Molecular genetics of the complement C3 convertases in lower vertebrates

Area of Science:

• Immunology
• Evolutionary Biology
• Genetics

Background:

• The complement system is crucial for innate immunity, involving complex gene families like C3/C4/C5 and B/C2 that form C3 convertases.
• Understanding the evolutionary history of these gene families in lower vertebrates provides insights into the development of the vertebrate immune system.

Purpose of the Study:

• To investigate the evolutionary trajectory of the C3/C4/C5 and B/C2 gene families in lower vertebrates at the cDNA level.
• To pinpoint the timing and location of gene duplication events within these complement gene families.

Main Methods:

• Comparative analysis of cDNA sequences from various lower vertebrate species, including cyclostomes, amphibians (Xenopus), and teleost fish (medaka).
• Identification and characterization of specific gene family members to infer evolutionary relationships and duplication events.

Main Results:

• Cyclostomes possess single members of the C3/C4/C5 and B/C2 gene families, indicating duplication occurred after their divergence.
• Typical C3 and C4 cDNAs were found in amphibians and teleosts, suggesting the C3/C4 gene duplication event predates the split between ray-finned and lobe-finned fish.
• Teleost B/C2 genes showed intermediate characteristics, unlike typical B cDNA in Xenopus, suggesting the B/C2 duplication might have occurred in the tetrapod lineage.
• Genetic linkage between C3/C4/C5 and B/C2 gene families was observed in Xenopus but not in medaka fish.

Conclusions:

• Gene duplication within the C3/C4/C5 and B/C2 families occurred in the lineage leading to jawed vertebrates.
• The C3/C4 gene duplication event is ancient, preceding the divergence of major fish lineages, while the B/C2 duplication appears to be a more recent event within tetrapods.
• The observed differences in gene linkage within the MHC region highlight evolutionary divergence in gene organization among vertebrates.