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Fig. 3. Segment-specific functions are cis-regulatory regions. Three ventral
nerve cords dissected out of Drosophila embryos immunostained for the
Abd-B protein are shown. Parasegment (PS) borders and parasegment labels
(PS10-PS13) are indicated according to the wild-type embryo. Below each cord
is a diagram representing the state of each regulatory domain in PS10 through
PS12, in which a black line represents a closed/silenced chomatin structure; a
colored oval represents an open/active chromatin domain. (A) A
wild-type embryonic ventral nerve cord has a distinct pattern of Abd-B
expression that begins in PS10 and increases in each parasegment posteriorly.
This is diagramed below as a parasegmentally regulated, sequential opening of
chromatin domains. (B) The CNS from an iab-7Sz
mutant in which the entire iab-7 domain is absent. Its PS12 develops
as a copy of PS11 (indicated by the similar staining pattern in PS11 and
PS12). (C) A deletion that removes the Fab-7 boundary causes a
fusion between iab-6 and iab-7, allowing the stronger
iab-7 enhancers to become initiated by the iab-6 initiator
element, resulting in an Abd-B expression pattern that is
characteristic of PS12 being initiated in PS11, and a homeotic transformation
of PS11 into PS12.
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