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Prashant P Sharma - One of the best experts on this subject based on the ideXlab platform.
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a critical appraisal of the placement of xiphosura chelicerata with account of known sources of phylogenetic error
Systematic Biology, 2019Co-Authors: Jesus A Ballesteros, Prashant P SharmaAbstract:Horseshoe crabs (Xiphosura) are traditionally regarded as sister group to the clade of terrestrial Chelicerates (Arachnida). This hypothesis has been challenged by recent phylogenomic analyses, but the non-monophyly of Arachnida has consistently been disregarded as artifactual. We re-evaluated the placement of Xiphosura among Chelicerates using the most complete phylogenetic data set to date, expanding outgroup sampling, and including data from whole genome sequencing projects. In spite of uncertainty in the placement of some arachnid clades, all analyses show Xiphosura consistently nested within Arachnida as the sister group to Ricinulei (hooded tick spiders). It is apparent that the radiation of arachnids is an old one and occurred over a brief period of time, resulting in several consecutive short internodes, and thus is a potential case for the confounding effects of incomplete lineage sorting (ILS). We simulated coalescent gene trees to explore the effects of increasing levels of ILS on the placement of horseshoe crabs. In addition, common sources of systematic error were evaluated, as well as the effects of fast-evolving partitions and the dynamics of problematic long branch orders. Our results indicated that the placement of horseshoe crabs cannot be explained by missing data, compositional biases, saturation, or ILS. Interrogation of the phylogenetic signal showed that the majority of loci favor the derived placement of Xiphosura over a monophyletic Arachnida. Our analyses support the inference that horseshoe crabs represent a group of aquatic arachnids, comparable to aquatic mites, breaking a long-standing paradigm in Chelicerate evolution and altering previous interpretations of the ancestral transition to the terrestrial habitat. Future studies testing Chelicerate relationships should approach the task with a sampling strategy where the monophyly of Arachnida is not held as the premise.
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Chelicerates.
Current biology : CB, 2018Co-Authors: Prashant P SharmaAbstract:Compared to other arthropods, such as crustaceans or insects, the term 'Chelicerate' often does not evoke a similar sense of recognition or familiarity. Yet the subphylum Chelicerata has been encountered by every living person today, frequently to the effect of fear, awe, or outright revulsion. Chelicerates include such familiar groups as spiders, scorpions, mites, and ticks, as well as an array of bizarre and unfamiliar forms, such as vinegaroons, camel spiders, and hooded tick spiders (Figure 1).
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A rticle Phylogenomic Interrogation of Arachnida Reveals Systemic Conflicts in Phylogenetic Signal
2016Co-Authors: Prashant P Sharma, Gonzalo Giribet, Stefan T. Kaluziak, Alicia R. Perez-porro, Vanessa L. Gonzalez, Ward C. Wheeler, Associate Nicolas VidalAbstract:Chelicerata represents one of the oldest groups of arthropods, with a fossil record extending to the Cambrian, and is sister group to the remaining extant arthropods, the mandibulates. Attempts to resolve the internal phylogeny of Chelicerates have achieved little consensus, due to marked discord in both morphological and molecular hypotheses of Chelicerate phylogeny. Themonophyly of Arachnida, the terrestrial Chelicerates, is generally accepted, but has garnered little support from molecular data, which have been limited either in breadth of taxonomic sampling or in depth of sequencing. To address the internal phylogeny of this group, we employed a phylogenomic approach, generating transcriptomic data for 17 species in combination with existing data, including two complete genomes. We analyzed multiple data sets contain-ing up to 1,235,912 sites across 3,644 loci, using alternative approaches to optimization of matrix composition. Here, we show that phylogenetic signal for the monophyly of Arachnida is restricted to the 500 slowest-evolving genes in the data set. Accelerated evolutionary rates in Acariformes, Pseudoscorpiones, and Parasitiformes potentially engender long-branch attraction artifacts, yielding nonmonophyly of Arachnida with increasing support upon incrementing the number of concatenated genes. Mutually exclusive hypotheses are supported by locus groups of variable evolutionary rate, revealing significant conflicts in phylogenetic signal. Analyses of gene-tree discordance indicate marked incongru-ence in relationships among Chelicerate orders, whereas derived relationships are demonstrably robust. Consistentl
