Gbdpp expression in cricket leg buds: spots in T1/T2 and rings in T3. Asterisks indicate non-specific staining.

The study of fruitflies has yielded huge dividends for our understanding of development, and the comparison of the fruitfly to other insects is proving equally beneficial for understanding morphological evolution. A recent report shows that the difference between the legs of fruitflies and crickets, for example, may depend only on the expression pattern of a single gene.

Crickets and fruitflies belong to two different superorders of the Insecta class. The two-spotted cricket (Gryllus bimaculatus) is a hemimetabolous insect — it hatches as a miniature adult and grows until it reaches its adult size. Holometabolous insects, such as the fruitfly Drosophila melanogaster , hatch as larvae that contain internal sacs ( imaginal discs) from which the adult appendages, such as the wings and legs, develop at metamorphosis.

Flies fly, whereas crickets have a spring in their step thanks to some specialised muscles that are present only in their third pair of legs (the T3 legs). So how do the developmental mechanisms controlling leg development compare in these two insects? Three developmental genes are required for patterning the fruitfly leg: hedgehog ( Dmhh), wingless ( Dmwg) and decapentaplegic ( Dmdpp). Niwa et al. have shown that orthologues of these three genes are also present in Gryllus, where their expression profiles in the embryo and the leg match very closely those seen in the developing fly. In the Drosophila leg imaginal disc and in the cricket leg bud, wg is expressed along the ventral side of the anterior–posterior boundary and Hh protein fills the posterior compartment. The one important exception is the expression of dpp in the leg. Instead of being expressed as a strong dorsal strip and a weak ventral one (as in the fly leg disc), Gbdpp expression first appears as dots along the dorsal side of the leg bud, and later turns into five circumferential rings. These rings are thought to coincide with the cricket's five leg segments along the proximo–distal axis. Curiously, this circumferential expression of dpp breaks down in legs T1 and T2, but is retained in the T3 pair of legs, in precisely those segments that will develop the strong jumping muscles.

This similar expression of the three key patterning genes is important as it suggests a fundamental conservation in the control of leg development, whereas the divergent distribution of dpp offers an explanation for differences in leg morphology. This study extends our knowledge of Drosophila to one more insect species, but there's still a million or so to go!