Antagonistic effects of whole-genome duplications and dysploidy on genome sizes in the pantropical monocot family Marantaceae: Consequences in the light of a new molecular phylogeny

Changes in chromosome number and structure, as well as alterations in genome size, are important cytological characters that often reflect speciation events. Due to a wide variety of previously reported chromosome numbers the species-rich arrowroot family (Marantaceae) is an ideal system to investigate the role of chromosomal changes in species diversification, especially of dysploidy and polyploidy. The mechanisms and direction of changes during evolution as well as ancestral states of chromosomal characters are largely unknown. This study provides a detailed survey of chromosomal and genome size variation in 43 Marantaceae accessions (37 species, 16 genera) from the whole distribution range. Phylogenetic tree mapping suggests that x = 13 is the ancestral basic chromosome number. Descending dysploidy to x = 9, 10, 11, and 12 occurred ten times in parallel within each of the five main clades. In contrast, ascending dysploidy to x = 14, occurred only once. Ploidy level variation was confined to basic numbers x = 9, 11, and 13 and was observed in four out of the five clades. The occurrence of triploids and pentaploids points towards heteroploid diploid-tetraploid and tetraploid-hexaploid hybridizations. Trends in genome evolution as exposed by regression analyses revealed a massive genome size increase after dysploidy - and a genome size decrease after polyploidy. A schematic illustration of possible origins and the modes of chromosomal changes in a biogeographical context is presented. Exemplified by the family Marantaceae, it is shown how variable chromosomal evolution can be and how it contributes to species richness and speciation in the plant kingdom.