Seedling Death in Interspecific Crosses with Cucumis africanus L.f.

Cucurbit Genetics Cooperative Report 4:58-60 (article 30) 1981

A. P. M. den Nijs, D. L. Visser and J. B. M. Custers
Institute for Horticultural Plant Breeding, P. O. Box 16, Wageningen, The Netherlands

Three interspecific crosses with Cucumis africanus L.f. out of our crossability analysis of African Cucumis species (4) set seed, most of which germinated. The seedlings, however, died from desiccation of the base of the stem after about four weeks of growth. This seedling death, as a special case of F1-hybrid breakdown, is described here in some detail as well as several attempts to overcome the barrier by making cuttings, grafting and in vitro culture.

Results of reciprocal crosses of C. africanus, C. myriocarpus Naud. and C. anguria L. are summarized in Table 1. seedling death was observed in 12 combinations of different accessions in three crosses with C. africanus. Individual progenies of two crosses with seedling death are listed in Table 2. The first cross, C. africanus x C. anguria, appeared to succeed in three out of eight combinations of accessions. One of the resulting progenies has thus far been positively identified as a hybrid, which was functionally male sterile and sparingly cross fertile. The reciprocal cross (5 combinations) never yielded fruit, despite good pollen tube penetration into the ovules (4). The second cross in Table 2 appeared less promising: six out of 14 different seed samples of C. africanus x C. myriocarpus did not germinate. The others germinated in part, and all seedlings died early.

We were unsuccessful in overcoming seedling death by rooting healthy tops of young plants as cuttings. Also, over 100 plantlets of C. africanus x C. anguria and C. myriocarpus x C. africanus grafted onto C. africanus died about two weeks later than non-grafted plants. In vitro culture of seeds and excised embryos was attempted to overcome seedling death. Surface-sterilized mature seeds of C. africanus x C. anguria (two samples, 61 seeds) and of C. africanus x C. myriocarpus ( three samples, 80 seeds) were incubated on MS medium, and embryos were isolated from half of them. Most seeds discharged a slight to dense cloud of some gray-white substance into the medium, especially those of C. africanus x C. myriocarpus. Most embryos developed a film of gray material around their radicle.

Only seeds and embryos without these symptoms of deterioration germinated and developed into plantlets. In the two progenies of C. africanus x C. anguria the germination percentages were 38 and 47% (Gbn 0162 x 0198 and 0162 x 0310), in those of C. africanus x C. myriocarpus 13.0 and 0% (Gbn 0162 x1763, 0162 x 0182, and 0181 x 0165). After approximately four weeks of aseptic culture (25 ± 1°C, 16 hrs. light, 1000 Lux), the base of the hypocotyl of the plantlets turned brown, narrowed and desiccated, toppling the 3 to 4 leaved shoot, while this and the roots still appeared healthy. Eventually all plantlets died. Kinetin added to the basal medium (0.1, 1, and 10 mg/l) did not stop the deterioration process. In vitro grafting onto plantlets of C. africanus succeeded, but soon the hybrid tissue just above the graft-union turned brown and the grafts died after about two weeks. There was no difference in the behavior of the hybrids of the two crosses tested.

We speculate that some vital compound (e.g. necessary in the primary metabolism) may be missing in the lethal seedling, with a gradual accumulation of a certain deleterious metabolite as a possible result. This might express itself first in the ‘oldest’ part of the plants, which enlarge first during seedling development.

A few combinations of C. africanus x C. anguria yielded vigorous F₁ hybrids, so there appears to be variability for crossability. This was not found for C. africanus x C. myriocarpus, but more accessions can be tested. A comparison of the present results with earlier relevant reports is puzzling. Both Deakin et al. (2) and Dane et al. (1) considered C. africanus closely related to both C. myriocarpus and C. anguria, since they obtained (at least sparingly) fertile F₁ plants in almost all possible combinations. Independent evidence from isozyme electrophoretic patterns (3, 5) links the three species closely together phylogenetically. Because different accessions of all three species were used by different authors, conclusions from any comparison remain tentative. We plan to extend the analysis to accessions also used in the earlier reports.

Table 1. Summary of crosses with three Cucumis species showing occurrence of seedling death.

	Male parent
Female parent	Cucumis myriocarpus	C. africanus	C. anguria
Cucumis myriocarpus	+	D(1)	-(4)
C. africanus	D(6), S(4)	+	+(3), D(5)
C. anguria	+(6)	-(5)	+

+ = vigorous offspring; – = occasional fruits but no seeds; D = seedling death, S = non-germinating seeds; numbers in parentheses refer to the number of combinations of different accessions.

Table 2. Behavior of individual progenies in two crosses with Cucumis africanus with seedling death. Legends as in Table 1.

Cucumis africanus x C. anguria; C. africanus accession used as female parent, C. anguria accession used as male parent.
Gbn	0198	0307	0310	1736	1758
0162	D, D, D	+	D, D	+	D
0181	D	–	–	–	–
0330	–	D	–	–	–
1457	+	–	–	–	–
C. africanus x C. myriocarpus; C. africanus accession used as female parent, C. myriocarpus accession used as male parent.
Gbn	0165	0182	0202	0203	1737	1763
0162	–	D, S	–	S	D, S	D
0181	S	D, D	S	S	D	–
1773	–	D, D	–	–	–	–

Gbn (Gene bank no.); 0307 = PI 196477, 0310 = PI 233646, and 1457 = PI 299570.

Literature Cited

1. Dane, F., D. W. Denna, and T. Tsuchiya. 1980. Evolutionary studies of wild species in the genus Cucumis. Z. Pflanzenzuchtig 85: 89-109.
2. Deakin, J. R., G. W. Bohn, and T. W. Whitaker. 1971. Interspecific hybridization in Cucumis. Econ. Bot. 25: 192-211.
3. Esquinas-Alcazar, J. T. 1977. Alloenzyme variation and relationships in the genus Cucumis. Ph.D. Thesis. University of California, Davis. 170 p.
4. Kho, Y. O., A. P. M. den Nijs and J. Franken. 1980. Interspecific hybridization in Cucumis L. II. The crossability of species, an investigation of in vivo pollen tube growth and seed set. Euphytica 29:661-672.
5. Puchalski, J. T., R. W. Robinson and J. W. Shail. 1978. Comparative electrophoresis of isozymes of Cucumis species. Cucurbit Genetics Coop. Rpt. 1:39.