Cucurbit Genetics Cooperative Report 2:43-44 (article 26) 1979
Emiel H. Oost and A. P. M. den Nijs
Institute for Horticultural Plant Breeding, Wageningen, The Netherlands
The African species Cucumis metuliferus carries resistance to root knot nematodes(Meloidogyne spp.), which would be an important asset for both melons and cucumbers. The species, however, could so far not be crossed with any other representative of the genus (1, 2, 3). In C. africanus, resistance has been found against cucumber green mottle mosaic virus (CGMMV). However, attempts to introduce this resistance into cucumber have failed so far.
The mentor pollen technique has proved helpful in overcoming crossing barriers between allied species in the genus Populus (4). The merits of this technique in crosses between C. Metuliferus, C. sativus, and C. africanus were evaluated in this study.
Fresh anthers with pollen of the three Cucumis species were irradiated with 100, 200, and 500 krad (cobalt source, 100 krad/4.5 minutes) during the summer of 1978. The lowest dose appeared to be sufficiently lethal, since no embryo development was found in selfings with 100 krad irradiated pollen. Pollen germination and pollen tube growth, however, appeared to be almost normal, although a rapid loss of viability was found (down to 10% within 12 hours). Irradiated pollen (anthers) of the mentor was mixed with unirradiated pollen (anthers) of one of the two other species in a volume ratio of about 2:1. Pollinations were made in five of the six possible combinations of reciprocal interspecific crosses. In vitro germination of all pollen was checked before pollination. After 48 hours, in vivo germination and pollen tube growth were examined under a U.V. microscope. Penetration of pollen tubes into ovules was evident in all pollinations with mentor pollen, either alone (as a control) or in mixtures with one of the other species. No difference was observed between these categories.
C. metuliferus and C. africanus, both non-parthenocarpic, yielded fruits only after pollination with own mentor pollen, independent of the presence of pollen of other species. Fruits were opened, starting from the 14th day after pollination, and the developing seeds were examined for embryo growth. Embryo sacs, some with globular structures (possible embryos), were found in C. metuliferus pollinated with C. metuliferus mentor pollen mixed with C. sativus pollen. Fourteen embryo sacs out of four C. metuliferus fruits from pollinations with mentor and sativus pollen were explanted on an artificial embryo medium, but all failed to grow. Pollinations of C. africanus with different mentor pollen combinations resulted only in some seeds with very small embryo sacs which exhibited no internal differentiation. In fruit of C. sativus, no embryo sacs were found. In all species, degeneration of nucleus and potential embryo sacs had already started in fruits opened 25 days after pollination.
Normal cross-pollinations on C. metuliferus and C. africanus never yielded fruits. The induced fruit set due to mentor pollen may have facilitated the development of those few seeds which resulted from cross- fertilization. If our embryos were of hybrid origin, embryo development has been rather slow. Degeneration occurred after 25 days but probably started sooner. Therefore, embryo culture seems essential, but appropriate media are still to be developed.
Literature Cited
- Dane, F. K. 1976. Evolutionary studies in the genus Cucumis. Ph.D. Thesis, Colorado State University, Fort Collins, CO.
- Deakin, J. R., G. W. Bohn, and T. W. Whitaker. 1971. Interspecific hybridization in Cucumis. Econ. Botany 25:195-211.
- Fassuliotis, G. 1977. Self-fertilization of Cucumis metuliferus Naud. and its cross- compatibility with C. melo, L. J. Amer. Soc. Hort. Sci. 102:336-339.
- Stettler, R. F. 1968. Irradiated mentor pollen: Its use in remote hybridization of black cottonwood. Nature 219:746-747.