Gene expression suggests conserved aspects of Hox gene regulation in arthropods and provides additional support for monophyletic Myriapoda
© Janssen and Budd; licensee BioMed Central Ltd. 2010
Received: 12 March 2010
Accepted: 5 July 2010
Published: 5 July 2010
Antisense transcripts of Ultrabithorax (aUbx) in the millipede Glomeris and the centipede Lithobius are expressed in patterns complementary to that of the Ubx sense transcripts. A similar complementary expression pattern has been described for non-coding RNAs (ncRNAs) of the bithoraxoid (bxd) locus in Drosophila, in which the transcription of bxd ncRNAs represses Ubx via transcriptional interference. We discuss our findings in the context of possibly conserved mechanisms of Ubx regulation in myriapods and the fly.
Bicistronic transcription of Ubx and Antennapedia (Antp) has been reported previously for a myriapod and a number of crustaceans. In this paper, we show that Ubx/Antp bicistronic transcripts also occur in Glomeris and an onychophoran, suggesting further conserved mechanisms of Hox gene regulation in arthropods.
Myriapod monophyly is supported by the expression of aUbx in all investigated myriapods, whereas in other arthropod classes, including the Onychophora, aUbx is not expressed. Of the two splice variants of Ubx/Antp only one could be isolated from myriapods, representing a possible further synapomorphy of the Myriapoda.
The Hox genes are expressed in broad overlapping domains along the anterior-posterior axis of developing arthropods, and specify the segment identity under the control of upstream acting segmentation genes [1, 2]. In Drosophila, the initially established expression patterns of the Hox genes are maintained by the trithorax (trxG) and Polycomb group (PcG) factors . These factors act through sets of response or maintenance elements (MEs), the best investigated of which are involved in the regulation of the Ultrabithorax (Ubx) gene [4, 5]. A number of non-coding RNAs (ncRNAs) have been reported for Drosophila, which are transcribed through MEs in the bithoraxoid (bxd) region located between Ubx and abd-A. The ncRNAs including bxd are expressed in similar patterns to those of the neighbouring Hox genes [6, 7]. Although it was initially thought that bxd would activate Ubx, a recent study suggests that transcription of ncRNAs promoted by Trithorax represses Ubx in cis by means of transcriptional interference . Elongated transcription of bxd-ncRNAs through the Ubx locus prevents the transcription of the latter in the same cells. However, in cells that do not express bxd Ubx is expressed . The expression patterns of bxd ncRNAs and Ubx are therefore complementary in Drosophila.
In organisms other than Drosophila, the mechanisms that regulate Ubx transcription are less well known. It is unclear whether MEs or bxd are conserved or if transcription of bxd interferes with the transcription of Ubx in a similar way to that in Drosophila. However, some evidence has recently accumulated suggesting that a similar mechanism could be involved in the regulation of Ubx outside Drosophila. Data from the beetle Tribolium show that ncRNAs of the Ubx region are expressed in patterns similar to those of the neighbouring Hox genes, resembling the observations in Drosophila . In the centipede Strigamia, the non-coding antisense transcript of Ubx is expressed in a pattern complementary to that of the coding Ubx sense transcript, suggesting that bidirectional transcription of a non-coding RNA, antisense Ubx, is also involved in the regulation of Ubx in this myriapod .
In this paper, we present data from two distant myriapod relatives - the millipede Glomeris marginata and the centipede Lithobius forficatus - which show conserved expression of antisense Ubx (aUbx) in a pattern complementary to that of Ubx in Myriapoda. Data from species of other arthropod groups and the onychophoran Euperipatoides kanangrensis reveal that aUbx expression does not represent an ancestral feature but a synapomorphy of the Myriapoda. The latter provides support for the still controversially discussed idea that the Myriapoda form a monophyletic group .
An mRNA that encodes a single protein, which describes the typical case for eukaryotic genes, is termed monocistronic, whereas mRNAs encoding two or several proteins are termed bicistronic and polycistronic respectively. We show here that bicistronic transcripts of Ubx and Antp (Ubx/Antp), as described for a number of crustaceans and the centipede Strigamia [9, 11], also exist in Glomeris and Euperipatoides. This finding suggests that bicistronic transcription is an ancestral feature that is likely to be involved also in arthropod Hox gene regulation by means of transcriptional interference and the blockade of Antp translation.
