CG13424/lms orthologs are present in dipteran, chordate, and cnidarian lineages

The Drosophila homeobox gene CG13424, subsequently named lateral muscles scarcer (lms), was reported to be expressed in specific somatic muscle founders (Berkeley Drosophila Genome Database, [33]), which suggested it as a candidate for a new regulator of muscle identity or differentiation. Database searches showed that orthologs of lms are present in the parasitoid wasp Nasonia, honey bee, the Cnidarian Nematostella, as well as in the chordates Branchiostoma and Ciona (Fig. 1A and data not shown), thus indicating an ancient origin of this gene. By contrast, in vertebrates and in C. elegans orthologs of lms are not present, suggesting that the corresponding genes have been lost in these lineages. Lms also contains a putative eh1 repression domain (in NK homeodomain proteins also known as TN domain) near its N-terminus (Fig. 1B) [34]. A phylogenetic tree for lms and its closest homologs in the above species as well as humans is shown in Fig. 1C, which indicates a sequence affinity of the Lms homeodomain with the NK homeodomain family. The Drosophila lms gene locus, which maps to 57A on the second chromosome, is shown in Fig. 1D.

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Figure 1. Sequence conservation of Lms and imprecise P-excisions at the lms locus.

(A) Sequence alignments of the homeodomains from Lms (D. melanogaster) with its predicted orthologs from the sequenced genomes of Nasonia vitripennis (parasitoid wasp), Branchiostoma floridae (amphioxus, shown is one of two paralogs also known as Nedxa and Nedxb [60]), Ciona intestinalis (vase tunicate), and Nematostella vectensis (Cnidaria). (B) Sequence alignments of the most closely related eh1 (TN) domains from Drosophila transcription factors with that from Lms. (C) Phylogenetic tree using the homeodomain sequences from Lms (D. melanogaster) and the most closely related homeodomains from Branchiostoma, Nasonia, Ciona, Nematostella, D. melanogaster, and humans, showing that Drosophila and humans lack any paralogs and orthologs, respectively, of lms. (D) lms gene locus with P-insertion GE11015 and deletions generated via imprecise excision of this P-element underneath. Predicted exons are boxed, sequences covered by the longest known EST RE33150 are shaded. The open reading frame starting from the first ATG of RE33150 is shown in dark grey, the eh1 (TN) domain checkered, and the homeodomain in black. The open reading frame extends towards the 5′ in the genomic sequence to another ATG that is embedded in a less favorable translation initiation sequence and may not be present in the transcripts.

https://doi.org/10.1371/journal.pone.0014323.g001

Expression pattern of lms during embryonic development

The embryonic expression pattern of lms was analyzed in greater detail by whole mount in situ hybridizations of wild type embryos and by comparing it to additional markers for the somatic mesoderm (Fig. 2). The gene lacks any maternal contribution and its expression occurs exclusively in the somatic mesoderm. During stage 11, lms mRNA is first detected in small clusters of mesodermal cells in each of the three thoracic hemisegments (Fig. 2A). During early stage 12, lms transcripts are also detected in abdominal segments, initially in a single mesodermal cell in each abdominal hemisegment (Fig. 2B). At late stage 12 and into early stage 13, the abdominal lms expression consists of clusters of three to four cells in each hemisegment and the size of the lms-positive thoracic cell clusters is increased (Fig. 2C). As shown in Fig. 2D in a stage 15 embryo, these lms-positive cell clusters develop into lateral somatic muscle fibers. Double stainings with a myosin heavy chain (MHC) antibody, which marks all somatic muscles, identifies the four lms positive syncytia in each abdominal segment as the Lateral Transverse muscles (LT1 - LT4, also known as muscles 21–24; Fig. 2E, [1]). LT1–LT4 are also positive for lms in the thoracic segments T2 and T3, whereas in T1, where LT muscles are absent [1], lms is expressed in muscle VT1 (see below, Fig. 2J).

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Figure 2. lms mRNA expression during embryonic lateral transverse muscle (LTM) development.

