Distinct functions of the Tribolium zerknüllt genes in serosa specification and dorsal closure.
BACKGROUND
In the long-germ insect Drosophila, a single extraembryonic membrane, the amnioserosa, covers the embryo at the dorsal side. In ancestral short-germ insects, an inner membrane, the amnion, covers the embryo ventrally, and an outer membrane, the serosa, completely surrounds the embryo. An early differentiation step partitions the uniform blastoderm into the anterior-dorsal serosa and the posterior-ventral germ rudiment giving rise to amnion and embryo proper. In Drosophila, amnioserosa formation depends on the dorsoventral patterning gene zerknüllt (zen), a derived Hox3 gene.


RESULTS
The short-germ beetle Tribolium castaneum possesses two zen homologs, Tc-zen1 and Tc-zen2. Tc-zen1 acts early and specifies the serosa. The loss of the serosa after Tc-zen1 RNAi is compensated by an expansion of the entire germ rudiment toward the anterior. Instead of the serosa, the amnion covers the embryo at the dorsal side, and later size regulation normalizes the early fate shifts, revealing a high degree of plasticity of short-germ development. Tc-zen2 acts later and initiates the amnion and serosa fusion required for dorsal closure. After Tc-zen2 RNAi, the amnion and serosa stay apart, and the embryo closes ventrally, assuming a completely everted (inside-out) topology.


CONCLUSIONS
In Tribolium, the duplication of the zen genes was accompanied by subfunctionalization. One of the paralogues, Tc-zen1, acts as an early anterior-posterior patterning gene by specifying the serosa. In absence of the serosa, Tribolium embryogenesis acquires features of long-germ development with a single extraembryonic membrane. We discuss implications for the evolution of insect development including the origin of the zen-derived anterior determinant bicoid.
van der Zee
Maurijn
M
Institut für Entwicklungsbiologie, Universität zu Köln, Gyrhofstrasse 17, D-50931 Köln, Germany.
Berns
Nicola
N
Roth
Siegfried
S
D000818:Animals; D019521:Body Patterning / Q000235:genetics*; D003001:Cloning, Molecular; D017931:DNA Primers; D029721:Drosophila Proteins; D004625:Embryo, Nonmammalian / Q000378:metabolism; D047108:Embryonic Development / Q000235:genetics*; D004848:Epithelium / Q000378:metabolism; D019143:Evolution, Molecular; D020440:Gene Duplication; D015870:Gene Expression; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism*; D017403:In Situ Hybridization; D020287:In Situ Nick-End Labeling; D019476:Insect Proteins; D018613:Microscopy, Confocal; D034622:RNA Interference; D012704:Serous Membrane / Q000196:embryology*; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*; D014231:Tribolium / Q000196:embryology* / Q000235:genetics
D017931:DNA Primers; D029721:Drosophila Proteins; D018398:Homeodomain Proteins; C400681:Hox3 protein, Ciona intestinalis; C499580:Hox3 protein, insect; D019476:Insect Proteins; D015534:Trans-Activators; C056478:bcd protein, Drosophila