Chromosome numbers, DNA content, morphological data, and nrITS sequence analyses in some species of Nassella (Trin.) E. Desv. and related genera (Stipeae, Poaceae)

Nassella is one of the largest genera in the tribe Stipeae with approximately 116 species, distributed from Canada to Uruguay and Argentina. Although there are strong morphological characters that distinguish this genus from the rest of the tribe, molecular phylogenetic analyses have shown that it is a polyphyletic genus closely related to Jarava and Amelichloa. Species boundaries for many Nassella taxa are somewhat diffuse and controversial, mainly due to morphological similarities with just a few morphological characters that differentiate one from the other. Previous chromosome counts in Nassella have shown the existence of a high chromosome number variability ranging from 28 to 60 suggesting that polyploidy might have played an important role in the evolution of the genus. The aim of this study was to assess the relationship between chromosome number variation and genome size focusing in 15 species of Nassella distributed in Uruguay and to establish the relationship among N. megapotamia (Spreng. ex Trin.) Barkworth, N. pauciciliata (Roseng. & Izag.) Barkworth, N. poeppigiana (Trin. & Rupr.) Barkworth and N. quinqueciliata (Roseng. & Izag.) Barkworth & Torres using cytogenetic, morphological, and molecular phylogenetic data. Chromosome numbers ranged from 2n = 26 to 2n = 46 confirming the high chromosome number variability reported for the tribe. This is the first record of nuclear DNA content for the genus and 2C-values range from 1.48 to 2.31 pg representing a 1.5-fold difference. There is no direct correspondence between increase in chromosome number and DNA content. This lack of correspondence could be due to DNA loss after polyploidization, or related to chromosomal rearrangements. Four Nassella species exhibited high cytological and morphological homogeneity. Our data based on chromosome number, molecular phylogenetic analysis, and the morphological approach including both optical and scanning electron microscopy strongly support that these four Nassella species (N. megapotamia, N. pauciciliata, N. poeppigiana, N. quinqueciliata) form a complex of species whose boundaries should be revised.