(A) Stage 11 embryo (lateral view) showing earliest lms mRNA expression in thoracic clusters of somatic mesodermal cells. (B) Early stage 12 embryo (ventral view) showing lms mRNA expression in thoracic and abdominal progenitors of lateral transverse muscles (LTMs). Sites of procephalic expression may correspond to progenitors of yet unidentified head muscles (see also C, D). (C) Late stage 12 embryo (lateral view) showing lms mRNA expression in founder cells that begin to fuse to form LTMs. (D) Stage 15 embryo (lateral view) showing lms mRNA expression in LTM fibers. (E) High magnification view of stage 16 embryo counter-stained with MHC antibodies (green), which identifies the lms expressing muscles (red) of the thoracic segments T2 and T3 and the abdominal segments as LTM 1–4 (muscles 21–24) (A1: first abdominal segment). (F) Stage 11 apME680-GFP embryo showing lateral high magnification view of segments T1 to A2 stained for lms mRNA (red) and GFP protein (anti-GFP; green). In T2 and T3, lms expression partially overlaps with the ap-GFP expressing clusters, and in abdominal segments, lms expression occurs in a single cell within the ap-GFP clusters at this stage (arrow). (G) Early stage 12 apME680-GFP embryo (T2–A5) showing coexpression of lms mRNA (red) and ap-GFP (green) in two muscle progenitors in each abdominal segment (circled), with the anterior cell displaying higher lms mRNA levels. (H) Mid stage 12 apME680-GFP embryo (T2 –A4, ventral view) showing co-expressed lms mRNA (red) and ap-GFP (green) in 2–3 LTM muscle progenitors and/or founder cells per abdominal hemisegment. An additional, lms-negative ap-GFP-labeled cell appears ventrally adjacent that will form a ventral acute muscle (asterisks; see also I). (I) Stage 14 apME680-GFP embryo (T2 – A6, laterally) showing co-expression of lms mRNA (red) and ap-GFP (green) in LTM muscle precursors. (J) Stage 15 apME680-GFP embryo (T1 – A4, laterally) showing coincidence of lms mRNA (red) and ap-GFP (green) expression in abdominal LTM1-4 muscles, partial overlap in LT1-4 of T2 and T3, and exclusive lms expression in muscle VT1 of T1. (K) Stage 12 lateral high magnification view of embryo stained for lms mRNA (green) and Kr protein (red). The presumed founder cell of LT2 expresses both lms and Kr (arrow heads). (L) Late stage 12 lateral high magnification view or rP298-lacZ embryo stained for lms mRNA (green) and LacZ. Each lms cell cluster contains four founder nuclei for the muscles LT1 – 4 (arrow heads).

https://doi.org/10.1371/journal.pone.0014323.g002

The muscle identity gene apterous (ap), encoding a LIM homeodomain containing transcription factor, is also expressed within the four LT muscles, as well as in two Ventral Acute muscles (VA2 and VA3) [35]. To compare the temporal and spatial expression patterns of lms and ap in these muscles and their progenitors, embryos carrying the apME680-GFP reporter were used for GFP/lms mRNA double stainings. The apME680 enhancer element recapitulates mesodermal ap expression in LT1–LT4 and in a single Ventral Acute muscle (VA) and their corresponding progenitors [35]. ap-GFP expression can be observed during stage 11 in small cell clusters within abdominal segments before lms expression is detectable (data not shown). During early stage 12, the single lms-positive cell observed in each abdominal hemisegment corresponds to one of the ap-GFP-positive cells within each abdominal cell cluster, whereas in T2 and T3 most of the lms-expressing cells are located dorsally adjacent to the ap-GFP-expressing cells (Fig. 2F). In T1, only lms expression but no ap-GFP can be detected (Fig. 2F). During mid stage 12, a second cell becomes positive for lms in each abdominal hemisegment and ap-GFP becomes restricted to the same two cells (Fig. 2G; the original, anterior cell has accumulated higher levels of lms mRNA). In T1 and T2, ap-GFP expression expands into the lms-positive cells (Fig. 2G). Double stainings for lms and the muscle identity factor Krüppel (Kr) at stage 12, which is expressed in the muscles LT2 & LT4 and their founders [6], show that the initial lms-positive cell corresponds to one of the Kr-positive cells (likely the LT2 founder or LT1/LT2 progenitor; Fig. 2K).

During late stage 12, lms and ap-GFP are co-expressed in up to four cells per abdominal hemisegment, which likely correspond to the four founder cells of the LT1 - 4 muscles (Fig. 2H). This assignment agrees with the presence of four founder nuclei labeled by duf (rP298)-LacZ within each segmental cluster of lms expression during late stage 12 (Fig. 2L, arrow heads). Upon myoblast fusion during stage 14, lms and ap-GFP are co-expressed in the thoracic and abdominal LT muscle precursors, although in the thorax ap-GFP still extends further ventrally (Fig. 2I; ap-GFP is also seen in lms-negative abdominal VA muscle precursors). Finally, in the fully-formed muscle fibers of the four Lateral Transverse muscles (LT1–LT4) from T2, T3, and each abdominal hemisegment, lms transcripts and apterous-GFP remain co-expressed, whereas in T1 only lms expression can be detected (Fig. 2J).

Taken together, these observations strongly indicate that lms and ap are co-expressed in the four founder cells of the Lateral Transverse Muscles LT1–LT4 and in the muscle fibers formed from them, although the spatial and temporal dynamics leading to this co-expression differs for the two genes.

Regulatory interactions among different muscle identity genes during LT muscle development

Apart from lms and ap, the muscle identity gene muscle segment homeobox (msh) is active in the founder cells of muscles LT1 - 4, and Krüppel (Kr) is expressed in LT2 & LT4 and their founders (among founders of additional muscles) [6], [13]. Conversely, the ladybird (lb) genes are expressed in the progenitors and founders of the segment boundary muscles (SBM) and lateral adult muscle precursors, which arise at positions directly adjacent to those of LT1 - 4 (Fig. 3A) [6], [7], [13]. To determine whether msh, ap, and lb exert any positive or negative regulatory inputs towards lms, we examined the effects of their loss or gain of function on lms expression.