Materials and methods
Species husbandry and embryo treatment
The general handling of G. marginata is described in Janssen et al. . The embryos were allowed to develop at room temperature (22 to 25°C). The developmental stage of the embryos was determined by 4'-6-diamidino-2-phenylindole (DAP) staining. Staging was performed as described previously [12, 13].
Specimens of L. forficatus were collected from a leaf litter stack in the backyard of the Evolutionary Biology Centre (EBC) in Uppsala/Sweden in spring (May/June). Around 50 centipedes were held at room temperature in a spacious plastic box filled with washed leaf litter (washing away small particles makes the later finding of the eggs easier). The adults were fed with pieces of common earthworms (Lumbricus) every few days. The often detritus-covered eggs were collected by hand and incubated in plastic dishes on damp paper tissues until they reached the desired developmental stage. Staging was performed as described previously . Generally, the handling was carried out similarly to the method described for Lithobius atkinsoni .
Primers used for PCR.
Primer sequence 5' → 3'
Lithobius forficatus Ubx
Against N-terminal part of ANTP
A fragment of Lithobius forficatus Ubx was isolated with gene-specific primers based on the published sequence of Lithobius atkinsoni Ubx . The isolated L. forficatus fragment is only 221 bp long, but works well in hybridization experiments.
Part of the bicistronic transcripts containing Ultrabithorax and Antennapedia (Ubx/Antp) were isolated from the brine shrimp Artemia (first PCR), the onychophoran Euperipatoides and the millipede Glomeris. The gene-specific primers used were directed against the homeodomains of Ubx (forward primer) and Antp (backward primer). Gene-specific primers to amplify a possible Tribolium Ubx/Antp transcript failed, even though we used the primers (Table 1) in all possible combinations including nested PCRs.
Sequences of the fragments were determined from both strands by sequencing (Big Dye Terminator Cycle Sequencing Kit; Perkin-Elmer Applied Biosystems, Foster City, CA, USA) chemistry on an automatic analyser (ABI3730XL; Perkin-Elmer Applied Biosystems) by a commercial sequencing service (Macrogen, Seoul, Korea). Sequences are available in GenBank under the accession numbers FN687748 (Gm-Ubx), FN687749 (Gm-Antp), FN687750 (Gm-Ubx/Antp _variant II), FN687751 (Ek-Ubx), FN687752 (Ek-Ubx/Antp _variant I), FN687753 (Ek-Ubx/Antp _variant II), FN687754 (Lf-Ubx) and FN687755 (Af-Ubx/Antp _variant II).
In situ hybridization and nuclear staining
Whole-mount in situ hybridization for all species was performed as described previously for Glomeris . Double whole-mount in situ hybridization and cell nuclei detection using DAPI was performed as described by Janssen et al. . Embryos were analyzed under a dissection microscope (Leica, Heerbrugg, Switzerland) equipped with a digital camera (Axiocam; Zeiss, Jena, Germany) or a DC100 (Leica) digital camera. Brightness, contrast and colour values were corrected in all images using image processing software (Adobe Photoshop CS2., V.0.1 for Apple Macintosh; Adobe Systems Inc. San Jose, CA, USA).
Ultrabithorax and Antennapedia transcripts
Partial sequences of the transcripts of all ten Hox genes of G. marginata were published previously . In all cases except fushi-tarazu, only part of the homeodomain and 3' UTR sequence was obtained. The published Ubx fragment neither ends in a poly-A tail nor has one of the typical polyadenylation sites and is therefore likely to be incomplete. Recent 3'-RACE experiments demonstrated the presence of additional 3' UTR transcript. The extended fragment ends in a poly-A tail, but lacks an obvious polyadenylation site close to this. The 3' UTR region contains nine possible polyadenylation sites more distant from the poly-A tail, allowing for the presence of transcripts with different 3' UTR length. Whether the recovered '3' UTR' sequence is a typical UTR that occurs in the monocistronic transcript of Ubx or if is merely the result of the bicistronic transcript of Ubx and Antp (see following section) is unclear.
Bicistronic transcript of Ultrabithorax and Antennapedia
For Glomeris, we identified an Ubx/Antp bicistronic transcript that encodes the Ubx homeodomain C-terminal to the upstream primer position and 38 bp of the Ubx 3' UTR, which is directly adjacent to the complete N-terminal part of the Antp homeodomain up to the downstream primer position (splice variant II; see below) (Figure 1B,B'). Whether the sequence C-terminal to this sequence is part of the fusion transcript is unclear; however, the sequence N-terminal to the described short fusion transcript has been independently recovered by 5' RACE using gene specific primers (GSPs) against the Antp homeodomain that amplified the Ubx/Antp fusion transcript instead of the Antp 5' transcript. This sequence is part of the Ubx transcript as proven by 5'-RACE PCR for Ubx.
