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First published online March 23, 2006
doi: 10.1242/10.1242/dev.02323

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The ABC of the BX-C: the bithorax complex explained

Department of Zoology and Animal Biology and National Research Centre `Frontiers in Genetics', University of Geneva, 30 quai E. Ansermet, 1211 Geneva-4, Switzerland.

View larger version (20K): [in a new window]   Fig. 1. Diagram of the BX-C. The multicolored bar represents the DNA of the BX-C. Map coordinate numbering follows the numbering established by the original Drosophila Genome Project sequencing of the BX-C (Martin et al., 1995). The three BX-C homeotic genes, Ubx, abd-A and Abd-B are indicated below this bar (with exons indicated by the black horizontal bars and introns indicated by the diagonal lines connecting the bars). The individual cis-regulatory domains are indicated by the different colored regions on this bar. The orange and red regions (abx/bx and bxd/pbx) control Ubx expression. The regions shaded in blue (iab-2, 3 and 4) control abd-A expression. And the regions shaded in green (iab-5 through iab-8) control Abd-B expression. The corresponding adult segments affected by mutations in each cis-regulatory region are indicated on the diagram of the adult fly using the same color code.

View larger version (27K): [in a new window]   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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View larger version (25K): [in a new window]   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.

View larger version (52K): [in a new window]   Fig. 4. Reporter constructs identify initiator and maintenance elements. Drosophila embryos immunostained for the ß-galactosidase protein. (A,C) Early embryos at germband extension, where the posterior parasegments have curved around towards the dorsal side; (B,D) later stage embryos. (A,B) Embryos in which lacZ expression is driven by an element from the iab-6 region. (A) In early embryos, lacZ expression is restricted to the posterior of the embryo, with its anterior border positioned at PS11. (B) At later stages of development, the repression of lacZ anterior to PS11 is lost, as the iab-6 element becomes active throughout the embryo. (C,D) Embryos in which lacZ expression is driven by a DNA fragment derived from iab-5. (C) In early embryos, the anterior border of lacZ expression is positioned at PS10. (D) Later in development, the anterior border of lacZ expression is maintained, indicating the presence of both an initiator and a maintenance element on this fragment. ant., anterior; post., posterior.

View larger version (58K): [in a new window]   Fig. 5. Enhancer trap transposons within the BX-C. The BX-C is represented as a multicolored bar, as shown in Fig. 1. Transposon insertions of enhancer trap constructs are indicated by triangles above or below the bar. Late-stage embryos are stained for expression of the lacZ reporter gene that is present on each enhancer trap construct. Embryos have been cut along the dorsal midline and flattened to make visualization easier. Numbers in brackets show parasegment borders [adapted from Bender and Hudson (Bender and Hudson, 2000), apart from the enhancer trap lines expressing lacZ in PS9 and PS10 (inserted at 125,489 and 113,864, respectively), which were supplied by D. Fitzgerald and W. Bender].