Modeling Biological Networks

• IV.6.v Subproject 5 – Gene Complexes in the Evolution of Development
  • IV.6.v.a Background and Overview
  • IV.6.v.b Preliminary Results
  • IV.6.v.c Specific Aims
• [ Other Subprojects ]

< Previous | Page 26 of 35 | Next >

IV.6.v Subproject 5 - Gene Complexes in the Evolution of Development:

We can apply the methods presented in Subprojects 1 - 4 to subsets of the genome of more complex organisms. The observations in these Subprojects make specific predictions about the network connectivity of key regulatory genes and in turn provide a methodology for seeking new key genes. These methods may have direct biological application in helping to unravel the genetic relation and evolutionary origins of different developmental programs. Our goal is to understand how two apparently incompatible and grossly divergent modes of development could evolve by mutation from a common precursor. Based on our earlier observations, we expect that small changes to key authority genes could result in complete reorganization of developmental programs.

The evolution of novel morphologies of complex organisms requires remodeling of developmental processes. We seek to define the mechanisms by which early development has evolved in two closely related and experimentally accessible animals. We exploit two sea urchin species of the genus Heliocidaris, which have evolved radically different patterns of development since their (4 mya) divergence. The changes illustrate the unexpected plasticity of embryological programs, and offer a model in which we can dissect their molecular and cellular mechanisms. H. tuberculata has a 90 µm diameter egg, and produces an indirect-developing feeding pluteus larva. Metamorphosis to the adult occurs in about 6 weeks. In contrast, the 430 µm H. erythrogramma egg generates a highly modified non-feeding direct-developing larva.

The H. erythrogramma larva has not simply lost larval feeding structures and gut, but has gained many new features specific to direct development that allow accelerated development of adult structures. Thus, evolution of development has involved coordinate changes in zygotic and maternal pathways.

We have taken advantage of the ease of making cross species hybrids in sea urchins. Hybrids between species that develop via pluteus larva can in many cases develop well, even to metamorphosis. These crosses can be across genera or families. In addition, hybrids between indirect-developing and direct-developing species are possible. We can fertilize eggs of H. erythrogramma with sperm of H. tuberculata, and these He x Ht hybrid embryos develop through metamorphosis. Hybrids exhibit restoration of feeding larval structures and paternal gene expression lost in the evolution of the direct-developing maternal species. These pluteus features are present in both the paternal species and in the immediate common ancestor of the two species. Hybrids produce a novel yet harmonious ontogeny, resulting from interaction of genomes encoding two disparate modes of development.

Cross-developmental mode hybrids provide a unique tool in dissecting developmental features in the evolution of development. They reveal that complex developmental processes are robust, even when two disparate genomes have to operate together in a hybrid ontogeny. Hybrids also show that a small number of regulatory genes determine evolution of expression of discrete larval features. This observation is consistent with the theoretical prediction that a small number of large-effect genetic changes may underlie initial events in macroevolution and with empirical observations on other organisms. We have identified candidate regulatory genes involved in the evolution of developmental mode, and tested their roles through functional assays to confirm the function of specific isolated genes in evolution of development of complex larval features.