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DOI: 10.1111/ede.12005 Evolution of the chelicera: a dachshund domain is retained in the deutocerebral appendage of Opiliones (Arthropoda, Chelicerata)
2016Co-Authors: Prashant P Sharma, Evelyn E. Schwager, B Cass, Ra G. Extavour, Gonzalo GiribetaAbstract:SUMMARY The proximo-distal axis of the arthropod leg is patterned by mutually antagonistic developmental expression domains of the genes extradenticle, homothorax, dachshund, and Distal-less. In the deutocerebral appendages (the an-tennae) of insects and crustaceans, the expression domain of dachshund is frequently either absent or, if present, is not required to pattern medial segments. By contrast, the dachshund domain is entirely absent in the deutocerebral ap-pendages of spiders, the chelicerae. It is unknown whether absence of dachshund expression in the spider chelicera is associated with the two-segmented morphology of this ap-pendage, or whether all Chelicerates lack the dachshund do-main in their chelicerae. We investigated gene expression in the harvestman Phalangium opilio, which bears the ple-siomorphic three-segmented chelicera observed in “primitive” Chelicerate orders. Consistent with patterns reported in spi-ders, in the harvestman chelicera homothorax, extradenticle, and Distal-less have broadly overlapping developmental do-mains, in contrast with mutually exclusive domains in the legs and pedipalps. However, unlike in spiders, the harvestman chelicera bears a distinct expression domain of dachshund in the proximal segment, the podomere that is putatively lost in derived arachnids. These data suggest that a tripar-tite proximo-distal domain structure is ancestral to all arthro-pod appendages, including deutocerebral appendages. As a corollary, these data also provide an intriguing putative ge-netic mechanism for the diversity of arachnid chelicerae: loss of developmental domains along the proximo-distal axis
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divergent patterning of the Chelicerate opisthosoma
2016Co-Authors: Prashant P Sharma, Evelyn E. Schwager, B Cass, Ra G. Extavour, Gonzalo GiribetaAbstract:has only been investigated in representatives of two arachnid orders to date: Acari (mites and ticks) and Araneae (spiders). Limited data are available for the “primitive ” arachnid orders, such as Scorpiones (scorpions) and Opiliones (harvestmen). Here, we present the first data on Hox gene expression in the harvestman Phalangium opilio. Ten Hox genes of this species were obtained from a de novo assembled developmental tran-scriptome using the Illumina GAII platform. All 10 genes are expressed in characteristic Hox-like expression patterns, and the expression of the anterior and central Hox genes is sim-ilar to those of other Chelicerates. However, intriguingly, the three posteriormost genes—Ultrabithorax, abdominal-A, and Abdominal-B—share an identical anterior expression bound-ary in the second opisthosomal segment, and their expres-sion domains extend through the opisthosoma to the posterior growth zone. The overlap in expression domains of the pos-terior Hox genes is correlated with the absence of opisthoso-mal organs posterior to the tubular tracheae, which occur on the second opisthosomal segment. Together with the stag-gered profile of posterior Hox genes in spiders, these data suggest the involvement of abdominal-A and Abdominal-B in the evolution of heteronomous patterning of the Chelicerate opisthosoma, providing a mechanism that helps explain the morphological diversity of Chelicerates
Ui-wook Hwang - One of the best experts on this subject based on the ideXlab platform.
