Phylogenetic relationships and chromosomal location of five distinct glycine receptor subunit genes in the teleost Danio rerio

Abstract.

Glycine receptors mediating synaptic inhibition are heteromeric proteins constituted of alpha and beta subunits. The mammalian GlyR subunits constitute a subgroup in the superfamily of ligand-gated ionic channels. To compare the evolutionary events in the mammalian and teleostean lineages for the receptor family, we first undertook systematic cloning of the constitutive subunits of the zebrafish glycine receptor. The isolation of two alpha subunits (αZ1 and αZ2) and one beta subunit (βZ) has been reported previously and we report here the characterization of two novel alpha subunits, αZ3 and αZ4, increasing the known zebrafish subunits number to four alpha and one beta. Establishment of phylogenetic relationships reveals that αZ1, αZ3 and βΖ are orthologous to mammalian α1, α3 and β subunits. However, two zebrafish GlyRα subunit genes are orthologous to the unique avian and mammalian α4 subunit revealing a duplication of the α 4 gene in zebrafish. Whole-mount in situ hybridization in 24-hours post fertilization (hpf) and 52-hpf embryos of the daughter gene products display very different expression patterns indicating distinct functions of the duplicated genes. Gene mapping reveals that the two duplicated genes are localized on two different linkage groups (LG5 and LG22) as would be daughter genes resulting from a large-scale duplication of the ancestral genome. Finally, we report that a linked pair of genes on human chromosome 4 (α 3 and β) is also linked on linkage group 1 in zebrafish (α Z3 and β Z) as a consequence of a mosaic conserved syntheny.