Jarosław Tyszka
On Growth and Form of Retaria: FORAMINIFERA
Anyone familiar with the vast array of skeletal forms in Radiolaria and Foraminifera is struck by their close phylogenetic relationship within the clade Retaria. Why do the skeletons of these unicellular organisms differ so fundamentally in both composition and morphology? What mechanisms drove such extraordinary diversification? Today, after two centuries of research, these questions remain largely unresolved. In this lecture, the author introduces this interdisciplinary field — at the intersection of foraminiferal morphogenesis and biomineralization — shaped by over a quarter-century of collaborative studies. Do we have a realistic chance of discovering shared morphogenetic mechanisms across Retaria?

Nicole Coots
Fluorescence Lifetime Imaging Microscopy (FLIM) as a method to elucidate trends in marine planktonic photosymbioses
The Retaria are an abundant planktonic protist lineage that play a substantial role in biogeochemical cycling, primary production, and food web dynamics in open ocean ecosystems. Their impact is, in large part, due to their capacity to host photosynthetic endosymbionts. Despite the importance of these relationships in the global oceans, our knowledge about the diversity of photosymbionts associated with the planktonic Retaria is still limited due to the difficulty of collecting and analyzing these cells. Here we apply a new protocol to assess retarian photosymbiont diversity: Fluorescence Lifetime Imaging Microscopy (FLIM) is an easy-to-use method to distinguish between different sources of fluorescence during confocal microscopy. We applied FISH probes that hybridize with the most commonly reported retarian photosymbiont lineages to environmental plankton samples and used FLIM to confirm the presence or absence of our FISH probes in retarian cells. By analyzing the fluorescence lifetime of 92 photosymbionts, we found that the retarian photosymbiosis is far more complex than originally described, where Retaria commonly host multiple algal lineages per cell, and regularly host algal lineages outside of the two most common lineages. These efforts demonstrate that FLIM is an overlooked tool in the study of host-microbe associations, with significant potential to be used across diverse host lineages and environmental samples.

Andrea Garvetto
Your parasitic relatives: the Phytomyxea
Long treated as fungi, Phytomyxea are a lineage of intracellular biotrophic parasites belonging within Rhizaria. Besides being well-supported in molecular phylogenies, the monophyly of Phytomyxea is strengthened by a set of consistent and conserved synapomorphies: cruciform nuclear division, a specific extrusome used to access host cells, a biphasic life cycle involving a generalist (sporangial) and a specialized (sporogenic) infection. This consistency is particularly interesting as Phytomyxea have been shown to be able to infect a broad and diverse array of organisms, spanning three distant major eukaryotic lineages (Heterokonta, Dinoflagellata and Angiosperms) in marine, freshwater and terrestrial habitats. In this seminar, I will provide the latest news in Phytomyxea biology using results from our group and from recent publications, to cover topics such as: biodiversity, comparative biology of infection, chronobiology and evolution within the Phytomyxea. Particular attention will be given to parallels and contrasts with Retaria biology, in order to foster discussion on the evolutionary history of this fascinating group of parasites.

Andrea Garvetto, Institute of Microbiology, University of Innsbruck, Innsbruck, Austria

Yana Eglit
A long-lost giant: the newest addition to Retaria?
Gymnosphaerids, the last unclassified remnants of now-defunct "Heliozoa", or "sun-animalcules", are among the last morphologically-described major groups in need of molecular phylogenetic placement. Rarely recorded in the literature, Gymnosphaerids are characterised by a conspicuous central microtubule organising body, large size, and a complex morphology. Wagnerella borealis, is a particularly striking gymnosphaerid cell up to 3 mm long that consists of a widened base, long stem, and a spherical head, all extensively covered in large siliceous spicules; it was originally described as a sponge. Wagnerella was last extensively studied by Zülzer (1909), who reported unusual features such as nuclear migration from base to head during the life cycle, and ‘budding’’ of the head. Upon finding a population, during two weeks in the field we collected and examined over two hundred cells to obtain molecular data and verified Zülzer’s microscopic observations. Preliminary analyses of the first single cell transcriptomes obtained for this group support its relationship with Retaria, a group containing other large complex protists like foraminifera and polycystine radiolarians. This is supported by some possibly shared ultrastructural features with Retaria previously studied by electron microscopy. In addition to corroborating the remarkable observations by Zülzer (1909), we also found more putative life history stages and construct a hypothetical life cycle.

Yana Eglit, Department of Molecular and Cellular Biology, University of Geneva, Geneva, Switzerland