Abstract
Gene or genome duplications increasing in organismal complexity are important events in evolution. On the basis of RT-PCR surveys, the tetrapod brains and the cartilaginous shark brain expressed four types of AMPAR subunits, and the brains of tilapia, pufferfish and zebrafish expressed eight or seven types of AMPAR subunits. Results of a similar RT-PCR survey showed that the teleost moray eel brain expressed seven kinds of AMPAR subunits, the jawless hagfish brain expressed two types of AMPAR subunits, and the cephalochordate amphioxus had three types of AMPAR subunits. Moreover, using the conserved amino acid sequences of AMPAR subunits as a query to search the urochordate ascidians database, one kind of ascidians AMPAR subunit was obtained. A phylogenetic tree of ascidians, amphioxus, shark, teleost, and tetrapod AMPAR subunits was contracted by Neighbor-joining analysis. This tree suggested that the vertebrate AMPAR subunit genes arose from a common ancestral gene. Subsequently, one or two gene duplication events might occur at either the cephalochordate or the jawless fish lineage. The gene structure of TMD1-TMD4 of chordate AMPAR subunits showed that the vertebrate AMPAR genes included two mutually exclusive exons, named flop and flip. On the other hand, there was only one flop/flip-coding exon in the non-vertebrate chordate AMPAR genes. On the basis of the phylogenetic relationships of flop and flip chordate sequences, the two vertebrate flop and flip-coding exon could result from a primordial exon causing by the tandem exon duplication. The mfold program prediction about the sequences around the Q/R site or the R/G site showed that the non-vertebrate chordate AMPAR transcripts could not form stable secondary structures. This result suggested that the modification of the AMPAR transcripts by Q/R or R/G editing were absent in the non-vertebrate chordate.