Phyletic patterns of early development in gastropod molluscs

A congruent topology for deep gastropod relationships

Gastropod molluscs are among the most diverse and abundant animals in the oceans, and are successful colonizers of terrestrial and freshwater environments. Past phylogenetic efforts to resolve gastropod relationships resulted in a range of conflicting hypotheses. Here, we use phylogenomics to address deep relationships between the five major gastropod lineages—Caenogastropoda, Heterobranchia, Neritimorpha, Patellogastropoda and Vetigastropoda—and provide one congruent and well-supported topology. We substantially expand taxon sampling for outgroups and for previously underrepresented gastropod lineages, presenting new transcriptomes for neritimorphs and patellogastropods. We conduct analyses under maximum-likelihood, Bayesian inference and a coalescent-based approach, accounting for the most pervasive sources of systematic errors in large datasets: compositional heterogeneity, site heterogeneity, heterotachy, variation in evolutionary rates among genes, matrix completeness, outgroup choice and gene tree conflict. We find that vetigastropods and patellogastropods are sister taxa, and that neritimorphs are the sister group to caenogastropods and heterobranchs. We name these two major unranked clades Psilogastropoda and Angiogastropoda, respectively. We additionally provide the first genomic-scale data for internal relationships of neritimorphs and patellogastropods. Our results highlight the need for reinterpreting the evolution of morphological and developmental characters in gastropods, especially for inferring their ancestral states.

The gastropod foregut — evolution viewed through a developmental lens

Comparative data on the developing gastropod foregut suggest that this multicomponent feeding complex consists of two developmental modules. Modularity is revealed by delayed development of the buccal cavity and radular sac (“ventral module”) relative to the dorsal food channel (“dorsal module”) in gastropods with feeding larvae compared with those that may have never had a feeding larval stage. If nonfeeding larvae like those of extant patellogastropods and vetigastropods are ancestral for gastropods, then the uncoupling and heterochronic offset of dorsal and ventral foregut modules allowed the post-metamorphic dorsal food channel to be co-opted as a simple but functional esophagus for feeding larvae. Furthermore, by reducing energy cost per ovum, the heterochronic offset may have given mothers the evolutionary option of increasing fecundity or investing in protective egg encapsulation material. A second developmental innovation was spatial separation of the dorsal and ventral foregut modules, as illustrated by distal foregut development in buccinid neogastropods and venom gland development in cone snails. Spatial uncoupling may have enhanced the evolvability of gastropod foreguts by allowing phenotypic variants of ventral module components to be selected within post-metamorphic ecological settings, without needing to be first tested for compatibility with larval feeding. Finally, we describe a case in which foregut modularity has helped facilitate a highly derived life history in which encapsulated embryos ingest nurse eggs.

Heterokairy: a significant form of developmental plasticity?

There is a current surge of research interest in the potential role of developmental plasticity in adaptation and evolution. Here we make a case that some of this research effort should explore the adaptive significance of heterokairy, a specific type of plasticity that describes environmentally driven, altered timing of development within a species. This emphasis seems warranted given the pervasive occurrence of heterochrony, altered developmental timing between species, in evolution. We briefly review studies investigating heterochrony within an adaptive context across animal taxa, including examples that explore links between heterokairy and heterochrony. We then outline how sequence heterokairy could be included within the research agenda for developmental plasticity. We suggest that the study of heterokairy may be particularly pertinent in (i) determining the importance of non-adaptive plasticity, and (ii) embedding concepts from comparative embryology such as developmental modularity and disassociation within a developmental plasticity framework.

