Fate Mapping and Study of Tissue-Specific Gene Expression
One obvious way to examine the tissue specificity of gene expression is to isolate the tissues and assay each for the protein or gene product in question. A slight modification of this approach is quite reasonable. The synthesis of messenger from the various tissues of a fly can be measured approximately by DNA-RNA hybridization. As we shall see in a later, DNA from desired genes can be obtained and then used in such in situ hybridization experiments. Remarkably, genetics experimentscalled fate mapping can also locate the tissues in which an altered gene is expressed. This approach does not require knowledge of the gene involved and therefore it is useful in initial steps of studies. Genetic engineering has also developed techniques for examination of tissue-specific gene expression, but those approaches first require isolating DNA or RNA of the gene in question. Fate mapping is useful when the gene involved is unknown. As an example of localizing the activity of a gene, imagine a mutant fly that is unable to flap its wings. This could be because of a defective wing, a defective wing muscle, a defective nerve to the muscle, or defective neurons in the brain. Fate mapping can determine which tissue is responsible for such altered behavior.
Fate mapping relies on the developmental pathway of the fly. A fertilized Drosophila egg contains a single cell whose nucleus undergoes about nine divisions. These nuclei then migrate to the surface of the egg to form the blastula stage, and three more divisions occur before cell walls are laid down. At this stage, different cells on the surface ultimately
become different parts of the adult fly, but cells lying near to each other frequently develop into adjacent parts on the fly. Therefore, a map can be drawn on the egg of the parts of the adult fly that each of these cells will become. If it were possible to associate a particular phenotype in an adult with a particular location on the egg, then the tissue responsible for the adult phenotype would be determined. This is possible!
The association of tissues in the mature fly with positions on the blastula utilizes selective chromosome loss during development of the blastula. Fly development is not greatly altered in a female egg cell if one of the X chromosomes contains a defect so that it begins the first nuclear replication a little late. Consequently, this chromosome often is not segregated into one of the two resultant daughter nuclei resulting from the first nuclear division in the egg. The final result of this chromosome loss is that about half of the cells of the blastula will be diploid XX and the others will be haploid X. Since the spatial orientation of the first cleavage with respect to the egg shell is not uniform from egg to egg, and because there is little mixing of the nuclei or cells during subsequent divisions, different sets of cells will be XX and X in different blastulas. Suppose that these two types of cells can be distinguished. This can be done by placing a recessive body color marker gene, for example yellow, on the stable X chromosome. Then cells of the fly possessing the XX genotype will be black and the X genotype will be yellow. The adult fly will have a mottled appearance.
The probability that two different body parts possess different colors will be proportional to the distance that their corresponding ancestor cells in the blastula state were separated. The greater their separation, the greater the chances that the line separating the two cell-types will fall between them. If they are close together, there is little chance they will be of different cell-type and therefore there is little chance they will possess different body color.
A collage appears when the body parts of the adult fly are mapped to the blastula. The map can then be used as follows to locate the tissue in which a recessive mutation is expressed. If the mutation is located on the X chromosome which is not lost during the development, then those tissues which can express the mutant phenotype will be haploid. For example, if the mutant phenotype always appears in flies with haploid second left legs and only appears in these flies, then it can be concluded that the tissue in which the mutation is expressed is the second left leg. More generally however, the frequency of association of the mutant phenotype with a number of landmarks gives the distance on the blastula between the landmarks and the tissue in question. Transferring these distances to the blastula fate map then reveals the tissue in question.