Monophyly of Machaerium and the sinking of Steinbachiella : a molecular phylogeny of the Jacarandá papilionoid legumes

Understanding the evolutionary history of taxonomically complex plant genera remains a central challenge in systematics, especially in species-rich tropical lineages shaped by intricate patterns of morphological variation and biogeographical diversification. Machaerium is one such lineage from among the largest woody genera within neotropical Papilionoideae (Leguminosae). It comprises nearly 130 species predominantly distributed across the American tropics, and showing exceptional diversity in Brazil, particularly in the rainforest biome of Amazonia and the Atlantic Forest, as well as in the seasonally dry vegetation of the Caatinga and the fire-prone savannas of the Cerrado. Molecular phylogenetic assessments have placed Machaerium within the pantropical Dalbergioid clade. However, despite these advances, infrageneric relationships within Machaerium are still poorly resolved. In this study, we address this gap by conducting maximum likelihood and Bayesian phylogenetic analyses of Machaerium using both plastid matK and nuclear ITS 5.8S DNA markers. Although our Sanger-based sequencing approach did not fully resolve the evolutionary relationships within Machaerium, it unequivocally supported the monophyly of the genus with the inclusion of Steinbachiella, which is deeply nested within Machaerium. Considering this well-supported phylogenetic placement, we propose the necessary nomenclatural changes to reflect the revised circumscription of the genus. A robust sister-group relationship between Machaerium and Ctenodon was consistently recovered in all combined analyses, further reinforcing their close evolutionary affinity within the Dalbergioid clade. The results of our study do not support the monophyly of all traditionally recognized sections (Machaerium sect. Machaerium, M. sect. Acutifolia, M. sect. Lineata, M. sect. Oblonga, and M. sect. Reticulata). However, despite the lack of strong infrageneric structure, we identified diagnostic morphological characters for a subset of the species previously assigned to sect. Lineata, such as the presence of spinescent stipules and a cladodromous pattern of secondary venation in the leaflets. Our findings underscore the need for a reassessment of infrageneric classification in Machaerium using high-throughput genomic sequencing integrated with morphological traits to advance the systematics and understanding of the evolutionary history in this ecologically diverse neotropical genus.