Cucurbit Genetics Cooperative Report 17:97-100 (article 29) 1994
The Crossability of Cucumis melo and C. metuliferus, an Investigation of In Vivo Pollen Tube Growth
A. Beharav and Y. Cohen
Bar-Ilan University, Dept. of Life Sciences, Ramat Gan 52900, ISRAEL
For many years viral disease in muskmelon (Cucumis melo) (2n=24) has not been of great concern to seedmen and growers (8). The development of resistant cultivars can eliminate the disease. Various accessions of Cucumis metuliferus (African horned cucumber) (2n=24) contain genes for some desirable characters that should be valuable additions to the gene pool of the muskmelon if they could be transferred (4, 9) and literature cited therein).
Numerous attempts to cross C. metuliferus with C. melo have failed. Occasional fruit set and ovary enlargement occurred when C. metuliferus was used as the female parent, but the fruits were small, misshapen, and lacked viable seeds (2, 4). Norton and Granberry (7) reported a successful cross with C. melo as the female parent, which gave viable F1 and F2 seeds, but attempts to duplicate the original cross were unsuccessful due to embryo abortion.
The nature of cross incompatibility between C. metuliferus and C. melo is controversial. Kho et al. (6) concluded from studies on in vivo pollen tube growth that it is inhibited in the stigma or style, while Fassuliotis (4) found a partial compatibility between the species that was indicated by a very small percentage of C. metuliferus ovules being fertilized by C. melo.
This paper presents observations on pollen tube growth in C. melo and C. metuliferus flowers after pollination within each of the species, and after cross-pollinations of the reciprocal crosses of the two species. The identification of the barriers to hybridization of those species led to attempts to develop procedures that would overcome the incompatibility.
C. melo ('Gylan' Genoecious E6/10) and C. metuliferus ('Italia') were grown in the field and female flowers were hand-pollinated on June 20th with male flowers of C. melo line PI124111F or C. metuliferus ('Italia'). Temperatures ranged from 18
˚ CD. Standard pollination procedures for Cucumis were used with special precautions to avoid contamination by pollen from foreign sources.
The fluorescence microscope procedure, as described by Kho and Baer (5) and modified by Tomer and Gottreich (10), was used to observe pollen tube growth.
Flowers were collected 0.5, 5.5, 24 and 48 hr after pollination and immediately fixed in AA (water 4, absolute alcohol 3, acetic acid 1). The flowers were washed in water and sectioned freehand longitudinally and transversely. Each section was softened in 8N NaOH for 1 hr, washed with water and stained for 5 min with 0.1% aniline blue w.s. dissolved in 0.1 M K3PO4. The stained tissues were then placed on a slide in glycerine and gently squashed by applying pressure on the cover slip. Observations were made at 390 (excitation) - 420 (emission) nm by use of Ft425 and LP450 filters. Photomigraphs were taken on Kodak film.
The stigma of each of the two species was receptive to its own and to the foreign pollen. the pollen of both species germinated within 30 min after application to the stigma (Fig. 1a, b).
Pollen germination after cross-pollination of the reciprocal crosses between species was comparable to that of a self or cross-pollination within species, but the pollen tube grow more slowly. It already showed after 30 min (Fig. 2a, b) and was very recognizable after 5.5 hr (Fig. 2c). Pollen tubes of C. melo PI 124111F reached the style of 'Gylan' E6/10 within 5.5 hr (Fig. 1c). By the 24th hr after pollination, they had entered the ovarian cavity (Fig. 1d) and fertilization took place within 48 hr, the epical end of the tube contained a highly fluorescent elongated callose plug within the ovules (Fig. 1e).
When C. melo was treated with C. metuliferus pollen, the pollen tubes were mostly arrested in the stigma or in the upper style (Fig. 2d). The pictures of arrested pollen tube growth (Fig. 2c, d) are similar to what is usually found as a result of incompatibility, namely thick (er) tubes, fully filled with callose (5,6). Thickened ends, bifurcated tubes and other deformations are sometimes found, in no particular combination. In contrast, the normally-growing tubes are slender, with dots of callose spread along the length of tubes (Fig. 1c, d).
We anticipate that using special pollination techniques such as irradiated mentor pollen (1,3), benzyladerine (1) and bud pollination, will be necessary to overcome the prefertilization barrier between the two species.
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