Capitella teleta gets left out: possible evolutionary shift causes loss of left tissues rather than increased neural tissue from dominant-negative BMPR1

BACKGROUND

The evolution of central nervous systems (CNSs) is a fascinating and complex topic; further work is needed to understand the genetic and developmental homology between organisms with a CNS. Research into a limited number of species suggests that CNSs may be homologous across Bilateria. This hypothesis is based in part on similar functions of BMP signaling in establishing fates along the dorsal-ventral (D-V) axis, including limiting neural specification to one ectodermal region. From an evolutionary-developmental perspective, the best way to understand a system is to explore it in a wide range of organisms to create a full picture.

METHODS

Here, we expand our understanding of BMP signaling in Spiralia, the third major clade of bilaterians, by examining phenotypes after expression of a dominant-negative BMP Receptor 1 and after knock-down of the putative BMP antagonist Chordin-like using CRISPR/Cas9 gene editing in the annelid Capitella teleta (Pleistoannelida).

RESULTS

Ectopic expression of the dominant-negative Ct-BMPR1 did not increase CNS tissue or alter overall D-V axis formation in the trunk. Instead, we observed a unique asymmetrical phenotype: a distinct loss of left tissues, including the left eye, brain, foregut, and trunk mesoderm. Adding ectopic BMP4 early during cleavage stages reversed the dominant-negative Ct-BMPR1 phenotype, leading to a similar loss or reduction of right tissues instead. Surprisingly, a similar asymmetrical loss of left tissues was evident from CRISPR knock-down of Ct-Chordin-like but concentrated in the trunk rather than the episphere.

CONCLUSIONS

Our data highlight a novel asymmetrical phenotype, giving us further insight into the complicated story of BMP's developmental role. We further solidify the hypothesis that the function of BMP signaling during the establishment of the D-V axis and CNS is fundamentally different in at least Pleistoannelida, possibly in Spiralia, and is not required for nervous system delimitation in this group.

Role of BMP signaling during early development of the annelid Capitella teleta

The mechanisms regulating nervous system development are still unknown for a wide variety of taxa. In insects and vertebrates, bone morphogenetic protein (BMP) signaling plays a key role in establishing the dorsal-ventral (D-V) axis and limiting the neuroectoderm to one side of that axis, leading to speculation about the conserved evolution of centralized nervous systems. Studies outside of insects and vertebrates show a more diverse picture of what, if any role, BMP signaling plays in neural development across Bilateria. This is especially true in the morphologically diverse Spiralia (≈Lophotrochozoa). Despite several studies of D-V axis formation and neural induction in spiralians, there is no consensus for how these two processes are related, or whether BMP signaling may have played an ancestral role in either process. To determine the function of BMP signaling during early development of the spiralian annelid Capitella teleta, we incubated embryos and larvae in BMP4 protein for different amounts of time. Adding exogenous BMP protein to early-cleaving C. teleta embryos had a striking effect on formation of the brain, eyes, foregut, and ventral midline in a time-dependent manner. However, adding BMP did not block brain or VNC formation or majorly disrupt the D-V axis. We identified three key time windows of BMP activity. 1) BMP treatment around birth of the 3rd-quartet micromeres caused the loss of the eyes, radialization of the brain, and a reduction of the foregut, which we interpret as a loss of A- and C-quadrant identities with a possible trans-fate switch to a D-quadrant identity. 2) Treatment after the birth of micromere 4d induced formation of a third ectopic brain lobe, eye, and foregut lobe, which we interpret as a trans-fate switch of B-quadrant micromeres to a C-quadrant identity. 3) Continuous BMP treatment from late cleavage (4d ​+ ​12 ​h) through mid-larval stages resulted in a modest expansion of Ct-chrdl expression in the dorsal ectoderm and a concomitant loss of the ventral midline (neurotroch ciliary band). Loss of the ventral midline was accompanied by a collapse of the bilaterally-symmetric ventral nerve cord, although the total amount of neural tissue was not greatly affected. Our results compared with those from other annelids and molluscs suggest that BMP signaling was not ancestrally involved in delimiting neural tissue to one region of the D-V axis. However, the effects of ectopic BMP on quadrant-identity during cleavage stages may represent a non-axial organizing signal that was present in the last common ancestor of annelids and mollusks. Furthermore, in the last common ancestor of annelids, BMP signaling may have functioned in patterning ectodermal fates along the D-V axis in the trunk. Ultimately, studies on a wider range of spiralian taxa are needed to determine the role of BMP signaling during neural induction and neural patterning in the last common ancestor of this group. Ultimately, these comparisons will give us insight into the evolutionary origins of centralized nervous systems and body plans.

Connecting pattern to process: Growth of spiral shell sculpture in the gastropod Nucella ostrina (Muricidae: Ocenebrinae)

Shell morphology is a well-suited and underused system to examine the development of novel forms. The three-dimensional structure produced (the shell) is separate from the largely two-dimensional tissue that secretes it (the mantle), allowing us to disentangle the pattern from the process. Despite knowing a great deal about the mechanics of shell secretion (process), and the variety of shell shapes that exist (pattern), no effort has been made to understand how the mantle changes to produce different shell shapes. We investigated this question in the dimorphic snail Nucella ostrina, which exhibits both smooth and ribbed shells to determine how ribs are formed by the mantle. Rib thickenings are produced only in the outer calcitic shell layer and secreted by the distal Outer Mantle Epithelium (OME) with increased acid phosphatase activity. The evenly thick inner aragonitic layers are secreted by the proximal OME which expresses acid phosphatase. Here we show that locally thicker ribs in N. ostrina are produced by changing the dimensions of the distal OME: elongation in the direction of growth and increased cell height. This should increase the amount of shell material secreted, producing locally thicker shell (ribs). Preliminary evidence suggests this mechanism may be widespread in gastropods.

