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First published online 10 June 2009
doi: 10.1242/dev.035451

Development 136, 2393-2402 (2009)
Published by The Company of Biologists 2009
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Differential and overlapping functions of two closely related Drosophila FGF8-like growth factors in mesoderm development

Anna Klingseisen1, Ivan B. N. Clark1, Tanja Gryzik2,* and H.-Arno J. Müller1,{dagger}

1 Division of Cell and Developmental Biology, College of Life Sciences, University of Dundee, Dundee DD1 5EH, UK.
2 Institut für Genetik, Heinrich Heine Universität Düsseldorf, 40225 Düsseldorf, Germany.

{dagger} Author for correspondence (e-mail: h.j.muller{at}dundee.ac.uk)

Accepted 14 May 2009

Thisbe (Ths) and Pyramus (Pyr), two closely related Drosophila homologues of the vertebrate fibroblast growth factor (FGF) 8/17/18 subfamily, are ligands for the FGF receptor Heartless (Htl). Both ligands are required for mesoderm development, but their differential expression patterns suggest distinct functions during development. We generated single mutants and found that ths or pyr loss-of-function mutations are semi-lethal and mutants exhibit much weaker phenotypes as compared with loss of both ligands or htl. Thus, pyr and ths display partial redundancy in their requirement in embryogenesis and viability. Nevertheless, we find that pyr and ths single mutants display defects in gastrulation and mesoderm differentiation. We show that localised expression of pyr is required for normal cell protrusions and high levels of MAPK activation in migrating mesoderm cells. The results support the model that Pyr acts as an instructive cue for mesoderm migration during gastrulation. Consistent with this function, mutations in pyr affect the normal segmental number of cardioblasts. Furthermore, Pyr is essential for the specification of even-skipped-positive mesodermal precursors and Pyr and Ths are both required for the specification of a subset of somatic muscles. The results demonstrate both independent and overlapping functions of two FGF8 homologues in mesoderm morphogenesis and differentiation. We propose that the integration of Pyr and Ths function is required for robustness of Htl-dependent mesoderm spreading and differentiation, but that the functions of Pyr have become more specific, possibly representing an early stage of functional divergence after gene duplication of a common ancestor.

Key words: Gastrulation, Mesoderm migration, Heart development, Fibroblast growth factor


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