How the fruit fly gets its stripes

In the fruit fly Drosophila melanogaster, mutations that produced odd-looking or inviable flies were known early in the twentieth century, but it was several decades before their nature began to be understood. Working at the European Molecular Biology Laboratory (Heidelberg, Germany) in the late 1970s, Christiane Nüsslein-Volhard and Eric Wieschaus were among the first to use such mutations to study a complex dilemma: how a multicellular animal develops from a single fertilized egg. At that time, the idea that mutations confer discrete, generally reproducible phenotypes was less established than it is now, and many biologists doubted it was possible to separate out the actions of individual genes during development. Nonetheless, Nüsslein-Volhard and Wieschaus set out systematically to identify mutations relating to one aspect of fly development: the division of the embryo into a linear series of segments.

In work that later garnered them the 1995 Nobel prize in Physiology and Medicine (along with Edward Lewis, an earlier pioneer of developmental genetics), Nüsslein-Volhard and Wieschaus treated flies with mutagens to generate a large number of offspring carrying independent mutations — affecting most of the genes in the fly genome. They mated the flies to produce descendants with two copies of each mutation and looked at larvae under a microscope to identify malformations that were caused by aberrant segmentation.

At the blastoderm stage, fly embryos become organized into 14 segments. By the time larvae hatch, each segment has a band of backward-pointing projections, called denticles, at its front end, whereas the back end is naked. Nüsslein-Volhard and Wieschaus used these features to distinguish individual segments and identify abnormal patterns of segmentation. By recombination mapping and complementation testing, they mapped the underlying mutations to 15 genes and found that they fitted into three distinct categories: segment-polarity, pair-rule and gap mutations. Segment-polarity mutations cause deletion of parallel parts of each segment. Pair-rule mutations (for instance, even skipped) cause deletions in alternate segments. Gap mutations cause deletion of groups of adjacent segments, corresponding to structurally distinct areas (for instance, the abdomen). The mutant embryos were identifiable shortly after the start of gastrulation, indicating that segmentation genes are active early in development.

What did these patterns of mutations say about the sequence of developmental events? Nüsslein-Volhard and Wieschaus proposed that gap genes help establish large-scale body patterns, whereas the segment-polarity and pair-rule genes control segmentation. The two-segment expression pattern of pair-rule genes, they suggested, could reflect an initial segmentation into seven double segments that later divide in half — perhaps avoiding errors that could arise in dividing the relatively few cells of the blastoderm evenly into 14 segments.

Later research showed that the fruit fly body plan is dictated by both maternal proteins and differential responses of embryonic genes to those proteins. The maternal proteins establish patterns of polarity in the unfertilized egg that carry forward into the embryo. Interactions between maternal and embryonic genes control location-specific cell differentiation to generate the larval and adult body plans.

Researchers have analysed the relationships between segmentation proteins to reveal complex regulatory interactions, generally bearing out Nüsslein-Volhard and Wieschaus's initial theories. Gap proteins are expressed within 2–3 hours after fertilization and divide the embryo into large areas along the head–tail axis. They also serve as transcription factors to regulate the expression of pair-rule genes. Pair-rule proteins set up boundaries within these areas, creating seven fundamental regions, and they also regulate other pair-rule genes or segment-polarity genes. Finally, segment-polarity proteins, which include transcription factors and cell signalling proteins, divide each of the seven regions into two to form the 14 larval segments. The role of segmentation genes is not confined to early embryogenesis — pair-rule genes, in particular, show complex patterns of expression later in development, and they help to control the differentiation of numerous body parts and tissues, the list of which continues to grow.