We also successfully isolated a splice version (splice variant I) of Ubx/Antp bicistronic transcripts from an onychophoran (Euperipatoides). This splice variant I is also described for a number of several crustaceans including the brine shrimp Artemia  (Figure 1B). For Euperipatoides and Artemia, we also isolated the shorter splice variant II of the bicistronic transcript described for Strigamia  (Figure 1B,B'). A splice variant I is not described for Strigamia and we could not isolate it from Glomeris either. We failed to detect any Ubx/Antp bicistronic transcripts in the beetle Tribolium (Insecta).
Extension and nature of the Ubx antisense (aUbx) transcript
The information on aUbx transcription is based on probes detecting the Ubx antisense strand during in situ hybridization experiments (Figure 1C). It was thus necessary to unravel the true extension of the aUbx transcript by in situ hybridization experiments with minimum size probes (around 300 bp for Glomeris) detecting aUbx complementary to the ends of the available Ubx fragments (Figure 1C). In all cases these sense probes detected the aUbx expression pattern (described below) suggesting their complete transcription. Whether the aUbx transcript extends the Ubx transcript is unclear; however, it does not extend into the transcripts of abdominal-A (abd-A) or Antennapedia (Antp), because in situ hybridization experiments with anti-abd-A and anti-Antp probes did not detect any transcription. The longest possible ORF of the aUbx transcript is 113aa long and encodes a repetitive sequence of the type (LLLLR/cSE) (Figure 1D).
Expression of aUbx
Complementary expression patterns of Ubx and aUbx
Transcript and expression of Lithobius Ubx and aUbx
Detection of aUbx in arthropods other than myriapods
We investigated the possible expression of aUbx in members of other arthropod classes and an onychophoran. Sense probes of the same length as the antisense probes used for the detection of Ubx in Tribolium (Insecta), the two known Ubx paralogs in Cupiennius (Chelicerata) , and Ubx in Euperipatoides (Onychophora) failed to detect any transcripts. In all cases, positive controls detecting the Ubx signal were successfully probed with antisense probes in parallel experiments (data not shown).
Conserved transcription and complementary expression of Ubx and aUbx supports myriapod monophyly
Sequence and expression data of Ultrabithorax are presently known from four myriapod species: the geophilomorph Strigamia maritima (Chilopoda) ; the lithobiomorph species L. atkinsoni and L. forficatus ( and this study); and the pill millipede G. marginata (Progoneata) . In all cases, the antisense DNA strand complementary to Ubx is transcribed and the expression pattern of the antisense transcripts (aUbx) is complementary to that of the sense transcript (coding transcript; Ubx) ( and this study). This finding suggests that complementary expression of sense and antisense transcripts generated from the Ubx locus is conserved between all myriapods.
Because aUbx expression has not yet been detected outside the Myriapoda, but has been detected in Chilopoda and Progoneata, it probably represents a synapomorphy for the Myriapoda, although this conclusion is dependent on the phylogenetic position of symphylans and pauropods [23–25]. This finding further supports myriapod monophyly, which is to date mainly based on nucleotide sequence data ([26, 27] morphological data are still controversial in this context [10, 25, 28, 29].
Similarities of Ubx regulation in Drosophila and myriapods: evidence for a conserved mechanism?
The fact that Ubx and aUbx are expressed in conserved and complex complementary patterns strongly suggests that one (or its transcription) is involved in the regulation of the other. Striking similarities to the situation in myriapods can be found in Drosophila, in which transcription of bxd non-coding RNAs (ncRNAs) upstream of Ubx prevents transcription of the latter. This repression is probably caused by transcriptional interference as the bxd transcript(s) elongate into the region of Ubx promoters and prevent the binding of the transcription machinery [4, 30]. As a result, bxd ncRNAs are expressed in a complementary pattern to that of Ubx, causing a mosaic-type expression pattern of Ubx within its overall expression domain [4, 6]
A similar situation is found in myriapods, in which a putative ncRNA, aUbx, is expressed in a complementary pattern to that of Ubx. Like the bxd ncRNAs in Drosophila, aUbx also precedes expression of Ubx, and also as in Drosophila, expression of Ubx in myriapods occurs in the anterior of each segment and expression of bxd and aUbx occur in the posterior of each segment (this study, [9, 31]).
