The Bitter and the Sweet: Inference of Homology and Evolution of Leaf Glands in Prunus (Rosaceae) through Anatomy, Micromorphology, and Ancestral-Character State Reconstruction
Distinctive and diagnostic leaf glands occur in diverse forms and positions in Prunus. However, barely any studies have been conducted to document and examine these forms. Leaf glands in Prunus species may occur as raised structures on the leaf margin, as flattened structures on the abaxial surface, or at the tips of leaf teeth. Dark punctate spots on the abaxial surface of some tropical species have been assumed to be glandular as well. Our results show that the raised glands on the leaf margin and the flat glands on the abaxial surface share similar anatomy with extrafloral nectaries, while glands on the leaf teeth are structurally similar to colleters. Leaf glands in Prunus can be divided into class 1A (raised, marginal) and class 1B (flat, laminar) extrafloral nectaries and class 2 (leaf-teeth colleters). The derived flat glands likely evolved as a result of an adaptive loss of leaf serrations in warmer climates, while the plesiomorphic glandular-teeth condition is indirectly selected for in cooler climates because of an adaptive advantage of serrated margins. The dark punctate spots are shown for the first time to be d-stomatal-type cork warts that do not perform a glandular function.
