Why six and seven are implausible ancestral angiosperm chromosome numbers

There is an ongoing shift about how best to infer ancestral chromosome numbers in plants. From 1938 to 2020, this was done by focusing on meiotic counts in extant species and using them to infer a hypothetical base chromosome number for larger clades, even when not found empirically. Since 2017, comparative-genomic studies provide a different approach that focuses on syntenic blocks in chromosome-level genome assemblies. For flowering plants, the two approaches have yielded drastically different results, namely either a base number around n = 7 or instead around 16.

Here I review how hypothetical base numbers and an assumed general evolutionary progression from lower to higher numbers-ideas that go back to benchmark papers by G. L. Stebbins-have hampered botanists' inference of plausible ancestral chromosome numbers.

Model-based studies of chromosome number evolution into the 2020 s have perpetuated some of the traditional biases by constraining numbers near or at the root of angiosperm phylogenies to be low, which may explain the discrepancies between their low inferred ancestral number (e.g., n = 7) and the higher number inferred from in silico comparison of syntenic blocks in chromosome-level genome assemblies (n = 16).

Data available at this point, including from chromosome counts in early-diverging lineages (which were not known in Stebbins's time), argue against the long-assumed general low-to-high trend for changes in chromosome number across flowering plants.