Cucurbit Genetics Cooperative Report 10:74-75 (article 38) 1987
Relationship ofCucurbitascabridifolia toC.foetidissima andC.pedatifolia: a case of natural interspecific hybridization
Thomas C. Andres
Department of Horticultural Science New York State Agricultural Experiment Station, Geneva, NY 14456
Cucurbitascabridifolia Bailey is a poorly known wild perennialgourd from southern Tamaulipas, Mexico (2). Recently T.C. Andres, J.J. Wyland,and M. Nee collected several populations ofC.scabridifolia-likeplants near the type locality. Based on field observations and an examinationof herbarium specimens,C.scabridifolia appears to be oneof a gradient of biotypes occurring between two other wild perennial gourdspecies,C.foetidissima HBK andC.pedatifoliaBailey. Table 1 lists five morphological characters which are distinct betweenC.foetidissima andC.pedatifolia, and showsthe generally intermediate position ofC.scabridifolia. Thesethree taxa are similar in other characters, such as in their flower andfruit morphology.
The various intermediate forms betweenC.foetidissima,C.scabridifolia, andC.pedatifolia has ledto considerable taxonomic confusion. For example, Bailey (2) described alobed-leaf form ofC.foetidissima which "may or maynot belong to this species".
The distribution of these intermediate types, includingC.scabridifolia,occurs in north-central Mexico, an area whereC.foetidissimaandC.pedatifolia overlap in range.C.pedatifolia,however, generally inhabits more arid regions south of the large range ofC.foetidissima, which extends northward into the U.S.
An experimental hybridization study was conducted to demonstrate thegenetic compatibility and thus, potential for natural hybridization to occurbetweenC.foetidissima andC.pedatifolia.Fully developed seeds, although in somewhat limited numbers, were successfullyobtained in the F1, F2 and backcross generations (1)The F1 plants showed hybrid vigor and bore numerous fruits. Theplants were intermediate in morphology between the two parent species, buthad generally more deeply lobed-leaves than typical ofC.scabridifolia.The phenotypes of the F2 plants were extremely variable, dueto Mendelian segregation of the genetic factors responsible for the interspecificdifferences. Some plants resembled the lobed-leaf forms ofC.foetidissimathat Bailey originally described, op. cit., others were extremely stuntedbush types, while still others contained deformed "virus-like syndromes"similar to those described for other interspecificCucurbita crossesby Whitaker and Bemis (4). Backcrosses of the F1 toC.foetidissima produced plants indistinguishable from the type specimenand the original description ofC.scabridifolia.
Apparently there are no pre-zygotic barriers to natural hybridizationbetweenC.foetidissima andC.pedatifolia.The two species occur within pollination range of each other, flower duringthe same period, and may be pollinated by the same species of bees.
Therefore,C.foetidissima evidently naturally hybridizeswithC.pedatifolia, andC.scabridifolia representsone of the hybrid derived biotypes.C.foetidissima andC.pedatifolia seem to be maintaining the essential integrity of theirseparate gene pools, despite hybridization between them, because of sterilitybarriers preventing extensive gene flow and also possibly due to naturalselection working against inferior F2 and backcross combinations.
Therefore, unlikeC.scabridifolia,C.foetidissimaandC.pedatifolia are legitimate species. Although a numericaltaxonomic study onCucurbita phenotypic relationships (3) groupedC.foetidissima andC.pedatifolia into separategroups, unrelated to any other species, they are genetically related.
Table 1. Morphological comparison betweenC.foetidissima,C.scabridifolia, andC.pedatifolia.
Literature Cited
- Andres, T.C., 1987. Hybridization ofCucurbitafoetidissima withC.pedatifolia,C.radicans, andC.ficifolia. Cucurbit Genetics Cooperative Report 10 (in press).
- Bailey, L.H. 1943. Species ofCucurbita. Gentes Herb. 6:265-322.
- Bemis, W.P., A.M. Rhodes, T.W. Whitaker, and S.G. Carmer. 1970. Numerical taxonomy applied toCucurbita relationships. Amer. J. Bot. 57:404-412.
- Whitaker, T.W. and W.P. Bemis. 1964. Virus-like syndromes ofCucurbita species hybrids. Heredity 19:229-236.
