Cytogenetic data remain limited for many lineages of Cactaceae, constraining our understanding of chromosome evolution in this diverse family. Here, we combine newly generated cytogenetic and genome size data for Armatocereus mataranus subsp. ancashensis with a comparative dataset of ribosomal DNA (rDNA) organization across Cactaceae to examine patterns of locus number and chromosomal distribution. rDNA loci were analysed usin...