Rozellida

History

Formation of the Rozellida concept

Rozellida were first detected as DNA sequences retrieved from a freshwater laboratory enclosure. Phylogenetic analysis of these sequences formed a unique terminal clade of then unknown affiliation provisionally called after the first clone in the clade: LKM11.

The term "Rozellida" was coined in 2010, as it was found that the formally described genus Rozella, previously considered a chytrid, is rather close to LKM11 and other newly-discovered environmental DNA sequences.

Additional members of the group were isolated in 2011 by a team led by Thomas Richards, from the Natural History Museum in London, and also an evolutionary geneticist at the University of Exeter, UK. The team used DNA techniques to disclose the existence of unknown genetic material dredged from the university pond. Once they had a few unknown sequences they fluorescently labeled small DNA sequences and let them bind to the matching DNA in the whole sample (fluorescence in situ hybridization). Under fluorescence microscopy, they could see that the possessor cells were ovoid in shape and 3–5 micrometres across. They then established that the Cryptomycota were present in other samples taken from further freshwater environments, soils and marine sediments.

The common characteristic of the clade members known as of 2011 is that they lack the chitinous cell walls present in almost all previously discovered fungi (including microsporidia) and which are a major feature of the kingdom. Without the chitin the Cryptomycota can be phagotrophic parasites that feed by attaching to, engulfing, or living inside other cells. Most known fungi feed by osmotrophy—taking in nutrients from outside the cell.

Despite their unconventional phagocytic feeding habits (typical fungi are osmotrophic), chitin has been observed in the inner layer of resting spores, and in immature resting spores for some species of Rozella, as indicated with calcofluor-white stain as well as the presence of a fungal-specific chitin synthase gene.

Inclusion of Microsporidia

In the 2010 article where "Rozellida" was proposed, the authors mentioned that some protein-based analyses suggest that Microsporidia could be closely related to Rozellida. Their own SSU rDNA analysis was unsutiable for this purpose, as Microsporidia is known to have very high mutation rates resulting in long-branch attraction. An rRNA analysis can only be done after there are more sequences, preferably those close related to both "Rozellida" and Microsporidia, to guide the algorithm into finding the true shape of the tree.

A 2014 rDNA analysis by Karpov et al. considers the group, which they now call "Cryptomycota", sister to Microsporidia. The same article lead to the concept of Opisthosporidia (Aphelid + Cryptomycota + Microsporidia), which as a whole was found in the analysis to be sister to fungi.

Also in 2014, Corsaro et al. discovered two new endonucleoparasites they termed Paramicrosporidium. Their SSU rDNA sequences allowed the authors to find that Microsporidia is nested in the Rozellids (which they call "Rozellomycota"). In the same year, they discovered a living Nucleophaga amoebae (described in 1895 and previously also considered a chytrid) and sequenced its SSU rDNA, placing it in Rozellomycota. Both of these genera are shown to branch out in the path connecting Rozella and Microsporidia. These two genera show morphological features similar to both Rozella and Microsporidia, and as a result provide some explanation for the large difference between the two.

In 2018, Tedersoo et al. performed a large phylogenetic study on fungi and related taxa to establish a new set of high-ranking taxa. Trees were built using SSU rDNA, RPB1 protein, and RPB2 protein. The study once again confirmed that Microsporidia belongs in Rozellomycota. After the 2020 Outline of Fungi and fungus-like taxa has accepted the conclusion (and provided an even bigger SSU rDNA tree), it is generally accepted that Microsporidia does, in fact, belong in the Rozellomycota.