An allotriploid derived from a amphidiploid x diploid mating in Cucumis

A fully fertile interspecific hybrid (Cucumis hytivus Chen and Kirkbride, 2n = 4x = 38) between Cucumis hystrix Chakr. (2n = 2x = 24) and C. sativus L. (2n = 2x = 14) was previously produced by means of F1 (2n = 19) embryo rescue and subsequent chromosome doubling. This amphidiploid, a new synthetic species, may serve as a genetic bridge in Cucumis, and thus is a source for broadening the genetic base of C. sativus. The identification and characterization of fertile progeny possessing lower ploidy levels would facilitate bridging among Cucumis species. Putative allotriploids (2n = 26) were recovered from C. hytivus x C. sativus matings by means of embryo culture, and experiments were designed to confirm their genetic constitution, describe their morphology, and establish an efficient protocol for their micropropagation. Apical and axillary buds of these putative allotriploid plants were used as explants to establish a micropropagation system for subsequent verification and characterization of ploidy. Of the array of micropropagationmedia tested, the most effective for the induction of adventitious buds (desginated Stage II) was a Murashige and Skoog (MS) growth media containing 13.3μM BA + 1.1μM NAA or containing 10 μm BA only. The mean number of adventitious buds per explant in the two media was 6.8 and 6.5, respectively. Shoots resulting from adventitious buds produced roots (Stage III) in relative abundance (39 of 42, 92.8%) on half-strength MS medium containing 1.0 μm IBA. The survivorship of rooted plantlets after acclimatization as assessed by relative production of leaves in plantlets (designated Stage IV) was 91.4% (148 of 162). The chromosome number in putative allotriploid plants as determined in mitotic root tip figures in all plants was 2n = 26, the number expected for allotriploids derived from such a mating. An examination of pollen viability in five samples of each plant by cytochemical staining revealed stainability to be< 10%. Compared to their parents, the allotriploid genotypes possess a high degree of parthenocarpy (84.8%) as measured by setting fruit in pollen-free conditions. While allotriploid fruit are black-spined and similar to the maternal parent C. hytivus, the dark green leaves typical of allotriploid plants mirrors that of the paternal C. sativus parent.