Growth patterns in Onychophora (velvet worms): lack of a localised posterior proliferation zone

Background  

During embryonic development of segmented animals, body segments are thought to arise from the so-called "posterior growth zone" and the occurrence of this "zone" has been used to support the homology of segmentation between arthropods, annelids, and vertebrates. However, the term "posterior growth zone" is used ambiguously in the literature, mostly referring to a region of increased proliferation at the posterior end of the embryo. To determine whether such a localised posterior proliferation zone is an ancestral feature of Panarthropoda (Onychophora + Tardigrada + Arthropoda), we examined cell division patterns in embryos of Onychophora.     

A revision of brain composition in Onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods

Background  

The composition of the arthropod head is one of the most contentious issues in animal evolution. In particular, controversy surrounds the homology and innervation of segmental cephalic appendages by the brain. Onychophora (velvet worms) play a crucial role in understanding the evolution of the arthropod brain, because they are close relatives of arthropods and have apparently changed little since the Early Cambrian. However, the segmental origins of their brain neuropils and the number of cephalic appendages innervated by the brain - key issues in clarifying brain composition in the last common ancestor of Onychophora and Arthropoda - remain unclear.     

Cryptic speciation in Brazilian Epiperipatus (Onychophora: Peripatidae) reveals an underestimated diversity among the peripatid velvet worms

Background

Taxonomical studies of the neotropical Peripatidae (Onychophora, velvet worms) have proven difficult, due to intraspecific variation and uniformity of morphological characters across this onychophoran subgroup. We therefore used molecular approaches, in addition to morphological methods, to explore the diversity of Epiperipatus from the Minas Gerais State of Brazil.

Methodology/Principal Findings

Our analyses revealed three new species. While Epiperipatus diadenoproctus sp. nov. can be distinguished from E. adenocryptus sp. nov. and E. paurognostus sp. nov. based on morphology and specific nucleotide positions in the mitochondrial cytochrome c oxidase subunit I (COI) and small ribosomal subunit RNA gene sequences (12S rRNA), anatomical differences between the two latter species are not evident. However, our phylogenetic analyses of molecular data suggest that they are cryptic species, with high Bayesian posterior probabilities and bootstrap and Bremer support values for each species clade. The sister group relationship of E. adenocryptus sp. nov. and E. paurognostus sp. nov. in our analyses correlates with the remarkable morphological similarity of these two species. To assess the species status of the new species, we performed a statistical parsimony network analysis based on 582 base pairs of the COI gene in our specimens, with the connection probability set to 95%. Our findings revealed no connections between groups of haplotypes, which have been recognized as allopatric lineages in our phylogenetic analyses, thus supporting our suggestion that they are separate species.

Conclusions/Significance

Our findings suggest high cryptic species diversity and endemism among the neotropical Peripatidae and demonstrate that the combination of morphological and molecular approaches is helpful for clarifying the taxonomy and species diversity of this apparently large and diverse onychophoran group.     

Neural development in Onychophora (velvet worms) suggests a step-wise evolution of segmentation in the nervous system of Panarthropoda

A fundamental question in biology is how animal segmentation arose during evolution. One particular challenge is to clarify whether segmental ganglia of the nervous system evolved once, twice, or several times within the Bilateria. As close relatives of arthropods, Onychophora play an important role in this debate since their nervous system displays a mixture of both segmental and non-segmental features. We present evidence that the onychophoran “ventral organs,” previously interpreted as segmental anlagen of the nervous system, do not contribute to nerve cord formation and therefore cannot be regarded as vestiges of segmental ganglia. The early axonal pathways in the central nervous system arise by an anterior-to-posterior cascade of axonogenesis from neuronal cell bodies, which are distributed irregularly along each presumptive ventral cord. This pattern contrasts with the strictly segmental neuromeres present in arthropod embryos and makes the assumption of a secondary loss of segmentation in the nervous system during the evolution of the Onychophora less plausible. We discuss the implications of these findings for the evolution of neural segmentation in the Panarthropoda (Arthropoda + Onychophora + Tardigrada). Our data best support the hypothesis that the ancestral panarthropod had only a partially segmented nervous system, which evolved progressively into the segmental chain of ganglia seen in extant tardigrades and arthropods.     

