Normal squamation pattern is coupled to segmented myotomes. (A) In euTH control fish, scales (numbered by column and lettered by row) formed in the center of myotomes bounded by vertical myosepta (magenta overlay). In hypoTH individuals, initial scale papillae (numbered and lettered) developed closer to myosepta, and multiple scales formed above some myotomes (e.g., A1, A2, A3 over two myotomes). As patterning unfolded, new scale papillae (yellow arrows) appeared within the same rows as initial papillae, associated with increasingly disordered patterns. (B) tbx6 mutants, which mis-patterned vertical myosepta (labelled with jam3b:jam3b-mCherry), had disrupted patterns of squamation, visualized by sp7:EGFP. In both wild-type control animals and tbx6 mutants, myosepta predict the location of successive scale papillae (yellow arrows). In the mutant, an apparently branched myotome lead to incusion of an extra papilla which disrupted hexagonal patterning. (C) Anterior-posterior distances between early scale papillae were slightly increased and more variable in tbx6 mutants (effect of TH status, F1,9.3 = 6.7, p = 0.0289; mixed model ANOVA with individual fish treated as a random effect and distances ln-transformed for analysis to stabilize variance of residuals among groups; n = 10 measurements from 5 sibling controls and 8 mutants). (D) Voronoi tessellation analysis showed a significant reduction in pattern fidelity in tbx6 mutants relative to wild-type, as assessed by the incidence of scales bounded by the modal number 6 neighbors found in wild-type vs. numbers of neighbors other than 6 (χ2 = 110.7, d.f. = 1, p < 0.0001, N = 572 scales of 10 sibling controls and N = 551 scales of 10 tbx6 mutants). Bars in plots, means ± 95% CI. Scale bars, (A) 500 μm, (B) 1 mm.
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