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complete mitochondrial genomes of carcinoscorpius rotundicauda and tachypleus tridentatus xiphosura arthropoda and implications for Chelicerate phylogenetic studies
International Journal of Biological Sciences, 2014Co-Authors: Su Youn Baek, Sang Myeon Park, Kuem Hee Jang, Cheon Young Chang, Eun Hwa Choi, Ui-wook HwangAbstract:Horseshoe crabs (order Xiphosura) are often referred to as an ancient order of marine Chelicerates and have been considered as keystone taxa for the understanding of Chelicerate evolution. However, the mitochondrial genome of this order is only available from a single species, Limulus polyphemus. In the present study, we analyzed the complete mitochondrial genomes from two Asian horseshoe crabs, Carcinoscorpius rotundicauda and Tachypleus tridentatus to offer novel data for the evolutionary relationship within Xiphosura and their position in the Chelicerate phylogeny. The mitochondrial genomes of C. rotundicauda (15,033 bp) and T. tridentatus (15,006 bp) encode 13 protein-coding genes, two ribosomal RNA (rRNA) genes, and 22 transfer RNA (tRNA) genes. Overall sequences and genome structure of two Asian species were highly similar to that of Limulus polyphemus, though clear differences among three were found in the stem-loop structure of the putative control region. In the phylogenetic analysis with complete mitochondrial genomes of 43 Chelicerate species, C. rotundicauda and T. tridentatus were recovered as a monophyly, while L. polyphemus solely formed an independent clade. Xiphosuran species were placed at the basal root of the tree, and major other Chelicerate taxa were clustered in a single monophyly, clearly confirming that horseshoe crabs composed an ancestral taxon among Chelicerates. By contrast, the phylogenetic tree without the information of Asian horseshoe crabs did not support monophyletic clustering of other Chelicerates. In conclusion, our analyses may provide more robust and reliable perspective on the study of evolutionary history for Chelicerates than earlier analyses with a single Atlantic species.
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complete mitochondrial genomes of carcinoscorpius rotundicauda and tachypleus tridentatus xiphosura arthropoda and implications for Chelicerate
2014Co-Authors: Su Youn Baek, Sang Myeon Park, Kuem Hee Jang, Cheon Young Chang, Eun Hwa Choi, Ui-wook HwangAbstract:Horseshoe crabs (order Xiphosura) are often referred to as an ancient order of marine Chelicerates and have been considered as keystone taxa for the understanding of Chelicerate evolution. However, the mitochondrial genome of this order is only available from a single species, Limulus polyphemus. In the present study, we analyzed the complete mitochondrial genomes from two Asian horseshoe crabs, Carcinoscorpius rotundicauda and Tachypleus tridentatus to offer novel data for the evolutionary relationship within Xiphosura and their position in the Chelicerate phylogeny. The mitochondrial genomes of C. rotundicauda (15,033 bp) and T. tridentatus (15,006 bp) encode 13 protein-coding genes, two ribosomal RNA (rRNA) genes, and 22 transfer RNA (tRNA) genes. Overall sequences and genome structure of two Asian species were highly similar to that of Limulus polyphemus, though clear differences among three were found in the stem-loop structure of the putative control region. In the phylogenetic analysis with complete mitochondrial genomes of 43 Chelicerate species, C. rotundicauda and T. tridentatus were recovered as a monophyly, while L. polyphemus solely formed an independent clade. Xiphosuran species were placed at the basal root of the tree, and major other Chelicerate taxa were clustered in a single monophyly, clearly confirming that horseshoe crabs composed an ancestral taxon among Chelicerates. By contrast, the phylogenetic tree without the information of Asian horseshoe crabs did not support monophyletic clustering of other Chelicerates. In conclusion, our analyses may provide more robust and reliable perspective on the study of evolutionary history for Chelicerates than earlier analyses with a single Atlantic species.
Su Youn Baek - One of the best experts on this subject based on the ideXlab platform.
