Unlocking the Secrets of Fruit Fly Development
The fruit fly was established as a model organism in the 1900s by Thomas Hunt Morgan, and it is still widely studied today. Claudia Mieko Mizutani, a professor at Case Western Reserve University, studies the fruit fly to try and explain the embryonic development of insects.
The embryo is divided into germ layers called the mesoderm, endoderm, and ectoderm, which develop into the different systems of the adult organism. Mizutani studies a subset of the ectoderm called the neuroectoderm. As its name implies, neuroectoderm develops into the body’s nervous system. Interestingly though, its size remains constant among a wide range of insect species.
The lab first looked at the mesoderm, which is located just below the neuroectoderm, to determine whether it influences the size of the neuroectoderm .To do this, they investigated the molecular differences between the mesoderm of different species of fruit flies including Drosophila busckii, Drosophila melanogaster, Drosophila simulans, and Drosophila sechellia. While each of these species has a uniquely sized mesoderm, the size of their neuroectoderms is the same.
The mesoderm is generated by activation of the Toll signaling pathways. A signaling molecule inside of the embryo called Dorsal influences the differentiation of mesoderm by activating mesoderm specific genes, such as the snail gene. Dorsal is normally kept outside of the nucleus by a protein called Cactus, but once the Toll signaling pathway is activated, Cactus is degraded and Dorsal is free to infiltrate the cell nucleus and change DNA expression.
The results showed that, among the different species of fruit flies, the number of nuclei, nuclei size, Dorsal signaling gradient, range of Toll signaling pathways, and nuclear density were all variable. Together, these differences allow for larger species to scale down the size of their mesoderms to maintain neuroectoderm size as well as for smaller species to generate larger mesoderms to maintain this same neuroectoderm size.
Mizutani found that the size of the neuroectoderm could be conserved through evolutionary history by altering the size of the mesoderm through a variety of mechanisms. Though Mizutani claims her work is unfinished, she said, “I really want to know how the size of the neuroectoderm is conserved amongst a variety of species. We have shown that the mesoderm changes size in different species in order to maintain neuroectoderm size, but the actual mechanism is still unresolved.”
