Decision letter: Quantitative system drift compensates for altered maternal inputs to the gap gene network of the scuttle fly Megaselia abdita

Abstract

Similar biological phenomena can result from different processes occurring in different organisms. For example, the early stages of how an insect develops from an egg can vary substantially between different species. Nonetheless, all insects have a body plan that develops in segments. The same outcome occurring as a result of different developmental steps is known as ‘system drift’, but the mechanisms underlying this phenomenon are largely unknown. How the body segments of the fruit fly Drosophila develop has been extensively studied. First, a female fruit fly adds messenger RNA (or mRNA) molecules copied from a number of genes into her egg cells. These mRNA molecules are then used to produce proteins whose concentration varies along the length of each egg. These proteins in turn switch on so-called ‘gap genes’ in differing amounts in different locations throughout the fruit fly embryo. The activity of these genes goes on to define the position and extent of specific segments along the fruit fly's body. Like the fruit fly, the scuttle fly Megaselia abdita has a segmented body. However, mothers of this species deposit somewhat different protein gradients into their eggs. How the regulation of development differs in the scuttle fly to compensate for this change is unknown. Now, Wotton et al. have studied, in detail, how gap genes are regulated in this less well-understood fly species to understand the mechanisms responsible for a specific example of system drift. In the fruit fly, gap genes normally switch-off (or reduce the expression of) other gap genes within the same developing body segment, and Wotton et al. found that the same kind of interactions tended to occur in the scuttle fly. As such, the overall structure of the gap gene network was fairly similar between scuttle and fruit flies. There were, however, differences in the strength of these interactions in the two fly species. These quantitative differences result in a different way of making the same segmental pattern in the embryo. In this way, Wotton et al. show how tinkering with the strength of specific gene interactions can provide an explanation for system drift.