Evolution of the basic chromosome number in Poaceae Barnh.

Abstract

Poaceae Barnh. is a large angiosperm taxon. It includes over 10000 species, belonging to 700 genera and 41 tribes. Basic chromosome numbers (BCNs) in species of 372 genera from 37 Poaceae tribes are compared in this paper. Although BCNs broadly vary in the family (x = 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13), this trait is conservative. Species within a particular genus have, as a rule, identical BCNs. In most genera, x = 10 (31.9%) or 7 (29.5%). Twenty-five tribes show no BCN differences among their genera: x = 13 in 1 tribe; 7, in 2; 11, in 3; 10, in 4, and 12, in 5. Other tribes have several BCNs each. The Poeae tribe is the most variable in this trait: x = 2, 3, 4, 5, 6, 7, 9, 10, and 13. A similarity of genetic maps in species belonging not only to different tribes but even to different Poaceae subfamilies has been established by using Restriction Fabric Length Polymorphism (RFLP) and Expressed Sequence Tag-Simple Sequence Repeat (EST-SSR) markers. There is also experimental evidence that the diversity in BCN in the Poaceae family was caused by repeated hybridization followed by polyploidization and dysploidization of ancestral species with few chromosomes. Studies of chromosome numbers and molecular mapping of Poaceae species with RFLP and EST-SSR markers indicate that the evolutionary transformations of haploid genomes and, as a consequence, their BCNs, via polyploidization and dysploidization were completed in general in ancestral species of the presently existing genera. The present BCN diversity in the family stems from the fact that the ancestors of different genera underwent different degrees of polyploidization and dysploidization. Phylogenetically, species of different genera should be regarded as parallel series descending from crosses of related species with few chromosomes rather than descendants of a single ancestor with a polyploid genome. It is suggested that further transformation of haploid genomes in ancestors by polyploidization and dysploidization was arrested by the transition of Poaceae to the stable sporophytic phase of the life cycle. Probably, the main factor responsible for this arrest was a significant modification of the cell nucleus architectonics associated with the transition to the stable diplophase.