Electric organ (EO) evolved independently in six different lineages of fish usually derived from skeletal muscle (SM) during

development. Despite their shared developmental origin, electric organs and skeletal muscle are morphologically and physiologically distinct tissues. The molecular mechanisms underlying the differences between EO and SM are not well understood, especially among the mormyrid electric fish from Africa.

We begin with the assumption that genes uniquely expressed in the electric organ are the most likely sites for evolutionary diversification (Zakon, 2006). Can genes in the electric organ be identified without a priori assumption? Using the developmental relationship between SM and EO, we apply the technique of supressive subtractive hybridization (Diatchenko, 1996) to characterize such genes in a non-biased manner. SSH is a widely utilized (c.a. 1000 publications in 5 years ranging from disease, microbiology, development), PCR-based method designed to selectively enrich for differentially expressed sequences while suppressing abundant genes in common.

We have performed Forward and reverse SSH on samples of EO and SM (pooled from five individual B. brachyistius, each subtraction resulting in a cDNA library enriched for mRNAs uniquely expressed in electric organ or skeletal muscle (SM). We randomly selected 143 EO library clones and 38 SM library clones for sequencing (average length = 647bp for all sequences), and attempted to identify each by BLAST searching publicly available NCBI sequence databases. Currently, we have detected 90 uniquely expressed transcripts, and have positively identified 25 unique transcripts homologous to known genes.

The majority of identifiable, uniquely expressed mRNAs in the electric organ encode proteins either (1) responsible for the transport of ions (i.e. Na+, K+ and Ca++ ) across membranes (e.g. homologues to alpha and beta subunits to Na+/K+ pumps, plasma membrane calcium pumps), or (2) proteins that bind directly to calcium intracellularly (i.e. S100, several parvalbumin isoforms, and troponin). We also provide evidence of differential expression in electric organ of two transcription factors (MEF2A, and a vertebrate homolouge of the Drosophila enhancer of rudimentary gene) as well as a unique transcript of myosin heavy chain not expressed in SM.