Worm Breeder's Gazette 12(5): 44 (February 1, 1993)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
mab-5 is a member of the C. elegans HOM-C complex, a set of genes evolutionarily related to the Drosophila Antennapedia and bithorax complexes. In Drosophila the expression of the HOM-C genes in specific regions along the anterior-posterior axis is regulated by positional information. The expression pattern of mab-5 has suggested the hypothesis that the C. elegans HOM-C genes might also be regulated by positional information. mab-5 is expressed specifically in the posterior of the worm, in a set of cells that have in common only their position within the body (they arise from many different points within the lineage, and comprise many different cell types.) In addition, certain cells (the QL neuroblast and the M mesoblast) migrate into the mab-5 region and then start to express mab-5 .
We have tested whether the expression of mab-5 in one cell, the M mesoblast, is dependent upon the position of this cell along the anterior-posterior axis of the anirnal. M is born during embryogenesis in the anterior of the animal, and migrates into the posterior. We looked at expression of an integrated mab-5 -lacZfusion in M during embryogenesis. At the time M was born, mab-5 -lacZwas expressed in specific cells in the posterior of the embryo, and no lacZ expression was seen in M (or in any other cell in the anterior). As M migrated into the posterior, it began to express mab-5 -lacZ.To test whether the expression of mab-5 in M was dependent on the position of the cell along the a-p axis, we blocked M migration in staged embryos, using cytochalasin D or colcemid. (Both are known to block cell migration in other systems.) If expression of mab-5 were dependent on the position of the cell, we would not expect to see any lacZ expression when M migration was inhibited.
Surprisingly, after embryos were incubated with either inhibitor, an anterior cell expressed lacZ, although the remainder of the staining pattern was like that of untreated embryos. We infer that this anterior cell is M since in different embryos, it could be found at various points along the M migration path. Thus, for the M cell, position along the a-p axis is not important for mab-5 expression. Instead, expression must be regulated by lineage-specific information and/or local cell-cell interactions.
Does this mean that positional information is not important in regulating mab-5 expression? At the moment, we do not know the answer to this question for any cell but M. It is possible that mab-5 expression is regulated by position in other cells, or at other times during development. Other experiments have suggested that position may be important in regulating mab 5 after hatching. Nevertheless, for at least this one cell the initial expression of mab-5 is independent of positional information. If mab-5 expression can be independent of positional information, why then is it expresse`d in a localized region of the body? One possibility is that the localized expression pattern has been conserved (from an ancestor of C. elegans in which it was set up using positional information) because expression of mab-5 in the wrong region of the body would cause abnormal development and would be selected against.