The most obvious difference between the expression of bxd ncRNAs in Drosophila and aUbx in myriapods is that aUbx (or its promoter) is located on the complementary DNA strand in myriapods and not oriented in a tandem position to Ubx on the same strand. How can this disparity be explained if we assume that aUbx expression in myriapods is homologous to bxd expression in Drosophila?
Alternative functions of aUbx expression
A number of theories have been suggested over the past few years to explain how noncoding antisense transcripts or bidirectional transcription may regulate the expression of the coding unit ( and references therein). A case of possible transcriptional interference displaying much similarity between Drosophila and myriapods has been discussed in the previous section. However, although this possibility appears to be likely, aUbx or its transcription could nevertheless also act differently. We therefore summarize and discuss some of those mechanisms in the light of our data.
First, transcription of the antisense strand can cause epigenetic modifications, methylation of sense-strand promoters, and conversion of the chromosome structure, causing repression of gene transcription on the sense strand . Epigenetic modification could explain or cause the complementary pattern of Ubx and aUbx if aUbx represses the transcription of Ubx in tissues or cells that are generally Ubx-competent.
Second, transcriptional interference can also occur via promoter collision, when RNA polymerases meet on opposite strands and cannot pass each other. This can cause the premature termination of one or both transcripts [30, 35].
Third, sense and antisense transcripts could form double-stranded (ds)RNA, a source for small interfering RNAs that would mediate RNA interference (RNAi) . The complementary expression pattern of Ubx and aUbx would be explainable by the rapid degeneration of Ubx due to perfectly matching miRNAs descendent from the possible Ubx-aUbx dsRNA .
The fact that aUbx is expressed significantly earlier than Ubx may also have important implications on the regulatory mechanisms discussed. It would guarantee the immediate binding of incorrectly expressed Ubx to pre-existing aUbx in an RNAi-based mechanism, or provide a head start for transcription of aUbx in cases of transcriptional interference. In the case of epigenetic modification, it would prevent the later transcription of Ubx by silencing its promoter(s).
A 21 bp repeat in the Ultrabithorax 3' UTR of Glomeris
We discovered a repetitive sequence of exactly 21 bp (Figure 1D) in the 3' UTR of Ubx. This sequence most probably represents a minisatellite (or short sequence repeat; SSR) common in bacterial and metazoan genomes . It may represent multiple recognition sites for micro (mi)RNAs . Alternatively, it could represent an ORF encoding a small 113 amino acid protein, possibly involved in the regulation of Ubx. The finding of an SSR could generally also be of interest for investigating population genetics in Glomeris .
Presence of Ubx/Antp bicistronic transcripts in myriapods, crustaceans and onychophorans, but not in insects?
The finding that bicistronic transcripts of Ubx and Antp (Ubx/Antp) are present in myriapods and crustaceans suggests that this represents a conserved state of at least the Mandibulata or potentially the Arthropoda. Despite this, we failed to isolate Ubx/Antp fusion transcripts from the beetle T. castaneum. The latter may merely represent a loss in higher insects that finally allowed the Hox complex to split between Ubx and Antp, as is the case in Drosophila melanogaster ; however, in Tribolium, the Hox cluster is still intact . Alternatively, it may represent the early loss of Ubx/Antp in the stem lineage of the insects or hexapods. If the hexapods have evolved from a crustacean ancestor (as in the Pancrustacea theory), a loss of Ubx/Antp may be present in the suggested recent sister-group crustacean orders Remipedia and/or Cephalocarida . The presence of Ubx/Antp fusion transcripts in an onychophoran shows that the evolutionary origin of bicistronic transcription of Ubx and Antp dates back to the common ancestor of onychophorans and euarthropods, suggesting that Ubx/Antp is also likely to occur in chelicerates.
Interestingly, only the short splice variant II (Figure 1B,B') has been isolated from myriapods. We therefore believe that variant I may be lacking in myriapods exclusively, again supporting myriapod monophyly. However, we are aware that negative results are less reliable arguments than positive results, and therefore we can only see the lack of splice variant I in myriapods as minor evidence for monophyletic Myriapoda.
The presence of the Ubx/Antp splice variant II in onychophorans, crustaceans and myriapods argues against a mere genomic rearrangement in a population of Ubx as suggested for the centipede Strigamia , but rather suggests an important and conserved role in Hox gene regulation across the Arthropoda.
