de Cs Agrarias, UNR, located at Zavalla (33 ˚ 01′ S; 60˚ 53’W), Argentina, in a randomized complete block design with three replication of eight plants, in a plant spacing of 1.4 m between rows and 0.80 m within plants in the row. The following variables were evaluated on a plot basis: total fruit number (TFN) (12 harvests), precocious fruit number (PFN) (first three harvests), days from sowing to first harvest (DFH), number (LN) and diameter (LD) (cm). These last three vegetative characters were evaluated in three plants 30 days after sowing. An ANOVA was performed, partitioning the genotype source of variation into parents, crosses and the contrast parents vs. crosses. The significance of this contrast was used as a test of heterosis (3). The CONTRAST statement of GLM, SAS, was used (7). In order to meet normal distribution of residuals TFN, PFN and DFH were transformed by square root and LN by loge.

The contrast parents vs. crosses was highly significant (p<0.01) for LN, LD and PH (F = 10.21, 11.8 and 10.54 respectively; 1,203 df) and significant (p<0.05) for TFN and PFN (F = 4.75 and 6.02; 1, 203 df). For these characters mean values and heterosis (best parent) of the five outstanding crosses are shown in Table 1. On average heterosis was superior for precocious fruit number, followed by total fruit number, leaf number and plant height. These results confirm, as it was reported in variety crosses, that at an intrapopulation level significant heterosis could be achieved, especially for early production. In breeding programs, in order to met maximum hybrid yield, both of the above components of heterosis should be pursued. In a population breeding scheme, where fruit color and plant habit traits can be fixed in the first selection cycles, the inbreeding and crossing method could be a useful tool in increasing population means for yield traits through hybrid or synthetic variety production.

Table 1.1 Means and heterosis (expressed as percentage of the best parent of each cross) of five outstanding crosses for total fruit number (TFN), precocious fruit number (PFN), and leaf number (LN).

Total fruit number			Precocious fruit number			Leaf number
Cross	Mean (fruits/ha)	Heterosis %	Cross	Mean (fruits/ha)	Heterosis %	Cross	Mean (no.)	Heterosis %
19 X 13	136,272	38.4	19 X 52	13,616	162.9	53 X 34	8.64	34.3
18 X 53	132,109	34.2	18 X 53	12,734	375.4	33 X 53	8.26	19.7
33 X 19	130,412	32.8	30 X 33	12,131	24.9	18 X 53	8.24	27.3
19 X 52	130,412,	41.1	7 X 4	10,926	129.2	19 X 33	7.80	9.24
53 X 19	127,778	29.8	33 X 30	10,825	11.5	53 X 7	7.78	20.5
Average	131,396	35.2		12,046	140.7		8.13	22.20

Table 1.2 Means and heterosis (expressed as percentage of the best parent of each cross) of five outstanding crosses for leaf diameter (LD) and plant height (PH).

Leaf diameter			Plant height
Cross	Mean (cm)	Heterosis (%)	Cross	Mean (cm)	Heterosis (%)
34 X 19	28.20	14.5	30 X 18	35.10	36.0
18 X 33	28.10	22.5	33 X 18	33.83	29.1
7 X 18	27.76	20.9	18 X 19	33.20	-4.4
18 X 52	27.30	13.4	34 X 53	33.06	25.1
53 X 7	27.16	18.3	33 X 34	32.60	24.4
Average	27.70	17.9		33.55	22.04

Acknowledgement: This research was supported in part by PID 202 UNR Grant.

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