Evolution of MHC class I genes in two ancient fish, paddlefish ( Polyodon spathula) and Chinese sturgeon ( Acipenser sinensis)

Here we present the first isolation of major histocompatibility complex (MHC) class I genes from two ancient fish, paddlefish ( Polyodon spathula) and Chinese sturgeon ( Acipenser sinensis). Seventeen sequences obtained showed high polymorphism and positive natural selection with d N/ d S > 1. Evolutionary relationships revealed that sequences from paddlefish and Chinese sturgeon distinguished from other vertebrate class I and had an intermingling of alleles, which indicates that Acipenseriformes have a common ancestral gene of class I and a trans-species polymorphism across Acipenseriformes. We also found clear evidence of recombination among class I genes of paddlefish and Chinese sturgeon.