Molecular Evidence of Fungal Signatures in the Marine Protist Corallochytrium limacisporum and its Implications in the Evolution of Animals and Fungi

Fungi, animals, and single-celled organisms belonging to the choanozoans together constitute the supergroup Opisthokonta. The latter are considered crucial in understanding the evolutionary origin of animals and fungi. The choanozoan Corallochytrium limacisporum is an enigmatic marine protist of considerable interest in opisthokontan evolution. Several isolates of the organism were obtained from a coral reef lagoon in the Lakshadweep group of islands of the Arabian Sea. The capability of these cultures to grow on media containing inorganic nitrogen sources prompted us to examine the possible presence of fungal signatures, namely the enzyme α-aminoadipate reductase (α-AAR) involved in the α-aminoadipate (AAA) pathway for synthesizing lysine and ergosterol, in one of the isolates. These features, as well as the sterol C-14 reductase gene involved in the sterol pathway of animals and fungi, were detected in the organism. Phylogenetic trees based on the α-AAR gene suggested that Corallochytrium limacisporum is a sister clade to fungi, while those based on the C-14 reductase gene did not adequately resolve whether the organism was more closely related to fungi or animals. While many studies indicate that Corallochytrium is a sister clade to animals, we suggest that further studies are required to examine whether this protist is in fact more closely related to fungi rather than to animals.