Cucurbit Genetics Cooperative Report 3:60-62 (article 33) 1980

Reciprocal Crosses Between Cucumis africanus L.f. and C. metuliferus Naud. I. Overcoming Barriers to Fertilization by Mentor Pollen AVG

A. P. M. den Nijs, J. B. M. Custers and A. J. Kooistra

Institute for Horticultural Plant Breeding, Wageningen, The Netherlands

Cucumis africanus L.f. carries resistance to cucumber green mottle mosaic virus, and C. metuliferus Naud. to root knot nematodes. We wish to introduce these resistances into the cultivated cucumber, C. sativus L. Crosses among these three species never succeeded (2, 5), but we recently succeeded in obtaining hybrids of C. metuliferus and C. africanus and likely also of the reciprocal cross.

Oost and den Nijs (4) reported on mentor pollen as a tool in interspecific hybridization in Cucumis. This technique was used in an extensive crossing program also encompassing the above mentioned species. The rhizobitoxine analog, amino-ethoxy-vinyl-glycine (AVG), was also used in attempts to overcome the crossing barriers, following the report of Natti and Loy (3) on its favorable effect on seed production in emasculated muskmelon.

For the present experiment we used two accessions of C. africanus (Gene bank nos. 0162 and 0181) and two of C. metuliferus (Gbn. 0164 and 1734). Plants were grown in the glasshouse in the summer season (± 23°C D/18°C N). Pollinations were made from the appearance of the first pistillate flower up to four months later, so age and carrying capacity of the plants varied greatly during the season. Mentor pollen was prepared following Oost and den Nijs (4), irradiation dose being 100 krad. Amino-ethoxy-vinyl-glycine was applied mixed in lanolin paste-water (7:3) at a concentration of 0.5 mg/ml. The mixture was smeared around the base of the flower directly after pollination, at an approximate rate of 0.1 ml per flower. Fruits that developed were dissected to check for ovules and embryos, starting two weeks after pollination. We have seen no indication of different behavior of accessions, so all data were pooled per species.

Results are presented in Table 1. Some conclusions are as follows.

1. No fruit set in controlled pollinations.
2. Mentor pollen effectively induced fruit set, but only ca. 1/3 of the fruits contained an embryo. In those fruits, the number of embryos was generally low. Controlled self-pollinations with only irradiated pollen also yielded many fruits (4) but ovules in these fruits always contained an embryo sac without an embryo.
3. Amino-ethoxy-vinyl-glycine induced only relatively few fruits to develop, but they all contained ovules with embryos. The number of such ovules was generally high.
4. The combined mentor pollen/AVG treatment resulted in fruit set comparable to the mentor pollen treatment alone. Most fruits contained ovules with embryos, an effect comparable with that of AVG alone. The number of pollinations with the combination treatment is thus far limited.

Many immature embryos from both young and maturing fruits were incubated on an artificial medium. In total, 235 embryos were successfully explanted of the C. africanus x C. metuliferus cross, and 51 of its reciprocal (1). A batch of mature seeds of C. africanus x C. metuliferus failed to germinate.

Thus far, in vitro culture has yielded plants of only C. metuliferus x C africanus. Ten plants have been transplanted into soil. They grew moderately in the glasshouse in autumn and produced light green leaves of intermediate shape. All staminate flower buds aborted, whereas pistillate flowers were small with shape and spines intermediate between those of the parents. Also, esterase isozyme patterns on polyacrylamide-gel-electropherograms of crude leaf extracts confirmed the hybrid nature of the plants. Their resistance spectrum is still being investigated. No fruits set so far following pollinations with pollen of various species, but it should be noted that the plants are now growing under unfavorable winter conditions.

Table 1. Effect of pollination technique on the results of reciprocal crosses between Cucumis africanus and C. metuliferus.

Treatment	Number of pollinated flowers	Number of developing fruits	Number of fruits containing embryos	Number of ovules with embryo/ Number of large ovules*
Cucumis africanus x C. metuliferus
Control	39	0	-	-
Mentor pollen	19	17	5	1/24; 1/12; 5/40; 30/57; 42/49
AVG	31	4	4	6/15; 19/40; 15/27; 29/35
Mentor pollen + AVG	7	5	4	1/25; 9/16; 40/50; 85/100
C. metuliferus x C. africanus
Control	52	0	-	-
Mentor pollen	6	6	2	1/20; 2/13
AVG	29	4	4	12/25; 10/20; 14/20; 22/30
Mentor pollen + AVG	5	4	2	2/19; 1/8

* Only (almost) full-sized ovules were examined.

Literature Cited

1. Custers, J. B. M. and G. van Ee. 1980. Reciprocal crosses between Cucumis africanus L.f. and C. metuliferus Naud. II. Embryo development in vivo and in vitro. Cucurbit Genetics Coop. Rpt. 3:50-51.
2. Deakin, J. R., L. W. Bohn and T. W. Whitaker. 1971. Interspecific hybridization in Cucumis. Econ. Bot. 25:195-211.
3. Natti, T. A. and J. B. Loy. 1978. Role of wound ethylene in fruit set of hand pollinated muskmelons. J. Amer. Soc. Hort. Sci. 103:834-836.
4. Oost, E. H. and A. P. M. den Nijs. 1979. Mentor pollen as a tool in interspecific hybridization in Cucumis. Cucurbit Genetics Coop. Rpt. 2:43-44.
5. Robinson, R. W. and E. Kowalewski. 1978. Interspecific hybridization of Cucumis. Cucurbit Genetics Coop. Rpt. 1:40.