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complete mitochondrial genomes of carcinoscorpius rotundicauda and tachypleus tridentatus xiphosura arthropoda and implications for Chelicerate phylogenetic studies
International Journal of Biological Sciences, 2014Co-Authors: Su Youn Baek, Sang Myeon Park, Kuem Hee Jang, Cheon Young Chang, Eun Hwa Choi, Ui-wook HwangAbstract:Horseshoe crabs (order Xiphosura) are often referred to as an ancient order of marine Chelicerates and have been considered as keystone taxa for the understanding of Chelicerate evolution. However, the mitochondrial genome of this order is only available from a single species, Limulus polyphemus. In the present study, we analyzed the complete mitochondrial genomes from two Asian horseshoe crabs, Carcinoscorpius rotundicauda and Tachypleus tridentatus to offer novel data for the evolutionary relationship within Xiphosura and their position in the Chelicerate phylogeny. The mitochondrial genomes of C. rotundicauda (15,033 bp) and T. tridentatus (15,006 bp) encode 13 protein-coding genes, two ribosomal RNA (rRNA) genes, and 22 transfer RNA (tRNA) genes. Overall sequences and genome structure of two Asian species were highly similar to that of Limulus polyphemus, though clear differences among three were found in the stem-loop structure of the putative control region. In the phylogenetic analysis with complete mitochondrial genomes of 43 Chelicerate species, C. rotundicauda and T. tridentatus were recovered as a monophyly, while L. polyphemus solely formed an independent clade. Xiphosuran species were placed at the basal root of the tree, and major other Chelicerate taxa were clustered in a single monophyly, clearly confirming that horseshoe crabs composed an ancestral taxon among Chelicerates. By contrast, the phylogenetic tree without the information of Asian horseshoe crabs did not support monophyletic clustering of other Chelicerates. In conclusion, our analyses may provide more robust and reliable perspective on the study of evolutionary history for Chelicerates than earlier analyses with a single Atlantic species.
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complete mitochondrial genomes of carcinoscorpius rotundicauda and tachypleus tridentatus xiphosura arthropoda and implications for Chelicerate
2014Co-Authors: Su Youn Baek, Sang Myeon Park, Kuem Hee Jang, Cheon Young Chang, Eun Hwa Choi, Ui-wook HwangAbstract:Horseshoe crabs (order Xiphosura) are often referred to as an ancient order of marine Chelicerates and have been considered as keystone taxa for the understanding of Chelicerate evolution. However, the mitochondrial genome of this order is only available from a single species, Limulus polyphemus. In the present study, we analyzed the complete mitochondrial genomes from two Asian horseshoe crabs, Carcinoscorpius rotundicauda and Tachypleus tridentatus to offer novel data for the evolutionary relationship within Xiphosura and their position in the Chelicerate phylogeny. The mitochondrial genomes of C. rotundicauda (15,033 bp) and T. tridentatus (15,006 bp) encode 13 protein-coding genes, two ribosomal RNA (rRNA) genes, and 22 transfer RNA (tRNA) genes. Overall sequences and genome structure of two Asian species were highly similar to that of Limulus polyphemus, though clear differences among three were found in the stem-loop structure of the putative control region. In the phylogenetic analysis with complete mitochondrial genomes of 43 Chelicerate species, C. rotundicauda and T. tridentatus were recovered as a monophyly, while L. polyphemus solely formed an independent clade. Xiphosuran species were placed at the basal root of the tree, and major other Chelicerate taxa were clustered in a single monophyly, clearly confirming that horseshoe crabs composed an ancestral taxon among Chelicerates. By contrast, the phylogenetic tree without the information of Asian horseshoe crabs did not support monophyletic clustering of other Chelicerates. In conclusion, our analyses may provide more robust and reliable perspective on the study of evolutionary history for Chelicerates than earlier analyses with a single Atlantic species.
Tharina Louise Bird - One of the best experts on this subject based on the ideXlab platform.
