MINSUK, S.B.*; RAFF, E.C.; RAFF, R.A.; Indiana University, Bloomington: Hen’s Teeth and Sea Urchin’s Toes

How are developmental processes transformed in evolution? The direct-developing sea urchin Heliocidaris erythrogramma (H.e.) has a radically different larval morphology from that of the indirect-developing H. tuberculata (H.t.) and their common ancestors, reflecting losses, gains, and transformations throughout ontogeny. By combining the divergent developmental programs of these two species we can observe their interaction. Previously, hybrid embryos revealed that H.e. eggs can express ancestral features in the presence of a paternal H.t. genome. But in order to discover what developmental potential is present in the H.e. embryo itself, we tested inductive interactions in chimaeric embryos. Intact larvae have at least two important mesoderm-ectoderm interactions: the patterning and morphogenesis of larval skeleton, and the induction of adult rudiment development. Since the large H.e. and the tiny H.t. embryos cannot be meaningfully combined, we made hybrid-plus-H.e. chimaeric embryos. Archenteron (presumptive endoderm and coelomic mesoderm) taken from the mid-gastrula of one embryo was implanted into ectoderm from another embryo of the same stage. Both embryos contributed mesenchyme cells. Ectoderm alone, and non-chimaeric recombinants, were used as controls. H.e. mesoderm induced rudiment development in hybrid ectoderm even more robustly than hybrid mesoderm did, demonstrating that rudiment induction is conserved despite radical changes in the larvae. Most dramatically, H.e. ectoderm, which never makes larval arms, was able to do so in combination with hybrid mesenchyme, revealing the mechanism of larval arm loss in H.e. Inducing ability has been lost by H.e. mesenchyme, and is rescued in hybrids. Hybrid mesenchyme can even induce H.e. ectoderm, which has therefore retained the ability to build larval arms and respond to appropriate signals.