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Figure 3. Effects of mutations of the muscle identity genes msh, lb, and ap on lms expression.

(A) to (D) show lateral and (E, F) dorso-lateral views of stage 13 embryos stained to reveal the effects of msh and lb loss of function mutations on expression of lms or ap. Triple-stained embryos for anti-Lb anti-Kr and lms transcripts are shown in (A and B) and double-stained for β3-tubulin (βTub) and ap transcripts in (C and D). (A) and (C) Wild type embryos. (B) Strongly reduced lms expression in homozygous msh^Δ68 embryos is associated with the loss of Kr staining in LT2 and LT4 muscles and with expanded Lb expression domain (SBM muscle). (D) In segments with reduced ap expression in the absence of msh, β3-tubulin-labeled muscle precursors are still detected. (E) and (F) show lms expression patterns in homozygous msh^Δ68 and in ladybird-deficient embryos (homozygous Df(3R)3/1). msh and lb mutations have opposite effects on lms expression. In msh null mutants (E) lms transcripts are either absent in all or in a subset of LT muscle precursors (asterisks) or the lms expression levels are reduced (arrow). Loss of lb (F) leads to an expanded expression of lms in lateral domains. Occasionally, ectopic expression of lms in more dorsal clusters of muscle cells can be detected (arrow). (G, H) Lateral views of stage 15 embryos (G: wild type, H: homozygous ap^UGO35; four abdominal hemisegments are shown with a focus on LT muscles) stained for lms transcripts and with anti-TM1 to reveal the muscle pattern, showing that in ap mutant embryos lms expression is largely unaffected.

https://doi.org/10.1371/journal.pone.0014323.g003

As shown in Fig. 3B (see also Table 1), deletion of msh results in a severe reduction of lms and a loss of Kr expression in the founders of the LT muscles. In addition, the expression of ap in the founders and precursors the LT muscles is strongly reduced (Fig. 3D, asterisks, compare with Fig. 3C). At later stages, when myoblast fusion has initiated, lms expression does appear in the precursors of the LT muscles in msh mutants, but the expression is still reduced or absent in many of the segments (Fig. 3E). These effects are in line with the reported loss of many LT muscles in msh mutants [13] and suggest that this loss may in part be due to the failure in the proper activation of these three muscle identity genes in their muscle founders.

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Table 1. Quantification of genetic effects on embryonic lms expression.

https://doi.org/10.1371/journal.pone.0014323.t001

In embryos carrying a deficiency for the two lb genes, lbe and lbl, lms expression in lateral clusters appears slightly expanded (Fig. 3F; Table 1). Occasionally, lms expression is also seen in a few ectopic mesodermal cells that lie close to the segmental borders, possibly at the positions of SBM myoblasts that have lost their identity in the lb^def context.

As shown in Fig. 3H for late stage ap^UGO35 null mutants, loss of ap function does not grossly affect lms expression in LT muscles (and their founders, data not shown). As has been reported, mutation of ap causes occasional losses of individual LT muscles [5] (see also below). As we observe that any changes of lms expression in ap mutants closely parallel these effects on the numbers of LT muscles, these changes could be an indirect consequence of mis-specification of individual muscle fates rather than direct effects of ap on lms expression. The mild effects of ap mutation on lms expression could either be due to the absence of regulatory inputs or to functional redundancy of ap with other regulators.

To complement the loss of function studies with gain of function experiments, we forced the expression of ap, lbe, Kr, and msh pan-mesodermally by using the 24B-Gal4 driver in combination with the corresponding UAS-constructs [36]. Panmesodermal expression of ap leads to malformation of the Lateral Transverse muscles (LT1 - 4, Fig. 4A) [17]. Additionally, a loss of the SBM (♯8) muscle can be detected (Fig. 4A, asterisks). Notably, ap misexpression results in lms expression in the three Ventral Acute muscles (VA1 - VA3, Fig. 4A, arrow). In the wildtype situation, no lms expression can be observed in these muscles.

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Figure 4. Effects of ectopic mesodermal expression of the muscle identity genes ap, lbe, Kr, and msh, on lms expression.

(A) Lateral view of stage 15 24B-GAL4;UAS-ap embryo stained with anti-β3-tubulin (βTub) and for lms transcripts. Four abdominal hemisegments are shown revealing that panmesodermal Ap expression leads to an ectopic activation of lms in ventral muscles (arrow). Asterisks indicate position of segment border muscle (SBM) that is barely detected in the Ap gain of function context. (B) Lateral view of stage 14 24B-GAL4; UAS-lbe embryo stained for lms transcripts and showing four abdominal segments. In the lbe gain of function context lms expression is lost or reduced (asterisks). (C) Lateral view of stage 15 24B-GAL4;UAS-Kr embryo stained with anti-β3-tubulin (βTub) and for lms transcripts, showing four abdominal segments. Kr gain of function leads to muscle pattern defects such as LT muscle bending (arrows) or LT muscle shortening (arrowheads) without affecting lms expression. (D) Lateral view of stage 14 24B-GAL4; UAS-msh embryo stained for lms transcripts. lms expression is ectopically induced in mesodermal cells at dorso-lateral and dorsal locations (arrows). (E) Scheme showing regulatory interactions between msh, lbe, ap and lms identity genes in lateral muscles. LT1-LT4 are in blue whereas SBM is in yellow (all interactions are meant to be cell-autonomous and shown as they occur during the muscular precluster, progenitor, and founder cell stages). It is currently not known whether the interactions shown are direct or whether they are indirect, e.g., being a result of cell fate transformations.

