Nutlet morphology in the genus Lappula (Boraginaceae) and implications for systematics

Abstract Nutlets exhibit the greatest morphological diversity within Lappula and have long served as a key criterion for classification within the genus. However, recent studies have pointed out that overemphasis on nutlet marginal glochids or wing features often leads to artificial infrageneric groupings and ambiguous species delimitations. These limitations highlight the need to reassess the taxonomic utility of nutlet traits through a more comprehensive approach. In this study, we examined 622 nutlets from 41 populations representing 30 Lappula taxa and conducted multivariate analyses of nine quantitative nutlet traits. Additionally, scanning electron microscopy (SEM) was employed to investigate nutlet surface micromorphology across 31 taxa. Our results reveal the widespread occurrence of heteromorphic nutlets within Lappula, newly documenting this feature in seven additional taxa. Statistical analyses identified three distinct phenotypic clusters based on quantitative traits, with nutlet length, width, thickness, and their derived ratios proving particularly informative for taxonomic delimitation. SEM observations identified four micromorphological types of nutlet surfaces, each showing high consistency at the species level. Both phenotypic clusters and micromorphological types exhibit strong phylogenetic signals, providing valuable systematic insights into evolutionary relationships within Lappula. This study significantly advances our understanding of nutlet morphological diversity and provides a critical foundation for future taxonomic revisions of the genus.