Two pathways to fleshiness: fruit ontogeny in Bromelioideae species (Bromeliaceae Juss.)

Capsule-type fruits are predominant in Bromeliaceae. However, the late divergent subfamily Bromelioideae generally presents fleshy fruits, which display a diverse range of morphologies even in related genera; however, there is almost no information regarding their anatomical and ontogenetic aspects. In this study we evaluate the fruit ontogeny of species from the major lineages of Bromelioideae, to better understand the possible paths in which fleshiness can be achieved in the subfamily. Flowers at pre-anthesis and anthesis, and fruits of Acanthostachys strobilacea, Billbergia pyramidalis, and Bromelia antiacantha were collected and processed with standard methods for analysis under light microscopy. Two basic ontogenetic pathways for achieving fleshiness were identified among the examined species. In the two former species, belonging to the core Bromelioideae, the pericarp development occurs solely through cell expansion and anticlinal cell division in the mesocarp. Conversely, in B. antiacantha, belonging to the tankless lineages, the pericarp development proceeds via both periclinal and anticlinal cell divisions of the outer and inner mesocarp. The analysis of fruit anatomy in the species included in this study highlights unprecedented traits of Bromelioideae fruits, such as the presence of the pericarp´s massive extra-carpellary portion in B. antiacantha, which contains many mucilage channels and fibrous tissue, and an intrusive septum in the carpellary portion, which acts as the main nutritive portion of the fruit. Additionally, in the present study, unique features of the ovary and the fruit, essential for characterizing the species were detailed, including the elongated placental tissue and the aliform projections of A. strobilacea. Our observations highlight distinct developmental processes to fleshy fruit formation in Bromelioideae. In this way, exploring the fruit ontogenetic diversity in Bromelioideae offers an exciting opportunity for future investigations. Besides, the unique fruit features identified bring out the importance of fruits as tools for genus delineation within this subfamily.