https://doi.org/10.1371/journal.pone.0014323.g004

Ectopic expression of lbe via 24B-Gal4 line results in enlarged and duplicated SBMs, increased numbers of lateral adult muscle precursors, and other muscle defects [7]. With regard to lms, we observe a severe reduction, and in some segments complete loss, of mRNA expression within the areas of the presumptive LT muscles (Fig. 4B; Table 1). It has been shown that increased SBM formation in ladybird mis-expressing embryos occurs at the expense of the LT musculature, which is replaced by unfused mono-nucleated myoblasts [7]. Based on these data and on the observed effects of ladybird loss and gain of function on lms, we conclude that ladybird may normally repress LT muscle founder genes, including lms, in SBM founders, whereas ectopic expression of ladybird is able to repress lms (and additional identity genes) in neighboring LT founders, leading to their failure to form LT muscles.

Pan-mesodermal Kr expression leads to mis-arrangements and patterning defects of the somatic musculature, including reduced numbers and altered shapes of the LT muscles (Fig. 4C). The remaining LT muscles are often bent at their ventral endings and others appear thicker and shortened (Fig. 4C, arrows and arrow head). In these embryos, lms mRNA is still expressed within the residual LT muscles. The absence of any ectopic expression of lms suggests that Kr is either not sufficient to activate lms on its own or it lacks any inputs altogether towards lms regulation.

Ectopic expression of msh via 24B-Gal4 leads to patterning defects in the dorsal musculature as well as disorganization of the ventral muscles [13]. Mis-expression of msh has also an effect on the expression of lms. Beside the regular expression of lms within the LT muscles, ectopic lms expression in lateral and in particular in dorsal areas of the somatic mesoderm is observed (Fig. 4D, arrows).

Taken together, misexpression of msh and to a lesser extent of ap causes ectopic expression of lms, suggesting that in the normal situation these two muscle-identity genes contribute to the transcriptional activation and maintenance of lms expression in the founders and precursors of the developing LT muscles. Conversely, our data suggest that ladybird normally has repressive inputs on lms expression, analogous to its reported effects on the muscle identity gene slouch, in order to prevent the inappropriate activation of these genes in the progenitors and founders of SBM muscles and lateral adult muscle precursors by yet undefined upstream regulators (Fig. 4E).

Generation of lms null alleles and consequences of loss of lms for LT muscle development

For the analysis of lms function during muscle development we generated null alleles by using the GE11015 P-element insertion in the lms locus (Fig. 1D). GE11015 is inserted 11 ntd. downstream of the computationally predicted start of the open reading frame, although there is currently no experimental evidence that this part of the gene locus is transcribed and that the ATG upstream of the insertion is being used as a translation start codon. The facts that the 5′ of the longest available EST begins ∼100 bp downstream of the insertion site, the second ATG of the predicted open reading frame has a better match to Drosophila consensus sequences for translation start sites, and the GE11015 strain is fully viable with normal embryonic expression of lms (data not shown) would suggest that GE11015 is inserted just upstream of the transcription start site of lms. From an imprecise excision screen with GE11015 we recovered several semilethal lines that we characterized by genomic PCR and sequencing of the deletion breakpoints. One allele, lms^S95, carries a deletion of both sides flanking the insertion site and removes the complete coding sequence of lms without affecting the coding sequences of the two adjacent genes located on either side. The deletion from a second allele, lms^S119, extends from the insertion site into the beginning of exon 2 (Fig. 1D). Based upon these molecular defects, both alleles are predicted to be functional nulls. Due to their weakness, adult homozygous flies for neither of the two alleles can be maintained as homozygous lines.

The embryonic muscle phenotypes of lms mutants were examined with general muscle markers and by crossing in apME680-GFP as a specific marker for the LT muscle pattern. In stage 16 embryos homozygous for lms^S95, we frequently observe the loss of one of the LT muscles in individual abdominal segments (Fig. 5B, compare with A). Generally, the LT muscles that are still present exhibit normal morphologies and attachments. However, in rarer cases, especially when two LT muscle fibers are missing in a hemisegment, the remaining LT muscles can be attached to segmental instead of intrasegmental attachment sites (Fig. 5C). The same range of phenotypes is also observed in embryos homozygous for lms^S119 (data not shown) and in embryos with the excision alleles in trans to the larger deficiency, Df(2R)exu2 (Fig. 5D). Overall, the observed muscle phenotypes are very reminiscent of the LT muscle phenotypes reported for ap mutant embryos [5] (Fig. 5E). Because lms and ap are co-expressed in developing LT muscles and mutations in both genes cause relatively mild LT muscle phenotypes, we generated ap, lms double mutant embryos to test whether the two genes exhibit partial functional redundancy in regulating LT muscle formation. However, in ap, lms double mutant embryos most of the LT muscles are also being formed (Fig. 5F). For a more detailed analysis of possible genetic interactions between lms and ap during LT muscle development, we undertook a quantitative analysis of the LT muscle phenotype of double mutant embryos and compared it to those of the single mutants. As shown in Fig. 5G, the expressivity of the LT muscle phenotype in lms single mutant embryos (green and yellow columns) is similar to that of ap single mutants (purple columns). In ap, lms double mutant embryos (red columns), the number of hemisegments with only three or two LT muscles present is significantly higher than for each of the single mutant genotypes. However, the effects appear additive rather than interactive, which indicates that the two genes function in parallel to ensure the formation of normal numbers of LT muscles.

