Elymus L. and Hordeum L. are the two largest genera in the wheat tribe (Triticeae Dum. ). The former comprises approximately 150 perennial and exclusively polyploid species and the latter encompasses about 40 perennial or annual taxa with different ploidy levels. Both of the genera are widely distributed in the temperate regions all over the world.Owing to the fact of a large number of species in each genus, wide geographic distribution,
various ecological habitats, and a great variation in morphology between and within species, the taxonomy at species level in the two genera, particularly in the genus Elymus, becomes extremely difficult. Based mainly on the morphological studies, the traditional taxonomists concluded that the two genera possessed comparatively distant biosystematic relationships. However, the data based on the cytological studies have shown that the H genome in Elymus has its origin from species in Hordeum. In order to shed light on the biosystematic relationships between the two genera, the present author conducted intergeneric hybridization between species in Elymus and Hordeum with different origins. Chromosome pairing behavior was observed and analyzed at metaphase I of meiosis in the parental species and their intergeneric hybrids. It was shown from the present study that comparatively high crossability occurred between species in the two genera when Elymus species were used as female, and that the hybrid plants could be easily obtained with the add of embryo culture techniques.The morphology of the intergeneric hybrids, which were generally weaker and smaller than the parents, was intermediate between the two parents, but more closely resembled that of the female parents, i.e., Elymus. The reproductive organs of the intergeneric hybrids developed incompletely in some hybrid combinations and abnormally in all hybrids with non-dehiscent anthers. All the hybrid plants were completely sterile. The results from cytological observation showed relatively low chromosome pairing at metaphase I of all the F1 hybrids, albeit with a large variation between different combinations. It is concluded from the present experimental data together with some other morphological and cytological observations obtained previously that the biosystematic relationships between the two genera Elymus and Hordeum is very complicated and can not be generalized in a simple way. Species which share the H genome in the two genera have comparatively close biosystematic relationships, but the H genomes in different species of the two genera have more or less differentiated during the evolutionary process. Species which do not contain the H genome in the two genera have rather distant phylogenetic relationships.