Abstract
The direction of visceral organ asymmetry is highly conserved during vertebrate evolution with heart development biased to the left and pancreas and liver development restricted to opposing sides of the midline. Here we show that reversals in visceral organ asymmetry have evolved in Astyanax mexicanus, a teleost species with interfertile surface-dwelling (surface fish) and cave-dwelling (cavefish) forms. Visceral organ asymmetry is conventional in surface fish but some cavefish have evolved reversals in heart, liver, and pancreas development. Corresponding changes in the normally left-sided expression of the Nodal-Pitx2/Lefty signaling system are also present in the cavefish lateral plate mesoderm (LPM). The Nodal antagonists lefty1 (lft1) and lefty2 (lft2), which confine Nodal signaling to the left LPM, are expressed in most surface fish, however, lft2, but not lft1, expression is absent during somitogenesis of most cavefish. Despite this difference, multiple lines of evidence suggested that evolutionary changes in L-R patterning are controlled upstream of Nodal-Pitx2/Lefty signaling. Accordingly, reciprocal hybridization of cavefish and surface fish showed that modifications of heart asymmetry are present in hybrids derived from cavefish mothers but not from surface fish mothers. The results indicate that changes in visceral asymmetry during cavefish evolution are influenced by maternal genetic effects.