Phylogenetic analysis of two single-copy nuclear genes revealed origin and complex relationships of polyploid species of Hordeum in Triticeae (Poaceae)

Abstract

Two single-copy nuclear genes, the second largest subunit of RNA polymerase II (<i>RPB2</i>) and thioredoxin-like gene (<i>HTL</i>), were used to explore the phylogeny and origin of polyploid species in <i>Hordeum</i>. Our results were partly in accord with previous studies, but disclosed additional complexity. Both <i>RPB2</i> and <i>HTL</i> trees confirmed the presence of <b>Xa</b> genome in <i>H. capense</i> and <i>H. secalinum</i>, and that <i>H. depressum</i> originated from <i>H. californicum</i> together with other American diploids, either <i>H. intercedens</i> or <i>H. pusillum</i>. American diploids solely contributed to the origin of <i>H. depressum</i>. The Asian diploids, either <i>iH. bogdanii</i> or <i>H. brevisubulatum</i>, contributed to the formation of American polyploids except <i>H. depressum</i>. <i>RPB2</i> and <i>HTL</i> sequences showed that <i>H. roshevitzii</i> did not contribute to the origin of American tetraploids. Our data showed a close relationship between the hexaploids <i>H. procerum</i> and <i>H. parodii</i> and the tetraploids <i>H. brachyantherum</i>, <i>H. fuegianum</i>, <i>H. guatemalense</i>, <i>H. jubatum</i>, and <i>H. tetraploidum</i>. The involvement of the diploid <i>H. pusillum</i> and the tetraploid <i>H. jubatum</i> in the formation of <i>H. arizonicum</i> was also indicated in the <i>HTL</i> phylogeny. Our results suggested a possible gene introgression of <b>W-</b> and <b>P-</b>genome species into the tetraploid <i>H. jubatum</i> and the hexaploid <i>H. procerum</i>.