Differential activity of Drosophila Hox genes induces myosin expression and can maintain compartment boundaries

Compartments are units of cell lineage that subdivide territories with different developmental potential. In Drosophila, the wing and haltere discs are subdivided into anterior and posterior (A/P) compartments, which require the activity of Hedgehog, and into dorsal and ventral (D/V) compartments, needing Notch signaling. There is enrichment in actomyosin proteins at the compartment boundaries, suggesting a role for these proteins in their maintenance. Compartments also develop in the mouse hindbrain rhombomeres, which are characterized by the expression of different Hox genes, a group of genes specifying different structures along their main axis of bilaterians. We show here that the Drosophila Hox gene Ultrabithorax can maintain the A/P and D/V compartment boundaries when Hedgehog or Notch signaling is compromised, and that the interaction of cells with and without Ultrabithorax expression induces high levels of non-muscle myosin II. In the absence of Ultrabithorax there is occasional mixing of cells from different segments. We also show a similar role in cell segregation for the Abdominal-B Hox gene. Our results suggest that the juxtaposition of cells with different Hox gene expression leads to their sorting out, probably through the accumulation of non-muscle myosin II at the boundary of the different cell territories. The increase in myosin expression seems to be a general mechanism used by Hox genes or signaling pathways to maintain the segregation of different groups of cells.Peer Reviewe

The elimination of an adult segment by the Hox gene Abdominal-B

Hox gene activity leads to morphological diversity of organs or structures in different species. One special case of Hox function is the elimination of a particular structure. The Abdominal-B Hox gene of Drosophila melanogaster provides an example of such activity, as this gene suppresses the formation of the seventh abdominal segment in the adult. This elimination occurs only in males, and is characteristic of more advanced Diptera. The elimination requires the differential expression or activity of genes that are downstream Abdominal-B, or that work togetherwith it, andwhich regulate cell proliferation or cell extrusion. Here,we reviewthe mechanisms responsible for such elimination and provide some new data on processes taking place within this segment. © 2015 Elsevier Ireland Ltd. All rights reserved.Ministerio de Economía y Competitividad (BFU2008-00632 and BFU2011-26075), the Consolider Program (CSD2007-0008) and an Institutional Grant from the Ramon Areces FoundationPeer Reviewe

<i>Ultrabithorax</i> can maintain a smooth D/V boundary in the absence of <i>Notch</i> signaling.

<p>(A, B) <i>ap</i>-Gal4 UAS-GFP wing (A) and haltere (B) discs, showing the smooth boundary between dorsal (D, in green) and ventral (V) compartments. (C, D) In <i>ap</i>-Gal4 UAS-GFP/<i>ap^UGO35</i> wing (C) and haltere (D) discs, this boundary is uneven. Note in D a group of dorsal cells in the ventral compartment (arrow). (E, F) In <i>ap</i>-Gal4 UAS-GFP/<i>ap^UGO35</i>; UAS-<i>Ubx</i>/<i>tub-</i>Gal80^ts wing discs (E), or in haltere discs of <i>ap</i>-Gal4 UAS-GFP/<i>ap^UGO35</i>; <i>Df109</i> UAS-<i>dsUbx</i>/<i>+</i> larvae (F), the straight D/V boundary is restored. See that the dorsal compartment in E is slightly reduced and that in F slightly enlarged. Scale bars are 40 µm in A, C and 30 µm in B, D, E and F.</p

Summary of the results obtained with clones of different genotypes in the haltere disc.

<p>Anterior compartments are to the left, and the red line marks the A/P compartment boundary. <i>Ubx</i> expression is marked in blue and hatching indicates absence of Hh signaling. The red arrows indicate rejection of the cells of the clone due to different <i>Ubx</i> expression and the green arrows rejection due to different Hh signaling. (A) <i>Ubx⁻</i> clones are segregated from the rest of the tissue. (B) Anterior <i>smo</i> clones cross from the A to the P compartment. (C) Anterior <i>smo Ubx</i> clones undergo rejection from both A and P cells (because of their lack of <i>Ubx</i>) and rejection from A cells due to the absence of <i>smo</i>. The end result is the crossing of the boundary. (D) Clones like those in B, but induced in a <i>bx</i> background are rejected by both A and P cells and do not cross the boundary.</p

Number of clones crossing or respecting the A/P boundary in haltere, second and third leg disc in wildtype and mutant combinations.

*<p>Small clones or clones where the crossing or not crossing was not evident.</p

Hedgehog signaling and <i>Ultrabithorax</i> provide specific cell affinities to the cells.

