Chromosome-number diversity and evolution in Nitrariaceae: a critical synthesis of cytological, genome-size and genomic evidence

Joan Simon Pallise , Maria Bosch Daniel

Chromosome-number data in Nitrariaceae are dispersed across historical cytological reports, regional compilations and recent flow-cytometric and genomic studies, and their biological interpretation is complicated by uneven sampling, taxonomic change and somatic chromosome-number variation. We critically synthesised chromosome-related evidence for the family using ChromoDB as the principal documentary dataset and Plants of the World Online as the contemporary taxonomic framework. The reviewed dataset comprised 218 records, more than 99% of which belonged to Nitraria and Peganum. A recurrent 12-based chromosome organisation emerged as the best-supported operational framework in both genera, with independently supported diploid and tetraploid states centred on 2n = 24 and 2n = 48. Genuine organismal polyploidy was demonstrated by concordant chromosome counts with flow-cytometric genome-size estimates in Nitraria and by regular meiotic behaviour in Peganum harmala. In contrast, several Nitraria species showed extensive somatic chromosome-number heterogeneity, including documented intraindividual mixoploidy; in other cases, multiple chromosome complements were associated with the same accession or voucher but the exact biological scale remained unresolved. Such additional chromosome classes were therefore not interpreted automatically as independent cytotypes. Genome-size data further showed that chromosome number and nuclear DNA content are only partially coupled. Discordant observations, including n = 6 and historical 2n = 22 in P. harmala, 2n = 18 in Nitraria retusa and an attributed 2n = 14 in Tetradiclis tenella, remain important but do not justify alternative basic numbers or specific dysploid pathways by arithmetic alone. The synthesis also shows that taxonomic reassignment and database structure can substantially alter the apparent chromosome spectrum of a species. We conclude that x = 12 is a robust operational framework for extant Nitraria and Peganum, but its ancestral status within Nitrariaceae remains unresolved. Future progress requires voucher-based, biologically resolved cytology integrated with meiosis, flow cytometry and chromosome-scale comparative genomics.