Unveiling the numbers: current state of knowledge of chromosome number counts in big plant genera.

Background: The plant nuclear genome is organized into discrete chromosomes, whose count is amongst the primary information for the description of an organism and our understanding of its differentiation and adaptation. Whilst species diversity is not evenly distributed across flowering plant genera, it is unknown whether the extraordinary taxonomic diversity exhibited by 86 big genera, collectively accounting for a quarter of all currently accepted angiosperm species, is supported by adequate chromosome number information.

Scope: Leveraging data from the Chromosome Count Database (CCDB), we assessed current knowledge of chromosome counts in big plant genera. We do not have any assumption concerning the overall amount of chromosome data collectively available for these genera, but we expected that current knowledge is not complete and it has been shaped by main historical events influencing cytogenetics and comparative genomics during the last century. We also hypothesized that chromosome counts are limited for those genera exhibiting higher taxonomic uncertainty and geographical or biological features that hamper plant sampling and reproduction.

Conclusions: Overall, the proportion of species with at least one chromosome count is low (14%) with temporal trend revealing an unequivocal decline in counts acquisition after the 90s, coinciding with advances in DNA amplification and sequencing and molecular phylogenetics. In addition, logistic regressions confirmed our expectations that the proportion of species with chromosome count per genus is lower for genera with a tropical distribution, being primarily trees or exhibiting specialised pollination to complete sexual reproduction. Contrary to our expectations, taxonomic uncertainty is strongly positively associated with the availability of chromosome counts per genus. We argue that a higher taxonomic uncertainty has promoted more sampling to aid the circumscription of plant species, highlighting the crucial role of chromosome variation in identifying genomic traits that have supported the diversification of big plant genera.