A world checklist of Onychophora (velvet worms), with notes on nomenclature and status of names

Currently, the number of valid species of Onychophora is uncertain. To facilitate taxonomic work on this understudied animal group, we present an updated checklist for the two extant onychophoran subgroups, Peripatidae and Peripatopsidae, along with an assessment of the status of each species. According to our study, 82 species of Peripatidae and 115 species of Peripatopsidae have been described thus far. However, among these 197 species, 20 are nomina dubia due to major taxonomic inconsistencies. Apart from nomina dubia, many of the valid species also require revision, in particular representatives of Paraperipatus within the Peripatopsidae, and nearly all species of Peripatidae. In addition to extant representatives, the record of unambiguous fossils includes three species with uncertain relationship to the extant taxa. For all species, we provide a list of synonyms, information on types and type localities, as well as remarks on taxonomic and nomenclatural problems and misspellings. According to recent evidence of high endemism and cryptic speciation among the Peripatidae and Peripatopsidae, previous synonyms are revised. Putative mutations, subspecies and variations are either raised to the species status or synonymised with corresponding taxa. In our revised checklist, we follow the rules and recommendations of the International Code of Zoological Nomenclature to clarify previous inconsistencies.     

Experimental taphonomy of velvet worms (Onychophora) and implications for the Cambrian "explosion,disparity and decimation" model

Experimental preservation of velvet worms (phylum Onychophora), a very rare but evolutionarily important group that has existed for more than 500 million years, showed that the absence of bucal parts, adhesive-expelling organs, gonopore, eyes, legs, claws, annulation and papillation in fossils may not represent absence in the living animals. In fossils, leg thickness and claw orientation can be unreliable. The experiments indicate that not only absence, but even presence of certain structures can simply be the result of tissue decomposition. Computer-aided photorealistic reconstructions of fossi onychophorans are presented. We recommend future researchers to conduct taphonomy experiments specially before analysing unusual fossils.     

Comparative anatomy of slime glands in onychophora (velvet worms)

Onychophorans use a unique hunting and defense strategy, which involves the ejection of an adhesive slime secretion produced by a pair of specialized glands. So far, a comparative study on the anatomy of these glands has not been carried out among different species. In this article, we compare anatomical features of slime glands in representatives of two major onychophoran subgroups, the Peripatopsidae and the Peripatidae, from different parts of the world. Our data show that the musculature of the reservoir is conserved whereas the composition of the secretory duct displays taxon‐specific variation. Major differences concern the arrangement of glandular endpieces, which are distributed along the duct in Peripatopsidae but condensed in numerous repeated rosettes in Peripatidae. In addition, there are differences in the attachment pattern of slime glands to the inner surface of the body wall and to the outer surface of the gut between the two major onychophoran subgroups. A tube‐like structure with a putative valve‐like function is found at the transition of the secretory duct and the reservoir in the five Peripatopsidae species studied whereas it is absent in the two representatives of Peripatidae. Our findings suggest that the arrangement of musculature in the reservoir of the slime gland has remained unchanged since the divergence of Peripatidae and Peripatopsidae, while the composition of the secretory duct has been altered in one of these groups. However, the direction of evolutionary changes in duct composition cannot be determined unambiguously due to current uncertainty regarding the phylogenetic relationships of Onychophora. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

Disparity, decimation and the Cambrian "explosion": comparison of early Cambrian and present faunal communities with emphasis on velvet worms (Onychophora)

The controversy about a Cambrian "explosion" of morphological disparity (followed by decimation), cladogenesis and fossilization is of central importance for the history of life. This paper revisits the controversy (with emphasis in onychophorans, which include emblematic organisms such as Hallucigenia), presents new data about the Chengjiang (Cambrian of China) faunal community and compares it and the Burgess Shale (Cambrian of Canada) with an ecologically similar but modern tropical marine site where onychophorans are absent, and with a modern neotropical terrestrial onychophoran community. Biovolume was estimated from material collected in Costa Rica and morphometric measurements were made on enlarged images of fossils. Cambrian tropical mudflats were characterized by the adaptive radiation of two contrasting groups: the vagile arthropods and the sessile poriferans. Arthropods were later replaced as the dominant benthic taxon by polychaetes. Vagility and the exoskeleton may explain the success of arthropods from the Cambrian to the modern marine and terrestrial communities, both in population and biovolume. Food ecological displacement was apparent in the B. Shale, but not in Chengjiang or the terrestrial community. When only hard parts were preserved, marine and terrestrial fossil deposits of tropical origin are even less representative than deposits produced by temperate taxa, Chengjiang being an exception. Nutrient limitations might explain why deposit feeding is less important in terrestrial onychophoran communities, where carnivory, scavenging and omnivory (associated with high motility and life over the substrate) became more important. Fossil morphometry supports the interpretation of "lobopod animals" as onychophorans, whose abundance in Chengjiang was equal to their abundance in modern communities. The extinction of marine onychophorans may reflect domination of the infaunal habitat by polychaetes. We conclude that (1) a mature ecological community structure was generalized during the Cambrian, and even biodiversity and equitability indices were surprisingly close to modern values; (2) the morphological diversity and geographic distribution of onychophorans indicate a significant pre-Cambrian evolutionary history which does not support the "explosion" hypothesis; (3) disparity among phyla was not as important as the explosion-decimation model predicts, but in the case of onychophorans, disparity within the phylum was greater than it is today and its reduction may have been associated with migration into the sediment when large predators evolved.     