Conserved regulatory aspects of Ubx/Antp expression
In crustaceans, bicistronic transcripts of Ubx/Antp are not (Daphnia) or only partially (only Ubx in Artemia) translated. Expression of the translated monocistronic transcripts, and therefore the protein, differs significantly from expression of Ubx/Antp . It is tempting to speculate that transcription of Ubx/Antp under control of the Ubx promoter interferes with the proper transcription of monocistronic Antp in these crustaceans.
The conserved appearance of Ubx/Antp in arthropods and onychophorans suggests their involvement in the regulation of Ubx, Antp or both Hox genes. In particular, repression of Antp via Ubx/Antp transcription appears likely, not least because the transcript is apparently spliced in such a way that it lacks most of its coding capacity (variant II).
For Glomeris and Euperipatoides, it is unclear whether the detected expression patterns of Ubx and Antp are a result of mono-or bicistronic transcription. However, in both, as in crustaceans , the Ubx/Antp transcript is probably under control of the Ubx promoter, as the expression pattern of Ubx/Antp is identical with that of Ubx (not shown). Thus, it is possible Ubx/Antp contributes to or even replaces monocistronic Ubx expression in Glomeris and Euperipatoides as it does in Artemia . If part of the detected mRNA expression patterns of Ubx and Antp  is a result of Ubx/Antp, it might not correlate with the protein pattern. Specific antibodies to detect UBX and ANTP protein are not available, and the crossreacting antibody FP6.87  does not detect UBX in Glomeris (data not shown). Further investigation is thus needed to unravel the role of Ubx/Antp transcription in arthropods.
Regulation of limb development in Glomeris
Ubx expression is likely to be involved in the delayed outgrowth of the walking legs posterior to T3 in Glomeris by repressing Distal-less (Dll) as shown for other arthropods [44–46]. The finding that aUbx, a possible repressor of Ubx (as discussed above), is strongly expressed in the tips of the legs in T2 and T3 further supports this view, suggesting that the absence of Ubx is indeed crucial for the accelerated development of walking legs in T1 to T3 in Glomeris . The exclusion of Ubx from the distal part of the legs possibly caused or supported by aUbx could represent a developmental novelty in the 'battle' of appendage growth in Ubx-expressing segments. In Strigamia and Lithobius, Ubx seems not to repress Dll, possibly because of a number of phosphorylation sites in the C-terminal end of the protein that interfere with the assumed repressor function of Ubx on Dll [19, 45]. Consequently, there is no need to keep the tips of the legs free from Ubx or, in other words, to express aUbx.
A number of conserved aspects of Ubx and Antp regulation are found across the Arthropoda. Repression of Ubx transcription, and thus formation of a complex segmental pattern of Ubx expression, may depend on transcriptional interference as shown for Drosophila, and suggested and visualized by aUbx expression in Glomeris. Furthermore, bicistronic transcription of Ubx and Antp and subsequent splicing of these transcripts as shown for Crustacea, Myriapoda and Onychophora, but possibly not Insecta, suggests that Ubx/Antp transcription is an important ancestral feature of Hox gene regulation as well. As shown for Crustacea, runthrough transcription and subsequent nontranslation of Ubx/Antp may compete with the proper transcription of the (translated) monocistronic Ubx and Antp transcripts , and thus transcriptional interference via Ubx/Antp transcription might contribute to a defined protein expression pattern within areas of ubiquitously expressed Hox gene mRNA. Presence of aUbx transcription and the possible lack of Ubx/Antp splice variant I in myriapods represent possible synapomorphies for the Myriapoda.
This work was mainly supported by the European Union via the Marie Curie Research and Training Network ZOONET (MRTN-CT-2004-005624). The work was also supported by the Swedish Research Council (VR) and the Swedish Royal Academy of Sciences (KVA). We thank WGM Damen for the Ubx1 and Ubx2 clones and embryos of the spider C. salei. Adults of the beetle T. castaneum to establish our own lab culture were provided by G. Bucher and N-M. Prpic-Schäper (Göttingen). cDNA of A. franciscana was provided by N-M. Prpic-Schäper. Live specimens of E.kanangrensis were collected with the most appreciated help of Noel Tait (Sydney). We would like to thank the three anonymous reviewers for their helpful comments on the manuscript.
