Biologists from the University of Kansas discovered an astonishing evolutionary progression that may redefine how scientists view and understand animal growth and development.
A genome sequencing of the Myxozoa, microscopic aquatic parasites that infect vertebrate and invertebrate hosts, revealed that these parasites are actually Cnidarians that had undergone extreme degeneration, scientists said. The parasite's features are closely related to those of jellyfish and corals.
In a study featured in the Proceedings of the National Academies of Sciences, researchers explained that Myxozoans are not primitive versions of jellyfish, but these parasites are the result of jellyfish becoming smaller and more simplified throughout the passing of time.
Despite the radical degeneration of the modern jellyfish's body structure and genome, the Myxozoa retained the essential trait of the jellyfish such as its stinger or nematocyst. Paulyn Cartwright, lead researcher of the study, said the parasite only has a few cells and does not have a gut or a mouth.
Cartwright explained that the parasite's genomes were 20 to 40 times smaller than average jellyfish genomes, making it one of the smallest animal genomes ever reported. The parasite has about 20 million base pairs, while the average Cnidarian has more than 300 million.
"Because they're so weird, it's difficult to imagine they were jellyfish," said Cartwright. Animals are usually classified as macroscopic multicellular organisms, but Myxozoas do not have those traits. "Myxozoa absolutely redefines what we think of as animal," she added.
Myxozoans can be found in both freshwater and marine habitats, and has a diverse clan of more than 2,000 microscopic parasites. Previously, these parasites were classified by scientists as Protists, but biologists questioned the classification. They noted that the Myxozoa contain a polar capsule, an elaborate structure that enable them to latch onto hosts. The structure is similar to that of a jellyfish stinger.
"Some people originally thought they were single-celled organisms," said Cartwright. But DNA sequencing revealed that these parasites were indeed animals and researchers believe that their findings could reveal a strategy in nature that is actually widespread albeit unknown.
"If it can happen once in evolution, it certainly can happen again," added Cartwright.