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. They have therefore been the subject of early studies of genome size as a fundamental biological trait of any organism. 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. 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. 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 (1.2–1.3 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.

Reductional dysploidy and genome size diversity in Pooideae, the largest subfamily of grasses (Poaceae)

“…Further insights into the genome organization and synteny in the chromosomes of Littledaleeae/Bromeae/Titiceae and Aveneae/Festuceae/Poeae would be of great value for understanding the evolutionary origin and development of these important grass groups.If the genome size of Brachypodieae of 0.3-0.5 pg/1Cx (Table2), which is similar to, or in some cases even smaller than that of rice (Oryza sativa; subf. Oryzoideae) at 0.5 pg/1Cx(Tkach et al 2024) could therefore be based on an autapomorphic genome miniaturization and does not represent the ancestral state of both Brachypodieae and Poeae, our data on the monoploid genome sizes (1Cx) of 'core Pooideae' are ambiguous with respect to a possible whole genome duplication at the origin of this lineage. Although 1Cx values < 1 pg are virtually absent and those < 2 pg are comparatively rare in the 'core Pooideae', is in contrast to most of the tribes of the 'early-diverging' lineages with the exception of Lygeeae and Meliceae, the overall predominantly rather small monoploid genome sizes (1Cx), especially of the tribe Poeae (Fig.2, middle column) do not support the hypothesis of a whole-genome duplication (WGD) for the ancestor of the 'core Pooideae', whereas such a WGD could not be excluded if only the mostly higher 1Cx values of Aveneae, Festuceae and especially Triticeae were considered.…”

“…the subfamilies Anomochlooideae and Pharoideae, which have monoploid genome sizes of about 1.2-1.3 pg and MCs of about 0.1 pg(Tkach et al 2024), while no genome size data are available for the subfamily Puelioideae, the next-diverging lineage in the phylogeny. Some of these 'early-diverging lineages' are also characterized by x = 12 (Pharus, Puelia), while the Anomochlooideae genus Streptochaeta has x = 11 and Anomochloa (2n = 36) may have x = 9 (seeTkach et al 2024).…”