Cucurbit Genetics Cooperative Report 7:94-95 (article 42) 1984

Embryo Size in Cucumis sativus x C. melo as Affected by Irradiation of the Pollen and Genotype of the Female Parent

Custers, J.B.M. and J.H.W. Bergervoet

Institute for Horticultural Plant Breeding, P.O. Box 16, 6700 AA Wageningen, The Netherlands

The cross Cucumis sativus x C. melo so far failed because the embryos cease their development at the globular- shaped stage (2). Embryo culture procedures suitable for non- hybrid proembryos and globular-shaped embryos of Cucumis spp. could not induce the hybrid globular-shaped embryos of C. sativus x C. melo to continue differentiation (3). The lack of capacity of progressive differentiation may in part be caused by the difference in basic chromosome number, i.e. x=7 in C. sativus and x=12 in C. melo. Davies (1), extrapolating from intervarietal crosses with irradiated pollen, has pointed out the possibility that irradiation of the pollen might also overcome embryo abortion caused by incompatibility of the two genomes involved. We surmise that pollen irradiation might induce selective elimination of the C. melo chromosomes from the hybrid cells of the young embryos of C. sativus x C. melo, which might improve the viability of these embryos. This contribution gives some preliminary data regarding this technique in this cross.

We used two accessions of C. sativus var. hardwickii (IVT Gene bank nos. 0777 and 1811 A) and one of C. melo ('Noy Yizreel', a cultivar from Israel with monogenic dominant powdery mildew resistance). The plants were grown in an insect-proof glasshouse (25 C D/18 C N) in the summer of 1983. The C. melo pollen was exposed to doses of 0, 10, 100 and 1000 Gy (1 Gy = 100 rad) of gamma radiation and used for pollination within two hours. The treatments were carried out on two dates. Fruits that developed were dissected three weeks after pollination. We assessed seed set (number of ovules > 6 mm per fruit), presence of endosperm and embryo (number of ovules with endosperm and with embryo respectively / 10 large ovules analyzed per fruit), endosperm size (measured from tip to haustorium), and embryo size (diameter or length).

Results of the experiment are in Table 1. In general the values of all the parameters decreased with increasing irradiation dose of the pollen. Mean seed sets, however, did not show significant differences at P = 0.05. Mean frequencies of ovules with endosperm and with embryo were significantly different at P = 0.05 only when the utmost irradiation doses were compared. Endosperm and embryo sizes were affected negatively by high doses of irradiation of the pollen, but clearly not by the low dose of 10 Gy. The difference between embryo sizes on plants of Gbn 0777 and Gbn 1811 A was most intriguing. Whereas the embryos on 0777 exhibited clearly tissue collapse, which was accompanied by yellow discolored dark areas in the endosperm, those on Gbn 1181 A appeared still firm and rather healthy and were surrounded by translucent endosperm. In a separate comparison of Gbn 1811 A with five other genotypes of C. sativus pollinated with non-irradiated pollen of C. melo, Gbn 1811 A also stood out because of the promising size of the hybrid embryos produced.

Table 1. Effects of pollen irradiation dose and female genotype on seed set, frequencies of ovules with endosperm and with embryo, and sizes of endosperm and embryo in the cross Cucumis sativus var. hardwickii x C. melo.
C. sativus (Gene bank no.)	C. melo pollen irradiation dose (Gy)	No. of fruits analyzed	Mean seed set	Mean frequency of ovules with		Mean size* (µ) of
				Endosperm	Embryo	Endosperm	Embryo
0777	0	5	71a**	100a	100a	375a	85a
	10	7	52a	100a	88ab	361a	75a
	100	4	42a	76a	43be	228b	46b
	1000	4	40a	8b	0c	205b	-
1811 A	0	2	82a	100a	100a	519a	211a
	10	3	107a	90ab	86ab	539a	228a
	100	2	87a	85ab	40ab	310b	63b
	1000	2	80a	15b	0b	274b	-
*Calculated on the basis of the number of endosperms and embryos found in the analyzed ovules.
**Per genotype mean values designated by the same letter are not significantly different from each other at P = 0.05.

It seems worthy to study more thoroughly the effects of Gbn 1811 A and of low dose irradiated pollen as its beneficial influence was not excluded. We intend to apply in vitro embryo culture in order to put to use the above effects on in vivo embryo size.

Literature Cited

1. Davies, R. 1981. Gene transfer in plants. Nature 291:531-532.
2. Niemirowicz-Szczytt, K. and B. Kubicki. 1979. Cross fertilization between cultivated species of genera Cucumis L. and Cucurbita L. Genetica Polonica 20:117-124.
3. Nijs, A.P.M. den and J.B.M. Custers. 1984. Resistance breeding of the cucumber by interspecific hybridization. In: R.W. Robinson (ed.), Biology and Chemistry of the Cucurbitaceae. (In Press)