How one comes to qualify a gene as morphogenic? Fleury didn’t included this information in his paper, neither in the text or the glossary despite his interest on morphogenesis. I’ve chosen to illustrate the concept using bicoid as it’s one of the examples used by the author.
What is it observed in embryos lacking maternal bicoid mRNA? Well, no head, two asses ; poor embryo, looks like some people I know, who seem to ass-stink when they speak.
Bicoid is a gene that is essential for normal axis formation in the fly, Drosophila. It is this gene product that basically tells the fly embryo which end is the front end—the cartoon to the right illustrates what mutant larvae look like. The top picture is a normal, or wild type, Drosophila larva. Students of the fly will recognize that this animal is facing to the left by the presence of the dark mouthparts; fly experts will be familiar with the stippling in the figure, which illustrates characteristic locations of bristles on the animal’s cuticle.
The picture below it is of an animal that lacks the bicoid gene product. It has no mouthparts, no head end at all; looking at the pattern of bristles, one can also see that the front end has the bristles found on the back end. (Yes, South Park fans, geneticists have created flies with two asses.)
The tricky part here is that that fly expressing the bicoid mutant phenotype may not carry the mutant gene. It could be genetically normal. What we know, though, from looking at it is that the poor two-assed fly’s mother was a mutant. We know that because Drosophila embryos do not synthesize the bicoid gene product at all, not even the wildtype flies, and they all inherit it directly from Mom. The only way they could be lacking it is if their mother failed to pack it into the egg.
You don’t need more information to qualify bicoid as a morphogen, in its absence the embryo fails to develop the head structures. How this happens is another story, a quite interesting one, but not relevant.
But you can get more information for the fun of it. What happens when you use bicoid from a wild type embryo to complement a bicoid– embryo?
One clever experiment: the role of the bicoid gene product has been tested with what is called a rescue experiment, illustrated above. At left on the top is a bicoid+, or wildtype larva, and on the right is a bicoid– larva that lacks any bicoid gene product. What if we injected it with bicoid? In the experiment, a little bit of bicoid+ cytoplasm is sucked out of the anterior end of a normal egg, and injected into the anterior end of an egg deficient for bicoid. That’s enough to do a partial rescue; it’s hard to get a perfect rescue, because dosage and localization are impossible to get exactly as they are in the intact egg.
The experiment at the bottom illustrates another interesting result: if the bicoid+ cytoplasm is injected into the middle of the egg, the embryo tries to form a head in the middle of it’s body, as indicated by the difficult-to-see jumble of mouthparts that form there.
So, not only bicoid mRNA is necessary to form the head but bicoid+ cytoplasm induce head parts formation when injected in bicoid– embryos, at the injection site.
The right-bottom embryo is a funny one, with two asses at the extremities and head-like structures at the middle.
Easy enough to understand what data lead to qualify a gene as morphogenic and what experiments consolidates them.