Labial[edit]
The lab gene is the most anteriorly expressed gene. It is expressed in the head, primarily in the intercalary segment (an appendageless segment between the antenna and mandible), and also in the midgut. Loss of function of lab results in the failure of the Drosophila embryo to internalize the mouth and head structures that initially develop on the outside of its body (a process called head involution). Failure of head involution disrupts or deletes the salivary glands and pharynx. The lab gene was initially so named because it disrupted the labial appendage; however, the lab gene is not expressed in the labial segment, and the labial appendage phenotype is likely a result of the broad disorganization resulting from the failure of head involution.[7]
Proboscipedia[edit]
The pb gene is responsible for the formation of the labial and maxillary palps. Some evidence shows pb interacts with Scr.[8]
Deformed[edit]
The Dfd gene is responsible for the formation of the maxillary and mandibular segments in the larval head.[9] The mutant phenotypes of Dfd' are similar to those of labial. Loss of function of Dfd in the embryo results in a failure of head involution (see labial gene), with a loss of larval head structures. Mutations in the adult have either deletions of parts of the head or transformations of head to thoracic identity.[7]
Sex combs reduced[edit]
The Scr gene is responsible for cephalic and thoracic development in Drosophila embryo and adult.[10]