A Comparative Study of Ryanodine Receptors (RyRs) Gene Expression Levels in the Basal Ray-Finned Fish, Bichir (Polypterus ornatipinnis) and the Derived Euteleost Zebrafish (Danio rerio)

Abstract

Ryanodine receptors (RyR) mediate the controlled release of intracellular stores of calcium from the sarcoplasmic reticulum. This release of calcium, triggered by membrane-depolarization, is responsible for initiating muscle contraction. Three RyR genes have been identified in mammals, two of which are expressed in specific tissues; RyR1 (skeletal muscle), RyR2 (cardiac muscle), and, RyR3, that is ubiquitously distributed. The RyR1 and RyR3 isoforms are co-expressed at equal levels in teleost fish skeletal muscle. Additionally, fish express fiber type-specific RyR1 paralogous genes in fast-twitch and slow-twitch muscles, termed RyR1-fast (RyR1b) and RyR1-slow (RyR1a) respectively. Bichir, (Polypterus ornatipinnis), a ray-finned fish, is considered to be one of the most basal extant species of this lineage. Utilizing a genomic survey of the genome of bichir for the presence of candidate RyR genes, 134 genomic clones were obtained. These cloned sequences were grouped into four cognate groups representing four RyR genes called RyR1a, RyR1b, RyR2, and RyR3 that phylogenetically cluster with their vertebrate orthologs. Quantitative real-time PCR and in situ hybridization show fibre type-specific expression of the RyR1a and RyR1b genes. However, the RyR3 gene is down regulated in bichir in contrast to derived teleosts including zebrafish (Danio rerio) in which the RyR1 and RyR3 genes are co-expressed at equivalent levels. Succinate dehydrogenase staining revealed that bichir and zebrafish possess similar red muscle fiber arrangements in their skeletal muscle; however, zebrafish possess a higher concentration of red muscle fibers, which could have contributed to the further diversification of the teleost lineage.