crocodile

DEVELOPMENTAL BIOLOGY

Embryonic

croc is expressed in the head anlagen of the blastoderm embryo under the control of the anterior, the dorsoventral and the terminal maternal organizer systems. croc is expressed initially in both the anterior and posterior regions of the embryo. In the anterior region, croc transcripts appear in a ventrally shifted "anterior cap," while the posterior expression domain consists of a transient ventrally located "posterior spot." During the cellular blastoderm stage the anterior cap retreats from the pole region and forms a tilted stripe covering the anlagen of the clypeolabrum and the anterior midgut including the esophagus and part of the intercalary segment on the blastoderm fate map. During gastrulation, the anterior cap retreats from the clypeolabral region. croc transcripts accumulate in cells associated with the developing foregut as well as a region corresponding to the intercalary segment anlage. In the posterior region, croc expression is reinitiated in an area corresponding to the developing mesoderm adjacent to the hindgut. During the extended germband stage, the anterior croc-expressing cells form a cluster of cells in association with the developing foregut that will eventually line the posterior pharynx wall. In addition, a number of croc expressing cells can be found in a metameric pattern within the developing mesoderm (Häcker, 1995). In the fully elongated germ band, additional sites of expression can be observed in a segmented pattern of precursor cells in the central nervous system (Häcker, 1992)

In Drosophila as well as many vertebrate systems, germ cells form extraembryonically and migrate into the embryo before navigating toward gonadal mesodermal cells. Just how the gonadal mesoderm attracts migratory germ cells is not well understood in any system. A genetic approach has been taken to identify genes required for germ cell migration in Drosophila. The role of zfh-1 is described in germ cell migration to the gonadal mesoderm. Zfh-1 protein is initially expressed in all mesodermal cells, but by stage 10, Zfh-1 levels have declined in most mesodermal cells, although high levels are maintained in extreme anterior and posterior mesodermal cells. The cells within the anterior cluster are likely to be hemocytes. During stage 10, Zfh-1-expressing mesodermal cells located at the posterior end of the embryo migrate anteriorly in two bilaterally symmetric groups between the endoderm and the interior of the dorsal mesoderm. These cells have been termed the 'caudal visceral mesoderm' as they contribute to the midgut musculature at later stages. Crocodile was used as a marker for the caudal visceral mesoderm. Croc is not expressed in the caudal visceral mesoderm in zfh-1 mutant embryos. Caudal visceral mesoderm is in close proximity to migratory germ cells during late stage 10. In zfh-1 mutant embryos, the initial association of germ cells with their final destination, gonadal mesoderm made up of the somatic gonadal precursors (SGP), is blocked. Instead, some germ cells remain attached to the gut, leading to a cluster of germ cells in the middle of the embryo during later stages of development (Broihier, 1998).

Effects of mutation or deletion

The croc mutant phenotype indicates that the croc wild-type gene is required to function as an early patterning gene in the anterior-most blastoderm head segment anlage and for the establishment of a specific head skeletal structure that derives from the non-adjacent intercalary segment at a later stage of embryogenesis. The clypeolabrum of croc mutant embryos is at least partially differentiated because the characteristic labral sensory organs and the labrum itself can be observed in croc mutant larvae. Internal clypeolabral structures were examined using ectodermal and mesodermal cell markers. Two dorsal rows of muscle attachment sites, apodemes, are present in croc mutant embryos, while the single ventral row of apodemes is missing. In addition, the palisade-like structure of the dorsopharyngeal muscles never forms, as revealed by antimyosin heavy chain antibody staining that labels muscle cells specifically. Thus croc is required for the stablishment of the normal dorso-pharyngeal muscle pattern (Häcker, 1995).

croc expression is maintained in intercalary derivatives, as eventually seen in a row of cells lining the posterior part of the pharynx where the posterior wall of the pharynx normally forms. Larval cuticle preparations show that the posterior wall of the pharynx is absent in croc mutants, and the ventral arm of the cephalopharyngeal skeleton is strongly reduced. Since the wg and en expression patterns are normal in the intercalary segment anlage of croc mutant embryos, it appears that croc plays a dual role at different levels during embryonic head development. Its early activity is transiently required for determinative events in etablishing the posterior part of the clypeolabrum, as reflected in the altered segment polarity gene expression patterns of en and wg, while its activity in the developing intercalary segment is not required for the establishment of the segment anlage per se, but rather for the differentiation of specific structural elements. No defects attributable to croc could be found in the posterior domain (Häcker, 1995).

Broihier, H. T., et al. (1998). zfh-1 is required for germ cell migration and gonadal mesoderm development in Drosophila. Development 125(4): 655-666.

