Viviparity in the sea star Cryptasterina hystera (Asterinidae)--conserved and modified features in reproduction and development

Hemal sinus basal laminae contact sites: a possible route between gonadal lumen and myoepithelial cells in the gonad of the sea star Patiria pectinifera

Sea stars are a group of marine invertebrates suitable for studying the hormonal regulation of reproduction and spawning. In spite of substantial progress in understanding how various substances such as 1-methyladenine act in their gonads, there are still many gaps concerning the fine details of their action. One such gap is how the gonadal wall contraction is induced. Recent literature data suggest that, upon 1-methyladenine stimulation, some cells within the gonadal lumen produce non-neuronal acetylcholine that, upon contact with the gonadal wall, induces contraction of myoepithelial cells. Our ultrastructural study of the gonads in the sea star Patiria pectinifera has shown, for the first time, that there are sites where the basal laminae bordering the hemal sinus directly contact one another and appear at this contact site as a single entity. These contact sites are often associated with hemidesmosome-like junctions that anchor male accessory cells or female follicle cells on one side of the site and myoepithelial cells on the opposite. We suggest that contraction-inducing substance is secreted from an accessory or follicle cell, passes through a basal lamina contact site, and on the opposite side of the contact site acts on a myoepithelial cell to induce its contraction.

Life-History Analysis of Asterinid Starfishes

AbstractThe starfish family Asterinidae shows a diversity of reproductive modes, and a number of species have sufficient life-history data that can be used for analysis, using life-cycle graphs. These include four species that reproduce by fission (Aquilonastra yairi, Nepanthia belcheri, Aquilonastra burtonii, and Ailsastra heteractis), a viviparous species (Parvulastra vivipara), two species with benthic egg masses (Asterina gibbosa and Asterina phylactica), one with planktonic larvae that do not feed (Cryptasterina pentagona), and one with larvae that feed in the plankton (Patiria miniata). Species are compared using adult and first-year survival and, for some species, the age at first reproduction, number of offspring (eggs or newly released juveniles), and individual growth parameters of the von Bertalanffy model. The sensitivity of population growth, fitness, to changes in these traits is shown by elasticity analysis, which aids in understanding possible consequences of environmental forces as well as possible directions of selection.

Nonspecific expression of fertilization genes in the crown-of-thorns Acanthaster cf. solaris: Unexpected evidence of hermaphroditism in a coral reef predator

The characterization of gene expression in gametes has advanced our understanding of the molecular basis for ecological variation in reproductive success and the evolution of reproductive isolation. These advances are especially significant for ecologically important keystone predators such as the coral-eating crown-of-thorns sea stars (COTS, Acanthaster) which are the most influential predator species in Indo-Pacific coral reef ecosystems and the focus of intensive management efforts. We used RNA-seq and transcriptome assemblies to characterize the expression of genes in mature COTS gonads. We described the sequence and domain organization of eight genes with sex-specific expression and well known functions in fertilization in other echinoderms. We found unexpected expression of genes in one ovary transcriptome that are characteristic of males and sperm, including genes that encode the sperm-specific guanylate cyclase receptor for an egg pheromone, and the sperm acrosomal protein bindin. In a reassembly of previously published RNA-seq data from COTS testes, we found a complementary pattern: strong expression of four genes that are otherwise well known to encode egg-specific fertilization proteins, including the egg receptor for bindin (EBR1) and the acrosome reaction-inducing substance in the egg coat (ARIS1, ARIS2, ARIS3). We also found histological evidence of both eggs and sperm developing in the same gonad in several COTS individuals from a parallel study. These results suggest the occurrence of hermaphrodites, and the potential for reproductive assurance via self-fertilization. Our findings have implications for management of COTS populations, especially in consideration of the large size and massive fecundity of these sea stars.

Culturing echinoderm larvae through metamorphosis

Echinoderms are favored study organisms not only in cell and developmental biology, but also physiology, larval biology, benthic ecology, population biology and paleontology, among other fields. However, many echinoderm embryology labs are not well-equipped to continue to rear the post-embryonic stages that result. This is unfortunate, as such labs are thus unable to address many intriguing biological phenomena, related to their own cell and developmental biology studies, that emerge during larval and juvenile stages. To facilitate broader studies of post-embryonic echinoderms, we provide here our collective experience rearing these organisms, with suggestions to try and pitfalls to avoid. Furthermore, we present information on rearing larvae from small laboratory to large aquaculture scales. Finally, we review taxon-specific approaches to larval rearing through metamorphosis in each of the four most commonly-studied echinoderm classes-asteroids, echinoids, holothuroids and ophiuroids.

