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Abstract

Desmidorchis retrospiciens is a member of the enormous Apocynaceae family, which has several genera and species that are morphologically challenging to identify. In this study, molecular characterization of a specimen from the Al-Baha region of Saudi Arabia was performed using two plastid loci, matK and rbcL, together with the nuclear ITS2 region.The sequences were identified through NCBI BLAST searches, and their relationships with reference taxa were assessed using Kimura 2-parameter (K2P) distances and neighbour-joining and maximum-likelihood phylogenetic analyses. BLASTn analysis showed 100% identity between the specimen and Desmidorchis awdeliana and Caralluma arabica for matK (mean K2P = 0.0032), Orbea sprengeri subsp. commutata for rbcL (mean K2P = 0.0024), and Boucerosia umbellata, Caralluma adscendens, and Caralluma umbellata for ITS2 (mean K2P = 0.036). NJ and ML phylogenetic analyses consistently grouped the specimen with Desmidorchis awdeliana, Caralluma arabica, and Orbea sprengeri subsp. commutata based on matK. The specimen also grouped with Orbea sprengeri subsp. commutata and Orbea wissmannii var. eremastrum based on rbcL. It further grouped with Boucerosia umbellata, Caralluma adscendens, and Caralluma umbellata based on ITS2. Each clade showed moderate to high bootstrap support. ITS2 showed markedly higher genetic divergence than the two plastid markers, consistent with its higher substitution rate and biparental inheritance. These findings corroborate earlier reports of taxonomic overlap among Desmidorchis, Caralluma, Orbea, and Boucerosia, and highlight the need for a multi-locus approach when studying such taxonomically complex succulent groups.

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