The South African Hordeum capense is more closely related to some American Hordeum species than to the European Hordeum secalinum: A perspective based on the 5S DNA units (Triticeae: Poaceae)

Headnote

Abstract: Several authors have proposed that the European Hordeum secalinum and the morphologically similar South African Hordeum capense are conspecific. In this paper we provide evidence that the two species differ in their 5S DNA unit class composition. We also report on the diversity of 5S DNA units in Hordeum muticum, a South American species. When the 5S rDNA unit class composition for these three species is compared with the unit class composition for all Hordeum species thus far investigated, it appears that H. capense is more closely related to the American Hordeum species containing the long Y2 unit class, than to H. secalinum, which lacks the long Y2 unit class but contains the long X2 unit class found in H. marinum. This analysis suggests H. capense may have originated from a stock common to the South American species, such as H. muticum.

Key words: 5S DNA unit class, Hordeum capense, Hordeum secalinum, Hordeum muticum, continental drift.

Introduction

Hordeum capense Thunb. is a perennial, mainly inbreeding wild barley plant confined to the highlands of South Africa. For the last hundred years it was thought to be conspecific with Hordeum secalinum Schreb., a mainly out-breeding barley with a geographic range from southern Sweden and Denmark, south along the Atlantic coast to Spain, with a few scattered locations in coastal Tunisia and Sardinia (Bothmer et al. 1991). As a result of urban encroachment, H. secalinum is rare in inland Europe and becoming quite scarce in coastal regions.

These two tetraploid species (4x = 2n = 28) are strikingly similar in appearance, especially in the morphology of the spike. Both are included in Section Stenostachys (Nevski 1941), which contains eight species, all with similar spike morphology. Six of these species are distributed in Asia (Hordeum bogdanii Wilensky, Hordeum brevisubulatum (Trin.) Link, Hordeum nevskianum (Bowden), Hordeum roshevitzii Bowden, Hordeum turkestanicum Nevski, Hordeum violaceum Boiss. and Hohen.), while one (Hordeum brachyantherum Nevski) is found in North America and eastern Asia (Aleutian Islands to Kamtchatka). Since H. capense and H. secalinum were thought to be conspecific because of their similarity, Stapf (1900) proposed that H. capense was "probably introduced in the Cape Colony" from Europe. In his worldwide taxonomic revision of Hordeum, Nevski (1941) followed Stapf in keeping it conspecific...