Allocation of cytoplasm to macromeres in embryos of annelids and molluscs is positively correlated with egg size

Evolutionary transitions between feeding and nonfeeding larval development have occurred many times in marine invertebrates, but the developmental changes underlying these frequent and ecologically important transitions are poorly known, especially in spiralians. We use phylogenetic comparative methods to test the hypothesis that evolutionary changes in egg size and larval nutritional mode are associated with parallel changes in allocation of cytoplasm to macromere cell lineages in diverse annelids and molluscs. Our analyses show that embryos of species with large eggs and nonfeeding larvae tend to allocate relatively more embryonic cytoplasm to macromeres at 3rd cleavage than do embryos of species with small eggs and feeding larvae. The association between egg size and allocation to macromeres in these spiralians may be driven by constraints associated with mitotic spindle positioning and size, or may be a result of "adaptation in cleavage" to maintain rapid cell cycles in micromeres, position yolk in cell lineages where it can be most efficiently used, or adjust allocation to ectoderm to accommodate changes in embryonic surface area/volume ratio.

Patterns and Drivers of Egg Pigment Intensity and Colour Diversity in the Ocean: A Meta-Analysis of Phylum Echinodermata

Egg pigmentation is proposed to serve numerous ecological, physiological, and adaptive functions in egg-laying animals. Despite the predominance and taxonomic diversity of egg layers, syntheses reviewing the putative functions and drivers of egg pigmentation have been relatively narrow in scope, centring almost exclusively on birds. Nonvertebrate and aquatic species are essentially overlooked, yet many of them produce maternally provisioned eggs in strikingly varied colours, from pale yellow to bright red or green. We explore the ways in which these colour patterns correlate with behavioural, morphological, geographic and phylogenetic variables in extant classes of Echinodermata, a phylum that has close phylogenetic ties with chordates and representatives in nearly all marine environments. Results of multivariate analyses show that intensely pigmented eggs are characteristic of pelagic or external development whereas pale eggs are commonly brooded internally. Of the five egg colours catalogued, orange and yellow are the most common. Yellow eggs are a primitive character, associated with all types of development (predominant in internal brooders), whereas green eggs are always pelagic, occur in the most derived orders of each class and are restricted to the Indo-Pacific Ocean. Orange eggs are geographically ubiquitous and may represent a 'universal' egg pigment that functions well under a diversity of environmental conditions. Finally, green occurs chiefly in the classes Holothuroidea and Ophiuroidea, orange in Asteroidea, yellow in Echinoidea, and brown in Holothuroidea. By examining an unprecedented combination of egg colours/intensities and reproductive strategies, this phylum-wide study sheds new light on the role and drivers of egg pigmentation, drawing parallels with theories developed from the study of more derived vertebrate taxa. The primary use of pigments (of any colour) to protect externally developing eggs from oxidative damage and predation is supported by the comparatively pale colour of equally large, internally brooded eggs. Secondarily, geographic location drives the evolution of egg colour diversity, presumably through the selection of better-adapted, more costly pigments in response to ecological pressure.

Matrotrophy and placentation in invertebrates: a new paradigm

Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy-placentotrophy-is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritive modes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent-offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and 'isolated' chambers with presumably more effective nutritional relationships, and (iii) internal secretory ('milk') glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels-positional, structural and physiological.

To brood or not to brood: Are marine invertebrates that protect their offspring more resilient to ocean acidification?

Anthropogenic atmospheric carbon dioxide (CO₂) is being absorbed by seawater resulting in increasingly acidic oceans, a process known as ocean acidification (OA). OA is thought to have largely deleterious effects on marine invertebrates, primarily impacting early life stages and consequently, their recruitment and species’ survival. Most research in this field has been limited to short-term, single-species and single-life stage studies, making it difficult to determine which taxa will be evolutionarily successful under OA conditions. We circumvent these limitations by relating the dominance and distribution of the known polychaete worm species living in a naturally acidic seawater vent system to their life history strategies. These data are coupled with breeding experiments, showing all dominant species in this natural system exhibit parental care. Our results provide evidence supporting the idea that long-term survival of marine species in acidic conditions is related to life history strategies where eggs are kept in protected maternal environments (brooders) or where larvae have no free swimming phases (direct developers). Our findings are the first to formally validate the hypothesis that species with life history strategies linked to parental care are more protected in an acidifying ocean compared to their relatives employing broadcast spawning and pelagic larval development.

