Homozygous embryos exhibit defects in posterior pattern. Injection of wild type RNA fails to rescue the defects.

Class 1 allele: Unrescued embryos (lacking maternal and zygotic phl activity) fail to differentiate into structured embryos and degenerate around 7 hrs of development. Rescued embryos (carrying wild type phl from their father) miss most structures posterior to abdominal segment 6 or 7 and anteriorly a portion of the cephalopharyngeal skeleton, labral sense organ and medial tooth are deleted. fkh expression (in wild type evident in hindgut, Malpighian tubule(s) and anal pads) entirely absent from class 1 embryos. Expression of tll, hkb, hb, fkh absent from both rescued and unrescued embryos, 7th ftz stripe deleted and 6th ftz stripe variably deleted/expanded. Conclude from 0% to 20--25% EL of blastoderm embryos deleted in class 1 alleles.

A hole is seen in the blastoderm layer below the pole cells in embryos. Ventral furrow formation extends posteriorly to approximately 5% egg length. The posterior midgut invagination does not occur. 10% of embryos twist along the dorsal-ventral axis at germband extension to produce corkscrew shaped embryos. Comparison of embryos derived from homozygous germline clones reveal a zygotic requirement for phl : "pole hole" embryos (which have zygotic phl function) undergo germ band shortening and show the "terminal class" phenotype upon completing development. Posterior phenotype: A deletion of terminal identities is observed and cells forming in this region assume more anterior fates. The remaining most posterior segments (typically A5, A6 and A7) are expanded. This altered segmental pattern is first seen at the blastoderm stage. Deformation of the CNS ladder is observed for many embryos after nerve cord condensation. Usually the anterior and posterior commissures are normal in the last complete terminal segment, but breaks in the longitudinal axis of the CNS are seen. In some cases the sensory cells of the PNS are displaced laterally in the expanded abdominal segments. In addition, fusion of the dorsal cell cluster along the dorsal midline is observed, and the positions of the lateral chordotonal organs are shifted dorsally in the most posterior complete segment of the embryo. Anterior phenotype: Several head defects are also apparent in mature "pole hole" embryos, these arise in structures derived from the labrum and acron. Much of the supraoesophageal ganglia (brain) is missing at 16 hours. The optic lobes are also altered. The labral sense organ (epiphysis) is missing. Cell death is seen in the head and tail regions. In contrast, although "null" embryos (which have no zygotic phl function) appear phenotypically equivalent to "pole hole" embryos until the completion of germband extension, development ceases at this elongated germband stage, and they do not undergo germband shortening. After 20 hours of development the embryos consist of a ball of embryonic tissue at the anterior egg pole, and an extruded mass of egg yolk at the posterior pole. The overall organisation and differentiation of organ systems within the embryo is abnormal, due to incomplete development and massive cell death. The development of the nervous system is severely affected.