Cucurbit Genetics Cooperative Report 10:69-71 (article 36) 1987

Cucurbita fraterna, the Closest Wild Relative and Progenitor of C. pepo

Thomas C. Andres

Department of Horticultural Science, New York State Agricultural Experiment Station, Geneva, NY 14456

Cucurbita fraterna Bailey is one of the least known species of Cucurbita. Our knowledge has been based entirely on the holotype, a single herbarium specimen collected by C.L. Lundell in 1937 that contains no pistillate flowers, fruits, seeds or roots. Recently, T.C. Andres, M. Nee, and J.J. Wyland collected the first complete specimens of C. fraterna, including seeds for germplasm, from two populations in northeastern Mexico. One of these populations represents the topotype (collected at the type locality near Llera, Tamaulipas). The other population was found 50 km northeast of this site. Some previously unidentified Cucurbita specimens collected in 1971 by J.V.A. Dieterle (nos. 3800 and 3804, deposited at the University of Michigan Herbarium), now appear to represent two additional populations of C. fraterna in Nuevo Leon, ca. 325 km northnorthwest of the type locality.

When Bailey (3) described C. fraterna, he believed it was closely related to C. texana (Scheele) Gray, hence the specific epithet from the Latin 'frater' (brother) of C. texana. (Another less closely related species, C. sororia Bailey, had already been named the 'soror' (sister) of C. texana). C. fraterna is similar to C. texana but differs in leaf shape and lobing, trichome morphology, and natural distribution.

C. texana has stimulated a considerable number of biosystematic investigations (5), since it has been considered the progenitor of C. pepo L. (1,2,3,4,6,11). This hypothesis is flawed because C. texana has a fairly restricted range and does not occur in Mexico, where the oldest archaeological records of C. pepo have been found (7,10) and where primitive landraces of C. pepo are still being grown today (12). Furthermore, C. texana contains only a small, atypical subset of the isozymes found in various C. pepo cultivars (9; Andres, unpub.). Ancestral species typically exhibit greater genetic diversity than recently derived species, although this may not necessarily be true in outcrossing species under artificial selection (8). Thus, no alternative hypothesis existed regarding the origin of C. pepo.

C. fraterna has fibrous roots and the plants are annual like the related taxa. The round fruits are similar in coloration and shape to C. texana, although most populations of C. texana also contain pyriform-shaped fruits. The round and pyriform fruit types resemble 'Miniature Ball Gourd' and 'Striped Pear Gourd', respectively, both being types of ornamental gourds, C. pepo var. ovifera (L.) Alef.

The fruits of all wild Cucurbita species and most ornamental gourds are extremely bitter, but a non-bitter form of C. fraterna was described on a herbarium label (Dieterle, no. 3804). Similarly, non-bitter C. sororia fruits have been found (R.A. Bye, personal communication).

The fruits of C. sororia differ from C. fraterna and C. texana by lacking very definite carpellary stripes (3). C. sororia appears to represent the wild progenitor of C. mixta Pang. (L.C. Merrick, personal communication). A population of C. sororia (Andres and Nee, collection no.177) was found growing near the type locality of C. fraterna. This species has not been previously reported in the state of Tamaulipas.

C. fraterna generally occurs in upland, seasonably dry thornscrub habitat whereas C. texana is restricted to river bottomland. C. texana fruits readily abscise from the peduncles earlier in development than those of C. fraterna and C. sororia. This may represent an adaptation for dispersal by floating during spring floods.

Accessions of C. fraterna, C. texana, C. pepo, C. sororia, and other Cucurbita species were grown in 1986 at Geneva, NY. Crosses were made and isozymes compared using horizontal starch gel electrophoresis. A total of approximately 25 loci representing ten enzymes was examined. The isozyme data indicate that considerably more allelic variation exists among the two sampled populations of C. fraterna than among 20 sampled populations of C. texana. Unlike C. texana, the alleles of C. fraterna are all commonly found in cultivars of C. pepo. A landrace of C. pepo with elongated, medium size Jack-O-Lantern like fruit, found near the populations of C. fraterna, was indistinguishable from C. fraterna in the enzymes sampled, and in its vegetative morphology. This suggests that C. pepo pumpkins may represent an early domesticated form of C. fraterna. The sympatric C. sororia population did share many alleles with C. fraterna, but also contained several novel alleles not found in C. pepo.

Based on this information, pollinations were made primarily between C. fraterna and various accessions of C. pepo, C. texana, C. sororia and, to a lesser extent, other less closely related species. C. fraterna was readily crossable with all types of C. pepo, including C. texana, but was much less compatible with other species of Cucurbita. C. sororia can be crossed with C. fraterna, but with poor seed set. The crossing data are therefore congruent with the isozyme data.

C. fraterna appears to be the original wild progenitor of C. pepo, based on the genetic evidence, occasional occurence of non-bitter fruits, proximity to archaeological sites of C. pepo, and similarity to traditional landraces of C. pepo. C. fraterna and C. texana may have originally been incipient species or ecotypes. But secondary contact with C. texana may have occurred when C. pepo (i.e., domesticated C. fraterna) spread northward by humans into eastern U.S., introducing new sources of genetic variability. If future evidence proves this to be the case, C. pepo would represent a compilospecies composed of the originally separate taxa, C. fraterna and C. texana. Furthermore, introgression may have occurred with populations of C. sororia. The astonishing range of cultivars present today in C. pepo may have arisen over a period of 10,000 years by a complex pattern of (A) multiple incipient domestications among the small, semi-isolated populations of C. fraterna, (B) dispersal to new areas, resulting in various genetic-environmental interactions, and (C) a generally reticulated pattern of occasional hybridization both among landraces and between these taxa and related wild taxa. Additional germplasm collecting and molecular analysis will help clarify these presumptive patterns of domestication.

Based on the evidence given, C. pepo should be classified as containing three subspecies: C. pepo ssp. pepo, C. pepo ssp. fraterna, and C. pepo ssp. texana.

Literature Cited

1. Bailey, L.H. 1929. The domesticated cucurbitas. Gentes Herbarum 2:61-115.
2. Bailey, L.H. 1930. Three discussions in Cucurbitaceae. Gentes Herbarum 2:175-186.
3. Bailey, L.H. 1943. Species of Cucurbita. Gentes Herbarum 6:265-322.
4. Carter, G.F. 1945. Plant Geography and Culture History in the American Southwest. no.5. New York: Viking Fund Publ. in Anthropol.
5. Decker, D.S. 1986. A biosystematic study of Cucurbita pepo. Doctoral Thesis. Texas A&M Univ., College Station.
6. Erwin, A.T. 1938. An interesting Texas cucurbit. Iowa State Coll. J. Sci. 12:253-261.
7. Heiser, C.B. 1985. Of Plants and People. Univ. Oklahoma Press, Norman.
8. Hutchinson, J.B. (ed.) 1965. Essays on Crop Plant Evolution. Cambridge Univ. Press, London.
9. Kirkpatrick, K.J., D.S. Decker, and H.D. Wilson. 1985. Allozyme differentiation in the Cucurbita pepo complex: C. pepo var. medullosa vs. C. texana. Econ. Bot. 39:289-299.
10. Whitaker, T.W. 1981. Archeological cucurbits. Econ. Bot. 35:460-466.
11. Whitaker. T.W. and W.P. Bemis. 1975. Origin and evolution of the cultivated Cucurbita. Bull. Torrey Bot. Club 102:362-368.
12. Whitaker, T.W. and R.J. Knight. 1980. Collecting cultivated and wild cucurbits in Mexico. Econ. Bot. 34:312-319.