Participatory Plant Breeding of Squash (Cucurbita sp.) Landraces in Mexico

C. Villanueva-Verduzco

Departamento de Fitotecnia, Universidad Autónoma Chapingo, Chapingo, Mexico 56230, Mexico

L.C. Merrick

Department of Agronomy, Iowa State University, Ames, IA 50011

A. Martínez-Mirafuentes, J.A. Martínez-Romero, I. Meneses-Márquez, E. Nájera-Martínez, M.L. Rodríguez-Mitzi, M.A. Sánchez-Hernández, and
J.L. Vildózola-Tenango

Departamento de Fitotecnia, Universidad Autónoma Chapingo, Chapingo, Mexico 56230, Mexico

Additional index words. Cucurbita argyrosperma, C. ficifolia, C. moschata, C. pepo, participatory plant breeding, phenotypic variation, morphological markers, in situ conservation, crop genetic resources, landraces, squash

Abstract. Four species of squash (Cucurbita argyrosperma, C. ficifolia, C. moschata, and C, pepo) are grown widely throughout Mexico, where most of their production continues to be with traditional, locally adapted crop varieties (landraces) rather than improved types. An on-farm, participatory plant breeding project was implemented to improve landrace populations of these squash species through collaboration between scientists and local farmers in three areas of central Mexicothe Chalco region (Mexico state), Cuautla valley (Morelos), and the Sierra Norte de Puebla (Puebla). The selection criteria were determined by joint decision between cooperating farmers and researchers, and include earliness, middle- or large-sized fruit, increased numbers of seeds or fruit, among others.

Squash (Cucurbita sp.) was one of the earliest domesticated crops of Mexico, where prehispanic cultures included it as a basic part of the diet (Lira, 1995; Merrick, 1995). In Mexico, squash is grown mainly for human consumption of seeds, or immature or mature fruitthe latter frequently prepared as dulce, a highly popular crystallized sweetbut flowers, vine tips, or leaves are also eaten or whole fruit fed to livestock (Whitaker, 1968). Four cultivated species in the genus are widely grown throughout Mexicotwo species, C. argyrosperma Huber and C. moschata (Duchesne ex Lam.) Duchesne ex Poir., generally at low to intermediate elevations, and two others, C. ficifolia Bouché and C. pepo L., generally at intermediate to high elevationswhile a fifth, C. maxima Duchesne ex Lam., is absent from most regions of the country (Lira and Montes, 1992; Whitaker and Knight, 1980). Most of Mexican squash production is under rainfed conditions, and throughout much of the

country the majority of cultivars utilized continue to be landraces or local traditional varieties rather than improved types (Merrick, 1989; Whitaker and Knight, 1980). Published agricultural statistics of annual production of squash in Mexico are generally poor estimates of actual production acreage since much of the production for local or distant marketsas well as home consumptionoccurs in relatively small parcels of land and thus does not enter into production estimates (Merrick, 1995; Whitaker, 1968). Production of improved varieties of squash in Mexico is basically restricted to certain varieties of C. pepo (mostly zucchini type cultivars) and C. maxima ('Kabosha' and others, mainly for export to Japanese markets) grown under irrigation in fertile valleys of the Central Plateau and coastal areas of northwestern Mexico. Despite its cultural significance and widespread production, there are no government breeding programs for squash in Mexico.

