Expression variations in ectodysplasin-A gene (eda) may contribute to morphological divergence of scales in haplochromine cichlids

Abstract

Background: Elasmoid scales are one of the most common dermal appendages and can be found in almost all species of bony fish differing greatly in their shape. Whilst the genetic underpinnings behind elasmoid scale development have been investigated, not much is known about the mechanisms involved in moulding of scales. To investigate the links between gene expression differences and morphological divergence, we inferred shape variation of scales from two different areas of the body (anterior and posterior) stemming from ten haplochromine cichlid species from different origins (Lake Tanganyika, Lake Malawi, Lake Victoria and riverine). Additionally, we investigated transcriptional differences of a set of genes known to be involved in scale development and morphogenesis in fish.

Results: We found that scales from the anterior and posterior part of the body strongly differ in their overall shape, and a separate look on scales from each body part revealed similar trajectories of shape differences considering the lake origin of single investigated species. Above all, nine as well as 11 out of 16 target genes showed expression differences between the lakes for the anterior and posterior dataset, respectively. Whereas in posterior scales four genes (dlx5, eda, rankl and shh) revealed significant correlations between expression and morphological differentiation, in anterior scales only one gene (eda) showed such a correlation. Furthermore, eda displayed the most significant expression difference between species of Lake Tanganyika and species of the other two younger lakes. Finally, we found genetic differences in downstream regions of eda gene (e.g., in the eda-tnfsf13b inter-genic region) that are associated with observed expression differences. This is reminiscent of a genetic difference in the eda-tnfsf13b inter-genic region which leads to gain or loss of armour plates in stickleback.

Conclusion: These findings provide evidence for cross-species transcriptional differences of an important morphogenetic factor, eda, which is involved in formation of ectodermal appendages. These expression differences appeared to be associated with morphological differences observed in the scales of haplochromine cichlids indicating potential role of eda mediated signal in divergent scale morphogenesis in fish.

Keywords: Adaptive radiation; African cichlids; East African lakes; Gene expression; Lake Malawi; Lake Tanganyika; Lake Victoria; Scale morphology.

Fig. 1

The haplochromine cichlid species and descriptions of the scale samples. a A simplified phylogenetic relatedness of the East African haplochromine cichlid species used in this study. b Positions of the anterior and posterior scales used in this study and landmarks used for the geometric morphometric analyses in both anterior (left) and posterior (right) scales shown as an example for Petrochromis famula. Blue dots represent major landmarks, white dots semi-landmarks and 1 mm scale bars are given below the images. c Principal component analysis (PCA) plots clearly separate scales from the anterior and posterior part of the body. Additional, warped outline drawings illustrate major shape changes along the axis (red) compared to the overall mean shape (grey)

Fig. 2

Morphospace of investigated scales from different species and body parts. Principal component analysis based on average shape of scales collected for the anterior a and posterior b part of the body and respective shape differences along the axis (grey: overall mean shape; red: shape change). Linear discriminant function analysis based on the first four PC-scores for anterior c and posterior d scales. All data points represent mean shapes obtained from 6 individually collected scales and shapes represent different lake origins. A.b. Astatotilapia burtoni, N.o. Neochromis omnicaeruleus, P.f. Petrochromis famula, P.p. P. polyodon, P.s. Paralabidochromis sauvage, S.d. Simochromis diagramma, S.f. Sciaenochromis fryeri, T.t. Tropheops tropheops, L.t. Labeotropheus trewavasae, M.z. Maylandia zebra

Fig. 3

The anterior versus posterior scales expression differences of the candidate target genes in haplochromine cichlids from three East African lakes. Comparisons of relative expression levels between anterior versus posterior scales for 16 candidate target genes in different lakes in East Africa at young adult stage. Significant differences between the are indicated by red asterisks (*P < 0.05; **P < 0.01). See Fig. 1A for corresponding species abbreviations

Fig. 4

The lake-based expression differences of the candidate target genes in haplochromine cichlids in this study. Comparisons of relative expression levels between the lakes, when all species of each lake were combined, within anterior or posterior scales for 16 candidate target genes. Significant differences between the are indicated by red asterisks (*P < 0.05; **P < 0.01; ***P < 0.001). See Fig. 1A for corresponding species abbreviations

Fig. 5

Correlation analyses of candidate target gene expressions and the anterior scale morphololgical divergence across the haplochromine species. A Pearson correlation coefficient (r) was used to assess the similarity between differences in expression level of the target genes and the major canical variate in the anterior scales across all species. See Fig. 1A for corresponding species abbreviations

Fig. 6

Correlation analyses of candidate target gene expressions and the posterior scale morphololgical divergence across the haplochromine species. A Pearson correlation coefficient (r) was used to assess the similarity between differences in expression level of the target genes and the major canical variate in the posterior scales across all species. See Fig. 1A for corresponding species abbreviations