Cross-tissue and cross-species analysis of gene expression in skeletal muscle and electric organ of African weakly-electric fish (Teleostei; Mormyridae).
BACKGROUND
African weakly-electric fishes of the family Mormyridae are able to produce and perceive weak electric signals (typically less than one volt in amplitude) owing to the presence of a specialized, muscle-derived electric organ (EO) in their tail region. Such electric signals, also known as Electric Organ Discharges (EODs), are used for objects/prey localization, for the identification of conspecifics, and in social and reproductive behaviour. This feature might have promoted the adaptive radiation of this family by acting as an effective pre-zygotic isolation mechanism. Despite the physiological and evolutionary importance of this trait, the investigation of the genetic basis of its function and modification has so far remained limited. In this study, we aim at: i) identifying constitutive differences in terms of gene expression between electric organ and skeletal muscle (SM) in two mormyrid species of the genus Campylomormyrus: C. compressirostris and C. tshokwe, and ii) exploring cross-specific patterns of gene expression within the two tissues among C. compressirostris, C. tshokwe, and the outgroup species Gnathonemus petersii.


RESULTS
Twelve paired-end (100 bp) strand-specific RNA-seq Illumina libraries were sequenced, producing circa 330 M quality-filtered short read pairs. The obtained reads were assembled de novo into four reference transcriptomes. In silico cross-tissue DE-analysis allowed us to identify 271 shared differentially expressed genes between EO and SM in C. compressirostris and C.tshokwe. Many of these genes correspond to myogenic factors, ion channels and pumps, and genes involved in several metabolic pathways. Cross-species analysis has revealed that the electric organ transcriptome is more variable in terms of gene expression levels across species than the skeletal muscle transcriptome.


CONCLUSIONS
The data obtained indicate that: i) the loss of contractile activity and the decoupling of the excitation-contraction processes are reflected by the down-regulation of the corresponding genes in the electric organ's transcriptome; ii) the metabolic activity of the EO might be specialized towards the production and turn-over of membrane structures; iii) several ion channels are highly expressed in the EO in order to increase excitability; iv) several myogenic factors might be down-regulated by transcription repressors in the EO.
Unit of Evolutionary Biology/Systematic Zoology, Institute of Biochemistry and Biology, University of Potsdam, Karl-Liebknecht-Strasse 24-25, Potsdam, Germany. lamanna@uni-potsdam.de.
Francesco
F
Lamanna
Department of Crop and Animal Sciences, Faculty of Horticulture and Agriculture, Humboldt University of Berlin, Philippstrasse 13, Berlin, Germany. frank.kirschbaum@staff.hu-berlin.de.
Frank
F
Kirschbaum
Museum für Naturkunde, Invalidenstraße 43, Berlin, Germany. isabelle.waurick@mfn-berlin.de.
Isabelle
I
Waurick
Max Delbrück Center for Molecular Medicine, Robert-Rössle-Strasse 10, Berlin-Buch, Germany. christoph.dieterich@age.mpg.de.|Max Planck Institute for Biology of Ageing, Joseph-Stelzmann-Strasse 9B, Cologne, Germany. christoph.dieterich@age.mpg.de.
Christoph
C
Dieterich
Unit of Evolutionary Biology/Systematic Zoology, Institute of Biochemistry and Biology, University of Potsdam, Karl-Liebknecht-Strasse 24-25, Potsdam, Germany. tiedeman@uni-potsdam.de.
Ralph
R
Tiedemann
D000818:Animals; D004555:Electric Fish / Q000235:genetics*; D004557:Electric Organ / Q000378:metabolism*; D005260:Female; D020869:Gene Expression Profiling; D005786:Gene Expression Regulation; D058977:Molecular Sequence Annotation; D018482:Muscle, Skeletal / Q000378:metabolism*; D009928:Organ Specificity / Q000235:genetics; D010641:Phenotype; D025341:Principal Component Analysis; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D017423:Sequence Analysis, RNA; D013045:Species Specificity; D059467:Transcriptome / Q000235:genetics
D012333:RNA, Messenger