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In 1996, Chapingo University (Universidad Autónoma Chapingo or UACH) initiated a participatory breeding program focused on landrace varieties of Cucurbita to address the need for a crop improvement program that focuses on varieties of squash of significance to the large sector of Mexican farmers in often marginal environments and whose production systems continue to be based largely on locally adapted crop germplasmand thus who typically do not benefit from breeding programs (Eyzaguirre and Iwanaga, 1996; Lamola and Bertram, 1994; Merrick, 1989; Qualset et al., 1997). The work is sponsored by the McKnight Foundation, and is part of a research and training project entitled "Conservation and Improvement of Crop Production in Mexico: A Farmer-Based Approach" (Anonymous, 1994; Villanueva et al., 1997). The so-called MILPA Projectnamed after milpa, the local name for the traditional, maize-based intercropping system common to Mexicois based in mid to high elevation regions of central Mexico and focuses on the genetic resource base of three main crop commoditiesmaize, squash, and beans (Phaseolus vulgaris L.)and associated edible weeds, known locally as quelites, which are agricultural or ruderal weeds harvested typically as leafy vegetables (Lamola and Bertram, 1994). The squash breeding program targets populations of four species of Cucurbita. The goal is to create a set of improved landrace populations by development of methods to improve productivity through on-farm based breeding techniques. The aim of the project is to achieve positive response to selection while at the same time maintaining high within-population diversity, a desirable characteristic in traditionally managed cropping systems (Alteri and Merrick, 1987; Eyzaguirre and Iwanaga, 1996; Qualset et al., 1997). Traits under selection are those chosen as important by the farmer cooperators. Results reported here consist of the initial characterization of variation and optimum sample size of selected characters of fruit and seeds, as well as response to selection from the first cycles of breeding work.

Materials and methods

Six of the nine study populations consist of landraces of C. ficifolia and C. pepo in the Chalco

valleya high elevation area (2240 m) 20 km outside of Mexico City in the state of Mexicoand C. moschata and C. argyrosperma in the neighboring, lower elevation (1600 m.) Cuautla valley in Morelos. Two additional populations of C. pepo in the Sierra Norte de Puebla in the state of Puebla (both >2000 m) were added later. Work was initiated by collection of a broad range of local germplasm and establishment of research sites with the assistance of farmer collaborators. All fields are under the farmer cooperators own management and consist of intercrops of squash and maize. A ninth study population consists of mixed germplasm from a collection of landraces of C. pepo from numerous sites in the Chalco region and now under selection at the Chapingo Experiment Station. In order to measure variation in certain fruit quality (flesh color and flavor) and quantitative traits (including fruit and seed weight, length and width, and peduncle thickness) and to calculate optimum sample sizes, random preliminary samples of 40 fruit were obtained from each population (Villanueva et al., 1997). Correlations and relative frequencies were calculated.

For the breeding efforts, study sites consisted of 2-ha plots to achieve adequate population sizes for measuring gain through selection over the limited time of the project (5 to 6 years). Each plot had the following structure: maize sown at 80 cm row ¥ 60 cm within-row spacing with three plants per hill; and squash rows interseeded every fifth maize row at 6 m within-row spacing with three plants per hill. Starting in 1996, visual mass selection has been applied annually with a selection pressure of 5%. The selection criteria were determined by joint decision between the cooperating agriculturists and the UACH researchers, and included such traits as earliness, middle- to large-sized fruit, increased numbers of seeds or fruit, or smooth rind texture. Criteria for C. argyrosperma included an increase in peduncule width because of the cooperating farmer's belief that it is positively correlated with production of seed, one of the main commodities of value in the latter species (Merrick, 1995). Selection was made while maintaining diversity, i.e., selected plants included varied combinations of fruit shapes, sizes and external colors (Villanueva et al., 1997).

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Results and discussion

Variation in qualitative and quantitative traits. Results of the survey of variation in two fruit quality traits (fruit color and flavor) reveal there is high phenotypic variation within and between landraces for the traits of interest. Mature fruit quality characters differed between species, probably largely as a result of domestication and selection criteria applied over time by farmers (Merrick, 1995; Villanueva et al., 1997). Table 1 shows that among three C. pepo landraces under study, the one from San Juan Coxtocan performed better than the others with 92.5% of its fruit flesh of a desirable color (bright yellow to bright orange), followed by the Ayotzingo (57.2%) and Juchitepec (42.5%) populations. Likewise, substantial variation was noted in C. pepo for flavor of fruit flesh, which ranged from insipid to very sweet. In C. argyrosperma, distasteful colors and flavors of fruit flesh were predominant possibly because the main consumption product of this squash species is seed. In C. ficifolia, no variation for flesh color was observed (white only), but variation was detected for flavor, with the majority of good quality (92.5% sweet to very sweet). Fruit flesh in C. moschata tended not to be of a desirable color (38.0% only). The latter species

exhibited variability for flavor, but was more likely to be insipid than were the C. pepo landraces included in the evaluation.

