| Establishing the Gradient and Coordinate Genes |
Development is a process where the products of some genes turn other genes on or off. But how does the process start? Even before fertilization, development is occurring. We normally think of an egg as a storehouse of energy supply and nutrients that the embryo will use as it develops. While this is true, the egg also supplies information to establish a molecular coordinate system. This coordinate system provides a way of telling "which end is up"; in other words the location of the embryo's head is determined even before the egg is fertilized.
Coordinate genes are named because they establish the primary coordinate system for what will become the embryo. One important example of a coordinate gene is bicoid, which is involved in establishing the anterior-posterior polarity in Drosophila. How does bicoid do this? To understand this process we need to first discuss how bicoid gets to the anterior part of the egg. Nurse cells surround the anterior region of the egg in Drosophila and other flies. Cytoplasmic bridges allow various substances - in this case mRNA from bicoid - to be transported from the nurse cells into the egg. The bicoid mRNA is then trapped by proteins produced by other genes. The result is a concentration gradient of bicoid mRNA: the anterior end has the highest concentration and the posterior end lacks it (Fig. 2). Translation of bicoid is inhibited until after fertilization, leading to a bicoid protein concentration gradient.
In addition to bicoid, other coordinate genes help establish an anterior-posterior polarity. Still other coordinate genes allow the establishment of a dorsal-ventral gradient. These coordinate genes, like bicoid, are sometimes called maternal effect genes. Maternal effect occurs when the phenotype of the individual is dependent on its mother's genotype, not its own. In cases of maternal effect, the transmission pattern of the alleles is the same as in standard Mendelian genetics but the action of the gene occurs a generation later. For example, consider a maternal effect gene where the mutant allele (m) is recessive to the wild-type allele. In the cross of homozygous, wild-type females to homozygous, mutant males, all the F1 offspring are heterozygotes and appear normal. In the reciprocal cross, all of the F1 offspring are heterozygotes but have the mutant phenotype (Fig. 3). Although the F1 offspring are genotypically identical in the reciprocal crosses, they are phenotypically different. This is because phenotype is due to the action of the mother's genotype. Maternal effect is not the same thing as maternal inheritance, such as in mitochondria, where the genetic material is transmitted only across maternal lines.