Paula Cisternas

Fixed, free, and fixed: the fickle phylogeny of extant Crinoidea (Echinodermata) and their Permian-Triassic origin.

Although the status of Crinoidea (sea lilies and featherstars) as sister group to all other living echinoderms is well-established, relationships among crinoids, particularly extant forms, are debated. All living species are currently placed in Articulata, which is generally accepted as the only crinoid group to survive the Permian-Triassic extinction event. Recent classifications have recognized five major extant taxa: Isocrinida, Hyocrinida, Bourgueticrinina, Comatulidina and Cyrtocrinida, plus several smaller groups with uncertain taxonomic status, e.g., Guillecrinus, Proisocrinus and Caledonicrinus. Here we infer the phylogeny of extant Crinoidea using three mitochondrial genes and two nuclear genes from 59 crinoid terminals that span the majority of extant crinoid diversity. Although there is poor support for some of the more basal nodes, and some tree topologies varied with the data used and mode of analysis, we obtain several robust results. Cyrtocrinida, Hyocrinida, Isocrinida are all recovered as clades, but two stalked crinoid groups, Bourgueticrinina and Guillecrinina, nest among the featherstars, lending support to an argument that they are paedomorphic forms. Hence, they are reduced to families within Comatulida. Proisocrinus is clearly shown to be part of Isocrinida, and Caledonicrinus may not be a bourgueticrinid. Among comatulids, tree topologies show little congruence with current taxonomy, indicating that much systematic revision is required. Relaxed molecular clock analyses with eight fossil calibration points recover Articulata with a median date to the most recent common ancestor at 231-252mya in the Middle to Upper Triassic. These analyses tend to support the hypothesis that the group is a radiation from a small clade that passed through the Permian-Triassic extinction event rather than several lineages that survived. Our tree topologies show various scenarios for the evolution of stalks and cirri in Articulata, so it is clear that further data and taxon sampling are needed to recover a more robust phylogeny of the group.

Reproduction and Larval Morphology of Broadcasting and Viviparous Species in the Cryptasterina Species Complex

The Cryptasterina group of asterinid sea stars in Australasia comprises cryptic species with derived life histories. C. pentagona and C. hystera have planktonic and intragonadal larvae, respectively. C. pentagona has the gonochoric, free-spawning mode of reproduction with a planktonic lecithotrophic brachiolaria larva. C. hystera is hermaphroditic with an intragonadal lecithotrophic brachiolaria, and the juveniles emerge through the gonopore. Both species have large lipid-rich buoyant eggs and well-developed brachiolariae. Early juveniles are sustained by maternal nutrients for several weeks while the digestive tract develops. C. hystera was reared in vitro through metamorphosis. Its brachiolariae exhibited the benthic exploration and settlement behavior typical of planktonic larvae, and they attached to the substratum with their brachiolar complex. These behaviors are unlikely to be used in the intragonadal environment. The presence of a buoyant egg and functional brachiolaria larva would not be expected in an intragonadal brooder and indicate the potential for life-history reversal to a planktonic existence. Life-history traits of species in the Cryptasterina group are compared with those of other asterinids in the genus Patiriella with viviparous development. Modifications of life-history traits and pathways associated with evolution of viviparity in the Asterinidae are assessed, and the presence of convergent adaptations and clade-specific features associated with this unusual mode of parental care are examined.

Development and distribution of the peptidergic system in larval and adult Patiriella: comparison of sea star bilateral and radial nervous systems.

Development of the larval peptidergic system in the sea star Patiriella regularis and structure of the adult nervous system in Patiriella species were documented in an immunofluorescence investigation using antisera to the sea star neuropeptide GFNSALMFamide 1 (S1) and confocal microscopy. P. regularis has planktotrophic development through bipinnaria and brachiolaria larvae. In early bipinnaria, two groups of immunoreactive cells appeared on either side of the anterior region and proliferated to form a pair of dorsolateral ganglia. The ganglia gave rise to fine varicose fibres that innervated the preoral and adoral ciliated bands. Peptidergic cells also innervated the postoral ciliated band, and a nerve tract connected the pre‐ and postoral bands. Fully developed bipinnaria had a well‐developed peptidergic system, the organisation of which reflected the bilateral larval body plan. As the brachiolar attachment complex differentiated at the anterior end, the ganglia became positioned on either side of the anterior projection, from which they innervated the complex. It is suggested, based on the distribution of S1‐like immunoreactivity in association with ciliary and attachment structures, that the peptidergic system functions in modulation of feeding, swimming, and settlement. The larval peptidergic system degenerates as the larval body is resorbed during metamorphosis. In adults, S1‐like immunoreactivity was intense in the axonal region of the ectoneural nervous system and in hyponeural perikarya. Immunoreactive cells in the neuroepithelium connected with the surface and may be sensory. Examination of immunoreactivity in several Patiriella species attests to the highly conserved organisation of the peptidergic system in adult asteroids. J. Comp. Neurol. 451:101–114, 2002.