<p>In <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057159#pone-0057159-g001" target="_blank">Figures 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0057159#pone-0057159-g002" target="_blank">2</a> anterior compartments (A) of the imaginal discs are to the left and posterior ones (P) to the right. (A) <i>Ubx</i> mutant clones, marked by the absence of <i>arm</i>-lacZ expression (in green), are round and tend to segregate from the surrounding tissue. (B) An <i>Ubx</i>-expressing clone (arrow), marked with <i>yellow</i> and induced in the second thoracic segment also segregates from the rest of the notum. (C, C′) A <i>smo</i> clone in the anterior compartment of the wing pouch, marked by the absence of GFP signal (in green), penetrates into the posterior compartment, which is marked by <i>en</i>-lacZ expression (in red). (D) <i>hh</i>-lacZ expression in the haltere disc. (E–E′′) A <i>smo</i> clone in the anterior compartment of the haltere pouch, marked as in C also penetrates into the posterior compartment, marked with <i>hh</i>-lacZ (in red, E′). Merged image in E′′. Scale bars are 30 µm in A, D, E′′ and 60 µm in C′.</p

Differences in the amount of different Hox genes cause accumulation of <i>sqh</i>-GFP in imaginal discs.

<p>(A–A′′) Z-stack of an <i>Abd-B</i> mutant clone induced in the genital disc and marked by the absence of lacZ expression (in blue in A′) showing increased <i>sqh</i>-GFP expression around it (in green, A, A′). (B, B′) Z-stack of a control <i>Abd-B</i> clone similarly marked but induced in the wing disc, showing there is no increase in <i>sqh</i> levels. (C) In <i>ap</i>-Gal4 UAS-GFP/<i>ap^UGO35</i>; UAS-<i>Abd-B</i>/<i>tub-</i>Gal80^ts wing discs, the D/V boundary is smooth (compare with Fig. 3C). (D) A similar result is obtained in <i>ap</i>-Gal4 UAS-GFP/<i>ap^UGO35</i>; UAS-O<i>Ubx</i><b>/</b><i>tub-</i>Gal80^ts.</p

Differences in amount of <i>Ultrabithorax</i> between adjacent cells induce accumulation of <i>spaghetti-squash, zipper</i> and <i>bazooka.</i>

<p>(A–C′′′) Z-stacks of <i>Ubx</i> clones induced in the haltere disc, marked by the absence of <i>arm</i>-lacZ (in grey in A, B and C, in blue in A′′′, B′′′ and C′′′), showing a ring of <i>sqh</i>-GFP, <i>zip</i>-GFP or <i>baz</i>-GFP (in green in A′, B′ and C′, respectively), and higher levels of F-actin (in red in A′′, B′′ and C′′) around the clones. Merged images in A′′′, B′′′ and C′′′. In D–D′′′ we show a sagital section of the clone shown in C–C′′′. Note the invagination of the clone and the accumulation of <i>baz</i>-GFP and F-actin in the border of the clone (arrowheads). (E–E′′′) Haltere disc of the <i>bx³ hh</i>-lacZ/<i>TM2, Ubx¹³⁰</i> genotype, showing accumulation of <i>sqh</i>-GFP (in green in E′, arrowhead) and F-actin (in red in E′′) at the A-P boundary, where compartments with (P compartment) and without (A compartment) <i>Ubx</i> abut (<i>Ubx</i> expression in grey in E and in blue in E′′′). Merged image in E′′′. (F–F′′) In <i>bx³ hh</i>-lacZ/<i>TM6B</i> haltere discs, by contrast, there is no accumulation of either <i>sqh</i>-GFP (in green in F′) or F-actin (in red in F′′) at the A-P boundary; <i>ß-galactosidase</i> expression is in grey in F. (G) <i>abx bx³ pbx</i>/<i>TM2, Ubx¹³⁰</i> adult showing a fusion of the T2 and T3 (transformed into the T2) segments (arrow). Scale bars are 10 µm in A′′′, B′′′ and C′′′, and 30 µm in E′′′ and F′′.</p

<i>Ultrabithorax</i> can maintain the A/P boundary in the absence of Hedgehog signaling.

<p> In all the panels of this Figure, the clones are marked by the lack of GFP in green, and the posterior compartment (to the right) is marked by either <i>hh</i>-lacZ or <i>en</i>-lacZ reporters in red. (A–B′′) Wing (A–A′′) and haltere (B–B′′) discs showing anterior clones double mutant for <i>smo</i> and <i>Ubx</i> that invade the posterior compartment. (C–C′′) <i>smo</i> clone induced in the anterior compartment of a <i>bx³/TM2, Ubx¹³⁰</i> haltere disc. See that it does not cross the compartment boundary. Note that a few cells in the A compartment weakly express <i>hh</i>-lacZ. (D–D′′) An anterior <i>smo</i> clone induced in the third leg disc cross the A/P compartment boundary. (E–E′′) A similar clone induced in the second leg disc does not cross the boundary. Scale bars are 30 µm except in A′′ (60 µm).</p