- Irish VF, Martinez-Arias A, Akam M: Spatial regulation of the Antennapedia and Ultrabithorax homeotic genes during Drosophila early development. EMBO J. 1989, 8: 1527-1537.PubMed CentralPubMedGoogle Scholar
- Carroll SB, DiNardo S, O'Farrell PH, White RA, Scott MP: Temporal and spatial relationships between segmentation and homeotic gene expression in Drosophila embryos: distributions of the fushi tarazu, engrailed, Sex combs reduced, Antennapedia, and Ultrabithorax proteins. Genes Dev. 1988, 2: 350-360. 10.1101/gad.2.3.350.View ArticlePubMedGoogle Scholar
- Grimaud C, Negre N, Cavalli G: From genetics to epigenetics: the tale of Polycomb group and trithoarax group genes. Chromosome Res. 2006, 14: 363-375. 10.1007/s10577-006-1069-y.View ArticlePubMedGoogle Scholar
- Petruk S, Sedkov Y, Riley KM, Hodgson J, Schweisguth F, Hirose S, Jaynes JB, Broch HW, Mazo A: Transcription of bxd noncoding RNAs promoted by trithorax represses Ubx in cis by transcriptional interference. Cell. 2006, 127: 1209-1221. 10.1016/j.cell.2006.10.039.PubMed CentralView ArticlePubMedGoogle Scholar
- Hodgson JW, Argiropoulos B, Brock HW: Site-specific recognition of a 70-base-pair element containing d(GA)(n) repeats mediates bithoraxoid polycomb group response element-dependent silencing. Mol Cell Biol. 2001, 21: 4528-4543. 10.1128/MCB.21.14.4528-4543.2001.PubMed CentralView ArticlePubMedGoogle Scholar
- Rank G, Prestel M, Paro R: Transcription through intergenetic chromosomal memory elements of the Drosophila Bithorax complex correlates with an epigenetic switch. Mol Cell Biol. 2002, 22: 8026-8034. 10.1128/MCB.22.22.8026-8034.2002.PubMed CentralView ArticlePubMedGoogle Scholar
- Bae E, Calhoun VC, Levine M, Lewis EB, Drewell RA: Characterization of the intergenic RNA profile at abdominal-A and Abdominal-B in the Drosophila bithorax complex. Proc Natl Acad Sci USA. 2002, 99: 16847-16852. 10.1073/pnas.222671299.PubMed CentralView ArticlePubMedGoogle Scholar
- Shippy TD, Ronshaugen M, Cande J, He J, Beeman RW, Levine M, Brown SJ, Denell RE: Analysis of the Tribolium homeotic complex: insight into mechanisms constraining insect Hox clusters. Dev Genes Evol. 2008, 218: 127-139. 10.1007/s00427-008-0213-4.PubMed CentralView ArticlePubMedGoogle Scholar
- Brena C, Chipman AD, Minelli A, Akam M: Expression of trunk Hox genes in the centipede Strigamia maritima: sense and anti-sense transcripts. Evol Dev. 2006, 8: 252-265. 10.1111/j.1525-142X.2006.00096.x.View ArticlePubMedGoogle Scholar
- Koch M: Monophyly of the Myriapoda? Reliability of current arguments. Proceedings of the 12th International Congress of Myriapodology. Afr Inverts. Edited by: Hamer M. 2003, 44: 137-153.Google Scholar
- Shiga Y, Sagawa K, Takai R, Sakaguchi H, Yamagata H, Hayashi S: Transcriptional readthrough of Hox genes Ubx and Antp and their divergent post-transcriptional control during crustacean evolution. Evol Dev. 2006, 8: 407-414. 10.1111/j.1525-142X.2006.00114.x.View ArticlePubMedGoogle Scholar
- Janssen R, Prpic N-M, Damen WGM: Gene expression suggests decoupled dorsal and ventral segmentation in the millipede Glomeris marginata (Myriapoda: Diplopoda). Dev Biol. 2004, 268: 89-104. 10.1016/j.ydbio.2003.12.021.View ArticlePubMedGoogle Scholar
- Dohle W: Die Embryonalentwicklung von Glomeris marginata (Villers) im Vergleich zur Entwicklung anderer Diplopoden. Zool Jb Anat. 1964, 81: 241-310.Google Scholar
- Kadner D, Stollewerk A: Neurogenesis in the chilopod Lithobius forficatus suggests more similarities to chelicerates than to insects. Dev Genes Evol. 2004, 214: 367-379. 10.1007/s00427-004-0419-z.View ArticlePubMedGoogle Scholar