In C. argyrosperma, small fruit (1.3 kg) were predominant, with the average seed yield representing 3.9% of total fruit weight. In contrast, in C. moschata, C. pepo and C. ficifolia, average seed yield represented a smaller percentage of the total fruit weight (3.4%, 2.8%, and 2.4%, respectively). Cucurbita pepo and C. moschata combine more favorable quality of fruit with good seed production because from those species both productsseeds and fruitare favored by farmers (Lira, 1995; Merrick, 1989). In C. ficifolia, selection is focused typically on fruit traits and not on seed yield (Tables 1 and 2).

Optimum sample size. Several of the characteristics of fruit and seed yield and fruit quality studied in the landraces showed great variability within and between varieties. Notable is the variation in seed weight per fruit, color and flavor of pulp and fruit weight (Table 2). The large sample size (up to n >400 fruit) required to study those characters is an indirect indication of the magnitude of variation, especially for seed yield and fruit weight which are among the most complex characters under selection in the project (Villanueva et al., 1997).

Table 1. Variation in mature fruit quality in landraces of Cucurbita species in the Chalco-Morelos region of Mexico, 1996.

Landrace

C. pepo C. ficifolia C. argyrosperma C. moschata

Ayotzingo, Juchitepec, San Juan. San Juan Achichipico, Yecapixtla,

Character Mex. Mex. Coxtocan, Mex. Coxtocan, Mex. Mor. Mor.

Fruit flesh color

White 100.0

Cream 5.1

Pale green 11.2

Yellow-green 61.2

Pale yellow 25.0 17.5 37.0

Yellow 17.8 40.0 7.5 22.5 25.0

Bright yellow 16.1 15.0 25.0 16.7

Orange 26.8 12.5 36.3 13.9

Bright orange 14.3 15.0 31.2 7.4

Fruit flesh flavor

Sour 7.5

Slightly sour 52.5

Insipid 31.7 35.0 41.2 7.5 32.5 54.7

Sweet 53.7 40.0 48.3 65.0 7.5 35.9

Very sweet 14.6 25.0 10.0 27.5 9.4

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Table 2. Descriptive statistics and optimum sample size for phenotypic characters in landraces of Cucurbita species in the Chalco-Morelos region of Mexico, 1996.

Landracez

C. pepo C. ficifolia C. C.

San Juan Los Reyes San Juan argyrosperma moschata

Ayotzingo, Juchitepec, Coxtocan, Acatlixhuayan, Coxtocan, Achichipico, Jantetelco,

Character Mex. Mex. Mex. Mex. Mex. Mor. Mor.

Fruit weight (kg) 2.5 ± 0 2.5 ± 1.0 5.2 ± 1.6 1.7 ± 0.3 4.5 ± 1.1 1.3 ± 0.4 3.6 ± 1.2

(236) (246) (145) (125) (86) (118) (171)

Fruit length (cm) 16.5 ± 3.6 26.4 ± 6.0 10.2 ± 1.6 23.4 ± 2.3 21.2 ± 5.0

(74) (80) (25) (15) (86)

Fruit width (cm) 15.8 ± 2.2 22.0 ± 3.5 17.4 ± 1.0 20.2 ± 2.0 20.3 ± 2.8

(30) (39) (6) (16) (30)

Fruit flesh thickness (cm) 2.1 ± 0.6 2.3 ± 0.6 2.9 ± 0.5 2.0 ± 0.1 2.5 ± 0.4 2.0 ± 0.4 2.9 ± 0.8

(11) (105) (46) (43) (41) (68) (123)

Fruit flesh color y 2.7 ± 1.3 1.7± 0.6 5.0± 0 5.0± 0

(357) (193) (1) (1)

Fruit flesh flavorx 3.2 ± 1.7 2.9± 0.6 4.0± 0 2.2± 0.6

(434) (66) (1) (115)

