Chromosome number overview in <i>Habenaria</i> (Habenariinae, Orchideae, Orchidaceae): a new smallest diploid number and insight on chromosome number evolution in a karyotypically variable genus

Abstract Orchids exhibit remarkable karyotypic variation, with chromosome numbers varying up to 20-fold across species. Although karyotype evolution plays an important role in plant evolution, Neotropical orchids, especially terrestrial taxa, remain poorly studied from a cytogenetic perspective. Despite the limited available data, Habenaria emerges as one of the most karyotypically variable genera. In this study we determined the chromosome number for nine Habenaria species and conducted a chromosome number survey to assess the role of chromosome number changes in the evolution of Habenariinae. Our findings expand the known chromosome number variation by reporting a new lowest chromosome number, 2n = 22, in H. aranifera and H. heptadactyla from Brazil. Considering the survey and our new data, chromosome numbers are now reported for 72 species, with only 35% of them being from Neotropical regions. We identified 22 distinct chromosome numbers, with 2n = 42 being the most prevalent, accounting for 50% of records and 66.2% of species. Descending dysploidy appears to be the dominant mechanism shaping karyotype evolution. However, polyploidy also plays a significant role, distinguishing Asian and African taxa (which show evidence of older polyploidization events with a base number x = 21) from the Neotropical clade (with more recent polyploidy events and a base number x = 10), which presents recurrent polyploidization events after the founder event from African to Neotropical regions. Our macroevolutionary analysis of chromosome number and ploidy levels supports the Ornstein–Uhlenbeck (OU) model as the best-fitting, suggesting that both traits are under stabilizing selective pressure and tend to evolve toward an optimal value. Tempo and mode of evolution suggested that both traits could be associated with speciation events, underscoring the crucial role of chromosomal evolution in orchids. The more ancient polyploidization events in Asian and African taxa and recent events in the Neotropical clade illustrate the important role of polyploidy and the founder event, which appear to be associated in Habenariinae.