Pancentromere analysis of Allium species reveals diverse centromere positions in onion and gigantic centromeres in garlic

In eukaryotes, centromeres interact with the kinetochore for distribution of genetic information in cell division, yet their sequence and size are diverse among species. However, their position on chromosomes is considered to be conserved within a species. In this study, we analyzed the centromeres of 3 Allium species, namely, Welsh onion (Allium fistulosum), onion (Allium cepa), and garlic (Allium sativum) via pancentromere analysis and repetitive sequence analysis of centromeres and their neighborhoods and revealed their mobility, sequence organization, and size. Among the 3 species, Welsh onion and garlic had stable centromeres, but the onion centromere appeared to be polymorphic and frequently differed in position by up to 28.0 Mb among cultivars and between multiple individuals of the same cultivar. This mobility was stabilized by hybridization with Welsh onions. Furthermore, these 3 species have very different centromere sequence organization, including differences in the existence and maturity of centromeric satellites, and differences in centromere size, with Welsh onion having a centromere of 1.9 Mb, and garlic having a centromere of ∼10.6 Mb, the largest of any organism with monocentric chromosomes analyzed to date. Our pancentromere analysis of these Allium species reveals the variation in sequence organization, size, and position of this important chromosomal region.