Maternally-regulated gastrulation as a source of variation contributing to cavefish forebrain evolution

Abstract

Sequential developmental events, starting from the moment of fertilization, are crucial for the acquisition of animal body plan. Subtle modifications in such early events are likely to have a major impact in later morphogenesis, bringing along morphological diversification. Here, comparing the blind cave and the surface morphotypes of Astyanax mexicanus fish, we found heterochronies during gastrulation, producing organizer and axial mesoderm tissues with different properties, including differences in expression of dkk1b, that may have contributed to cavefish brain evolution. These variations observed during gastrulation depend fully on maternal factors, whereas later phenotypic differences in neural development became progressively hidden when zygotic genes take the control over development. Transcriptomic analysis of fertilized eggs from both morphotypes and reciprocal F1 hybrids showed a strong and specific maternal signature. Our work strongly suggests that maternal effect genes and developmental heterochronies occurring during gastrulation have impacted morphological brain change during cavefish evolution.