Immune system cell markers in the northern brown bandicoot, Isoodon macrourus.

This is the first study to report the presence of T and B lymphocyte markers and antigen presenting-like molecules in a marsupial bandicoot. Intra-cytoplasmic markers for CD3 and CD5, as well as surface Thy-1.1 and CD1a molecules were located in lymphocytes of T dependent regions of immuno-lymphoid tissue in the northern brown bandicoot using immunohistochemical techniques. Similarly, intra-cytoplasmic domains of CD79a, CD79b molecules and surface IgG molecules enabled characterisation of B lymphocytes and plasma cells. The phenotypic expression of these molecules parallels findings in eutherians, suggesting firstly the conservation of lineage epitopes for T and B subsets and secondly, the potential for similar functional properties of immune system cells between marsupials and eutherians. In addition, the presence of MHC class II and CD1a molecules on dendritic-like cells may indicate similar mechanisms for antigen processing and presentation as reported in eutherians. The use of such immune system cell markers will enable functional studies to characterise the marsupial immune system as well as ontogeny studies of immune competence.

Engrailed is expressed in larval development and in the radial nervous system of Patiriella sea stars

We documented expression of the pan-metazoan neurogenic gene engrailed in larval and juvenile Patiriella sea stars to determine if this gene patterns bilateral and radial echinoderm nervous systems. Engrailed homologues, containing conserved En protein domains, were cloned from the radial nerve cord. During development, engrailed was expressed in ectodermal (nervous system) and mesodermal (coeloms) derivatives. In larvae, engrailed was expressed in cells lining the larval and future adult coeloms. Engrailed was not expressed in the larval nervous system. As adult-specific developmental programs were switched on during metamorphosis, engrailed was expressed in the central nervous system and peripheral nervous system (PNS), paralleling the pattern of neuropeptide immunolocalisation. Engrailed was first seen in the developing nerve ring and appeared to be up-regulated as the nervous system developed. Expression of engrailed in the nerve plexus of the tube feet, the lobes of the hydrocoel along the adult arm axis, is similar to the reiterated pattern of expression seen in other animals. Engrailed expression in developing nervous tissue reflects its conserved role in neurogenesis, but its broad expression in the adult nervous system of Patiriella differs from the localised expression seen in other bilaterians. The role of engrailed in patterning repeated PNS structures indicates that it may be important in patterning the fivefold organisation of the ambulacrae, a defining feature of the Echinodermata.

Evolution of larval form in the sea star genus Patiriella: Conservation and change in the larval nervous system

The organization of the peptidergic system in the larvae of Patiriella species with divergent ontogenies was compared to determine which aspects of neurogenesis are conserved and which are altered in the evolution of development in these sea stars. P. regularis has ancestral‐type feeding bipinnaria and brachiolaria larvae and the organization of the nervous system, in association with feeding structures, paralleled the bilateral larval body plan. P. calcar and P. exigua have non‐feeding planktonic and benthic brachiolariae, respectively, and there was no trace of the neuronal architecture involved with feeding. The nervous system in the attachment stage brachiolaria was similar in all three species and neuronal organization reflected larval symmetry. Delayed expression of peptidergic lineages to the brachiolaria stage in the lecithotrophs indicates heterochronic change in the timing of neurogenesis or deletion of the ancestral early neurogenic program. The bipinnarial program is suggested to be a developmental module autonomous from the brachiolar one. With a divergence time of less than 10 Ma, the evolution of development in Patiriella has resulted in extensive reduction in the complexity of the larval nervous system in parallel with simplification in larval form. There is, however, strong conservation in the morphology and neuronal architecture of structures involved with settlement.

Expression of Hox4 during development of the pentamerous juvenile sea star, Parvulastra exigua

Expression of Hox4 during development of the bilateral larva and pentameral juvenile sea star was investigated in Parvulastra exigua. The role of Hox4, possibly the anterior-most gene in the echinoderm Hox cluster, in the formation of the echinoderm adult body plan has not been examined previously. In the larva of P. exigua, PeHox4 is expressed in the developing coeloms—the anterior and the right and left coeloms that generate the aboral and oral coeloms of the juvenile. At the rudiment stage, PeHox4 was expressed in the five primary lobes of the hydrocoel that give rise to primary podia, the foundation of the adult body plan. This suggests a role for this gene in the development of the echinoderm body plan. In contrast to other bilaterians, Hox4 was not expressed in the developing asteroid central nervous system.

