• This investigation of the Institute of Evolutionary Biology (CSIC-UPF), published in the journal Current Biology, finds the first genetic evidence of parallel evolution between fungi and close relatives to animals
• They show that several microorganisms evolutionarily close to animals and fungi evolved independently and in parallel with the same result: loss flagellum, in some cases, and development of a chitin cell wall, in others
• From this, scientists have been able to conclude how was the common ancestor of animals, fungi and other unicellular organisms (known as "LOCA" for "last opisthokont common ancestor"): an unicellular amoeboid with flagellum that ate bacteria

Since long ago, scientists believe that different organisms subject to the same environmental selective pressure can evolve independently and reach a similar result. This is what is known as a convergent or parallel evolution. Although many possible examples have been exposed, there was genetic evidence of only a few cases.

This week, in an investigation leaded by Iñaki Ruiz Trillo, an ICREA research professor at the Institute of Evolutionary Biology (CSIC-UPF) and published in Current Biology, a journal of CellPress, scientists provide the first genetic evidence of convergent evolution in organisms close to animals and fungi. The microorganisms studied belong to the opisthokonts, one of the major lineages of eukaryotes, which includes animals, fungi and some unicellular forms.

Scientists have sequenced and analyzed the transcriptome (messenger RNA) of several of these unicellular organisms (protists), which are poorly known and difficult to find in nature. The results show that the flagellum (organelle used to move) was independently lost several times along the opisthokont evolution that ultimately lead to fungi and animals. The authors infer the presence of genes associated with the flagellum in relatives of non-flagellates ones, an evidence that the flagellum was secondarily lost.

In addition, Iñaki Ruiz-Trillo says, these genes have been found in two organisms that were previously thought to be non flagellated like, Ministeria Vibrans and Chorallochytrium limacisporum. After the genetic evidence, scientists observed Ministeria Vibrans under confocal microscopy and saw that it has a structure that had been overlooked until then. It is, explains Ruiz-Trillo, "the structure of an appendix composed by tubulin and with the classic form of a flagellum, which confirms, beyond doubt and contrary to what was previously believed, that these organisms are flagellate, although only in specific moments of their life cycle".

Similarly, this study shows genetic evidence of organisms that evolved in parallel to form membranes of chitin, both in fungi and unicellular relatives of animals, and these changes have been placed on the phylogenetic tree.

Evolutionary implications: a common ancestor

With this research, scientists have redrawn the phylogenetic tree of the opisthokonts. and conclude that the ancestor of animals, fungi and other unicellular organisms (LOCA, “ Last Opisthokont Common Ancestor”) was a unicellular microorganism with filopodia (small protrusions or elongations on the surface), and a flagellum, that ate bacteria. It also had a large variety of chitin syntheases genes (the genes involved in the contruction of chitin cell walls).

From this ancestor, the osmotrophic life style of fungi and other closely related animal lineages evolved independently by creating chitin cell walls. Moreover, the flagellum of the ancestral opisthokont was lost in parallel in up to five lineages, both in fungi and other lineages closer to animals (but it was preserved in animals in the case of sperm).

Ancient, rare and exceptional organisms

The unicellular organisms studied by the team are extremely rare and difficult to find in nature. This is the case Corallochytrium limacisporum, an ancient lineage that was once found in waters of coral reefs and the researchers were able to isolate again from coral reefs in Hawaii and India. Or Ministeria Vibrans, of which little is known about its ecology and measures a barely three microns (five to ten times smaller than a human cell).

For this study, scientists have relied on samples from several laboratories around the world. They have cultured the organism and sequenced their transcriptome (the genes that are expressed). Phylogenetically, they are close relatives but ancestral of animals, whose evolutionary lines diverged more than 800 million years ago. They can be very helpful in order to decipher how unicellular organisms became multicellular.

Reference article: Phylogenomics reveals convergent evolution of lifestyles in close relatives of animals and fungui. G. Torruella, A. de Mendoza, X. Grau-Bové, M. Antó, M.A. Chaplin, J. del Campo, L. Eme, G. Pérez-Cordón, C.M. Whipps, K.M. Nichols, R. Paley, A.J. Roger, A. Sitja-Bobadilla, S. Donachie, and I. Ruiz-Trillo. Current Biology (10 September)