- Hughes CL, Kaufman TC: Exploring the myriapod body plan: expression patterns of the ten Hox genes in a centipede. Development. 2002, 19: 1225-1238.Google Scholar
- Damen WGM, Hausdorf M, Seyfarth EA, Tautz D: The expression pattern of Hox genes in the spider Cupiennius salei suggests a conserved mode of head segmentation in arthropods. Proc Natl Acad Sci USA. 1998, 95: 10665-10670. 10.1073/pnas.95.18.10665.PubMed CentralView ArticlePubMedGoogle Scholar
- Eriksson BJ, Tait NN, Budd GE, Akam M: The involvement of engrailed and wingless during segmentation in the onychophoran Euperiaptoides kanangrensis (Peripatopsidae: Onychophora) (Reid 1996). Dev Genes Evol. 2009, 219: 249-264. 10.1007/s00427-009-0287-7.View ArticlePubMedGoogle Scholar
- Wolff C, Sommer R, Schröder R, Glaser G, Tautz D: Conserved and divergent expression aspects of the Drosophila segmentation gene hunchback in the short germ band embryo of the flour beetle Tribolium. Development. 1995, 121: 4227-4236.PubMedGoogle Scholar
- Janssen R, Damen WGM: The ten Hox genes of the millipede Glomeris marginata. Dev Genes Evol. 2006, 216: 451-465. 10.1007/s00427-006-0092-5.View ArticlePubMedGoogle Scholar
- Prpic N-M, Tautz D: The expression of the proximodistal axis patterning genes Distal-less and dachshund in the appendages of Glomeris marginata (Myriapoda: Diplopoda) suggests a special role of these genes in patterning the head appendages. Dev Biol. 2003, 260: 97-112. 10.1016/S0012-1606(03)00217-3.View ArticlePubMedGoogle Scholar
- Janssen R, Budd GE, Damen WG, Prpic N-M: Evidence for Wg-independent tergite boundary formation in the millipede Glomeris marginata. Dev Genes Evol. 2008, 218: 361-370. 10.1007/s00427-008-0231-2.View ArticlePubMedGoogle Scholar
- Cook CE, Smithe ML, Telford MJ, Bastianello A, Akam M: Hox genes and the phylogeny of the arthropods. Curr Biol. 2001, 11: 759-763. 10.1016/S0960-9822(01)00222-6.View ArticlePubMedGoogle Scholar
- Dohle W: Progoneata. Spezielle Zoologie Teil 1: Einzeller Und Wirbellose Tiere. Edited by: Westheide W, Rieger R. 1996, Stuttgart, Jena: Gustav Fischer Verlag, 592-600.Google Scholar
- Edgecombe GD: Arthropod phylogeny: An overview from the perspective of morphology, molecular data and the fossil record. Arth Struct Dev. 2010, 39: 74-87. 10.1016/j.asd.2009.10.002.View ArticleGoogle Scholar
- Shear WA, Edgecombe GD: The geological record and phylogeny of the Myriapoda. Arthropod Struct Dev. 2010, 39: 174-190. 10.1016/j.asd.2009.11.002.View ArticlePubMedGoogle Scholar
- Regier JC, Wilson HM, Shultz JW: Phylogenetic analysis of Myriapoda using three nuclear protein-coding genes. Mol Phyl Evol. 2005, 34: 147-158. 10.1016/j.ympev.2004.09.005.View ArticleGoogle Scholar
- Gai Y-H, Song D-X, Sun H-Y, Zhou K-Y: Myriapod monophyly and relationships among myriapod classes based on nearly complete 28 S and 18 S rDNA sequences". Zool Sci. 2006, 23: 1101-1108. 10.2108/zsj.23.1101.View ArticlePubMedGoogle Scholar
- Loesel R, Strausfeld NJ: Common design in a unique midline neuropil in the brains of arthropods. Arth Struct Dev. 2002, 31: 77-91. 10.1016/S1467-8039(02)00017-8.View ArticleGoogle Scholar
- Strausfeld NJ, Strausfeld CM, Loesel R, Rowell D, Stowe S: Arthropod phylogeny: onychophoran brain organization suggests an archaic relationship with a chelicerate stem lineage. Proc R Soc B. 2006, 273: 1857-1866. 10.1098/rspb.2006.3536.PubMed CentralView ArticlePubMedGoogle Scholar
