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.
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•Exogenous BMP protein does not block neural specification in Capitella teleta.•Exogenous BMP protein does not disrupt dorsal-ventral axis formation.•Exogenous BMP protein causes trans-fate switching in a time-dependent manner.•BMP protein added at 3q causes a loss of eyes, radialized brain and reduced foregut.•BMP protein added after 4q causes ectopic 3rd eyes, brain lobes, and foregut lobes.
Many marine animals depend upon a larval phase of their life cycle to locate suitable habitat, and larvae use light detection to influence swimming behaviour and dispersal. Light detection is ...mediated by the opsin genes, which encode light-sensitive transmembrane proteins. Previous studies suggest that r-opsins in the eyes mediate locomotory behaviour in marine protostomes, but few have provided direct evidence through gene mutagenesis. Larvae of the marine annelid Capitella teleta have simple eyespots and are positively phototactic, although the molecular components that mediate this behaviour are unknown. Here, we characterize the spatio-temporal expression of the rhabdomeric opsin genes in C. teleta and show that a single rhabdomeric opsin gene, Ct-r-opsin1, is expressed in the larval photoreceptor cells. To investigate its function, Ct-r-opsin1 was disrupted using CRISPR/CAS9 mutagenesis. Polymerase chain reaction amplification and DNA sequencing demonstrated efficient editing of the Ct-r-opsin1 locus. In addition, the pattern of Ct-r-opsin1 expression in photoreceptor cells was altered. Notably, there was a significant decrease in larval phototaxis, although the eyespot photoreceptor cell and associated pigment cell formed normally and persisted in Ct-r-opsin1-mutant animals. The loss of phototaxis owing to mutations in Ct-r-opsin1 is similar to that observed when the entire photoreceptor and pigment cell are deleted, demonstrating that a single r-opsin gene is sufficient to mediate phototaxis in C. teleta. These results establish the feasibility of gene editing in animals like C. teleta, and extend previous work on the development, evolution and function of the C. teleta visual system . Our study represents one example of disruption of animal behaviour by gene editing through CRISPR/CAS9 mutagenesis, and has broad implications for performing genome editing studies in a wide variety of other understudied animals.
Over the last few decades, the annelid Capitella teleta has been used increasingly as a study system for investigations of development and regeneration. Its favorable properties include an ability to ...continuously maintain a laboratory culture, availability of a sequenced genome, a stereotypic cleavage program of early development, substantial regeneration abilities, and established experimental and functional genomics techniques. With this review I tell of my adventure of establishing the Capitella teleta as an emerging model and share examples of a few of the contributions our work has made to the fields of evo-devo and developmental biology. I highlight examples of conservation in developmental programs as well as surprising deviations from existing paradigms that highlight the importance of leveraging biological diversity to shift thinking in the field. The story for each study system is unique, and every animal has its own advantages and disadvantages as an experimental system. Just like most progress in science, it takes strategy, hard work and determination to develop tools and resources for a less studied animal, but luck and serendipity also play a role. I include a few narratives to personalize the science, share details of the story that are not included in typical publications, and provide perspective for investigators who are interested in developing their own study organism.
Hsp70/110s are a subfamily of heat shock proteins that play crucial roles against different types of environmental stressors and pathogenic organisms. To understand the evolution and divergence ...patterns of the Hsp70/110 gene family in polychaetes, genome-wide data from
Capitella teleta
and
Owenia fusiformis
were systematically analyzed based on bioinformatics methods. A total of 32 Hsp70/110 genes were identified in
C. teleta
, while only 11 Hsp70/110 genes were found in
O. fusiformis
. The phylogenetic tree supported that these genes were divided into 9 subfamilies. Selective pressure analysis showed that the Hsp70/110 gene family in polychaetes has evolved under purifying selection. Furthermore, diverse expression profiles were displayed for different Hsp70/110 genes in the two polychaetes. These results provide a theoretical basis for further studies of the molecular evolutionary patterns and function of the Hsp70/110 gene family in polychaetes.
Diverse architectures of nervous systems (NSs) such as a plexus in cnidarians or a more centralized nervous system (CNS) in insects and vertebrates are present across Metazoa, but it is unclear what ...selection pressures drove evolution and diversification of NSs. One underlying aspect of this diversity lies in the cellular and molecular mechanisms driving neurogenesis, i.e. generation of neurons from neural precursor cells (NPCs). In cnidarians, vertebrates, and arthropods, homologs of SoxB and bHLH proneural genes control different steps of neurogenesis, suggesting that some neurogenic mechanisms may be conserved. However, data are lacking for spiralian taxa.
