Phylogenetic relationships among the polyploid and diploid Aegilops species inferred from the nuclear 5S rDNA sequences (Poaceae: Triticeae)

Phylogenetic inferences of the polyploid Aegilops taxa were drawn based upon the analysis of 909 nuclear 5S rDNA sequences obtained from 15 Aegilops polyploid taxa (531 sequences new to this paper) and 378 sequences from our previous study on the diploid taxa. The 531 sequences can be split into two orthologous groups (unit classes), the long AE1 and short AE1 previously identified in the diploid set. An examination of the relationships between unit classes and their associated haplomes suggests that U haplome sequences found in Ae. umbellulata are the closest to the T sequences found in Amblyopyrum muticum and that sequences of the polyploid species expected to be the M type found in Ae. comos are more similar to the T haplome sequences, except in the three hexaploids Ae. glumiaristata, Ae. juvenalis, and Ae. vavilovii and the tetraploid Ae. crassa where they are found to be similar to the M haplome sequences. These three hexaploid taxa likely originated from the tetraploid Ae. crassa (DM), while the closest taxon to the fourth hexaploid, Ae. recta, is the tetraploid Ae. neglecta (UM). Based upon the distribution of the unit classes, several reticulate phylogenies depicting evolutionary relationships among diploid, tetraploid, and hexaploid taxa were constructed; however, none of these widely used methods could depict the expected reticulate relationship as previously drawn from cytogenetic analyses in this group of allopolyploid species. These results suggest that evolutionary relationships derived from models based upon the assumption of bifurcating species require careful interpretation when these same models are applied to species with reticulate evolution.