Ultrastructural study of pollen and tapetum development in Hydrocleys nymphoides, Alisma plantago-aquatica, and Sagittaria montevidensis (Alismataceae).

The Alismataceae family, widely distributed across tropical temperate swamps and wetlands, includes 15 genera post-merger with Limnocharitaceae. In Argentina, six genera are represented across three clades. Embryological characters, notably the male gametophyte and anther, are crucial in taxonomy due to their stability against environmental changes. This study aims to analyze the ultrastructure of the tapetum and pollen grain development in three economically and ecologically important species representing each clade: Sagittaria montevidensis (Clade A), Hydrocleys nymphoides (Clade B), and Alisma plantago-aquatica (Clade C). Anthers at different developmental stages were processed according to classic techniques for their observation with bright-field and transmission electron microscopy. The three studied species within the Alismataceae family exhibit similar reproductive characteristics. Seven stages of pollen grain development were identified. The microsporogenesis is successive with a regular meiosis. The ultrastructure of the tapetal cells shows similarities to other species with plasmodial tapetum. During the microspore tetrad stage, there is tapetal hyperactivity and an increase in secretion processes. In the free microspore stage, the tapetal cells lose their walls and increase the amount of rough endoplasmic reticulum forming a network of cisternae that extend into evaginations. Later cells completely invade the anther locule and fuse to form a tapetal plasmodium. No peritapetal membrane with orbicules was observed. Pollen is released at the tricellular stage. The pollen grain wall presents an ectexine with a basal layer, columellae, and tectum with supratectal spines while an endexine is not observed in any of the three species. This research enhances the understanding of tapetal cell interactions with developing pollen grains and contributes to the knowledge of the ultrastructure of plasmodial tapetum. Moreover, these findings highlight evolutionary reproductive patterns in Alismataceae, suggesting the plasmodial tapetum as a synapomorphy for the order.