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Cheliceral morphology of Solifugae (Arachnida): Primary homology, terminology, and character survey
Bulletin of the American Museum of Natural History, 2015Co-Authors: Tharina Louise BirdAbstract:ABSTRACT Arachnids of the order Solifugae (solifuges, false spiders, sun spiders, camel spiders, Walzenspinne, wind spiders) possess the largest jaws for body size among the Chelicerata. The chelicerae provide the most important character systems for solifuge systematics, including dentition and the male cheliceral flagellum, both used extensively for species delimitation and diagnosis. However, the terminology used for cheliceral characters is not standardized and often contradictory, in part because it fails to represent homologous structures among taxa. Misinterpretation of character homology may introduce errors in phylogenetic analyses concerning relationships within Solifugae and among the orders of Chelicerata. This contribution presents the first comprehensive analysis of cheliceral morphology across the order Solifugae, the aims of which were to provide a broad survey of cheliceral characters for solifuge systematics, to identify and reinterpret structures based on primary homology, to revise the...
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High resolution images for 'Cheliceral morphology in Solifugae (Arachnida) : primary homology, terminology, and character survey. (Bulletin of the American Museum of Natural History, no. 394)'
American Museum of Natural History Research Library, 2015Co-Authors: Lorenzo Prendini, Robert A. Wharton, Tharina Louise BirdAbstract:355 pages : illustrations (some color) ; 26 cm.Arachnids of the order Solifugae (solifuges, false spiders, sun spiders, camel spiders, Walzenspinne, wind spiders) possess the largest jaws for body size among the Chelicerata. The chelicerae provide the most important character systems for solifuge systematics, including dentition and the male cheliceral flagellum, both used extensively for species delimitation and diagnosis. However, the terminology used for cheliceral characters is not standardized and often contradictory, in part because it fails to represent homologous structures among taxa. Misinterpretation of character homology may introduce errors in phylogenetic analyses concerning relationships within Solifugae and among the orders of Chelicerata. This contribution presents the first comprehensive analysis of cheliceral morphology across the order Solifugae, the aims of which were to provide a broad survey of cheliceral characters for solifuge systematics, to identify and reinterpret structures based on primary homology, to revise the terminology to be consistent with homology hypotheses, and to provide a guide to terminological synonyms and character interpretations in the literature. Chelicerae were studied in 188 exemplar species (17% of the total), representing all 12 solifuge families, 17 of the 19 subfamilies, 64 genera (46% of the total), and the full range of variation in cheliceral morphology across the order. In total, 157 species representing 49 genera and 17 subfamilies are illustrated. Hypotheses of character transformation, particularly concerning the male flagellum, and a standardized terminology, are presented. The functional morphology of the chelicerae is discussed and the role of sexually dimorphic modifications to the male chelicerae in mating behavior emphasized. The revised terminology, based on hypotheses of primary homology, will facilitate solifuge revisionary systematics and provide a stronger basis for reconstructing phylogenetic relationships within the order Solifugae and testing the phylogenetic position of the order within Chelicerata
Gonzalo Giribet - One of the best experts on this subject based on the ideXlab platform.
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The Phylogeny and Evolutionary History of Arthropods
Current Biology, 2019Co-Authors: Gonzalo Giribet, Gregory D. EdgecombeAbstract:Arthropods are the most diverse animal phylum, and their phylogenetic relationships have been debated for centuries. With the advent of molecular phylogenetics, arthropods were found to be monophyletic and placed within a clade of molting animals, the ecdysozoans, with nematodes and six other phyla. Molecular phylogenetics also provided a new framework for relationships between the major arthropod groups, such as the clade Pancrustacea, which comprises insects and crustaceans. Phylogenomics based on second-generation genomics and transcriptomics has further resolved puzzles such as the exact position of myriapods or the closest crustacean relatives of hexapods. It is now broadly recognized that extant arthropods are split into Chelicerates and mandibulates, and relationships within the two mandibulate clades (myriapods and pancrustaceans) are stabilizing. Notably, the phylogeny of insects is now understood with considerable confidence, whereas relationships among Chelicerate orders remain poorly resolved. The evolutionary history of arthropods is illuminated by a rich record of fossils, often with exquisite preservation, but current analyses conflict over whether certain fossil groups are stem- or crown-group arthropods. Molecular time-trees calibrated with fossils estimate the origins of arthropods to be in the Ediacaran, while most other deep nodes date to the Cambrian. The earliest stem-group arthropods were lobopodians, worm-like animals with annulated appendages. Confidently placing some key extinct clades on the arthropod tree of life may require less ambiguous interpretation of fossil structures and better integration of morphological data into the phylogeny.
