RUI: Embryonic Control of Metamorphic Remodeling in Neural Crest-Derived Skeleton of Xenopus Laevis

James Madison University, Harrisonburg VA

Investigators

Abstract

Many animals develop two distinct body forms. In amphibians, the earlier, larval body form is designed for life in water, and the later, adult form, for life on land. The intervening metamorphosis involves loss of some larval features, remodeling of others, and gain of adult features. All changes are regulated by thyroid hormones (TH). One long-standing hypothesis for explaining the widespread occurrence and long-term persistence of metamorphosis is that metamorphosing animals have evolved separate mechanisms for developing larval and adult features. This decoupling of larval and adult development frees the two stages to evolve different body forms that are well suited to their respective environments. By exploiting two environments over their lifetimes, the members of one species thus face less competition with each other for essential resources. However, there have been few attempts to test this "developmental decoupling" hypothesis on amphibians and none that specifically address whether the larval and adult features are controlled by the same programs of gene expression. Hox genes, which are expressed in different combinations in the cells that form the lower jaw, throat and gill cartilages, trigger genetic programs that produce the distinct shapes of these larval cartilages. The study will address whether these genetic programs also control how these cartilages respond to TH. In tadpoles, the lower jaw responds with cell proliferation, the gill cartilages with cell death, and the intervening throat cartilage with both. The study will use grafting experiments to produce tadpoles with a lower jaw cartilage in place of a gill cartilage, and vice versa and the tadpoles will subsequently be exposed to TH to determine if the TH response of the out-of-place cartilage is affected by its new environment, or whether it retains the response expected from its Hox gene expression prior to grafting. In addition, eggs will also be injected with either a blocker of Hox gene activity or an extra Hox gene to produce tadpoles with a lower jaw cartilage in the shape of a throat cartilage, or vice versa and these tadpoles will also be exposed to TH to determine if the cartilage with a transformed larval shape also has a transformed TH response. Determination of how TH response is controlled will aid in understanding whether metamorphosis in amphibians represents a true decoupling of larval and adult development.