Abstract
Hordeum murinum L. is a species complex composed of related taxa, including the subspecies glaucum, murinum and leporinum. However, the phylogenetic relationships between the different taxa and their cytotypes, and the origin of the polyploid forms, remain points of controversy. The present work reports a comparative karyotype analysis of seven accessions of the H. murinum complex representing all subspecies and cytotypes. The karyotypes were determined by examining the distribution of the repetitive Triticeae DNA sequences pTa71, pTa794, pSc119.2, pAs1 and pHch950, the simple sequence repeats (SSRs) (AG)10, (AAC)5, (AAG)5, (ACT)5, (ATC)5, and (CCCTAAA)3 via in situ hybridization. The chromosomes of the three subgenomes involved in the polyploids were identified. All tetraploids of all subspecies shared the same two subgenomes (thus suggesting them to in fact belong to the same taxon), the result of hybridization between two diploid ancestors. One of the subgenomes present in all tetraploids of all subspecies was found to be very similar (though not identical) to the chromosome complement of the diploid glaucum. The hexaploid form of leporinum came about through a cross between a tetraploid and a third diploid form. Exclusively bivalent associations among homologous chromosomes were observed when analyzing pollen mother cells of tetraploid taxa. In conclusion, the present results identify all the individual chromosomes within the H. murinum complex, reveal its genome structure and phylogeny, and explain the appearance of the different cytotypes. Three cryptic species are proposed according to ploidy level that may deserve full taxonomic recognition.
Highlights
Polyploidy has been important in the evolution of the family Poaceae [1]
Since there is no single diagnostic morphological characteristic that distinguishes the three forms - which are confused - most authors refer to the murinum complex [9,10]
Material representing all three subspecies and cytotypes of the H. murinum complex was obtained from the IPK Germplasm Bank (Gatersleben, Germany)
Summary
Polyploidy has been important in the evolution of the family Poaceae [1]. It has played an important role in the diversification of the genus Hordeum, which contains diploid, tetraploid and hexaploid taxa. C-banding karyotypes and the meiotic behaviour of Hordeum hybrids suggest the existence of four basic diploid genomes [2,3,4]: H, I, Xa and Xu (following the nomenclature of Wang et al [5] and Linde-Laursen et al [6]). The species H. murinum L., which possesses the Xu-genome, is usually recognized as having three subspecies: glaucum (Steud.) Tzvelev (2n = 2x = 14), murinum (2n = 4x = 28), and leporinum (Link) Arcang. Since there is no single diagnostic morphological characteristic that distinguishes the three forms - which are confused - most authors refer to the murinum complex [9,10]
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