Reinvestigation of epithelial lining of the genital coelomic sinus in asteroids. An ultrastructural study

Ultrastructural study of gonadal muscles in sea star, Asterina pectinifera, showed that myoepithelial cells were located only in the epithelial lining of the genital coelomic sinus. No myoepithelial cells were found in the visceral peritoneal epithelium or within connective tissue layer of the outer sac. Morphology of the myoepithelial cells in gonads of A. pectinifera varies during the reproductive cycle. During the gametogenic phase of the reproductive cycle, the myoepithelial cells get an elongated, spindle-like shape having a length of 20–30 m. In prespawning gonads, many of the myoepithelial cells form cytoplasmic extensions of 3–5 m in length, filled with myofilaments and penetrating into the underlying connective tissue of the outer sac or haemal sinus. Besides, myoepithelial cells, simultaneously anchored in the inner and outer sacs, were also observed. These changes result in development of more elaborated musculature and increase in contractility of the gonadal wall in prespawning gonads as compared to that during other stages of the reproductive cycle.

The evolution of pregnancy

Although viviparity has evolved many times in the animal kingdom, it remains relatively uncommon-scorpions and therian mammals being rare examples of entirely viviparous major taxa. Viviparity is a specialised form of intra-species parasitism which biases parental investment towards fertilised eggs, temporally spreads that investment, and also temporarily protects offspring from many selection pressures. Importantly, the mammalian viviparity appeared at a relatively late stage in the process of vertebrate evolution. Because of this, viviparity was 'superimposed' on complex pre-existing cardiovascular, respiratory, metabolic and immune systems, and has altered them dramatically. Also, pregnancy has exerted pervasive effects on gene expression in mammals, including genetic imprinting, X inactivation, sex determination, and the ectopic expression in the extra-embryonic membranes of many genes previously expressed in the gonads, brain, pituitary and immune system. Finally, although lactation probably pre-dated viviparity in mammalian evolution, the two have co-evolved as alternative strategies of offspring nutrition ever since.

Extraordinarily rapid life-history divergence between Cryptasterina sea star species

Life history plays a critical role in governing microevolutionary processes such as gene flow and adaptation, as well as macroevolutionary processes such speciation. Here, we use multilocus phylogeographic analyses to examine a speciation event involving spectacular life-history differences between sister species of sea stars. Cryptasterina hystera has evolved a suite of derived life-history traits (including internal self-fertilization and brood protection) that differ from its sister species Cryptasterina pentagona, a gonochoric broadcast spawner. We show that these species have only been reproductively isolated for approximately 6000 years (95% highest posterior density of 905-22 628), and that this life-history change may be responsible for dramatic genetic consequences, including low nucleotide diversity, zero heterozygosity and no gene flow. The rapid divergence of these species rules out some mechanisms of isolation such as adaptation to microhabitats in sympatry, or slow divergence by genetic drift during prolonged isolation. We hypothesize that the large phenotypic differences between species relative to the short divergence time suggests that the life-history differences observed may be direct responses to disruptive selection between populations. We speculate that local environmental or demographic differences at the southern range margin are possible mechanisms of selection driving one of the fastest known marine speciation events.

Bindin from a sea star

The genetic basis for the evolution of development includes genes that encode proteins expressed on the surfaces of sperm and eggs. Previous studies of the sperm acrosomal protein bindin have helped to characterize the adaptive evolution of gamete compatibility and speciation in sea urchins. The absence of evidence for bindin expression in taxa other than the Echinoidea has limited such studies to sea urchins, and led to the suggestion that bindin might be a sea urchin-specific molecule. Here we characterize the gene that encodes bindin in a broadcast-spawning asterinid sea star (Patiria miniata). We describe the sequence and domain structure of a full-length bindin cDNA and its single intron. In comparison with sea urchins, P. miniata bindin is larger but the two molecules share several general features of their domain structure and some sequence features of two domains. Our results extend the known evolutionary history of bindin from the Mesozoic (among the crown group sea urchins) into the early Paleozoic (and the common ancestor of eleutherozoans), and present new opportunities for understanding the role of bindin molecular evolution in sexual selection, life history evolution, and speciation among sea stars.

