Prototheca richardsi, a pathogen of anuran larvae, is related to a clade of protistan parasites near the animal-fungal divergence.

Prototheca richardsi is a protist of uncertain taxonomy which mediates growth inhibition in anuran larvae. Cells of P. richardsi were isolated from tadpole faeces and DNA was purified by Qiagen chromatography. Nuclear small-subunit (18S) rDNA (ssu-rDNA) was amplified by PCR using universal primers, cloned, and six clones (two from each of three separate isolates) were sequenced. All clones yielded an essentially identical sequence of 1802 nucleotides. In situ hybridization of fluorescent Prototheca-specific oligonucleotide probes, designed using the derived 18S rDNA sequence, confirmed that the sequence was indeed from P. richardsi cells and not from other components of tadpole faeces. The P. richardsi sequence was aligned with ssu-rDNA from a range of other eukaryotes, and phylogenetic analyses were carried out using several inference methods. P. richardsi consistently and stably grouped within a novel clade that contains rDNAs from an apparently heterodisperse group of parasitic micro-organisms assigned to the class Ichthyosporea. P. richardsi is evidently misplaced in the genus Prototheca, and the authors propose its inclusion in a new genus Anurofeca.