Illinois Natural History Survey - University of Illinois

Acanthocephalans and Rotifers Provide Clues for the Study of Evolution of Animal Parasites

The success enjoyed by animal parasites is expressed in the independent evolution of parasitism in nearly every phylum of animals. Many of these animal parasites are helminths, more commonly referred to as worms. The phyla Platyhelminthes (flatworms), Nematoda (roundworms), and Arthropoda (referring specifically to tongue worms) contain both free-living and obligately parasitic species. The formerly recognized phylum Acanthocephala contains only obligate parasites. These groups have been studied in order to examine the evolution of parasitism in animals. Hypotheses concerning the evolution of parasites have been developed through comparisons of related parasitic and free-living organisms. More recently, these comparisons have utilized phylogenetic methods, which attempt to uncover true evolutionary relationships among organisms.

The phylogenetic method groups related organisms by the presence of shared derived characters, referred to as synapo-morphies. The resulting hypothesis of evolutionary relationship is termed a phylogeny. The development of phylogenetic hypotheses of parasitic helminths and their free-living relatives has been hampered by the large number of species in particular groups, and extreme morphological and ecological diversification, which severely limit the number of characters available for a phylogenetic analysis.

Scientists at the Illinois Natural History Survey, in collaboration with scientists at the University of California-Davis and Duquesne University, have been examining the phylogenetic relationships of the Acanthocephala, a fascinating group of obligately parasitic helminths. The characters used to construct the phylogenetic hypotheses of the Acanthocephala are DNA sequence data from the nuclear genome. In addition, nuclear DNA sequences were collected from many other animal phyla in order to determine the evolutionary origin of the Acanthocephala.

Echinorhynchus salmoides collected from an
introduced rainbow trout in Lake Michigan.

The Acanthocephala have long been regarded as an independent phylum of approximately 850 species, all of which display complex life cycles. As larvae, acanthocephalans are obligate parasities of arthropods; as sexually mature adults, they parasitize vertebrates, residing in the alimentary tract of the host. Determining the phylogenetic relations of the Acanthocephala to other animal phyla has been enigmatic owing to the extreme adaptations to its parasitic mode. The word acanthocephala means "thorny head" and refers to the ubiquitous presence of a retractable proboscis armed with recurved hooks. This proboscis allows the acanthocephalan to adhere to the intestinal wall of the vertebrate host. All acanthocephalans lack a mouth and a gut, possessing instead a specialized tegument to absorb nutrients from food material passing through the host intestine. Superficially, the Acanthocephala resemble another group of obligately parasitic helminths, the cestodes (tapeworms). However, features common to both acanthocephalans and cestodes, a hooked head region and lack of digestive tract, are hypothesized to be a result of independent evolution within these two groups.

Phylogenetic hypothesis of animal phyla resulting from analysis of nuclear DNA sequences.

Phylogenetic analysis of nuclear DNA sequence data from 15 animal phyla, including the Acanthocephala, revealed a very surprising relationship. All analyses produce a phylogenetic hypothesis that places the Acanthocephala within the Rotifera. In other words, some rotifers are more closely related to acanthocephalans than they are to other rotifers. The conclusion from this relationship is that the Acanthocephala appear to have evolved from free-living rotifers. This result is supported by the presence of unique morphological similarities in the two groups. Both rotifers and acanthocephalans possess a syncytial epidermis (not differentiated into cells), a unique skeletal lamina, and flagellum in the anterior position on sperm cells.

The result of this investigation represents the first time that a free-living nearest relative of a parasitic helminth group has been identified. With this elucidated relationship, the "acanth research group" at the Natural History Survey will revise higher-level classifications (placing the Acanthocephala into the phylum Rotifera), examine the adaptive evolution of the Acanthocephala, examine further homology between acanthocephalan and rotifer morphological features, test hypotheses concerning the evolution of parasitism in the Acanthocephala, and examine obligately parasitic rotifers to determine if they represent the nearest relative of the Acanthocephala.

Thomas J. Near, Center for Biodiversity

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Last Modified 11/05/96

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