Pattern formation inDrosophila melanogaster: The effects of mutations on polarity in the developing leg

Developmental effects of a temperature-sensitive RNA polymerase II mutation in Drosophila melanogaster

Drosophila melanogaster possessing a temperature-sensitive (ts) mutation that maps to an X-linked locus ( RpII215 ) (the locus has also been called l(1)L5 and Ultrabithorax -like or Ubl ) encoding a subunit of RNA polymerase II are fertile at 22 degrees C but become sterile when shifted to 29 degrees C. Homozygous RpII215ts adult females shifted to 29 degrees C lay structurally normal eggs for 24 hr, after which increasing numbers of eggs are abnormal. Eggs left to develop at 29 degrees C die as morphologically normal late embryos or first instar larvae when produced by females maintained at 29 degrees C for less than 6 hr. However, eggs produced by females undergoing oogenesis at 29 degrees C for longer than 6 hr develop abnormally, displaying holes primarily in their ventral cuticle and possessing an abnormal pharyngeal apparatus. As exposure of females to 29 degrees C lengthens there is an increase in the severity of these defects. Some of the eggs can be rescued by either mating RpII215ts females to wild-type males or shifting the eggs to 22 degrees C. The percentage of eggs rescued decreases with increased length of oogenesis at 29 degrees C, up to 20 hr, at which point they are no longer rescuable. The terminal phenotype of eggs that fail to be rescued by the above procedure is less extreme than that of eggs for which no rescue attempt was made. Holes in the ventral cuticle are reduced or absent, but pattern formation is disrupted such that segments are often missing, incorrectly oriented or fused. Because the RpII215 locus encodes a subunit of RNA polymerase II, the developmental defects described above are most likely due to reduced or aberrant transcription during oogenesis and early embryogenesis. This postulated effect on transcription results, in part, from the maternal loading of a gene product(s) that is thermolabile in eggs.

Intercalary regeneration in legs of crayfish: proximal segments

An arthropod leg represents a protuberance of the body segmental integument which bears distinctive markers in both the mediolateral and the anteroposterior axes. To clarify the biaxial organization of the body segmental morphogenetic field, and to study the relation among the whole-limb, limb segmental, and body segmental fields previously recognized in arthropods, we have grafted a proximal leg segment into the ventral midline in crayfish. After this operation the majority of animals regenerated a mirror-symmetric pair of supernumerary legs at the host site. Some of these legs had the most proximal segment, the coxa, partially fused to the adjacent body surface. Minority patterns of regeneration included one midline leg with a gill, three midline legs with a gill, and two normal legs with a third double-half leg. These results are compatible with the principle that intercalary regeneration restores the continuity of positional information.