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Cucurbit Genetics Cooperative Report 10:74-75 (article 38) 1987

Relationship of Cucurbita scabridifolia to C. foetidissima and C. pedatifolia: a case of natural interspecific hybridization

Thomas C. Andres

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

Cucurbita scabridifolia Bailey is a poorly known wild perennial gourd from southern Tamaulipas, Mexico (2). Recently T.C. Andres, J.J. Wyland, and M. Nee collected several populations of C. scabridifolia-like plants near the type locality. Based on field observations and an examination of herbarium specimens, C. scabridifolia appears to be one of a gradient of biotypes occurring between two other wild perennial gourd species, C. foetidissima HBK and C. pedatifolia Bailey. Table 1 lists five morphological characters which are distinct between C. foetidissima and C. pedatifolia, and shows the generally intermediate position of C. scabridifolia. These three taxa are similar in other characters, such as in their flower and fruit morphology.

The various intermediate forms between C. foetidissima, C. scabridifolia, and C. pedatifolia has led to considerable taxonomic confusion. For example, Bailey (2) described a lobed-leaf form of C. foetidissima which "may or may not belong to this species".

The distribution of these intermediate types, including C. scabridifolia, occurs in north-central Mexico, an area where C. foetidissima and C. pedatifolia overlap in range. C. pedatifolia, however, generally inhabits more arid regions south of the large range of C. foetidissima, which extends northward into the U.S.

An experimental hybridization study was conducted to demonstrate the genetic compatibility and thus, potential for natural hybridization to occur between C. foetidissima and C. pedatifolia. Fully developed seeds, although in somewhat limited numbers, were successfully obtained in the F1, F2 and backcross generations (1) The F1 plants showed hybrid vigor and bore numerous fruits. The plants were intermediate in morphology between the two parent species, but had generally more deeply lobed-leaves than typical of C. scabridifolia. The phenotypes of the F2 plants were extremely variable, due to Mendelian segregation of the genetic factors responsible for the interspecific differences. Some plants resembled the lobed-leaf forms of C. foetidissima that Bailey originally described, op. cit., others were extremely stunted bush types, while still others contained deformed "virus-like syndromes" similar to those described for other interspecific Cucurbita crosses by Whitaker and Bemis (4). Backcrosses of the F1 to C. foetidissima produced plants indistinguishable from the type specimen and the original description of C. scabridifolia.

Apparently there are no pre-zygotic barriers to natural hybridization between C. foetidissima and C. pedatifolia. The two species occur within pollination range of each other, flower during the same period, and may be pollinated by the same species of bees.

Therefore, C. foetidissima evidently naturally hybridizes with C. pedatifolia, and C. scabridifolia represents one of the hybrid derived biotypes. C. foetidissima and C. pedatifolia seem to be maintaining the essential integrity of their separate gene pools, despite hybridization between them, because of sterility barriers preventing extensive gene flow and also possibly due to natural selection working against inferior F2 and backcross combinations.

Therefore, unlike C. scabridifolia, C. foetidissima and C. pedatifolia are legitimate species. Although a numerical taxonomic study on Cucurbita phenotypic relationships (3) grouped C. foetidissima and C. pedatifolia into separate groups, unrelated to any other species, they are genetically related.

Table 1. Morphological comparison between C. foetidissima, C. scabridifolia, and C. pedatifolia.

Literature Cited

  1. Andres, T.C., 1987. Hybridization of Cucurbita foetidissima with C. pedatifolia, C. radicans, and C. ficifolia. Cucurbit Genetics Cooperative Report 10 (in press).
  2. Bailey, L.H. 1943. Species of Cucurbita. Gentes Herb. 6:265-322.
  3. Bemis, W.P., A.M. Rhodes, T.W. Whitaker, and S.G. Carmer. 1970. Numerical taxonomy applied to Cucurbita relationships. Amer. J. Bot. 57:404-412.
  4. Whitaker, T.W. and W.P. Bemis. 1964. Virus-like syndromes of Cucurbita species hybrids. Heredity 19:229-236.
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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
Created by T.C. Wehner and T. Ng, 1 June 2005; design by C.T. Glenn;
send questions to T.C. Wehner; last revised on 14 December, 2009