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).


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crocodile: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation

date revised: 15 December 2010

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