Incompatibility in diploid and tetraploid crosses of Cucumis sativus and Cucumis metuliferus

The African horned cucumber ( Cucumis metuliferus Naud.; 2x = 2n = 24) contains genes that can confer resistance to many important cucumber ( C. sativus L.; 2x = 2n = 14) pests [ e. g., root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood]. Cucumber is highly susceptible to this root-knot nematode species, and a recent screening of C. sativus accessions in the U. S. National Plant Germplasm collection did not identify sources of resistance. Thus, autotetraploids of Cucumis sativus and C. metuliferus were created to recover fertile resistant interspecific progeny. Autotetraploids were obtained at the highest rate when seeds were immersed in 0.5% colchicine for a period of 6 to 8 hrs. Treatment durations less than 6 hrs produced few tetraploids, and durations of 10 hrs or more were lethal to seeds or developing seedlings. Crosses between C. sativus and C. metuliferus were made using diploid and tetraploid lines in all possible combinations, including reciprocals. Fruit development occurred in crosses when diploid and tetraploid C. sativus were used as the female parent. However, seeds developed only in fruit of C. sativus (4n) x C. metuliferus (2n) crossings. Seeds from these crosses, however, were flat and not viable. No fruit development occurred in crosses where C. metuliferus was used as the female parent.