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
- Andres, T.C., 1987. Hybridization of Cucurbita foetidissima
with C. pedatifolia, C. radicans, and C.
ficifolia. Cucurbit Genetics Cooperative Report 10 (in press).
- Bailey, L.H. 1943. Species of Cucurbita. Gentes Herb.
6:265-322.
- 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.
- Whitaker, T.W. and W.P. Bemis. 1964. Virus-like syndromes of Cucurbita species hybrids. Heredity 19:229-236.
