Role of BMP signaling during early development of the annelidCapitella teleta

Abstract

The mechanisms regulating nervous system development are still unknown for a wide variety of taxa. In insects and vertebrates, bone morphogenetic protein (BMP) signaling is known to play a key role in both neural specification and dorsal-ventral (D-V) axis formation, leading to speculation about the conserved evolution of nervous systems. Studies outside 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. Here we incubated larvae of the sedentary annelid Capitella teleta in BMP4 protein at various cleavage stages to determine the role of BMP signaling during early development. Adding exogenous BMP protein to early-cleaving C. teleta embryos had a striking effect on formation of the brain, eyes, and foregut in a time-dependent manner. However, adding BMP did not block neural specification of the brain or VNC or block formation of the D-V axis. We identified three key time windows of BMP activity, and hypothesize that BMP may cause trans-fate switching of blastomere quadrant identities in at least one time window. 1. Early treatment around 2q caused the loss of the eyes, radialization of the brain, and a reduction of the foregut, which we interpret as a loss of A-, B- and C-quadrant identities with a possible trans-fate switch to a D-quadrant identity. 2. Treatment after 4q 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 early cleavage 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 VNC although the total amount of neural tissue did not appear to be greatly affected. Our results compared to those from other annelids and molluscs suggest that BMP signaling was not ancestrally involved in delimiting neural tissue or establishing the D-V axis in the last common ancestor of annelids. However, the effects of ectopic BMP on quadrant-identity during cleavage stages may represent a very early ‘organizing’ function in the context of spiralian development. Ultimately, studies on a wider range of spiralian taxa are needed to determine if the ability of BMP signaling to block neural induction and help establish the D-V axis was lost within Annelida or if BMP signaling gained these functions multiple times across Bilateria. Ultimately, these comparisons will give us insight into the evolutionary origins of centralized nervous systems and body plans.

Molluscan dorsal-ventral patterning relying onbmp2/4andchordinprovides insights into spiralian development and bilaterian evolution

“…While the DV patterning mechanism is considered conserved across bilaterian clades (12), investigations on several spiralian representatives have revealed different results (13)(14)(15)(16)(17)(18). Meanwhile, in various spiralian phyla, DV patterning is deeply integrated into a highly specialized organizer-driven developmental mode (23,(26)(27)(28)(29), with the underlying molecular mechanism largely unknown.…”

“…A major issue in the studies of spiralian DV patterning is that researchers have reported very different results using different systems. Researchers concluded that spiralian DV patterning relies on bmp2/4 (without known roles of chordin) (19,20), other BMP ligand and antagonist (13), or even non-BMP signaling (14)(15)(16)(17)(18). Taken together, a paradox rises: while it is widely accepted that the common ancestor of bilaterians utilizes bmp2/4 and chordin for DV patterning (12), this character is not revealed in the six spiralian representatives spanning three spiralian phyla (three annelids, two mollusks and one platyhelminth, Fig.…”

“…The very different reports in spiralian DV patterning undoubtedly suggest the diversity of the underlying mechanisms (13)(14)(15)(16)(17)(18)(19)(20). However, it is notable that the experimental designs of these studies vary in many aspects: strategies to influence BMP signaling (MOs, small molecule inhibitors, exogenous BMP proteins), indicators of DV patterning (cell lineages, characteristic tissues, marker gene expression), and the developmental stages investigated (early embryos, larvae).…”

“…This hypothesis can be tested in more spiralian lineages, which would be important to understand the unique developmental mode in spiralians (25,26,63). Nevertheless, we want to express a cautious attitude to infer the conservation/prevalence of this BMP2/4-based organizer function given that BMP signaling has been suggested to have no roles in DV patterning in two annelids and a mollusk (14,15,17,18).…”

“…Moreover, current studies on several representative species have revealed quite diverse DV patterning mechanisms. These spiralians could use other BMP molecules (the leech annelid Helobdella robusta) (13) or even do not employ BMP signaling in DV patterning (the annelids Capitella teleta, Chaetopterus pergamentaceus and the mollusk Crepidula fornicata) (14)(15)(16)(17)(18). The roles of bmp2/4 in DV patterning were proven in two spiralians (the platyhelminth Dugesia japonica and the mollusk Tritia obsolete) (19,20), but it is unknown whether bmp2/4 coordinates with chordin to induce correct DV patterning (as seen in most nonspiralian animals (9,12)).…”