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A rticle Phylogenomic Interrogation of Arachnida Reveals Systemic Conflicts in Phylogenetic Signal
2016Co-Authors: Prashant P Sharma, Gonzalo Giribet, Stefan T. Kaluziak, Alicia R. Perez-porro, Vanessa L. Gonzalez, Ward C. Wheeler, Associate Nicolas VidalAbstract:Chelicerata represents one of the oldest groups of arthropods, with a fossil record extending to the Cambrian, and is sister group to the remaining extant arthropods, the mandibulates. Attempts to resolve the internal phylogeny of Chelicerates have achieved little consensus, due to marked discord in both morphological and molecular hypotheses of Chelicerate phylogeny. Themonophyly of Arachnida, the terrestrial Chelicerates, is generally accepted, but has garnered little support from molecular data, which have been limited either in breadth of taxonomic sampling or in depth of sequencing. To address the internal phylogeny of this group, we employed a phylogenomic approach, generating transcriptomic data for 17 species in combination with existing data, including two complete genomes. We analyzed multiple data sets contain-ing up to 1,235,912 sites across 3,644 loci, using alternative approaches to optimization of matrix composition. Here, we show that phylogenetic signal for the monophyly of Arachnida is restricted to the 500 slowest-evolving genes in the data set. Accelerated evolutionary rates in Acariformes, Pseudoscorpiones, and Parasitiformes potentially engender long-branch attraction artifacts, yielding nonmonophyly of Arachnida with increasing support upon incrementing the number of concatenated genes. Mutually exclusive hypotheses are supported by locus groups of variable evolutionary rate, revealing significant conflicts in phylogenetic signal. Analyses of gene-tree discordance indicate marked incongru-ence in relationships among Chelicerate orders, whereas derived relationships are demonstrably robust. Consistentl
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Phylogenomic Interrogation of Arachnida Reveals Systemic Conflicts in Phylogenetic Signal
Molecular biology and evolution, 2014Co-Authors: Prashant P Sharma, Stefan T. Kaluziak, Alicia R. Perez-porro, Vanessa L. Gonzalez, Ward C. Wheeler, Gustavo Hormiga, Gonzalo GiribetAbstract:Chelicerata represents one of the oldest groups of arthropods, with a fossil record extending to the Cambrian, and is sister group to the remaining extant arthropods, the mandibulates. Attempts to resolve the internal phylogeny of Chelicerates have achieved little consensus, due to marked discord in both morphological and molecular hypotheses of Chelicerate phylogeny. The monophyly of Arachnida, the terrestrial Chelicerates, is generally accepted, but has garnered little support from molecular data, which have been limited either in breadth of taxonomic sampling or in depth of sequencing. To address the internal phylogeny of this group, we employed a phylogenomic approach, generating transcriptomic data for 17 species in combination with existing data, including two complete genomes. We analyzed multiple data sets containing up to 1,235,912 sites across 3,644 loci, using alternative approaches to optimization of matrix composition. Here, we show that phylogenetic signal for the monophyly of Arachnida is restricted to the 500 slowest-evolving genes in the data set. Accelerated evolutionary rates in Acariformes, Pseudoscorpiones, and Parasitiformes potentially engender longbranch attraction artifacts, yielding nonmonophyly of Arachnida with increasing support upon incrementing the number of concatenated genes. Mutually exclusive hypotheses are supported by locus groups of variable evolutionary rate, revealing significant conflicts in phylogenetic signal. Analyses of gene-tree discordance indicate marked incongruence in relationships among Chelicerate orders, whereas derived relationships are demonstrably robust. Consistently recovered and supported relationships include the monophyly of Chelicerata, Euchelicerata, Tetrapulmonata, and all orders represented by multiple terminals. Relationships supported by subsets of slow-evolving genes include Ricinulei+Solifugae; a clade comprised of Ricinulei, Opiliones, and Solifugae; and a clade comprised of Tetrapulmonata, Scorpiones, and Pseudoscorpiones. We demonstrate that outgroup selection without regard for branch length distribution exacerbates long-branch attraction artifacts and does not mitigate gene-tree discordance, regardless of high gene representation for outgroups that are model organisms. Arachnopulmonata (new name) is proposed for the clade comprising Scorpiones+Tetrapulmonata (previously named Pulmonata).
