CHEN, S; FRIEDRICH, M; Wayne State University; Wayne State University: Analysis of cell proliferation during embryonic leg development in the grasshopper Schistocerca americana

Exhibiting an extaordinary range of morphological diversification, insect appendages serve as a useful paradigm to study molecular developmental mechanisms in the evolution of animal form. A particularly striking example is the hind leg of orthopteran insects such as grasshopper, which evolved into specialized jumping leg. While the first two grasshopper leg pairs (T1 and T2) conform to the insect leg bauplan, femur and tibia of the hind leg pair (T3) are greatly enlarged and modified. These differences correlate with the specific expression of the selector gene Ultrabithorax throughout embryonic development of the T3 leg. As a first attempt to explore the mechanism leading to differential T3 leg growth, we analyzed the relationship between length increase and ectodermal cell proliferation in tibia and femur by quantitative analysis of tissue preparations double labeled with propidium iodide and anti-phospho-histone 3 antibody. We find that although an increase of T3 over T1/T2 length becomes manifest from 35% of development, the time window of most pronounced differential T3 length increase extends from 50 to 70% of development. Surprisingly, in all legs mitotic activity consistently decreases from 45% of development and arrests completely at 65% of development. In both tibia and femur ectoderm, a slightly higher ratio of mitotic cells is observed in T3 compared to T1/T2 throughout most but not all stages. Our results indicate that differential mitotic activity may contribute to the differential T3 leg growth during the initial phase of leg development while much of the length increase during the second half of embryogenesis is caused by mechanisms different from cell proliferation.