To that end, we characterized NPCs and their daughters at different stages of neurogenesis in the spiralian annelid Capitella teleta. We assessed cellular division patterns in the neuroectoderm using static and pulse-chase labeling with thymidine analogs (EdU and BrdU), which enabled identification of NPCs that underwent multiple rounds of division. Actively-dividing brain NPCs were found to be apically-localized, whereas actively-dividing NPCs for the ventral nerve cord (VNC) were found apically, basally, and closer to the ventral midline. We used lineage tracing to characterize the changing boundary of the trunk neuroectoderm. Finally, to start to generate a genetic hierarchy, we performed double-fluorescent in-situ hybridization (FISH) and single-FISH plus EdU labeling for neurogenic gene homologs. In the brain and VNC, Ct-soxB1 and Ct-neurogenin were expressed in a large proportion of apically-localized, EdU
NPCs. In contrast, Ct-ash1 was expressed in a small subset of apically-localized, EdU
NPCs and subsurface, EdU
cells, but not in Ct-neuroD
or Ct-elav1
cells, which also were subsurface.
Our data suggest a putative genetic hierarchy with Ct-soxB1 and Ct-neurogenin at the top, followed by Ct-ash1, then Ct-neuroD, and finally Ct-elav1. Comparison of our data with that from Platynereis dumerilii revealed expression of neurogenin homologs in proliferating NPCs in annelids, which appears different than the expression of vertebrate neurogenin homologs in cells that are exiting the cell cycle. Furthermore, differences between neurogenesis in the head versus trunk of C. teleta suggest that these two tissues may be independent developmental modules, possibly with differing evolutionary trajectories.
Spiralian development is characterized by stereotypic cell geometry and spindle orientation in early cleavage stage embryos, as well as conservation of ultimate fates of descendent clones. Diverse ...taxa such as molluscs, annelids, flatworms, and nemerteans exhibit spiralian development, but it is a mystery how such a conserved developmental program gives rise to such diverse body plans. This review highlights examples of variation during early development among spiralians, emphasizing recent experimental studies in the annelid Capitella teleta Blake, Grassle and Eckelbarger, 2009. Intracellular fate mapping studies in C. teleta reveal that many of its cells’ fates are shared among spiralians, but it also has a novel origin for trunk mesoderm (3c and 3d micromeres). Studies have identified an inductive signal in spiralians that has “organizing activity” and that influences cell fates in the surrounding embryo. Capitella teleta also has an organizing activity; however, surprisingly, it is localized to a different cell, it signals at a different developmental stage, and likely utilizes a distinct molecular signaling pathway compared with that in molluscs. A model is presented to provide a mechanistic explanation of evolutionary changes in the cellular identity of the organizer. Detailed experimental investigations in spiralian embryos demonstrate variation in developmental features that may influence the evolution of novel forms.
The segmented worms, or annelids, are a clade within the Lophotrochozoa, one of the three bilaterian superclades. Annelids have long been models for regeneration studies due to their impressive ...regenerative abilities. Furthermore, the group exhibits variation in adult regeneration abilities with some species able to replace anterior segments, posterior segments, both or neither. Successful regeneration includes regrowth of complex organ systems, including the centralized nervous system, gut, musculature, nephridia and gonads. Here, regenerative capabilities of the annelid
are reviewed.
exhibits robust posterior regeneration and benefits from having an available sequenced genome and functional genomic tools available to study the molecular and cellular control of the regeneration response. The highly stereotypic developmental program of
provides opportunities to study adult regeneration and generate robust comparisons between development and regeneration.
The population dynamics of the capitellid polychaete
Capitella
aff.
teleta
were studied in Gamo Lagoon, located in northeast Japan, for the subsequent 2 years from 2016, when a series of restoration ...works was conducted following the 2011 Great East Japan Earthquake and tsunami.