Challenges and opportunities in developmental integrative physiology

This review explores challenges and opportunities in developmental physiology outlined by a symposium at the 2014 American Physiological Society Intersociety Meeting: Comparative Approaches to Grand Challenges in Physiology. Across animal taxa, adverse embryonic/fetal environmental conditions can alter morphological and physiological phenotypes in juveniles or adults, and capacities for developmental plasticity are common phenomena. Human neonates with body sizes at the extremes of perinatal growth are at an increased risk of adult disease, particularly hypertension and cardiovascular disease. There are many rewarding areas of current and future research in comparative developmental physiology. We present key mechanisms, models, and experimental designs that can be used across taxa to investigate patterns in, and implications of, the development of animal phenotypes. Intraspecific variation in the timing of developmental events can be increased through developmental plasticity (heterokairy), and could provide the raw material for selection to produce heterochrony--an evolutionary change in the timing of developmental events. Epigenetics and critical windows research recognizes that in ovo or fetal development represent a vulnerable period in the life history of an animal, when the developing organism may be unable to actively mitigate environmental perturbations. 'Critical windows' are periods of susceptibility or vulnerability to environmental or maternal challenges, periods when recovery from challenge is possible, and periods when the phenotype or epigenome has been altered. Developmental plasticity may allow survival in an altered environment, but it also has possible long-term consequences for the animal. "Catch-up growth" in humans after the critical perinatal window has closed elicits adult obesity and exacerbates a programmed hypertensive phenotype (one of many examples of "fetal programing"). Grand challenges for developmental physiology include integrating variation in developmental timing within and across generations, applying multiple stressor dosages and stressor exposure at different developmental timepoints, assessment of epigenetic and parental influences, developing new animal models and techniques, and assessing and implementing these designs and models in human health and development.

Sequence and comparison of mitochondrial genomes in the genus Nerita (Gastropoda: Neritimorpha: Neritidae) and phylogenetic considerations among gastropods

In the present study, we determined the mitochondrial DNA (mtDNA) sequence of three Neritas, Nerita versicolor, Nerita tessellata, and Nerita fulgurans. We present an analysis of the features of their gene content and genome organization and compare these within the genus Nerita, and among the main gastropod groups. The new sequences were used in a phylogenetic analysis including all available gastropod mitochondrial genomes. Genomic lengths were quite conserved, being 15,866bp for N. versicolor, 15,741bp for N. tessellata and 15,343bp for N. fulgurans. Intergenic regions were generally short; genes are transcribed from both strands and have a nucleotide composition high in A and T. The high similarity in nucleotide content of the different sequences, gene composition, as well as an identical genomic organization among the Nerita species compared in this study, indicates a high degree of conservation within this diverse genus. Values ​​of Ka/Ks of the 13 protein coding genes (PCGs) of Nerita species ranged from 0 to 0.18, and suggested different selection pressures in gene sequences. Bayesian phylogenetic analyses using concatenated DNA sequences of the 13 PCGs and the two rRNAs, and of amino acid sequences strongly supported Neritimorpha and Vetigastropoda as sister groups.

The other gastropod larvae: larval morphogenesis in a marine neritimorph

Two of the three major gastropod clades with feeding larvae are sister groups and larval morphogenesis for members of these clades, the Caenogastropoda and Heterobranchia, has been well studied. The third clade, the Neritimorpha, has an unstable phylogenetic position and little is known about development of their planktotrophic larvae. Information about larval morphology of neritimorphs and resolution of their controversial phylogenetic placement is critically important for understanding evolution of larval feeding within the Gastropoda. We describe larval morphogenesis to metamorphic competence for laboratory-reared larvae of Nerita melanotragus (Smith, 1884) (Neritimorpha: Neritidae). Preliminary observations suggest that prehatch larvae are capable of delayed hatching, possibly by entering a diapause state. Our description of larval morphogenesis, as based on tissue sections for light and transmission electron microscopy, scanning electron microscopy, three-dimensional-reconstructions of sectioned tissue, and labeling of muscles with fluorphore-tagged phalloidin, revealed four features that are unprecedented among both feeding and nonfeeding gastropod larvae. Larvae of N. melanotragus have muscles on the left and right side that both meet current criteria of a larval retractor muscle; shell-anchored muscles with oblique striations that project inside the visceral nerve loop to insert mainly on the velar lobes. They also have left and right digestive glands of similar size and a left and right hypobranchial gland. A larval "heart" is absent, but water circulation through the mantle cavity may be facilitated by large circular orifices, lined by patches of motile cilia, leading in and out of the mantle cavity. Comparison of larval traits among all three groups of gastropods with feeding larvae indicates that larvae of N. melanotragus have many unique characteristics, but they show more similarities to caenogastropod than to heterobranch larvae. These results are a significant step toward the goal of identifying primitive versus derived larval traits among feeding gastropod larvae.

