Chapter 19, Problem 22P

In addition to the maternal effect genes that establish anterior/posterior polarity in the Drosophila embryo (like bicoid and nanos), other maternal effect genes, including dorsal, pelle, and Toll, independently determine dorsal/ventral polarity. The dorsal gene encodes a transcription factor (Dorsal), originally deposited in the egg cytoplasm, that determines ventralness in a concentration-dependent manner. As shown in the following figure(Wild type), a gradient of Dorsal nuclear localization exists in early embryos: Cells whose nuclei have the highest Dorsal protein concentration become the ventral-most cells, cells whose nuclei have no Dorsal protein aredorsal-most, and lateral cells “learn” their positions and fates through the particular intermediate Dorsal proteinlevels in their nuclei. The figure shows sections through blastoderm embryos, where D = dorsal and V = ventral

Embryos from mothers homozygous for dorsal null mutations are dorsalized-every cell along the dorsal/ventral axis “thinks” it is the ventral-most cell. Embryos from mothers homozygous for loss-of-function pelle mutations or heterozygous for gain-of-function (constitutively active) Toll^D> mutations show altered patterns of Dorsal protein localization, as shown in the figure. Pelle and Toll expression are unaltered in dorsal loss-of-function mutants (not shown).

a.	Describe the alterations in Dorsal protein localization in embryos produced by pelle-/pelle- or TollD/Toll+mutant mothers.
b	Based on the information given, order the dorsal, pelle, and Toll genes in a pathway.
c.	The two kinds of embryos described in part (a) die just before hatching. Describe their morphological mutant phenotypes