Capitella
aff.
teleta
was found to be widely distributed from the estuary side, where the levee was located, to the innermost part but was more abundant in the innermost part, which is rich in organic matter. In the lagoon, the daily maximum water level dropped from 2017 to 2018 during the reconstruction of a flow-conducting levee, which blocked water flow and isolated the inner part of the lagoon. Although the density decreased drastically for approximately 11 months under diurnal hypoxia and strongly reducing conditions, small-sized new recruits were observed and the population recovered quickly after the daily maximum water level increased. In Gamo Lagoon,
C
. aff.
teleta
inhabiting the innermost part and estuary side of the lagoon contributed to maintaining the population by dispersing planktonic larvae between them. Thus, the maximum water level had a significant effect on the maintenance of the
C
. aff.
teleta
population in the lagoon, and sufficiently high water levels enable the dispersion of planktonic larvae to help recover the population quickly, suggesting that it is important to keep the water area connected.
How nervous systems evolved remains an unresolved question. Previous studies in vertebrates and arthropods revealed that homologous genes regulate important neurogenic processes such as cell ...proliferation and differentiation. However, the mechanisms through which such homologs regulate neurogenesis across different bilaterian clades are variable, making inferences about nervous system evolution difficult. A better understanding of neurogenesis in the third major bilaterian clade, Spiralia, would greatly contribute to our ability to deduce the ancestral mechanism of neurogenesis.
Using whole-mount in situ hybridization, we examined spatiotemporal gene expression for homologs of
,
,
,
-
,
, and
in embryos and larvae of the spiralian annelid
, which has a central nervous system (CNS) comprising a brain and ventral nerve cord. For all homologs examined, we found expression in the neuroectoderm and/or CNS during neurogenesis. Furthermore, the onset of expression and localization within the developing neural tissue for each of these genes indicates putative roles in separate phases of neurogenesis, e.g., in neural precursor cells (NPCs) versus in cells that have exited the cell cycle.
-
,
-
, and
-
are the earliest genes expressed in surface cells in the anterior and ventral neuroectoderm, while
-
expression initiates slightly later in surface neuroectoderm.
-
is expressed in single cells in neural and non-neural ectoderm, while
-
and
-
are localized to differentiating neural cells in the brain and ventral nerve cord.
These results suggest that the genes investigated in this article are involved in a neurogenic gene regulatory network in
. We propose that Ct-SoxB1, Ct-SoxB, and Ct-Ngn are involved in maintaining NPCs in a proliferative state. Ct-Pros may function in division of NPCs, Ct-Ash1 may promote cell cycle exit and ingression of NPC daughter cells, and Ct-NeuroD and Ct-Msi may control neuronal differentiation. Our results support the idea of a common genetic toolkit driving neural development whose molecular architecture has been rearranged within and across clades during evolution. Future functional studies should help elucidate the role of these homologs during
neurogenesis and identify which aspects of bilaterian neurogenesis may have been ancestral or were derived within Spiralia.
To date, specimens belonging to the genus
Capitella
Blainville, 1828 collected from Korean waters have been identified as
C. capitata
sensu Eisig, 1887 for over 35 years, depending almost totally on ...the description of Paik (Bull Korean Fish Soc 13:89–92,
1980
), but consisted of the capitellid species examined in this study. Although the characteristics of 9 thoracic segments, thoracic chaetigers 1–7 with capillaries, and abdominal segments without branchiae or capillary chaetae correspond to
C. capitata
(Fabricius
1780
), Korean capitellid specimens differ in a prostomium without deep dorsal depression, chaetigers 8–9 without neuropodial capillaries, chaetiger 8–9 of males without constriction, and high numbers of genital spines in males. Capitellid specimens from Korean waters resemble
C. teleta
Blake, Grassle & Eckelbarger,
2009
rather than
C. capitata
by the following morphological characteristics: presence of eyes, weak dorsal depression on prostomium, chaetigers 8–9 with neuropodial hooded hooks only, inflated chaetigers 8–9 of males, and 3–4 rows of teeth above the main fang. For precise identification of the Korean capitellid specimens, genetic analysis was performed, and the result revealed
Capitella
specimens of Korea genetically matched well with
C. teleta
of Ainan (0.4%) and
Capitella
spp. (0.2%) of Italy in partial cytochrome c oxidase subunit I (COI) gene comparison. Although there is little genetic difference between
C. teleta
from different areas, morphological differences in the shape of the prostomium and pygidium and the number of chaetae per fascicle suggest geographical variations in morphology caused by their adjacent environments. This result suggests the possibility of the existence of a species complex of
C. teleta
. In this study, genetic and morphological features of
C. teleta
of Korea are provided. In addition, a comprehensive comparison with previous records of
C. capitata
and
C. teleta
are conducted, and newly discovered intraspecific variations and the geographic distribution of
C. teleta
are discussed.
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