Metabolic and water loss rates of two cryptic species in the African velvet worm genus Opisthopatus (Onychophora)

Velvet worms (Onychophora) are characterised by a dearth of mechanisms to retain water, yet recently identified cryptic species are located in areas with seemingly different climates. Using flow-through respirometry, this study determined the metabolic, water loss and cuticular water loss rates of two cryptic species of Opisthopatus cinctipes s.l. from locations that differ in their current climate. When controlling for trial temperature and body mass, velvet worms from the drier and warmer site had significantly lower water loss rates than the wetter and cooler site. Mass-corrected metabolic rate and cuticular water loss did not differ significantly between the two sites. The scaling exponent for the relationship between log metabolic rate and log body mass for O. cinctipes s.l. declined with an increase in temperature from 5 to 15 °C. Females in the two cryptic Opisthopatus species had higher metabolic, water loss and cuticular water loss rates than males, which may represent the increased energetic demands of embryonic growth and development in these viviparous taxa.     

Genome size in Dipsacaceae and Morina longifolia (Morinaceae)

Dipsacaceae and Morinaceae have for a long time been regarded as separate but related families, whereas according to APG III they are included within the larger family Caprifoliaceae. Although genome size studies seldom provide conclusive characters for higher level systematics, they can yield useful information at a lower taxonomy level. In this study, we used DNA flow cytometry (supplemented by Feulgen densitometry) for measurement of genome size variation in the Dipsacaceae genera Cephalaria, Dipsacus, Knautia, Lomelosia, Pterocephalus, Scabiosa, Sixalix, Succisa, and Succisella, and Morina of the Morinaceae. At the monoploid level the Dipsacaceae genera (x = 7–10) vary 5.94-fold between 0.902 and 5.362 pg DNA (1Cx-value), whereas Morina longifolia (x = 17) has only 0.670 pg DNA. At the holoploid level 11.58-fold variation occurs between 0.902 and 10.446 pg DNA (1C-value). In Knautia sect. Trichera ploidy levels 2x, 4x, 6x are accompanied by corresponding increments of C-values, but genome downsizing is observed. In Knautia sect. Tricheroides the only investigated species K. integrifolia (2n = 20) has only 0.60-fold the mean genome size of sect. Trichera. Scabiosa canescens (2n = 2x = 16) has approximately double the C-value of all other Austrian Scabiosa species at the diploid level (pseudopolyploidy). These values raise concern against DNA-ploidy estimations at the interspecific level when chromosome numbers are unknown. The species sorted into two major clades of an existing phylogenetic tree of Dipsacaceae differ characteristically in their range of Cx-values. The Knautia–Cephalaria–Dipsacus–Succisella clade has the great majority of its Cx-values larger than those of the Scabiosa–Pterocephalus–Lomelosia clade.     

C-banding and DNA content in three species of Belostoma (Heteroptera) with large differences in chromosome size and number

C-banding was carried out on Belostoma elegans (2n=26+X₁X₂Y) (), B. micantulum (2n=14+XY) () and B. oxyurum (2n=6+XY) () (Belostomatidae, Heteroptera). The C-bands always have a telomeric localization and no interstitial bands were detected. An inverse relationship between chromosome size and chromosome number exists, and besides, an inverse relationship between chromosome size and the size of the C-bands was observed. The DNA content was determined in all three species. B. elegans has a C content of 1.55±0.06 pg, B. micantulum has 0.88±0.04 pg and B. oxyurum had 0.53±0.04 pg.Considering the male meiotic characteristics, the chromosome complement and the results of C-banding and DNA content, the karyotype of B. oxyurum probably originated through autosomal fusions. The karyotype of B. micantulum and B. elegans could have originated through autosomal fusions or fragmentations respectively; with the information available up to now it is not possible to discard any of the two pathways.     

The chromosome number of Heliconia (Musaceae)

The chromosome number 2n = 24 was determined in 31 taxa of Heliconia , representing nearly the entire morphological spectrum of the genus. The karyotype is a graded series of rather small chromosomes ranging between 0.7 and 1.7 urn. One pair has satellites. It is concluded that morphological chromosome differentiation played no great part in the diversification of the genus.     

Estimation of chromosome number and size by pulsed-field gel electrophoresis (PFGE) in medically important Candida species.

