PRICE, A.L.; PATEL, N.H.; University of Chicago, Illinois; Univ. of California, Berkeley: With or without you…The role of mesoderm in ectoderm segmentation in crustaceans

The segmental characteristics of the ectoderm and mesoderm as well as the proximity of the germ layers during morphogenesis suggests that signaling may be required between these two layers for proper segmentation. This hypothesis has been tested in both fruit flies and leeches. In the fruitfly, D. melanogaster, twist or snail mutants that do not form mesoderm still display ectodermal segmentation. In contrast, ablation studies in leeches show that mesoderm is necessary for proper segmentation of ectoderm. One reason for these differences may be that these two animals show very different modes of development. In D.melanogaster, segments are established molecularly in a syncitial blastoderm without growth whereas the body axis of leeches elongates by the asymmetric division of teloblasts and segmentation occurs in an anterior to posterior progression. Most arthropods do not develop like D. melanogaster, but instead elongate by anteroposterior growth in a cellular environment. We are working with a crustacean, Parhyale hawaiensis, which displays more typical elongation of the germ band during a period of growth with segmentation occurring sequentially from anterior to posterior. In Parhyale, ectodermal parasegments form as cells organize into rows and undergo stereotypical divisions, simultaneously the mesoderm develops by asymmetric division of mesoteloblasts. By ablating mesoteloblasts, we show that segmentation of ectoderm occurs in the absence of mesoderm. In addition, it has been suggested that there may be signaling between ectoderm and mesoderm during outgrowth of limbs. We show that growth of the limbs occurs in the absence of mesoderm in Parhyale.