Comparative genomic and phylogenetic analyses of the intelectin gene family: Implications for their origin and evolution

• No intelectin homologs are found in invertebrate metazoans but 12 in amphioxus and 21 in ascidians.
• The intelectin gene family undergoes extensive lineage-specific duplication.
• Vertebrate intelectins have four strikingly well-conserved exons.
• Clear proto-orthologies were difficult to be established among mammal, frog, fish and amphioxus.
• An origin for FReD unique within the intelectin gene family is placed prior to metazoan emergence.

Intelectin is a newly characterized gene family involved in early embryogenesis, host-pathogen interactions and iron metabolism. In this study, we searched the genomes of metazoans by extensive BLAST survey and found no intelectin homologs in invertebrate metazoans but 12 in amphioxus Branchiostoma floridae and 21 in ascidians Ciona intestinalis. Some ascidians oocyte cortical granule lectins (CGLs) have unknown insertion sequences between fibrinogen-related domain (FReD) and Intelectin Domain, the boundaries of which are equivalent to exon structures. In addition to ascidians intelectins/CGLs located in the base, phylogenetic tree comprises four main clades representing mammal, frog, fish, and amphioxus, indicating that intelectin genes undergo extensive lineage-specific duplication or gene conversion. However, genomic neighborhood surrounding analysis shows that clear proto-orthologies are difficult to be established among these counterparts. In addition, sequence comparison and phylogenetic analysis of FReDs from intelectins and other fibrinogen-like proteins from choanoflagellate, anemone, frog and human indicate FReDs of intelectins are unique. Likewise, these choanoflagellate and anemone genes may be close to intelectin gene.