Cucurbit Genetics Cooperative Report 7:94-95 (article
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
*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.
- Davies, R. 1981. Gene transfer in plants. Nature 291:531-532.
- Niemirowicz-Szczytt, K. and B. Kubicki. 1979. Cross
fertilization between cultivated species of genera Cucumis L. and Cucurbita L. Genetica Polonica 20:117-124.
- 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