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Wheat Information Service
Number 76: 59-60 (1993)


Reproductive barriers between Hordeum and Secale species

J. Fujigaki¹ and T. Tozu<sup>2

1</sup>Junior College, Tokyo University of Agriculture, Sakuragaoka, Setagaya, Tokyo 156, Japan
²Professor emeritus, Shizuoka University, Shizuoka 422, Japan


Intergeneric as well as interspecific hybridizations have been attempted to reveal phylogenetical relationships among genera in the tribe Triticeae and a number of successful hybrids have been produced (Sears 1948, Sakamoto 1973, Sharma and Gill 1983). These hybrids have potential values to widen genetic variation for cereal breeding. The hybrid between Hordeum and Secale was firstly reported by Brink et al (1944), then several successful hybrids have been reported (Fedak 1985 for review). There still remain problems to be solved concerning reproductive barriers between the two genera.

In the present report, several combinations of crosses between Hordeum and Secale species were conducted to estimate crossability between the two genera. Abortive process of the hybrid seeds between them was studied cytohistologically. Then the reproductive barriers were discussed.

Five Japanese two-rowed cultivars; H. vulgare cv. Hatakaze, Golden Melon, Satsuki-nijo, Nitta-nijo, Akagi-nijo, and five wild Hordeum species; spontaneum (two accessions), murinum, stebbinssii, glaucum, bulbosum (4x, two acces.) were used as female parents. Six different species of Secale; africanum, kuprijanovii, segetale (two acces.), vavilovii (two acces.), ancestrale, cereale (two acces.) were employed as male.

The female parents were emasculated about two days before anthesis and pollen grains were applied to the stigmas directly from anthers of the male parent. One day after pollination, one drop of 100 ppm solution of giberellic acid (GA₃) was applied to each floret. After surface sterilization of the immature caryopses they were cultured directly on modified Murashige and Skoog medium.

Some of the fertilized caryopses were harvested at 3, 6, 9, 12 and 15 days after pollination and they were studied on the paraffin sections.

In total 5,058 caryopses from 19,623 florets pollinated in 24 cross combinations were scored by their size and shape as fertilized ones with rye pollen, because the self-fertilized ones became plump and large but the crossed ones much smaller in size and slender in shape at ten to 14 days after pollination. Japanese two-rowed cultivars used in this experiment could be crossed with rye species, though the rates of obtaining the fertilized caryopses ranged from 1.5% for Akagi-nijo x S. vavilovii to 60.1% for Hatakaze x S. vavilovii. When Hatakaze was used as the female parent in the crosses, it always showed fairly good crossability, 58.2%, 60.1% and 55.3%, with different rye species. Conversely Nitta-nijo showed low crossability (1.5 - 17.9%) and wild Hordeum species had much less crossability (0.0 - 4.5%) with rye.

Considerable genetic variation is known to exist for crossability among cultivars as well as wild species in the same genus (Sharma and Gill, 1983). In wheat two genes, Kr1 on 5BL and Kr2 on 5AL, were reported to control effectively the crossability with rye (Krolow 1970, Lange and Riley 1973, Sitch et al 1985), and rye carries a single gene for crossability (Tanner and Falk 1981). Kr1 and Kr2 repress crossability independently and the wheat cultivars carrying the both genes have low or no crossability with rye, while non-carriers show high crossability and those carrying one of the two are intermediate. The present result showed similar pattern in the crossability, thus it is suggested that Hordeum species also have two crossability genes interacting against Secale pollen.

Through the immature caryopsis culture only four caryopses from Satsuki-nijo x S. africanum generated shoots and roots. However, one died before and another one after transplanting into soil. Finally two hybrid plants grew up, though those were weak at the early seedling stage and showed leaf chlorosis. It was clearly showed that response to the immature caryopsis culture is quite different among different cross combinations and is independent of crossability.

Cytohistological study on seed abortion revealed that after fertilization of barley egg cell with rye pollen the zygote developed up to globular embryo stage, while in endosperm nuclear divisions occurred but cytokinesis did not follow. Abnormal nuclear divisions and nucellus enlargement were also observed. At nine to 12 days after pollination the embryo and endosperm seemed to degenerate.

There are several reproductive barriers such as prevention of fertilization, hybrid breakdown, hybrid weakness or inviability, hybrid sterility, etc. (Hadley and Openshaw 1980). The present result suggests the crossability of Hordeum species is controlled by two genes against rye. The cytohistological study and the hybrids rescued by the immature caryopsis culture clearly show that fertilization occurs and the zygotes grow up to globular stage, but gradually break down in the course of development. Especially endosperm development is quite abnormal, eg, no cytokinesis occurs following the nuclear divisions. It seems to be a main cause of the seed abortion. The hybrids were weak and showed leaf chlorosis. This suggests complementary gene actions might occur in the hybrids.

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