Intragenomic and Interspecific Polymorphism of 35S rDNA Internal Transcribed Spacer 1 (ITS1) in Selenicereus and Related Species (Cactaceae: Cactoideae: Phyllocacteae)

Locus-specific Illumina NGS was used to analyze the polymorphism of the partial 18S rDNA, spacer ITS1, and partial 5.8S rDNA sequences across 25 cactus species (53 accessions) of the subtribes Hylocereinae and Echinocereinae. To ensure data reliability, only ribotypes (zero-radius operational taxonomic units, ZOTUs) with a minimum read depth of 10 reads within each genome-read pool were retained. A single ribotype was identified in 18 analyzed genomes. In 23 accessions, two or three ribotypes were detected, typically consisting of a major ribotype (98–99%) and one or two minor ribotypes. In the remaining samples, two distinct ribotypes were present in approximately equal proportions, suggesting that these individuals are recent hybrids. Ribotype patterns clearly distinguish the genus Deamia from Selenicereus, Hylocereus, and Epiphyllum. Putative pseudogenes with deletions in the 18S and 5.8S rDNA, as well as deletions and insertions in the ITS1 spacers, were detected. Their abundance was generally low (usually <1%), except in two Deamia species (5–15%). In ITS1 of some minor ribotypes, deletions eliminated cleavage sites B1, B2, and A3, which are important for pre-rRNA processing. Consequently, the products of these putative pseudogenes could not participate in ribosome formation. In addition, pseudogenic copies exhibited a strong transition bias and A/T enrichment in the ITS1 region.