Rhodelphidia

''Rhodelphis'' is a single-celled archaeplastid that lives in aquatic environments and is the sister group to red algae and possibly Picozoa. While red algae have no flagellated stages and are generally photoautotrophic, ''Rhodelphis'' is a flagellated predator containing a non-photosynthetic plastid. This group is important to the understanding of plastid evolution because they provide insight into the morphology and biochemistry of early archaeplastids. ''Rhodelphis'' contains a remnant plastid that is not capable of photosynthesis, but may play a role in biochemical pathways in the cell like heme synthesis and iron-sulfur clustering. The plastid does not have a genome, but genes are targeted to it from the nucleus. ''Rhodelphis'' is ovoid with a tapered anterior end bearing two perpendicularly-oriented flagella.

History

''Rhodelphis'' was described by Gawryluk ''et al.'' in 2019 along with species, ''R. marinus'' and ''R. limneticus,'' which were first collected in 2015 and 2016, respectively.

Habitat and Ecology

Although ''Rhodelphis'' only contains two known species, those were isolated from very different aquatic habitats. ''R. marinus'' was found in Vietnam inhabiting coral sand in the shallow waters off the coast of a small island, Bay Canh. The other species, ''R. limneticus,'' was found in a freshwater sample from Lake Trubin in Ukraine, living among debris. ''Rhodelphis'' is a heterotrophic predator that feeds on bacteria and smaller flagellates, but little is known about its role in aquatic ecosystems.

Description

Morphology

''Rhodelphis'' is an ovoid unicellular organism with a diameter of 10-13 µm. The cells do not contain any pigments, so they appear mostly clear under a microscope and are covered in umbrella-shaped glycostyles. The cells are motile and can swim around using their two flagella. Originating just below the anterior end of the cell, the flagella are perpendicular to one another and are of approximately equal length. The posteriorly oriented flagellum is covered in hair-like mastigonemes. No ostensible feeding apparatus is present, but phagocytosis of prey takes place at the posterior end.

Although it has not been identified using microscopy, evidence of plastid import proteins has revealed ''Rhodelphis’'' non-photosynthetic remnant plastid. The plastid has retained some function in iron-sulfur cluster assembly and heme biosynthesis, but it does not synthesize fatty acids or isoprenoids—''Rhodelphis'' uses different pathways in the cytosol for this. ''Rhodelphis'' also contains mitochondria with tubular cristae that possess an iron-sulfur cluster biosynthesis pathway and play a part in heme biosynthesis.

Genetics

''Rhodelphis'' is the sister group to the red algae, but the two groups differ substantially in their genetic makeup. ''Rhodelphis’'' genome is far larger than red algal genomes and its genes contain far more introns. In addition, genomic analyses revealed that ''Rhodelphis'' contains many sequences that are absent in red algae, such as those that encode flagellar proteins and components required for phagocytosis.

The common ancestor of red algae and ''Rhodelphis'' resulted from a primary endosymbiotic event early in the evolution of archaeplastids. ''Rhodelphis'' was found to contain plastid-targeted proteins as well as homologs to protein-transporters found in chloroplasts. The genes that were targeted to the plastids matched those found in red algae. Despite the targeting of proteins from the nucleus to the plastid, ''Rhodelphis'' contains only two proteins that could be involved in photosynthesis, and it seems that the plastid genome has been completely lost.

Importance

''Rhodelphis'' is part of the archaeplastids, a group that shares a common ancestor that was able to obtain a primary plastid. Since there are few intermediates of primary endosymbiotic events, the discovery of ''Rhodelphis'' may be able to provide insight into the type of organisms that may have taken up plastids in the first place, the ancestors to all archaeplastids. Additionally, it shows some of the steps that were taken early on in plastid evolution such as protein targeting and a transition from phagotrophy to mixotrophy.  These discoveries are important to understanding the evolution of green algae, plants and red algae, which are integral primary producers across the globe.

List of species

* ''Rhodelphis limneticus''
* ''Rhodelphis marinus''

See also

* Picozoa, a proposed sister of Rhodelphyta and red algae

External links

* Nicoletta Lanese; Denis Tikhonenkov (image)
Image Of The Day: Predatory Protists — The protist ''Rhodelphis limneticus'' bears little resemblance to its close genetic relative, red algae
On: The Scientist. Jul 19, 2019.

References

{{Taxonbar, from=Q66018128

Archaeplastida