Search

Rhodophyta (red algae) is one of three lineages of Archaeplastida.sup.1, a supergroup that is united by the primary endosymbiotic origin of plastids in eukaryotes.sup.2,3. Red algae are a diverse and species-rich group, members of which are typically photoautotrophic, but are united by a number of highly derived characteristics: they have relatively small intron-poor genomes, reduced metabolism and lack cytoskeletal structures that are associated with motility, flagella and centrioles. This suggests that marked gene loss occurred around their origin.sup.4; however, this is difficult to reconstruct because they differ so much from the other archaeplastid lineages, and the relationships between these lineages are unclear. Here we describe the novel eukaryotic phylum Rhodelphidia and, using phylogenomics, demonstrate that it is a closely related sister to red algae. However, the characteristics of the two Rhodelphis species described here are nearly opposite to those that define red algae: they are non-photosynthetic, flagellate predators with gene-rich genomes, along with a relic genome-lacking primary plastid that probably participates in haem synthesis. Overall, these findings alter our views of the origins of Rhodophyta, and Archaeplastida evolution as a whole, as they indicate that mixotrophic feeding--that is, a combination of predation and phototrophy--persisted well into the evolution of the group. Species of the eukaryotic phylum Rhodelphidia are non-photosynthetic, flagellate predators with gene-rich genomes, in contrast to their closely related sister lineage--the red algae--which are immotile, typically photoautotrophic and have relatively small intron-poor genomes and reduced metabolism., Author(s): Ryan M. R. Gawryluk [sup.1] [sup.3] , Denis V. Tikhonenkov [sup.1] [sup.2] , Elisabeth Hehenberger [sup.1] [sup.4] , Filip Husnik [sup.1] , Alexander P. Mylnikov [sup.2] , Patrick J. [...]

Apicomplexa is a group of obligate intracellular parasites that includes the causative agents of human diseases such as malaria and toxoplasmosis. Apicomplexans evolved from free-living phototrophic ancestors, but how this transition to parasitism occurred remains unknown. One potential clue lies in coral reefs, of which environmental DNA surveys have uncovered several lineages of uncharacterized basally branching apicomplexans.sup.1,2. Reef-building corals have a well-studied symbiotic relationship with photosynthetic Symbiodiniaceae dinoflagellates (for example, Symbiodinium.sup.3), but the identification of other key microbial symbionts of corals has proven to be challenging.sup.4,5. Here we use community surveys, genomics and microscopy analyses to identify an apicomplexan lineage--which we informally name 'corallicolids'--that was found at a high prevalence (over 80% of samples, 70% of genera) across all major groups of corals. Corallicolids were the second most abundant coral-associated microeukaryotes after the Symbiodiniaceae, and are therefore core members of the coral microbiome. In situ fluorescence and electron microscopy confirmed that corallicolids live intracellularly within the tissues of the coral gastric cavity, and that they possess apicomplexan ultrastructural features. We sequenced the genome of the corallicolid plastid, which lacked all genes for photosystem proteins; this indicates that corallicolids probably contain a non-photosynthetic plastid (an apicoplast.sup.6). However, the corallicolid plastid differs from all other known apicoplasts because it retains the four ancestral genes that are involved in chlorophyll biosynthesis. Corallicolids thus share characteristics with both their parasitic and their free-living relatives, which suggests that they are evolutionary intermediates and implies the existence of a unique biochemistry during the transition from phototrophy to parasitism. A newly identified lineage of apicomplexans, named corallicolids, are intracellular symbionts of many coral species, and possesses a plastid that retains genes for chlorophyll biosynthesis despite lacking photosystem genes., Author(s): Waldan K. Kwong [sup.1] , Javier del Campo [sup.1] , Varsha Mathur [sup.1] , Mark J. A. Vermeij [sup.2] [sup.3] , Patrick J. Keeling [sup.1] Author Affiliations: (1) Department [...]

Multicellularity is often considered a prerequisite for morphological complexity, as seen in the camera-type eyes found in several groups of animals. A notable exception exists in single-celled eukaryotes called dinoflagellates, some of which have an eye-like 'ocelloid' consisting of subcellular analogues to a cornea, lens, iris, and retina (1). These planktonic cells are uncultivated and rarely encountered in environmental samples, obscuring the function and evolutionary origin of the ocelloid. Here we show, using a combination of electron microscopy, tomography, isolated-organelle genomics, and single-cell genomics, that ocelloids are built from pre-existing organelles, including a cornea-like layer made of mitochondria and a retinal body made of anastomosing plastids. We find that the retinal body forms the central core of a network of peridinin-type plastids, which in dinoflagellates and their relatives originated through an ancient endosymbiosis with a red alga (2). As such, the ocelloid is a chimaeric structure, incorporating organelles with different endosymbiotic histories. The anatomical complexity of single-celled organisms may be limited by the components available for differentiation, but the ocelloid shows that preexisting organelles can be assembled into a structure so complex that it was initially mistaken for a multicellular eye (3). Although mitochondria and plastids are acknowledged chiefly for their metabolic roles, they can also be building blocks for greater structural complexity., Many organisms can orient to light. In some single-celled eukaryotes, such as Chlamydomonas and many dinoflagellates, an 'eyespot' directs photons onto photoreceptors on the flagellum, allowing the cell to respond [...]

One of the parallels between archaeology and evolutionary biology is that the most ordinary-looking objects can sometimes be the most revealing. An ancient ornament of unrivalled artistry might reveal little [...]