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Fig. 2. The additivity of segment-specific functions. Diagrams of two
Drosophila larvae, anterior to the top. (A) A wild-type larva;
(B) a larva mutant for the bxd/pbx segment-specific function.
The diagram next to each larva represents the presence or absence of the
segment-specific functions that are required to determine a particular
segment/parasegment (moving across them horizontally). Underneath these are
the three BX-C homeotic genes, Ubx, abd-A and
Abd-B. In embryos that lack the entire BX-C, all segments posterior
to the second thoracic segment (T2) develop as T2; thus T2 represents the
ground state in this model. Because mutations in individual segment-specific
functions always cause homeotic transformations towards the last unaffected,
more-anterior segment, Ed Lewis proposed that segment-specific functions act
in an additive fashion (Lewis,
1978). (A) The wild-type larva shows the segment-specific
functions required for the proper development of each segment/parasegment. (B)
The mutant larva lacks the bxd/pbx function and therefore has its A1
segment transformed into a copy of T3. Note that ventral pits (a
characteristic of T3; arrows) are present in all of the more-posterior
segments, indicating that the bxd/pbx segment-specific functions are
also required in more-posterior segments (asterisks).
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