Seedling diversity, development and evolution in Bromeliaceae Juss. (Poales)

Understanding post-seminal development is essential for species conservation and provides key insights into reproductive biology and macroevolution. Despite their remarkable structural diversity among monocots, seedlings remain an understudied developmental stage, although they are critical for juvenile sporophyte survival and establishment. In Bromeliaceae, one of the major adaptive radiations of monocots, knowledge of seedling development remains fragmented and taxonomically biased. This study comparatively characterizes the ontogeny and morphoanatomy of bromeliad seedlings, providing the first broad phylogenetic assessment for the family.

A total of 33 species from 16 genera representing seven subfamilies were analyzed, including Brocchinia, the sister genus to all other bromeliads, as well as rare, endemic, and threatened lineages, such as Hechtioideae and Navioideae, that had never been sampled before. Post-seminal development was monitored using light and scanning electron microscopy. Characters were optimized on a consensus supertree using maximum parsimony and maximum likelihood.

We described and compared 31 informative morphological, micromorphological, and anatomical characters of the eophylls, cotyledons, and roots. Our analyses indicate that a compact, early-photosynthetic cotyledon and peltate trichomes are plesiomorphic features of the family. In contrast, a laminar cotyledonary hypophyll and uniseriate glandular trichomes represent derived conditions shared by most lineages. Anatomical features, including the presence of a water-storage hypodermis, stomatal position and variation in sclerenchyma associated with vascular bundles, also show taxonomic and ecological significance.

Through a phylogenetically representative analysis of seedling ontogeny, anatomy, and micromorphology, including previously unsampled taxa, we provide the first evolutionary assessment of this life stage across major lineages of Bromeliaceae. Our findings establish a novel comparative framework for future investigations of seedling development and evolution in Bromeliaceae and other tropical monocots, while expanding current understanding of the evolution of adaptive strategies associated with vegetative organs in the family.