Clark, K. L., et al. (1993). Co-crystal structure of the HNF-3/fork head DNA-recognition motif resembles histone H5. Nature 364 (6436): 412-420.

Grossniklaus, U., Cadigan, K. M. and Gehring, W. J. (1994). Three maternal coordinate systems cooperate in the patterning of the Drosophila head. Development 120: 3155-3171.

Häcker, U., et al. (1992). Developmentally regulated Drosophila gene family encoding the forkhead domain. Proc Natl Acad Sci 89: 8734-58.

Häcker, U., et al. (1995). The Drosophila fork head domain protein crocodile is required for the establishment of head structures. EMBO J 14: 5306-5317.

Hiemisch, H., Schutz, G. and Kaestner, K. H. (1998a). The mouse Fkh1/Mf1 gene: cDNA sequence, chromosomal localization and expression in adult tissues. Gene 220(1-2): 77-82.

Hiemisch, H., et al. (1998b). Expression of the mouse Fkh1/Mf1 and Mfh1 genes in late gestation embryos is restricted to mesoderm derivatives. Mech. Dev. 73(1): 129-32.

Hong, H. K., Lass, J. H. and Chakravarti, A. (1999). Pleiotropic skeletal and ocular phenotypes of the mouse mutation congenital hydrocephalus (ch/Mf1) arise from a winged helix/forkhead transcriptionfactor gene. Hum. Mol. Genet. 8(4): 625-37.

Jeong, J., et al. (2004). Hedgehog signaling in the neural crest cells regulates the patterning and growth of facial primordia. Genes Dev. 18: 937-951. 15107405

Kaestner, K. H., et al. (1993). Six members of the mouse forkhead gene family are developmentally regulated. Proc Natl Acad Sci U S A 90 (16): 7628-7631.

Kaestner, K. H., et al. (1996). Clustered arrangement of winged helix genes fkh-6 and MFH-1: possible implications for mesoderm development. Development 122 (6): 1751-1758.

Kidson, S. H., et al. (1999). The Forkhead/Winged-helix gene, Mf1, is necessary for the normal development of the cornea and formation of the anterior chamber in the mouse eye. Dev. Biol. 211(2): 306-22.

Koster, M., Dillinger, K. and Knochel, W. (1998). Expression pattern of the winged helix factor XFD-11 during Xenopus embryogenesis. Mech. Dev. 76(1-2): 169-73.

Kume, T., et al. (1998). The forkhead/winged helix gene Mf1 is disrupted in the pleiotropic mouse mutation congenital hydrocephalus. Cell 93(6): 985-96.

Mears, A. J., et al. (1998). Mutations of the forkhead/winged-helix gene, FKHL7, in patients with Axenfeld-Rieger anomaly. Am. J. Hum. Genet. 63(5): 1316-28.

Miura, N., et al. (1993). MFH-1, a new member of the fork head domain family, is expressed in developing mesenchyme. FEBS Lett 326 (1-3): 171-176.

Miura, N., et al. (1997). Isolation of the mouse (MFH-1) and human (FKHL 14) mesenchyme fork head-1 genes reveals conservation of their gene and protein structures Genomics 41 (3): 489-492.

Nishimura, D. Y., et al. (1998). The forkhead transcription factor gene FKHL7 is responsible for glaucoma phenotypes which map to 6p25. Nat. Genet. 19(2): 140-7.

Nissen, R. M., et al. (2003). Zebrafish foxi one modulates cellular responses to Fgf signaling required for the integrity of ear and jaw patterning. Development 130: 2543-2554. 12702667

Sasaki, H. and Hogan, B. L. M. (1993). Differentail expression of multiple fork head related genes during gastrulation and axial pattern formation in the mouse embryo. Development 118: 47-59.

Scheucher, M., et al. (1996). Transcription of four different fork headHNF-3 related genes (XFD-4, 6, 9 and 10) in Xenopus laevis embryos. Roux's Arch. Dev. Biol. 204: 203-211

Weigel, D. (1989). The homeotic gene forkhead encodes a nuclear protein and is expressed in the terminal regions of the Drosophila embryo. Cell 57:645-658. 89249328

White, J. T., et al. (2010). Notch signaling, wt1 and foxc2 are key regulators of the podocyte gene regulatory network in Xenopus. Development 137(11): 1863-73. PubMed Citation: 20431116

Winnier, G. E., Hargett, L. and Hogan, B. L. (1997). The winged helix transcription factor MFH1 is required for proliferation and patterning of paraxial mesoderm in the mouse embryo. Genes Dev. 11(7): 926-40.

date revised: 15 December 2010

Home page: The Interactive Fly © 1997 Thomas B. Brody, Ph.D.

The Interactive Fly resides on the
Society for Developmental Biology's Web server.