- Mazo A, Hodgson JW, Petruk S, Sedkov Y, Brock HW: Transcriptional interference: an unexpected layer of complexity in gene regulation. J Cell Sci. 2007, 120: 2755-2761. 10.1242/jcs.007633.View ArticlePubMedGoogle Scholar
- Petruk S, Sedkov Y, Brock HW, Mazo A: A model for initiation of mosaic Hox gene expression patterns by non-coding RNAs in early embryos. RNA Biol. 2007, 4: 1-View ArticlePubMedGoogle Scholar
- Douris V, Telford MJ, Averof M: Evidence for multiple independent origins of trans-splicing in Metazoa. Mol Biol Evol. 2010, 27: 684-693. 10.1093/molbev/msp286.View ArticlePubMedGoogle Scholar
- Osato N, Suzuki Y, Ikeo K, Gojobori T: Transcriptional interferences in cis natural antisense transcripts of humans and mice. Genetics. 2007, 176: 1299-306. 10.1534/genetics.106.069484.PubMed CentralView ArticlePubMedGoogle Scholar
- Tufarelli C, Stanley JA, Garrick D, Sharpe JA, Ayyub H, Wood WG, Higgs DR: Transcription of antisense RNA leading to gene silencing and methylation as a novel cause of human genetic disease. Nat Genet. 2003, 34: 157-165. 10.1038/ng1157.View ArticlePubMedGoogle Scholar
- Crampton N, Bonass WA, Kirkham J, Rivetti C, Thomson NH: Collision events between RNA polymerases in convergent transcription studied by atomic force microscopy. Nucleic Acids Res. 2006, 34: 5416-5425. 10.1093/nar/gkl668.PubMed CentralView ArticlePubMedGoogle Scholar
- Okamura K, Balla S, Martin R, Liu N, Lai EC: Two distinct mechanisms generate endogenous siRNAs from bidirectional transcription in Drosophila melanogaster. Nat Struct Mol Biol. 2008, 15: 998-10.1038/nsmb0908-998c.View ArticlePubMedGoogle Scholar
- Wienholds E, Plasterk RHA: MicroRNA function in animal development. FEBS Lett. 2009, 579: 5911-5922. 10.1016/j.febslet.2005.07.070.View ArticleGoogle Scholar
- Mouton L, Nong G, Preston JF, Ebert D: Variable-number tandem repeats as molecular markers for biotypes of Pasteuria ramose in Daphnia spp. App Environ Microbiol. 2007, 73: 3715-3718. 10.1128/AEM.02398-06.View ArticleGoogle Scholar
- Lai EC: Micro RNAs are complementary to 3' UTR sequence motifs that mediate negative post-transcriptional regulation. Nat Genet. 2002, 30: 363-364. 10.1038/ng865.View ArticlePubMedGoogle Scholar
- Ellis JR, Burke JM: EST-SSRs as a resource for population genetic analyses. Heredity. 2007, 99: 125-132. 10.1038/sj.hdy.6801001.View ArticlePubMedGoogle Scholar
- Kaufman TC, Lewis R, Wakimoto B: Cytogenetic analysis of chromosome 3 in Drosophila melanogaster: The homoeotic gene complex in polytene chromosome interval 84A-B. Genetics. 1980, 94: 115-133.PubMed CentralPubMedGoogle Scholar
- Regier JC, Shultz JW, Zwick A, Hussey A, Ball B, Wetzer R, Martin JW, Cunningham CW: Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature. 2010, 463: 1079-1083. 10.1038/nature08742.View ArticlePubMedGoogle Scholar
- Kelsh R, Weinzierl RO, White RA, Akam M: Homeotic gene expression in the locust Schistocerca: an antibody that detects conserved epitopes in Ultrabithorax and abdominal-B. Dev Genet. 1994, 15: 19-31. 10.1002/dvg.1020150104.View ArticlePubMedGoogle Scholar
- Mann RS, Hogness DS: Functional dissection of Ultrabithorax protein in D. melanogaster. Cell. 1990, 60: 597-610. 10.1016/0092-8674(90)90663-Y.View ArticlePubMedGoogle Scholar
- Ronshaugen M, McGinnis N, McGinnis W: Hox protein mutation and macroevolution of the insect body plan. Nature. 2002, 415: 914-917. 10.1038/nature716.View ArticlePubMedGoogle Scholar
- Galant R, Carroll SB: Evolution of a transcriptional repression domain in an insect Hox protein. Nature. 2002, 415: 910-913. 10.1038/nature717.View ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.