TH regulates squamation by relieving repressive activities of unliganded TRs. (A) Euthyroid (euTH) individuals homozygous for a presumptive null allele of thyroid hormone receptor gene thrab had an overtly normal pattern of squamation. Whereas hypoTH individuals heterozygous for the thrab mutation developed numerous, miniaturized scales, hypoTH animals homozygous mutant for thrab had a normal pattern of squamation. Visualized in sp7:EGFP transgenics (n ≥ 6 each genotype). (B) In euTH fish (n = 16 heterozygotes, n = 7 homozygous mutants), loss of thrab did not significantly affect standard length (SL) at squamation onset whereas in hypoTH fish (n = 6 heterozygotes, n = 16 homozygous mutants) loss of thrab led to a significant rescue towards the SL at squamation onset of euTH fish. Shared letters above groups indicate means that were not significantly different from one another (p > 0.05) in post hoc Tukey-Kramer comparisons (overall ANOVA, genotype x TH status interaction F1,41 = 183.4, p < 0.0001) (C) thrab mutation did not affect adult scale number in euTH fish (n = 6 each genotype), but consistent with the rescue of delayed squamation onset, thrab mutation significantly rescued adult scale number in hypoTH fish (n = 5 each genotype) towards wild-type levels. Scale bar, (A) 1 mm.
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