An integrative approach identifies developmental sequence heterochronies in freshwater basommatophoran snails

Adopting an integrative approach to the study of sequence heterochrony, we compared the timing of developmental events encompassing a mixture of developmental stages and functional traits in the embryos of 12 species of basommatophoran snails in an explicit phylogenetic framework. PARSIMOV analysis demonstrated clear functional heterochronies associated both with basal branches within the phylogeny and with terminal speciation events. A consensus of changes inferred under both accelerated transformation and delayed transformation optimizations identified four heterochronies where the direction of movement was known plus six twin heterochronies where the relative movements of the two events could not be assigned. On average, 0.5 and 0.58 events were inferred to have changed their position in the developmental sequence on internal and terminal branches of the phylogeny, respectively; these values are comparable with frequencies of sequence heterochrony reported in mammals. Directional heterochronies such as the early occurrence of body flexing in relation to the ontogeny of the eye spots, heart beat, and free swimming events occurred convergently and/or at different levels (i.e., familial, generic, and species) within the phylogeny. Such a functional approach to the study of developmental sequences has highlighted the possibility that heterochrony may have played a prominent role in the evolution of this group of invertebrates.

Complete mitochondrial DNA sequence of a Conoidean gastropod, Lophiotoma (Xenuroturris) cerithiformis: gene order and gastropod phylogeny

We have determined the first complete nucleotide sequence of the mitochondrial genome of a venomous mollusc, the Conoidean gastropod, Lophiotoma (Xenuroturris) cerithiformis. It is 15,380 nucleotide pairs (ntp) and encodes 13 proteins, two ribosomal RNAs and 22 tRNAs of the mitochondrion's own protein synthesizing system. The protein mRNAs, ribosomal RNAs and 13 of the tRNAs are transcribed from the same strand, the remaining tRNAs from the other strand. The longest segment of unassigned sequence is 139 ntp and includes a 82 ntp segment that is a perfect inverted repeat sequence of 37 ntp separated by 8 nt. The gene arrangement of L. cerithiformis mtDNA shows remarkable similarity to the gene arrangements of mtDNAs of the vetigastropod Haliotis rubra, the polyplacophoran Katharina tunicata and the cephalopod Octopus vulgaris, but differs dramatically from the gene arrangements found in the mtDNAs of pulmonate and opisthobranch gastropods, as well as mtDNAs of bivalves and scaphopods. A single sixteen gene inversion that distinguishes L. cerithiformis mtDNA from mtDNAs of H. rubra, K. tunicata and O. vulgaris is shared by mtDNA of a littorinomorph gastropod Littorina saxitalis, suggesting a close relationship of conoidean and littorinomorph gastropods.

Origin of the Eumetazoa: testing ecological predictions of molecular clocks against the Proterozoic fossil record

Molecular clocks have the potential to shed light on the timing of early metazoan divergences, but differing algorithms and calibration points yield conspicuously discordant results. We argue here that competing molecular clock hypotheses should be testable in the fossil record, on the principle that fundamentally new grades of animal organization will have ecosystem-wide impacts. Using a set of seven nuclear-encoded protein sequences, we demonstrate the paraphyly of Porifera and calculate sponge/eumetazoan and cnidarian/bilaterian divergence times by using both distance [minimum evolution (ME)] and maximum likelihood (ML) molecular clocks; ME brackets the appearance of Eumetazoa between 634 and 604 Ma, whereas ML suggests it was between 867 and 748 Ma. Significantly, the ME, but not the ML, estimate is coincident with a major regime change in the Proterozoic acritarch record, including: (i) disappearance of low-diversity, evolutionarily static, pre-Ediacaran acanthomorphs; (ii) radiation of the high-diversity, short-lived Doushantuo-Pertatataka microbiota; and (iii) an order-of-magnitude increase in evolutionary turnover rate. We interpret this turnover as a consequence of the novel ecological challenges accompanying the evolution of the eumetazoan nervous system and gut. Thus, the more readily preserved microfossil record provides positive evidence for the absence of pre-Ediacaran eumetazoans and strongly supports the veracity, and therefore more general application, of the ME molecular clock.