Nervous system development in feeding and nonfeeding asteroid larvae and the early juvenile

Larval and juvenile nervous systems (NS) of three asterinid sea stars with contrasting feeding and nonfeeding modes of development were characterized using the echinoderm-specific synaptotagmin antibody. In the feeding bipinnaria and brachiolaria larvae of Patiriella regularis, the species with ancestral-type development, an extensive NS was associated with the ciliary bands (CBs) and attachment complex. Lecithotrophic planktonic (Meridastra calcar) and benthic (Parvulastra exigua) brachiolariae lacked CBs and the associated NS, but had an extensive NS in the attachment complex. The similarity in the distribution and morphology of synaptotagmin immunoreactive neurons and the anatomy of the NS in the attachment complex of these closely related sea stars suggests conservation of neurogenesis in settlement-stage larvae regardless of larval feeding mode. Nerve cells were prominent on the brachia of all three species. In advanced brachiolariae the larval nervous system was localized to the adhesive disc as the larval body resorbed during metamorphosis. The structures and tissues that contained larval neurons degenerated during metamorphosis. There was no evidence that the larval NS persists through metamorphosis. In juvenile development, synaptotagmin IR was first evident in the NS of the tube feet. As the central nervous system developed, synaptotagmin IR reflected the histological organization of the adult NS. The juvenile NS formed de novo with a temporal lapse between histogenesis and synaptotagmin IR. We evaluated the ontogeny of NS organization in the change in body plan from the bilateral larva to the radial juvenile.

A new brooding species of the biscuit star Tosia (Echinodermata:Asteroidea:Goniasteridae), distinguished by molecular, morphological and larval characters

The biscuit star Tosia australis Gray, 1840 is a well known component of the shallow rocky reef fauna of south-eastern Australia. The putative T. australis species complex was subjected to reproductive, morphometric and molecular analyses. Molecular analyses of the data from three markers (mitochondrial COI and 16S rRNA and the nuclear non-coding region ITS2) confirmed the presence of a cryptic species, the morphology of which does not agree with any of the existing nominal species. Two separate reproductive modes were observed within the complex and documented via scanning electron microscopy. T. neossia, sp. nov., described herein from south-eastern Australia, is shown to release gametes from gonopores on the actinal surface. Embryos develop first into non-feeding, non-swimming brachiolaria, and then into tripod brachiolaria before metamorphosis. No surface cilia are present at any point throughout development of T. neossia. T. australis sensu stricto is shown to release gametes from the abactinal surface. Embryos develop into non-feeding, swimming brachiolaria before metamorphosis. Whereas T. australis var. astrologorum is confirmed as synonymous with T. australis, the status of the putative Western Australian taxon T. nobilis remains unresolved.

Discovery and cross-amplification of microsatellite polymorphisms in asterinid sea stars

Variation in tandem repeats of two- to six-base nucleotide motifs (microsatellites) can be used to obtain inexpensive and highly informative multi-locus data on population genetics.We developed and tested a large set of cross-amplifiable sea star (Asterinidae) microsatellite markers from a mixed pool of genomic DNA from eight species. We describe cloned sequences, primers, and PCR conditions, and characterize population-level variation for some species and markers. A few clones containing microsatellites showed considerable similarity to sequences (including genes of known function) in other sea stars and in sea urchins (from the Strongylocentrotus purpuratus complete genome). The pooled genomic DNA method was an efficient way to sample microsatellites from many species: we cloned 2-10 microsatellites from each of eight species, and most could be cross-amplified in 1-7 other species. At 12 loci in two species, we found 1-10 alleles per microsatellite, with a broad range of inbreeding coefficients. Measures of polymorphism were negatively correlated with the extent of cross-amplification.

Morphological and genetic variation indicate cryptic species within Lamarck's little sea star, Parvulastra (=Patiriella) exigua

The asterinid sea star Parvulastra exigua (Lamarck) is a common member of temperate intertidal marine communities from geographically widespread sites around the southern hemisphere. Individuals from Australian populations lay benthic egg masses (through orally directed gonopores) from which nonplanktonic offspring hatch and metamorphose without a dispersing planktonic larval phase. Scattered reports in the taxonomic literature refer to a similar form in southern Africa with aborally directed gonopores (and possibly broadcast spawning of planktonic eggs and larvae); such differences would be consistent with cryptic species variation. Surveys of morphology and mtDNA sequences have revealed cryptic species diversity in other asterinid genera. Here we summarize the taxonomic history of Lamarck's "Astérie exiguë" and survey morphological variation (the location of the gonopores) for evidence that some P. exigua populations include cryptic species with a different mode of reproduction. We found strong evidence for multiple species in the form of two phenotypes and modes of reproduction (oral and aboral gonopore locations) in populations from southern Africa and islands in the Atlantic and Indian oceans. Both modes of reproduction have broad geographic ranges. These results are consistent with previously published genetic data that indicate multiple species in African and island (but not Australian) populations.