Background and Aims Myxospermy (seed mucilage extrusion upon hydration) is widespread, yet its evolutionary integration with seed structure and ecological significance in xeric environments remain poorly understood. Using Helianthemum (Cistaceae), which spans broad climatic gradients, we analysed seed-coat anatomy and quantified seed trait variation within a Structure–Form–Function framework to identify integrated seed syndromes, reconstruct their evolutionary history, and test macroclimatic associations. Methods We quantified mucilage biochemical composition and 18 continuous variables describing seed-coat microstructure, geometry, and hydration performance across 82 taxa (75% of the genus) using microscopy, staining, and image analysis. Multivariate approaches identified integrated seed syndromes. Their evolution was reconstructed on a time-calibrated phylogenomic tree using stochastic character mapping and transition-rate models. Phylogenetically informed models tested associations between seed phenotypes and climatic gradients. Key results Three integrated syndromes emerged: the ancestral Pectic Simple Syndrome (PSS), and two independently derived complex types—the Pectic Hydrating Syndrome (PHS) and Cellulosic Expansive Syndrome (CES). They differ in mucilage composition, seed-coat architecture, and geometry: PSS provides a non-expanding baseline, whereas the derived complex syndromes maximize either volumetric water retention (PHS) or spatial soil anchorage (CES). Evolutionary transitions were dynamic and frequently reversible. Furthermore, while these discrete syndromes showed weak climatic segregation, the underlying continuous traits—seed size, sphericity, and hydration capacity—were strongly structured by aridity and thermal variability gradients. Conclusions Myxospermy in Helianthemum constitutes a highly integrated seed syndrome shaped by coordinated structural and functional evolution. Rather than being deterministic key innovations, specialized mucilage architectures are evolutionarily labile, reflecting environmental filtering acting on integrated phenotypes. Ancestral mucilage production likely functioned as an exaptation, facilitating ecological sorting across heterogeneous drylands.
Julia et al. (Thu,) studied this question.