Karyotype diversity analysis and nuclear genome size estimation for Pennisetum Rich. (Poaceae) ornamental grasses reveal genetic relationship and chromosomal evolution

•Genetic backgrounds and relationships of nine Pennisetum ornamental grasses were revealed.•Chromosomal and genomic characteristics of four Pennisetum cultivars were first reported.•Traditional classification system of Pennisetum genus was assessed both on morphological and cytological levels.•Karyotypic and genomic differences were observed among taxa with different basic chromosome numbers.•Chromosomal evolution of Pennisetum genus from a symmetrical to an asymmetrical order was affirmed.

So far no successful artificial hybridizations among ornamental grasses in the Pennisetum Rich. (Poaceae) genus that are widely applied in landscape design have been reported because of unclear genetic backgrounds and uncertain evolutionary and genetic relationships among one another, severely restricting the number of ornamental cultivars and their gardening utility. The present study aims to reveal genetic backgrounds and relationships for nine Pennisetum ornamental grasses via karyotype diversity analysis; discuss the morphological classification system proposed in previous studies on morphological, chromosomal and genomic levels; and verify the chromosomal evolution of Pennisetum genus using the combined karyotypic and genomic data. Our results indicated that (1) traditional morphological classification system of Pennisetum genus is reasonable, both from the morphological and cytological points of view; (2) hybridization among Pennisetum alopecuroides, P. alopecuroides ‘Little Bunny’, P. alopecuroides ‘Moudry’ and P. orientale may be easier to obtain a hit; (3) chromosomes with lower basic chromosome number are more asymmetry than those chromosomes with higher basic chromosome number, and taxa with different basic chromosome numbers may have experienced different way of speciation; and (4) chromosomal evolution in Pennisetum genus progressed from a symmetrical to an asymmetrical order in the course of evolution; and the structure of the genome of Pennisetum species involved in the reduction of chromosome number and an increase in their sizes and DNA contents. The present study guides the breeding programs and genome projects of Pennisetum ornamental grasses practically, and provides references for the evolutionary studies on Pennisetum genus theoretically.