Adding a new dimension to investigations of early radiolarian evolution

Deconstructing Difflugia: The tangled evolution of lobose testate amoebae shells (Amoebozoa: Arcellinida) illustrates the importance of convergent evolution in protist phylogeny

Protists, the micro-eukaryotes that are neither plants, animals nor fungi build up the greatest part of eukaryotic diversity on Earth. Yet, their evolutionary histories and patterns are still mostly ignored, and their complexity overlooked. Protists are often assumed to keep stable morphologies for long periods of time (morphological stasis). In this work, we test this paradigm taking Arcellinida testate amoebae as a model. We build a taxon-rich phylogeny based on two mitochondrial (COI and NADH) and one nuclear (SSU) gene, and reconstruct morphological evolution among clades. In addition, we prove the existence of mitochondrial mRNA editing for the COI gene. The trees show a lack of conservatism of shell outlines within the main clades, as well as a widespread occurrence of morphological convergences between far-related taxa. Our results refute, therefore, a widespread morphological stasis, which may be an artefact resulting from low taxon coverage. As a corollary, we also revise the groups systematics, notably by emending the large and highly polyphyletic genus Difflugia. These results lead, amongst others, to the erection of a new infraorder Cylindrothecina, as well as two new genera Cylindrifflugia and Golemanskia.

A Morpho-molecular Perspective on the Diversity and Evolution of Spumellaria (Radiolaria)

Spumellaria (Radiolaria, Rhizaria) are holoplanktonic amoeboid protists, ubiquitous and abundant in the global ocean. Their silicified skeleton preserves very well in sediments, displaying an excellent fossil record extremely valuable for paleo-environmental reconstruction studies, from where most of their extant diversity and ecology have been inferred. This study represents a comprehensive classification of Spumellaria based on the combination of ribosomal taxonomic marker genes (rDNA) and morphological characteristics. In contrast to established taxonomic knowledge, we demonstrate that symmetry of the skeleton takes more importance than internal structures at high classification ranks. Such reconsideration allows gathering different morphologies with concentric structure and spherical or radial symmetry believed to belong to other Radiolaria orders from the fossil record, as for some Entactinaria families. Our calibrated molecular clock dates the origin of Spumellaria in the middle Cambrian (ca. 515 Ma), among the first radiolarian representatives in the fossil record. This study allows a direct connection between living specimens and extinct morphologies from the Cambrian, bringing both a standpoint for future molecular environmental surveys and a better understanding for paleo-environmental reconstruction analysis.