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Cucurbit Genetics Cooperative Report 7:76-77 (article 33) 1984

Evidence of Gene Flow Between Cultivated Cucurbita mixta and a Field Edge Population of Wild Cucurbita at Onavas, Sonora

Nabhan, Gary P.

Office of Arid Lands Studies, University of Arizona, Tucson, AZ 85721

In an earlier-written note appearing in this newsletter, Merrick and Nabhan (1) suggest that natural hybridization occasionally occurs between wild Cucurbita sororia group populations and domesticated C. mixta in Mexico. Whitaker (2) has recently discussed an "undescribed" wild Cucurbita that climbs into trees and shrubs in the Rio Mayo watershed of Sonora, which is completely cross-compatible with and an apparent miniature of cultivated C. mixta. Because Whitaker (2) hypothesized that this taxon is either the possible ancestral type of C. mixta, or an impoverished feral form of it, further evaluation of wild mixta-like cucurbits in Sonora appears crucial to understanding C. mixta evolution.

The following is a progress report of NSF-funded population genetic and ethnobiological studies of cucurbits begun at Onavas, a Pima Indian village where native agriculture is of considerable antiquity (3). Fifteen miles east of Onavas, Dr. Amadeo Rea and I began observing and collecting wild Cucurbita cf. palmeri in thornscrub vegetation away from any farms. Upon reaching Onavas, we noticed an increase in density of cucurbit fruits within 150 m of floodplain field edges. Both wild and weedy cucurbits are called chicoyote in Spanish or adavi in Piman, and we enlisted local teenagers to collect fruit of this folk taxon of which 200 were scored for at least some of the traits discussed below.

Within the 200 fruits evaluated from Onavas, marginless seeds were found in 28. This trait is extremely rare in wild mesophytic gourds of the C. sororia complex, including C. palmeri, although it is found in domesticated C. mixta land races from Piman villages, such as Pennington's 1977 collection (NS/S E01-009). Upon observing this trait in Onavas gourds, Whitaker (pers. comm.) suggested that we check to see if their flesh was bitter due to the Bi wild type gene. Surprisingly, 26 of the 28 fruit with marginless seed had sweet, edible flesh, in contrast to the rest of the collection, many if not most of which were bitter. (Only four of dozens of the other fruits were found to be sweet before our tongues went on strike!)

To determine if the marginless seeded fruit could be considered a subpopulation distinct from the rest of the population, each fruit was scored for several characteristics, then chi square tests were run on the enumeration data from these two samples. Fruit with marginless seeds were not significantly different in mean fruit circumference, peduncle bristliness, nor skin coloration from the remainder of the fruit. However, the marginless-seeded fruit have peduncles which are significantly more corky and more persistent than those of the other fruit; (the null hypotheses were rejected at the .05 level of significance).

Peduncle corkiness, flesh sweetness, and marginless seeds are all traits known in C. mixta land races that are infrequent or unknown in wild mesophytic gourds from natural habitats in Sonora; this suggests gene exchange with the domesticates. The Onavas field edge population (GN 84-16) as a whole was extremely variable in its combinations of traits which I assume represent both wild-type and introgressed domesticate-type gene expression. It encompassed traits found in sample GN 84-15 collected 15 miles away from the fields, including yellow, oblong-shaped fruit averaging 25.8 cm in circumference, with bristly fluted peduncles and flat, tan seed 10 mm long. At the other end of the field edge population spectrum was a sweet, green and white striped fruit 35 cm in circumference, teardrop shaped, with white, inflated, etched seed. Though mixta-like, these latter seeds were still smaller than the 20 mm length of "wild mixta" seed from Alamos grown by Whitaker in 1970.

In Pima fields at Onavas where land races of C. mixta and C. moschata are grown, they are surrounded by these wild gourds. Pima elder Pedro Estrella believes that when wild chicoyotes grow near squashes, their bitterness "inoculates" the squash, making them unpalatable. This site will be intensively studied during the 1984 growing season.

Literature Cited

  1. Merrick, L.C. and G. P. Nabhan. 1984. Natural hybridization of wild Cucurbita sororia group and domesticated C. mixta in southern Sonora, Mexico. Cucurbit Genetics Coop. Rept. 7:73-75.
  2. Whitaker, T. W. 1980. Cucurbitaceas americanas utiles al hombre. Comision de Investigaciones Cientificas, Buenas Aires. 42 p.
  3. Pennington, C. 1980. The Pima Bajo of Central Sonora. Volume 1, The Material Culture. University of Utah Press, Salt Lake City. 410 p.
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