Botanists interested in changes in chromosome number have often relied on inferring a basic number, x, but this has been done without consistent (reproducible) reference to species’ relationships and frequencies of particular numbers in a group. Cusimano et al. reconstruct ancestral numbers in the large monocot family Araceae using likelihood models that treat polyploidy, chromosome fusion and chromosome fission as events with particular probabilities. The results show that chromosome fusion is common, whereas polyploidization is mostly restricted to recent Araceae groups, and suggest that the algebraic approach to the calculation of x may not be reliable, at least when applied to large clades.
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