Antitropical distributions and species delimitation in a group of ophiocomid brittle stars (Echinodermata: Ophiuroidea: Ophiocomidae).

In this paper we examine the phylogeny and biogeography of the temperate genera of the Ophiocomidae (Echinodermata: Ophiuroidea) which have an interesting asymmetrical anti-tropical distribution, with two genera (Ophiocomina and Ophiopteris) previously considered to have a separate species in both the North and South hemispheres, and the third (Clarkcoma) diversifying in the southern Australian/New Zealand region. Our phylogeny, generated from one mitochondrial and two nuclear markers, revealed that Ophiopteris is sister to a mixed Ophiocomina/Clarkcoma clade. Ophiocomina was polyphyletic, with O. nigra and an undescribed species from the South Atlantic Ocean sister to a clade including Clarkcoma species and O. australis. The phylogeny also revealed a number of recently diverged lineages occurring within Clarkcoma, some of which are considered to be cryptic species due to the similarity in morphology combined with the apparent absence of interbreeding in a sympatric distribution, while the status of others is less certain. The phylogeny provides support for two transequatorial events in the group under study. A molecular clock analysis places both events in the middle to late Miocene. The analysis excludes a tectonic vicariance hypothesis for the antitropical distribution associated with the breakup of Pangaea and also excludes the hypothesis of more recent gene flow associated with Plio/Pleistocene glacial cycling.

Transcriptomic analysis of Nodal- and BMP-associated genes during juvenile development of the sea urchin Heliocidaris erythrogramma

Understanding the unusual radial body plan of echinoderms and its relationship to the bilateral plan of other deuterostomes remains a challenge. The molecular processes of embryonic and early larval development in sea urchins are well characterised, but those giving rise to the adult and its radial body remain poorly studied. We used the developmental transcriptome generated for Heliocidaris erythrogramma, a species that forms the juvenile soon after gastrulation, to investigate changes in gene expression underlying radial body development. As coelomogenesis is key to the development of pentamery and juvenile formation on the left side of the larva, we focussed on genes associated with the nodal and BMP2/4 network that pattern this asymmetry. We identified 46 genes associated with this Nodal and BMP2/4 signalling network, and determined their expression profiles from the gastrula, through to rudiment development, metamorphosis and the fully formed juvenile. Genes associated with Nodal signalling shared similar expression profiles, indicating that they may have a regulatory relationship in patterning morphogenesis of the juvenile sea urchin. Similarly, many genes associated with BMP2/4 signalling had similar expression profiles through juvenile development. Further examination of the roles of Nodal- and BMP2/4-associated genes is required to determine function and whether the gene expression profiles seen in H. erythrogramma are due to ongoing activity of gene networks established during early development, or to redeployment of regulatory cassettes to pattern the adult radial body plan.

Nodal and BMP expression during the transition to pentamery in the sea urchin Heliocidaris erythrogramma : insights into patterning the enigmatic echinoderm body plan

BackgroundThe molecular mechanisms underlying the development of the unusual echinoderm pentameral body plan and their likeness to mechanisms underlying the development of the bilateral plans of other deuterostomes are of interest in tracing body plan evolution. In this first study of the spatial expression of genes associated with Nodal and BMP2/4 signalling during the transition to pentamery in sea urchins, we investigate Heliocidaris erythrogramma, a species that provides access to the developing adult rudiment within days of fertilization.ResultsBMP2/4, and the putative downstream genes, Six1/2, Eya, Tbx2/3 and Msx were expressed in the earliest morphological manifestation of pentamery during development, the five hydrocoele lobes. The formation of the vestibular ectoderm, the specialized region overlying the left coelom that forms adult ectoderm, involved the expression of putative Nodal target genes Chordin, Gsc and BMP2/4 and putative BMP2/4 target genes Dlx, Msx and Tbx. The expression of Nodal, Lefty and Pitx2 in the right ectoderm, and Pitx2 in the right coelom, was as previously observed in other sea urchins.ConclusionThat genes associated with Nodal and BMP2/4 signalling are expressed in the hydrocoele lobes, indicates that they have a role in the developmental transition to pentamery, contributing to our understanding of how the most unusual body plan in the Bilateria may have evolved. We suggest that the Nodal and BMP2/4 signalling cascades might have been duplicated or split during the evolution to pentamery.