Unifoliolate Leaves Are Compound in Neotropical Rutaceae: Novel Evidence from Lateral Leaflet Traces

Unifoliolate compound leaves have a single leaflet and are common in Galipeinae, the most diverse lineage of Neotropical Rutaceae. In Galipeine, the apical end of the petiole is swollen (apical swollen) has been interpreted as evidence of a compound nature. Additionally, recent studies have also identified traces of lateral leaflets that undergo early abortion. Here, we analyzed leaf ontogeny and vascularization in nine genera to investigate the structural organization and possible compound origin of unifoliolate leaves in Galipeinae. Shoot apices and mature leaves were examined using light microscopy and scanning electron microscopy, and selected species were examined using micro-computed tomography, providing a broader understanding of the vascular architecture in the apical swelling. Two vascularization patterns were identified at the apical swollen: (Type 1) vascular plexus formed by traces of early aborted lateral leaflets (Erythrochiton brasiliensis and Conchocarpus macrophyllus and others species); (Type 2) a closed vascular (C. ruber, C. fontanesianus and C. albiflorus). The leaflet traces present in type 1 provide novel anatomical evidence of an ancestral compound condition, whereas type 2 suggests simple leaf origins. Our results indicate that swelling apical alone may not be a definitive criterion for classifying leaves as a unifoliolate compound in Galipeinae.