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Figure 5. Embryonic LT muscle phenotypes in lms mutants and ap, lms double mutants.

(A) to (F) show representative examples of phenotypes in three abdominal segments from stage 16 embryos with the denoted genotypes that were stained with anti tropomyosin (Tm1) or anti β3-tubulin (βTub) as indicated to visualize muscle patterns. (A) Wild type embryo with four LT muscles (arrows) in every segment. (B) Homozygous lms^S95 embryo with a LT4 and a LT3 muscle missing in the segments depicted on the left hand and right hand side, respectively (arrow heads). (Note that due to the slightly more dorsal orientation of the embryo as compared to embryo in (A), the VL muscles are not included in the Z-scan.) (C) lms^S95 with stronger than average disruptions of LT muscles. In left hand segment, one LT muscle is missing (arrow heads), and in segment in center, LT1 is connected to an abnormal attachment site (arrow) whereas the remaining LT muscles are replaced by aberrant syncytia with undefined identities (asterisk). (D) lms^S95/Df(2R)exu2 embryo with an LT1 muscles missing (arrow heads). (E) Homozygous ap^UGO35 embryo with mis-arranged LT muscles, causing a gap (arrow heads), and possible transformation of LT4 (asterisk) into a second LT3 muscle. (F) Homozygous ap^UGO35, lms^S95 double mutant embryo with only three LT muscles in each of the left and right hand segments (arrow heads; perhaps with the regular LT3 missing and LT4 transformed into LT3). (G) Numerical evaluation of LT muscle phenotypes. Genotypes are color-coded as shown, with “n” denoting the assessed number of abdominal segments for each genotype. Confidence intervals were calculated at 95% confidence levels. Note that sporadic occurrences of a fifth LT muscle have also been reported for controls and may be genetic background-dependent [7].

https://doi.org/10.1371/journal.pone.0014323.g005

lms function during adult muscle development in Drosophila

To examine whether lms also fulfils a role during the development of the adult musculature, we stained imaginal discs for lms mRNA expression. We used discs from a line expressing GFP driven by regulatory regions of the Him gene, which is expressed in all adepithelial cells of the wing and leg discs that will form the adult musculatures of the thorax and its appendices [30], [37], [38], [39], [40]. As shown in Fig. 6A, during late third instar the Him-GFP-labeled adepithelial cells cover the entire notum and hinge area of the wing disc. By contrast, lms is only expressed in the adepithelial cells within a narrow zone along the distal border of this area, which is located near the hinge region of the disc. The adepithelial cells of this zone are known to give rise to the direct flight muscles (DFMs) and, as seen in Fig. 6A, lms expression occurs predominantly within the anterior ∼1/2 of this zone. Likewise, lms is also restricted to a small area of adepithelial cells in third instar leg discs (Fig. 6B). The lms expressing cells are located in the stalk region of the disc outside of the leg proper area. Because clonal analysis had suggested that direct flight muscle 51 is derived from the leg disc, it is conceivable that some of this expression corresponds to myoblasts of presumptive DFM 51 in mesothoracic leg discs [38]. However, the observed expression in similar domains in all leg discs (including those of T1 and T3) would suggest that most of these cells give rise to muscles other than the DFMs, possibly including some that extend from the thorax into the coxae.

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Figure 6. Expression of lms in adepithelial cells of wing and leg discs.

Shown are 3rd instar wing and leg discs. (A, B) show GFP-expressing adepithelial cells associated with wing (A) and leg (B) imaginal discs dissected from the transgenic Him-GFP larvae and stained with anti-GFP (red) and for lms transcripts (green). (A) lms transcripts accumulate specifically in a subset of adepithelial cells located in most distal positions of the thoracic part of the wing disc (arrow). (B) In the late 3^rd instar leg discs from Him-GFP larvae lms is expressed in a restricted subpopulation of adepithelial cells (arrow) located outside of the leg disc proper within the stalk region. (C) Highly restricted lms expression in leg discs at the same position as in (B) (arrow) is already detected in early 3^rd instar larvae.

https://doi.org/10.1371/journal.pone.0014323.g006

The restricted expression pattern of lms in wing discs is interesting in light of previous reports that the distal adepithelial myoblasts giving rise to direct flight muscles are marked by high levels of expression of the homeobox gene cut, whereas the remaining myoblasts giving rise to the indirect flight muscles are marked by the expression of vg and low levels of cut during late third instar [30]. Therefore we performed additional experiments to compare the spatial expression and clarify the regulatory relationships among cut, vg, and lms. In wild type wing discs, most of the lms expression occurs in distal adepithelial cells that are adjacent to the Vg-expressing cells (Fig. 7A, arrow), except for a small area of overlap (Fig. 7A, asterisk). Accordingly, lms shows extensive co-expression with high-level Cut in the anterior ∼1/2 of the distal domain, albeit with a slight extension into the low-Cut domain (Fig. 7B). Together with the published data, these observations confirm that lms expression is largely restricted to a subset of the adepithelial myoblasts giving rise to direct flight muscles (Fig. 7J).