Inferring microevolution from museum collections and resampling: lessons learned from Cepaea

Natural history collections are an important and largely untapped source of long-term data on evolutionary changes in wild populations. Here, we utilize three large geo-referenced sets of samples of the common European land-snail Cepaea nemoralis stored in the collection of Naturalis Biodiversity Center in Leiden, the Netherlands. Resampling of these populations allowed us to gain insight into changes occurring over 95, 69, and 50 years. Cepaea nemoralis is polymorphic for the colour and banding of the shell; the mode of inheritance of these patterns is known, and the polymorphism is under both thermal and predatory selection. At two sites the general direction of changes was towards lighter shells (yellow and less heavily banded), which is consistent with predictions based on on-going climatic change. At one site no directional changes were detected. At all sites there were significant shifts in morph frequencies between years, and our study contributes to the recognition that short-term changes in the states of populations often exceed long-term trends. Our interpretation was limited by the few time points available in the studied collections. We therefore stress the need for natural history collections to routinely collect large samples of common species, to allow much more reliable hind-casting of evolutionary responses to environmental change.

Shaving a Shell: Effect of Manipulated Sculpture and Feeding on Shell Growth and Sculpture Development in Nucella lamellosa (Muricidae: Ocenebrinae)

Gastropod shell sculpture offers a novel tool for studying morphological patterning. Existing shell features may be manipulated experimentally to test how alteration affects subsequent shell growth and form. Axial sculpture occurs in many gastropod groups, and spacing of sculpture may be regular or irregular. But how gastropods control sculpture placement during shell growth is unknown. We studied the growth and positioning of axial lamellae in the muricid Nucella lamellosa, and compared these to the superficially similar axial varices seen in other muricids. First, we tested whether the feeding rate had any effect on the rate of addition or positioning of new lamellae. Second, we tested what effect previous shell sculpture had on lamellar placement, and shell growth in general, by removing all shell sculpture and allowing snails to grow over the "shaved" shell surface. Lamellar growth appeared to be relatively plastic; spacing was highly variable both within and among individual snails, and 1-2 weeks were required to complete the addition of a new lamella. Body growth rate was the primary determinant of lamellar growth; past lamellae had no effect on placement of new lamellae or rate of shell length increase. Feeding rate and body size affected only growth in shell length, and had no direct effect on spacing or on the rate of addition of new lamellae. The growth of axial lamellae in N. lamellosa differed from that of varices by exhibiting neither a) regular spacing nor b) a growth hiatus after completion of a lamella. Significantly, despite the obvious impediment of previous sculpture to future shell growth, removal of this sculpture had no observable effect on the rate of body growth or on any aspect of subsequent lamellar growth.

Phylogenetic reconstruction and shell evolution of the Diplommatinidae (Gastropoda: Caenogastropoda)

The fascinating and often unlikely shell shapes in the terrestrial micromollusc family Diplommatinidae (Gastropoda: Caenogastropoda) provide a particularly attractive set of multiple morphological traits to investigate evolutionary patterns of shape variation. Here, a molecular phylogenetic reconstruction, based on five genes and 2700 bp, was undertaken for this family, integrated with ancestral state reconstruction and phylogenetic PCA of discrete and quantitative traits, respectively. We found strong support for the Diplommatininae as a monophyletic group, separating the Cochlostomatidae into a separate family. Five main clades appear within the Diplommatininae, corresponding with both coiling direction and biogeographic patterns. A Belau clade (A) with highly diverse (but always sinistral) morphology comprised Hungerfordia, Palaina, and some Diplommatina. Arinia (dextral) and Opisthostoma (sinistroid) are sister groups in clade B. Clade C and D solely contain sinistral Diplommatina that are robust and little ornamented (clade C) or slender and sculptured (clade D). Clade E is dextral but biogeographically diverse with species from all sampled regions save the Caroline Islands. Adelopoma, Diplommatina, Palaina, and Hungerfordia require revision to allow taxonomy to reflect phylogeny, whereas Opisthostoma is clearly monophyletic. Ancestral state reconstruction suggests a sinistral origin for the Diplommatinidae, with three reversals to dextrality.

Differential progression of magnetization transfer imaging changes depending on severity of cerebral hypoxic-ischemic injury

We hypothesized that magnetic resonance magnetization transfer (MT) imaging would be sensitive for detecting cerebral ischemic injury in white matter of neonatal brain. We compared the progression of changes in T(2) and the MT ratio (MTR) after cerebral hypoxic-ischemic insults of differing severity in neonatal rats. Magnetization transfer imaging parameters were first optimized, and then MTR and T(2) maps were acquired at various times after a mild (rather selective white matter) or substantial insult produced by unilateral cerebral hypoxia-ischemia. Depending on insult severity, time after insult, and region (e.g., subcortical white matter or cortex), cerebral hypoxia-ischemia produced reductions in MTR and an increase in T(2). The exception was acutely at 1 to 5 h at which time points MTR was reduced ipsilaterally in white matter, whereas T(2) was not affected significantly. Progression of imaging changes differed in rats grouped according to whether gross damage was present after chronic recovery. Behavioral changes were generally associated with chronic reductions in MTR and gross brain damage. Magnetization transfer imaging was capable of early detection of hypoxic-ischemic injury and particularly sensitive for identifying the progression of cerebral injury in white matter. Magnetization transfer ratio has potential for assisting with early diagnosis and treatment assessment for infants affected by perinatal hypoxia-ischemia.