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Evolution of the chelicera: a dachshund domain is retained in the deutocerebral appendage of Opiliones (Arthropoda, Chelicerata)
Evolution & development, 2012Co-Authors: Prashant P Sharma, Evelyn E. Schwager, Cassandra G. Extavour, Gonzalo GiribetAbstract:The proximo-distal axis of the arthropod leg is patterned by mutually antagonistic developmental expression domains of the genes extradenticle, homothorax, dachshund, and Distal-less. In the deutocerebral appendages (the antennae) of insects and crustaceans, the expression domain of dachshund is frequently either absent or, if present, is not required to pattern medial segments. By contrast, the dachshund domain is entirely absent in the deutocerebral appendages of spiders, the chelicerae. It is unknown whether absence of dachshund expression in the spider chelicera is associated with the two-segmented morphology of this appendage, or whether all Chelicerates lack the dachshund domain in their chelicerae. We investigated gene expression in the harvestman Phalangium opilio, which bears the plesiomorphic three-segmented chelicera observed in "primitive" Chelicerate orders. Consistent with patterns reported in spiders, in the harvestman chelicera homothorax, extradenticle, and Distal-less have broadly overlapping developmental domains, in contrast with mutually exclusive domains in the legs and pedipalps. However, unlike in spiders, the harvestman chelicera bears a distinct expression domain of dachshund in the proximal segment, the podomere that is putatively lost in derived arachnids. These data suggest that a tripartite proximo-distal domain structure is ancestral to all arthropod appendages, including deutocerebral appendages. As a corollary, these data also provide an intriguing putative genetic mechanism for the diversity of arachnid chelicerae: loss of developmental domains along the proximo-distal axis.
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hox gene expression in the harvestman phalangium opilio reveals divergent patterning of the Chelicerate opisthosoma
Evolution & Development, 2012Co-Authors: Prashant P Sharma, Evelyn E. Schwager, Cassandra G. Extavour, Gonzalo GiribetAbstract:Among Chelicerates, Hox gene expression has only been investigated in representatives of two arachnid orders to date: Acari (mites and ticks) and Araneae (spiders). Limited data are available for the "primitive" arachnid orders, such as Scorpiones (scorpions) and Opiliones (harvestmen). Here, we present the first data on Hox gene expression in the harvestman Phalangium opilio. Ten Hox genes of this species were obtained from a de novo assembled developmental transcriptome using the Illumina GAII platform. All 10 genes are expressed in characteristic Hox-like expression patterns, and the expression of the anterior and central Hox genes is similar to those of other Chelicerates. However, intriguingly, the three posteriormost genes-Ultrabithorax, abdominal-A, and Abdominal-B-share an identical anterior expression boundary in the second opisthosomal segment, and their expression domains extend through the opisthosoma to the posterior growth zone. The overlap in expression domains of the posterior Hox genes is correlated with the absence of opisthosomal organs posterior to the tubular tracheae, which occur on the second opisthosomal segment. Together with the staggered profile of posterior Hox genes in spiders, these data suggest the involvement of abdominal-A and Abdominal-B in the evolution of heteronomous patterning of the Chelicerate opisthosoma, providing a mechanism that helps explain the morphological diversity of Chelicerates.