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Figure 7. Regulatory interactions between lms and vg during patterning of adepithelial cells in wing discs.

Shown are high magnification views centering on the wing hinge areas of 3rd instar wing discs (distal is up, proximal is down; anterior to the left; areas shown correspond to dashed rectangles in J, K). A-C: wild type; D-F: lms^S95/Df(2R)exu2; G-I: 1151-GAL4>vg). (A) lms mRNA expression (green) occurs in areas distally adjacent to the areas of Vg expression (red) in the adepithelial cell layer (arrows indicate border between the two domains), although there is also a small region of overlap (asterisk). (B) lms expression within the area displaying high levels of Cut protein (“high Cut domain”), which forms direct flight muscles. (C) Normal expression of Vg in presumptive indirect flight muscle myoblasts and high-level Cut expression in adjacent direct flight muscle myoblasts, respectively. (D, E, F) In lms mutant wing discs, Vg expression is expanded into the Cut domain. (G, H, I) lms mRNA expression in a largely normal pattern in wing disc with ectopic vg expression in all adepithelial cells. (J, K) Schematic drawings of Vg and Cut expression in wildtype and lms mutant disc, respectively, illustrating the expansion of Cut expression into anterior portions of the Vg domain upon loss of lms activity (area shown in panels A – I is indicated by dashed rectangle; blue dots in J represents high-level lms expression and region outlined with blue dotted line low-level lms expression area).

https://doi.org/10.1371/journal.pone.0014323.g007

To test whether the expression of cut and vg is regulated by lms, we stained wing discs from lms mutant third instar larvae for Cut and Vg proteins. As shown in Fig. 7D-F, Vg expression is strongly expanded into the high-Cut domain in these discs such that almost all nuclei with high-Cut also contain Vg. Hence, lms contributes to the negative regulation of vg in the myoblasts of the presumptive direct flight muscles and high-Cut, which is unaltered in lms mutants, does not appear to be sufficient to repress vg (Fig. 7K). Next, we tested whether vg has any regulatory effects on lms expression. For this purpose, we forced expression of vg in all adepithelial cells with the 1151-Gal4 driver [25], [41]. Although we do observe a wing posture phenotype under these conditions (see also Fig. 8D), we do not detect any obvious repression of lms expression with ectopic vg (Fig. 7G-I). Thus it appears that vg is not sufficient to exert repressive effects on lms that could have fully explained the largely complementary expression of the two genes in the wing discs.

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Figure 8. Wing posture phenotype and morphological analysis of direct flight musculature in adult lms mutant flies.

(A) Control fly (yw). (B) lms^S95 homozygous mutant fly. (C) Homozygous lms^S119 fly. (D) 1151-Gal4; UAS-vg fly with ectopic expression of vg in DFM myoblasts. (E) Plastic section showing DFMs from control fly (Df(2R)exu2/+). (F) Section showing DFMs from lms^S95/Df(2R)exu2 fly. (G) Section from lms^S119/Df(2R)exu2 fly at higher magnification showing the striated, non-fibrillar DFMs 49, 50, 51, 53 that appear normal. (H) and (J – M) show 3D-reconstructions obtained from stacks of images acquired by ultramicroscopy from whole mount flies. In some cases, certain areas from individual layers that covered important muscles underneath have been omitted for better clarity (see movies S2, S3, S4 with the complete reconstruction). (H) Control fly (yw) scanned from the inside. The muscles are numbered according to Miller, 1950 [46]. (I) Schematic drawing of direct flight muscle pattern as seen in (H) (maroon: outer muscle layer; orange: intermediate layer; yellow: inner layer). Black lines demark external cuticle and sclerites. (J) lms^S95/Df(2R)exu2 mutant fly scanned from the inside. (K) lms^S95/Df(2R)exu2 mutant fly scanned from the outside. (L) lms^S119/Df(2R)exu2 mutant fly scanned from the inside. (M) lms^S119/Df(2R)exu2 mutant (same fly as in G) scanned from the outside.

