With the aim of correlating the pericarp structure with current phylogenies of Myrteae, this study describes the ontogeny in five species included in five out of the six South American clades of the tribe. In these taxa, the outer and inner ovarian epidermis gives rise to the exocarp and the endocarp, respectively, both with 1 layer. In the mesocarp, derived from the ovarian mesophyll, secretory cavities are arranged into a circle just below the exocarp and near the endocarp in Campomanesia adamantium; only below the exocarp in Eugenia pitanga and Myrcia multiflora; more internally in Myrciaria cuspidata, and below the exocarp and throughout the mesophyll in Myrceugenia alpigena. The promising traits for phylogenetic studies in the group include: direction of elongation of pericarp layers, regions that develop most in relation to the circle of larger vascular bundles, differentiation of spongy and sclerenchymatous tissues and position of secretory cavities.

The subtribe Eugeniinae comprises of two genera, Eugenia (ca. 1,100 species) and Myrcianthes (ca. 40 species). Eugenia is the largest genus of neotropical Myrtaceae and its latest classification proposes 11 sections. This study describes the seed anatomy of forty-one species of Eugeniinae in order to provide possible diagnostic characteristics. Following standard anatomical techniques, flower buds, flowers, and fruits were processed and analyzed using microtome sections and light microscopy. The phylogeny used the regions ITS, rpl16, psbA-trnH, trnL-rpl32, and trnQ-rps16, following recent studies in the group. Ancestral character reconstruction uncovered that: (1) the ancestral ovule in Eugeniinae was campylotropous (98.9% probability), bitegmic (98.5% probability), and unitegmic ovules arose on more than one lineage independently within Eugenia; (2) the pachychalazal seed-coat appeared with a 92% probability of being the ancestral type; (3) non-lignified seed-coat (24,5% probability) and aerenchymatous mesotesta (45.8% probability) are diagnostic characters in Myrcianthes pungens (aerenchymatous mesotesta present in the developing seed-coat) and in the species of E. sect. Pseudeugenia until the species of E. sect. Schizocalomyrtus and it is the type of seed-coat that predominates in most basal sections on the tree; (4) the partial sclerification (only in the exotesta—exotestal seed-coat) is mainly observed in species of E. sect. Excelsae, E. sect. Jossinia (group X), and E. sect. Racemosae (22.2% probability); (5) and in the species of the recent lineages of Eugenia, with a probability of 27.2%, predominate the exomesotestal or testal construction of the seed-coat [character observed in almost all species analyzed of E. sect. Jossinia (group Y) and E. sect. Umbellatae]. A dehiscent fruit is considered as a plesiomorphic state in Myrtaceae; the ancestor of this family had seeds with a completely sclerified testa, and the other testa types described for the current species with dehiscent and indehiscent fruits are simplified versions of this ancestral type. Perhaps, this means that the sclerified layers in the seed-coat have remained in whole or in part as a plesiomorphic condition for taxa with a capsule and bacca. Maintaining the plesiomorphic condition may have represented a selective advantage at some point in the evolutionary history of the family and its groups.