Development of the trypanosomatid Phytomonas vermiformis in the vernal shieldbug Peribalus strictus vernalis

Flagellates of the genus Phytomonas (Trypanosomatidae) are plant parasites, several species of which damage agriculturally important crops, yet their development in insect vectors (true bugs) remains poorly characterized. Here, we detail the life cycle of Phytomonas vermiformis in its hemipteran host, the vernal shieldbug Peribalus strictus vernalis, using transmission electron microscopy, providing the most comprehensive description of a phytomonad development to date. This species shows several distinctive features: formation of pseudocysts during traversal of the intestinal epithelium, presence of giant pseudocysts containing hundreds of promastigotes within salivary gland epitheliocytes, and extensive production of intracellular endomastigotes in the salivary glands. By integrating these findings with previously published data, we conducted a phylogeny-guided comparative analysis of developmental traits across Phytomonas, revealing evolutionary patterns such as independent losses of midgut proliferation and multiple modes of salivary gland colonization. The results expose major gaps in our understanding of phytomonad development, the resolution of which is crucial for uncovering the adaptations underlying dixenous parasitism in this group.