https://doi.org/10.1371/journal.pone.0014323.g008

lms mutant flies frequently show a held-out wing phenotype, in which the wings are held at various angles (typically ∼45°) from the body axis instead of parallel to it (Fig. 8B, C, compare to A). The phenotype is seen both in flies that are homozygous for the lms null alleles and in flies that carry an lms mutation in trans to larger deficiencies at the locus, although the penetrance and expressivity can vary presumably due to genetic background effects (e.g., lms^S119/lms^S119 or lms^S119/Df(2R)BSC400 escaper flies raised at 18^oC: ∼80% penetrance of wing posture defects; lms^S119/Df(2R)exu2: ∼40% penetrance). The same held-out phenotype is also seen upon RNAi knockdown of lms in adepithelial cells (1151-GAL4>lms-IR; data not shown). The flies with normal wing postures or with mildly held-out wings from the above-described genotypes are able to fly, but most individuals with more strongly held-out wings show poor flying capabilities or are unable to fly. In a flying assay with a 500 ml graduated cylinder according to Benzer [42], ∼45% of lms^S119/Df(2R)exu2 flies with held-out wings (n = 66) landed on the bottom, whereas for the lms^S119/Df(2R)exu2 flies with normal wing postures (n = 77) and yw control flies (n = 168) this number was only ∼8 and 18%, respectively (data not shown). Likewise, when lms^S119/lms^S119 and lms^S95/Df(2R)BSC400 escaper flies with held-out wings were dropped from a height of 100 cm, all of them landed within an area of ∼25 cm diameter. When these flies were kept in an open dish or on a tip, they walked and jumped when touched, but did not fly away (e.g., see Movie S1).

The observed wing posture phenotype is similar to that of flies with ectopic vg expression in adepithelial myoblasts, albeit somewhat milder (Fig. 8D). The effects of ectopic vg have been connected to disruptions of direct flight muscles [30]. Because of the similarity of these wing phenotypes, the known role of direct flight muscles in controlling the wing posture [43], and the observed expression of lms within the domain of the myoblasts that give rise to direct flight muscles, we surmised that lms has a specific function in regulating DFM development.

Initially, we examined the direct flight musculature in dissected flies and plastic sections, which did not reveal any obvious defects in the pattern and ultrastructure (Fig. 8E – G). Therefore, we decided to analyze the DFMs in fixed whole mount thoraces via ultramicroscopy and 3D reconstruction of the scanned images, which produces detailed views of the musculature and other internal structures of the fly [44], [45]. As shown in Fig. 8H, I in a view from the inside of the thorax towards the wing attachment (see Movies S2, S3, S4), this technique has allowed us to reconstruct the morphology of Drosophila DFMs with great detail comparable to scanning electron micrographs [46], [47], [48] while providing the additional advantage of 3D views. Next, we used this method to examine the direct flight musculatures in lms mutant flies that featured strong held-out-wing phenotypes. Figures 8J and L show 3D reconstructions of the direct flight muscles from lms^S95/Df(2R)exu2 and lms^S119/Df(2R)exu2 flies, respectively, viewed from the inside like the control in Fig. 8H, and panels K and J depict these muscles from the same flies as viewed from the outside. Importantly, all muscles are present with their characteristic shapes, arrangements, and connections to the proper attachment sites in these mutant flies. Although there are minor differences in muscle size (e.g., compare reduced thickness of muscles 53 from the mutants in Fig. 8J with the corresponding muscle 53 from control in Fig. 8H), given the low sample size of flies that can be processed with this method (see Materials and Methods) it is unclear whether these subtle differences are related to the absence of lms function.

Drosophila stocks and genetics

The GE11010 P-element insertion line, which contains an insertion 11nt downstream of the predicted start codon of the lms open reading frame (or 5′ to it if the first ATG of the longest cDNA available, EST RE33150, is being used), was purchased from Genexel (Korea) through a CNRS license. Df(2R)exu2 and Df(2R)BSC400 containing deletions uncovering the lms locus was obtained from the Bloomington Stock Center, as were the lines used for the excision screen. Df(3R)3/1 (deleting bap, lbl, lbe and C15) was used as a ladybird deficiency [54]. apME680-GFP and ap^UGO35 were a gift form J. Botas (Baylor, Houston, TX [35]). 1151-Gal4 driving expression in the adepithelial cells during larval development was a gift from K. VijayRaghavan (National Centre for Biological Sciences, Bangalore) [25], [41]. Him-GFP was a gift from J. W. Posakony (Univ. California, San Diego) [37], [40]. UAS-vg was a gift from H. Skaer (Cambridge University, UK) [30]. UAS-msh, msh^Δ68 mutants, msh-lacZ, and rP298-lacZ were obtained from A. Nose (Tokyo University of Pharmacy and Life Sciences, Japan) [13], [55]. UAS-ap was a gift from J. Thomas (Salk Institute, La Jolla, CA) [5]. UAS-Kr was kindly provided by the Jäckle lab (MPI f. biophys. Chemie, Göttingen) [6]. Pan-mesodermal expression of muscle-identity genes were achieved with the 24B-Gal4 driver line [36] and the embryos were left to develop at 28°C for 12 hours before fixation. lacZ- or GFP-marked balancers were used throughout for the identification of homozygous mutant embryos.

For the generation of lms^S95 ap^UGO35 double mutants, 50 female flies transheterozygous for the two mutations were crossed to Sco/SM6. To identify recombined chromosomes the lines derived from individual balanced offspring were tested for lethality in trans to ap^UGO35 and via PCR for the lms deletion.