The chromosomal DNAs of eight medically important Candida species, C. albicans, C. stellatoidea, C. tropicalis, C. parapsilosis, C. krusei, C. guilliermondii, C. kefyr and C. glabrata, were analysed by pulsed-field gel electrophoresis under various conditions. The corresponding bands in the gels were assigned by three kinds of DNA probe which hybridized to DNA of all the species: rDNA, TUB2 and PEP4. The best conditions for separating the chromosomal DNAs were investigated and the numbers and molecular sizes of the chromosome bands were determined for each species. The chromosomal DNAs of the species were separated into 5-14 bands ranging in size from 0.5 to 4.5 Mb. Based on the quantification of the chromosome band intensities using a laser fluorescent gel scanner, the chromosome numbers were estimated. The apparent average total number of chromosomes per cell was 16 for C. albicans, 16 for C. stellatoidea, 12 for C. tropicalis, 14 for C. parapsilosis, 8 for C. krusei, 8 for C. guilliermondii, 18 for C.kefyr, and 14 for C. glabrata; the total chromosomal DNA size of each species per cell was calculated at about 31 Mb, 33 Mb, 31 Mb, 26 Mb, 20 Mb, 12 Mb, 29 Mb and 14 Mb, respectively.     

Genome size and chromosome number in Echinops (Asteraceae, Cardueae) in the Aegean and Balkan regions: technical aspects of nuclear DNA amount assessment and genome evolution in a phylogenetic frame

This work focuses on the representatives of genus Echinops (Asteraceae, Cardueae) in the Aegean and Balkan regions, from the perspective of their genome evolution. Chromosome numbers were determined by orcein staining in 14 populations of nine taxa, and DNA contents were assessed by flow cytometry in 24 populations of nine taxa. A molecular phylogeny based on the internal transcribed spacer (ITS) and trnL-trnF and including first sequences for two taxa (Echinops sphaerocephalus subsp. taygeteus and E.?spinosissimus subsp. neumayeri) provided a framework for discussing genome changes. From a methodological point of view, similar C-DNA value estimates were obtained when measuring, for a same population, fresh leaves from adult plants collected in the field and from cultivated seedlings. Conversely, despite giving the appearance of being correct (e.g., low coefficient of variation), genome size assessed using silica gel-preserved material differs significantly from values obtained for the same populations with fresh material. Nevertheless, silica gel-preserved material may still provide rough estimates of genome size for, e.g., inferring ploidy level. Suitable—non-silica gel-based—DNA amounts assessed for 23 populations range from 2C?=?6.52?pg (E.?spinosissimus subsp. neumayeri) to 2C?=?9.37?pg (E.?bannaticus). Chromosome counts were established for the first time for Echinops graecus (2n?=?32), E.?sphaerocephalus subsp. albidus (2n?=?32), E.?sphaerocephalus subsp. taygeteus (2n?=?ca.?30), and E.?spinosissimus subsp. neumayeri (2n?=?28). Genome size and chromosome number are confirmed as crucial parameters for deciphering lineage diversification within the genus Echinops.     

Genome size variation in Chenopodium quinoa (Chenopodiaceae)

The extent and significance of intraspecific genome size variation were analysed in quinoa (Chenopodium quinoa Willd.), a pseudocereal important for human consumption in the Andean region of South America. Flow cytometry, with propidium iodide as the DNA stain, was used to estimate the genome size of 20 quinoa accessions from Ecuador, Peru, Bolivia, Argentina, Chile and the USA. Limited genome size variation was found among the analysed accessions. The differences between the accessions were statistically significant but the maximum inter-accession difference between the populations with the largest and the smallest genome reached only 5.9%. The largest genome was found in population C4 from Chile (mean 3.077 pg/2C) and the smallest in the Peruvian population P2 (mean 2.905 pg/2C). The variation was not correlated with collection site; however, the quinoa accessions analysed in this study belonged to three distinct geographical groups: northern highland, southern highland and lowland.     

Genome size and numerical polymorphism for the B chromosome in races of maize (Zea mays ssp. mays, Poaceae)

Twenty-one native populations (1120 individuals) of maize from Northern Argentina were studied. These populations, which belong to 13 native races, were cultivated at different altitudes (80-3620 m). Nineteen of the populations analyzed showed B chromosome (Bs) numerical polymorphism. The frequency of individuals with Bs varied from 0 to 94%. The number of Bs per plant varied from 0 to 8 Bs, with the predominant doses being 0, 1, 2, and 3. Those populations with varying number of Bs showed a positive and statistically significant correlation of mean number of Bs with altitude. The DNA content, in plants without Bs (A-DNA)(2n = 20), of 17 populations of the 21 studied was determined. A 36% variation (5.0-6.8 pg) in A-DNA content was found. A significant negative correlation between A-DNA content and altitude of cultivation and between A-DNA content and mean number of Bs was found. This indicates that there is a close interrelationship between the DNA content of A chromosomes and doses of Bs. These results suggest that there is a maximum limit to the mass of nuclear DNA so that Bs are tolerated as long as this maximum limit is not exceeded.