Mosaic heterochrony reveals developmental evidence for species delimitation between Ageratum conyzoides L. and Conoclinium coelestinum (L.) DC. (Eupatorieae, Asteraceae)

Background: Morphological convergence within Eupatorieae (Asteraceae) complicates species delimitation, particularly between Ageratum conyzoides L. and Conoclinium coelestinum (L.) DC., whose mature morphological characters often overlap. We investigated whether developmental anatomy provides reliable taxonomic evidence beyond mature morphology.Methods: Plants collected from ecologically distinct populations were cultivated under standardized conditions and monitored throughout vegetative and reproductive development. Developmental anatomy, quantitative morphometrics, trichome density, and phytochemical profiles were compared using light microscopy and statistical analyses, with developmental trajectories interpreted within a heterochrony framework.Results: Early developmental stages were largely conserved between species, indicating a shared developmental foundation. Divergence emerged during later ontogeny through trait-specific heterochronic shifts. Conoclinium coelestinum exhibited delayed but greater internode elongation and longer roots at maturity, whereas Ageratum conyzoides showed accelerated trichome development and earlier xylem lignification. Ten reproducible developmental diagnostic characters were identified. These developmental differences were complemented by distinct phytochemical profiles, with 9 and 22 volatile compounds detected in A. conyzoides and C. coelestinum, respectively.Conclusions: Developmental anatomy provides robust evidence for distinguishing these morphologically similar taxa. Integrating ontogenetic, anatomical, morphometric, and phytochemical data supports the recognition of A. conyzoides and C. coelestinum as distinct species and demonstrates the value of developmental characters in resolving species boundaries within Eupatorieae.