Seed weight per fruit (g) 71.3± 31.7 111.7± 29.6 51.5± 20.3 122.3± 31.6

(314) (108) (238) (103)

Seed length (cm) 2.5± 0.2 2.4 ± 0.2 1.9± 0.1 2.4± 0.2 2.0± 0.2

(9) (11) (9) (11) (16)

Seed width (cm) 1.0± 0.1 1.0 ± 0.1 1.1± 0.1 0.8± 0.01 1.2± 0.1

(11) (16) (13) (23) (11)

Peduncle thickness (cm) 2.3± 0.6

(15)

zMean ± sd (calculated optimum sample size in parentheses).

yFruit flesh color scale: 1 = bright orange, 2 = orange, 3 = bright yellow, 4 = yellow, 5 = pale yellow, 6 = white.

xFruit flesh flavor scale: 1 = very sweet; 2 = sweet; 3 = insipid.

Morphological markers. Phenotype correlation analysis (a = 0.5) indicated that fruit weight in the four species was dependent on fruit length (r =0.65** to 0.82**), fruit width (r = 0.51** to 0.87**), fruit flesh thickness (r = 0.41** to 0.57**) and seed weight (r = 0.40** to 0.70**). For C. moschata alone the seed

weight per fruit depended on fruit weight, length, and width. In C. argyrosperma, the seed yield was highly and positively correlated with fruit weight (0.70**), pulp thickness (0.39**), and fruit stem thickness (0.52**), and seed length (0.47**) and width (0.41**).

Table 3. Differential of or response to selection in landraces of four Cucurbita species via on-farm participatory breeding, 1996­97.

D Fruit Fruit Fruit Fruit flesh Fruit Fruit Seed Seed Seed Peduncle

or wt length width thickness flesh flesh length width wt thickness

Landrace Rz (kg) (cm) (cm) (cm) colory flavorx (cm) (cm) (g) (cm)

C. pepo

Juchitepec, Mex. D 0.40 ­0.05 0.30 0.17 0.32 ­0.32 ­0.03 ­0.01 12.80

Ayotzingo, Mex. R 0.38 0.40 1.35 0.95 ­0.03 ­0.01 33.80

San Juan C., Mex. R ­2.35 ­1.05 2.10 1.05

Los Reyes A., Mex. D 0.31 1.03 1.14 0.26 ­1.78 ­2.60

E1 Tejocote, Pue. D ­0.76 10.60

E1 Socorro, Pue. D ­0.51 12.10

Chapingo, Mex.w D 0.37 ­0.04 ­0.01 ­0.01 6.37

C. argyrosperma

Achichipico, Mor. R 0.18 0.03 0.32 0.06 94.05 0.49

C. moschata

Jantetelco, Mor. R 0.39 ­1.51 0.93 0.73 0.82 0.47 77.20

zD = differential of selection (data from two selection cycles), R = response to selection (data from one selection cycle only).

yFruit flesh color scale: 1 = bright orange, 2 = orange, 3 = bright yellow, 4 = yellow, 5 = pale yellow, 6 = white.

xFruit flesh flavor scale: 1 = very sweet, 2 = sweet, 3 = insipid.

wExperimental population at UACH experiment station, created from a mixture of central Mexican landrace populations of C. pepo.

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Response to on-farm selection. For the first cycles of selection (1996­97), depending on the trait under selection and the locality, either negative or positive response to selection was observed among the study sites (Table 3). For C. pepo varieties, the differentials of selection for fruit weight ranged from 310 to 763 g/fruit, whereas seed yield ranged from 6.7 to 33.8 g. A relatively large reduction of weight per fruit measured for one landrace (C. pepo from San Juan Coxtocan) resulted from a farmer-initiated change from the selection criteria of large fruit size and good fruit quality in 1996 to higher seed yield in 1997. The response to selection for fruit and seed yield in C. argyrosperma was 180 g/fruit and 94.05 g of seeds/fruit. In C. moschata the corresponding gain was 395 and 77.2 g, respectively. For the latter landrace population, a gain in 0.73 cm was noted for fruit flesh thickness.

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