Generation of lms deletions

For P-excision mutagenesis, males from the GE11015 homozygous strain were crossed with virgins from a strain containing the Δ2–3 transposase on a CyO chromosome. Males carrying the P-element insertion together with the transposase were crossed with w; Sco/CyO. White eyed male progeny (either carrying Sco or CyO) were crossed individually with w; Df(2R)exu2/SM6, eve-lacZ females. For each of these single crosses, the lethality was scored and a balanced GE11015-excision/SM6, eve-lacZ or, if viable homozygous, stock was established. Breakpoints were determined molecularly by genomic PCR and sequencing.

Embryo fixation

Embryos were collected on agar in Petri dishes, dechorionated for 2.5 min in 100% bleach (DanKlorix, Colgate-Palmolive), washed with H₂O for 2 min and fixed by shaking for 20–30 min in 800 µl 5X Buffer B (50 mM K-Phosphate pH6.8, 225 mM KCl, 75 mM NaCl, 65 mM MgCl₂), 800 µl formaldehyde, 2.5 ml H₂O and 8 ml heptane. After removing the vitelline membrane by shaking the embryos 30 sec in methanol and two washes in methanol embryos were stored at 4°C until staining.

In situ hybridization

4 µg of the RE33150 EST clone of lms, obtained from the Riken embryo library, were digested with EcoRI, gel purified with the QIAquick Gel Extraction Kit (Qiagen) and resuspended in 30 µl of DEPC-H₂O. 10 µl of this preparation were used for a 2 h transcription reaction at 37°C using 3 µl of T3 RNA Polymerase in 30 µl final volume. 0.5 µl of the reaction was tested on a gel and the remaining probe was treated with 10 µl of 5X Carbonate Buffer (300 mM Na₂CO₃, 200 mM NaHCO₃, pH 10.2) and 20 µl DEPC-H₂O for 20 min at 65–70°C. 50 µl of 2X Stop Solution (0.2 M NaAc, pH 6.0) were added. The probe was precipitated after addition of 15 µl of 4 M LiCl, 2 µl of tRNA (50 mg/ml) and 300 µl of 100% EtOH, washed with 500 µl of 70% EtOH in DEPC-H₂O, dissolved in 70 µl of Hybridization Solution (50% formamide, 5X SSC, 50 µg/ml heparin, 100 µg/ml tRNA, 100 µg/ml ssDNA, 0.1% Tween, pH 6.5) and used at 1∶2000 dilution for in situ hybridization.

Immunohistochemistry

Whole mount embryo in situ hybridization, double fluorescent in situ hybridization and antibody labeling were performed as described in [9] and detected with Fast Red reagent (Sigma-Aldrich) or tyramide reagents (PerkinElmer). Immuno-stainings of larval imaginal discs were performed as described in [56]. In situ hybridization on larval discs was performed according to [57]. The stainings were either scanned with a Leica confocal microscope or recorded by using a Zeiss Apotome microscope. Ultramicroscopic images of adult flies were generated according to [44], [45]. Images were assembled using Photoshop.

The following primary antibodies were used: rabbit anti-βgalactosidase (Cappel/MP Biomedicals, CA); rabbit anti-GFP (Molecular Probes, OR); rat anti-Tropomyosin (Babraham, UK), mouse anti-Cut (Developmental Studies Hybridoma Bank, DSHB), rabbit anti-Vg (gift from S Carroll; HHMI/Univ. of Wisconsin, Madison, WI), rabbit anti-myosin (D. Kiehart, Duke Univ., NC), anti-β3-Tubulin (R. Renkawitz-Pohl, Univ. Marburg), mouse anti-Lbe ([58]), rabbit anti-Kr (1∶2000; gift from P. Carrera and G. Vorbrüggen, MPI Göttingen). For fluorescent detection, FITC-, Cy3-, or Cy5-conjugated secondary antibodies were used. Secondary antibodies were obtained from Jackson Laboratories.

Ultramicroscopy

Ultramicroscopy (UM) is a microscopy technique allowing three-dimensional reconstructions of up to cm-sized specimen with micrometer resolution using a laser light sheet [44], [45]. Using this technique, we performed three-dimensional reconstructions of the inner anatomy of chemically cleared entire Drosophila flies [59]. Flies were anaesthetised by ether, fixed in 4% paraformaldehyde overnight, and dehydrated in an ascending ethanol series (50%, 70%, 96%, 2×100% for 6 h). Afterwards, they were incubated in a clearing solution consisting of 2 parts benzyl benzoate and 1 part benzylalcohol (Sigma-Aldrich, Germany) for 3 days. Imaging the flies' inner morphology was performed using autofluorescence of Drosophila muscles. Autofluorescence was excited at 488 nm laser light sheet having a beam diameter of approx. 3.2 µm at the focus. Images were recorded with a 10X objective (NA 0.30) using a CCD camera (CoolSnap K4, Roper Scientific) as described in [44]. Two individuals from each genotype were analyzed (yw, lms^S95/Df(2R)exu2, lms^S119/Df(2R)exu2) and the mutant flies utilized showed strong held-out wing phenotypes.