Life cycle evolution in the digenea: a new perspective from phylogeny.

Area of Science:

• Parasitology
• Evolutionary Biology
• Molecular Phylogenetics

Background:

• The digenean flatworm life cycle is complex and diverse.
• Understanding the evolutionary origins of these life cycles is crucial for parasite biology.

Purpose of the Study:

• To reconstruct the evolutionary history of the digenean life cycle using molecular phylogenetics.
• To infer the ancestral character states and evolutionary pathways of key life cycle traits.

Main Methods:

• Construction of a new molecular phylogeny using small and large subunit ribosomal RNA genes.
• Mapping of character states onto the phylogeny.
• Parsimony-based inference of character evolution.

Main Results:

• The plesiomorphic (ancestral) digenean life cycle involved external miracidial penetration of gastropods and direct consumption of infected snails by teleost definitive hosts.
• External encystment and the use of second intermediate hosts are inferred as derived traits, repeatedly adopted across digenean evolution.
• A basal dichotomy is proposed between 'Diplostomida' and 'Plagiorchiida', with the most primitive life cycles found in basal Plagiorchiida and basal Diplostomida associated with tetrapods.
• Blood flukes (Schistosomatoidea) likely evolved their two-host life cycles by abbreviating three-host cycles.

Conclusions:

• The study provides a novel phylogenetic framework for understanding digenean life cycle evolution.
• Derived life cycle strategies, including the use of second intermediate hosts and tetrapod definitive hosts, have evolved multiple times independently.
• The proposed hypothesis suggests the blood-fluke life cycle may be plesiomorphic within Diplostomida, challenging previous assumptions.