Phylogenetics and reticulation among koelerioid clades, part I: Contraction of Trisetum, expansion of Acrospelion, Graphephorum, and Tzveleviochloa; Graciliotrisetum gen. nov. and resurrection of Aegialina (Poaceae, Pooideae, Poeae, Aveninae)

Koelerioid grasses (subtribe Aveninae, tribe Poeae; Pooideae) resolve into two major clades, here called Koelerioid Clades A and B. Phylogenetic relationships among koelerioid grasses are investigated using plastid DNA sequences of <italic>rpl32‐trnL, rps16‐trnK, rps16</italic> intron, and ITS regions, focusing on <italic>Trisetum</italic>, <italic>Acrospelion</italic>, and some annual species (<italic>Rostraria</italic> p.p. and <italic>Trisetaria</italic> p.p.) closely related to <italic>Trisetum flavescens</italic> in Koelerioid Clade A. Phylogenetic analyses of several selected data sets performed for 80 taxa and using maximum likelihood and Bayesian methods, revealed mostly congruent topologies in the nuclear and plastid trees, but also reticulation affecting several lineages. <italic>Trisetum</italic> is restricted to one species, <italic>T. flavescens</italic>, which is a sister to the clade formed by <italic>Trisetum gracile</italic> and <italic>Trisetaria aurea</italic>. The latter two species are classified here in the genus <italic>Graciliotrisetum</italic> gen. nov. The sister clade includes three species of <italic>Rostraria</italic> and <italic>Trisetaria lapalmae</italic>, all of which are classified here in a resurrected genus, <italic>Aegialina</italic>, which includes four species. <italic>Acrospelion</italic> is enlarged to include 13 species after the addition of other species formerly classified in <italic>Trisetum</italic> sect. <italic>Trisetum</italic> and <italic>T</italic>. sect. <italic>Acrospelion</italic>. We also transfer <italic>Trisetum ambiguum</italic>, <italic>Trisetum longiglume</italic>, and <italic>Koeleria mendocinensis</italic> to <italic>Graphephorum</italic>; and <italic>Helictotrichon delavayi</italic> to <italic>Tzveleviochloa</italic>, expanding these genera to eight and six species, respectively. We evaluate cases of reticulate evolution between Koelerioid Clades A and B and within Koelerioid Clade A, which probably gave rise to <italic>Graphephorum</italic>, <italic>Rostraria cristata</italic>, and <italic>Rostraria obtusiflora</italic>. Finally, we comment on polyploidy and biogeographic patterns in koelerioid grasses. We propose the following 26 new combinations: <italic>Acrospelion alpestre, Acrospelion altaicum, Acrospelion argenteum, Acrospelion bertolonii, Acrospelion buschianum, Acrospelion buschianum</italic> subsp. <italic>transcaucasicum, Acrospelion fuscum, Acrospelion laconicum, Acrospelion macrotrichum, Acrospelion rigidum, Acrospelion rigidum</italic> subsp. <italic>teberdense, Acrospelion tamonanteae, Acrospelion velutinum, Aegialina lapalmae, Aegialina pubescens, Aegialina pumila, Aegialina pumila</italic> subsp. <italic>fuscescens, Aegialina salzmannii, Aegialina salzmannii</italic> subsp. <italic>cossoniana, Graciliotrisetum aureum, Graciliotrisetum gracile, Graphephorum ambiguum, Graphephorum longiglume, Graphephorum mendocinense, Graphephorum orthochaetum</italic>, and <italic>Tzveleviochloa delavayi.</italic> Lectotypes are designated for the names <italic>Aegialitis tenuis, Aira melicoides, Avena aspera</italic> var. <italic>parviflora, Avena delavayi, Koeleria grisebachii</italic> var. <italic>mendocinensis, Koeleria pubescens</italic> subsp. <italic>cossoniana, Koeleria pumila, Koeleria salzmannii, Phalaris pubescens, Trisetum aureum, Trisetum cernuum, Trisetum fuscescens, Trisetum longiglume</italic>, and <italic>Trisetum wolfii</italic>; and we designate one neotype for <italic>Alopecurus litoreus</italic>. [ABSTRACT FROM AUTHOR]