Genome sizes of grasses (Poaceae), chromosomal evolution, paleogenomics and the ancestral grass karyotype (AGK)

Abstract

Grasses are one of the largest angiosperm families, widespread and economically important. Variation in genome size has functional consequences and is an essential parameter for understanding evolutionary patterns. In this study, we report the nuclear genome sizes (2C values) of 32 species and subspecies from 27 genera of Poaceae, including most of its subfamilies, examined by flow cytometry. Obtained genome sizes were analyzed together with the chromosome numbers to give information on the size of monoploid chromosome sets with the chromosome base number x and the mean chromosome size and then supplemented with the previously published data to obtain a deeper insight into the genome size evolution in grasses. Monoploid genomes of < 0.6 pg/1Cx and chromosomes of < 0.1 pg are presumably characteristic of the subfamilies Arundinoideae, Chloridoideae, Micrairoideae and the Oryzoideae. The larger 1Cx values (1.2–1.8 pg) of the evolutionarily ‘early diverging’ subfamilies Anomochlooideae and Pharoideae are discussed in context with the origin of grasses and the pan-grass whole-genome duplication. The data indicate that the ancestral grass had a monoploid genome of this size, which is less than half the size previously assumed. Genome size data and available chromosome numbers support the concept of the ancestral grass karyotype (AGK) with x = 12. The AGK seems to have been conserved in some grass subfamilies (Bambusoideae, Oryzoideae, Pharoideae, parts of the Pooideae), while the major genome rearrangements are lineage-specific and occurred after the separation of the BOP and the PACMAD clades, i.e. when the diversification of their subfamilies had begun.