The making of a maggot brain

Abstract

The way neurons in the brain rewire in larvae as they turn to adult fruit flies sheds light on how complete metamorphosis was 'invented' over the course of evolution.

Keywords: D. melanogaster; brain; developmental biology; evolutionary biology; interneurons; metamorphosis; mushroom body; neurons; trans-differentiation.

Figure 1.. Evolutionary origins of metamorphosis.

(A) Complete metamorphosis – where an animal goes through a larval and pupal life stage before developing into an adult – appeared relatively late in the insect evolutionary family tree during the advent of winged insects from the Pterygota family. It is the defining feature of a group of insects known as the Holometabola, which includes butterflies and flies (shown on the right). Hemimetabolous insects, such as grasshoppers (shown on the left), on the other hand, emerge as nymphs that look like adults (except for their lack of wings and genitalia), and gradually develop into mature adults after a series of molts. (B) Top: Development in evolutionarily ancestral insects, such as grasshoppers, follows a straight trajectory (black line). From the egg hatches a miniature and sexually immature version of the adult that eventually develops via multiple growth molts (not shown). Bottom: In insects with complete metamorphosis, development involves a detour away from the ancestral trajectory (magenta) to establish a larva. Development resumes back towards the evolutionarily ancestral trajectory (green) via a pupal stage (not shown), giving rise to the adult fly. The horizontal grey arrow indicates the so-called imaginal discs – cells that are stored and arrested early during development – to be available later for establishing the adult fly.