Several hypotheses exist concerning the putative homology of various parts of larval and adult nervous systems found among bilaterian animals. Based on similar expression patterns of evolutionarily conserved transcription factors, some authors have concluded that the structure of the adult nervous system of the last common ancestor of the Bilateria included an anterior brain with three divisions and a distinct longitudinal ventral nerve cord [1–3], with bilaterians that exhibit a more diffusely organized central nervous system (for example, hemichordates) having acquired this characteristic secondarily . Other hypotheses suggest that the last common ancestor of all bilaterians was more similar to extant acoelomorph flatworms that have an anterior compact brain with a centralized neuropil and parallel dorsal, ventral, and lateral longitudinal nerve cords [5, 6], and that adult bilaterian 'brains' have evolved independently several times . Although the phylogenetic position of the acoelomorph flatworms remains contentious , having a single anterior compact neuronal center may still be plesiomorphic for bilaterian animals. The origin of the anterior bilaterian nervous system may be an amalgamation of neural ectodermal domains that are positioned within the oral and aboral regions of a planula-like ancestor [9, 10] that possessed only an intraepithelial nerve net. The majority of information on bilaterian neural development focuses on animals that form a centralized subepithelial nervous system and much less information is known about animals with diverse forms of intraepithelial nervous systems (so called 'skin brains' see ), whose significance in protostome evolution is rarely addressed.
Confounding issues regarding the origin of the adult bilaterian nervous system pertain to its spatial proximity and integration with components of a larval nervous system, particularly, the development of ciliated apical tuft cells, which numerous larval forms exhibit, and their relationship to the larval apical (sensory) organ where many neuronal cell bodies of larval forms are concentrated. Despite some shared developmental and structural features there is no uniform consensus regarding the homology of the larval aboral organs of cnidarian planulae with the larval apical organs of various bilaterians [12–15]. One obvious difference is that some paired-class homeobox genes involved with the development of larval apical organ and adult brain in bilaterians (such as homeobrain, rx, and orthopedia) are only expressed within oral ectoderm of cnidarians . How the different ectodermal domains of a planula-like ancestor became coupled to the bilaterian anterior region remains an open question, but the resulting cellular domains within it are a combination of several different ciliary and neuronal cell types that may have been co-opted into unique apical structures several times (for example, see [17–19]). The neuronal compositions of bilaterian larval apical organs are clearly diverse, and the putative homology of various neurotransmitter-expressing cell types among evolutionarily distant larval types remains controversial. Furthermore, since similar morphologies among disparate larval forms may be the result of convergent evolutionary forces [20, 21], testing these ideas requires finding novel methods and broad taxonomic sampling to evaluate the homology of these intriguing larval structures.
The larval apical organs of phoronids and brachiopods are relevant to the evolutionary reconstruction of bilaterian brains. Although the evolutionary relationships within phoronids and brachiopods [22, 23] as well as their exact sister group position are still under debate [24–26], phoronids and brachiopods clearly reside within the assemblage of protostome animals known as the Lophotrochozoa or Spiralia. In light if this, developmental and structural traits (cleavage patterns, mesoderm formation, morphology of the coelomic cavities, and ciliated bands comprised of monociliated cells) that once aligned phoronids and brachiopods with deuterostomes have been largely disproven or interpreted as the result of convergent evolution [27–30]. The presence of numerous (thirty or more) serotonergic cells in the larval apical organs of phoronids (previously referred to as the apical ganglion, but usage of this term has been criticized, see ) has also been interpreted as a deuterostome-like trait [32, 33]. However, further investigation showed that the types of serotonergic cells within the actinotroch apical organ correspond more to serotonergic cell types within the apical organs of annelids and mollusks [34, 35]. The apical organs of phoronid larvae also differ from those of echinoderm and hemichordate larvae in that apical organs of actinotrochs are comprised of a tombstone or U-shaped field of neuronal cells that send processes into a central neuropil [34, 36], and these neuronal cell bodies do not originate within ciliated bands. Similar structural features are found in the larval apical organs of brachiopods, although neurotransmitter expression within apical neuronal cell types varies among systematic groups [37–39].
Considering all of these structural and biochemical differences among bilaterian larval apical organs, evaluating the homology of cell types and the complex neuronal centers they make up becomes problematic as there are no universally agreed upon criteria for discriminating homologous neuronal cell types (but see ). Some studies have tried to make a connection between the expression of select patterning genes and the specification of neuronal cells with conserved neurotransmitter expression types (for example, serotonin or vasotocin, see [41, 42]), however the complete gene regulatory networks that specify the great majority of anterior larval neurosecretory cell types remain unknown. What has been shown more recently is the broad conservation of genes involved in the specification of both larval and adult anterior neural ectoderm, such as Six3/6, homeobrain, and NK2.1[3, 43–45]. Even if a direct connection between the expression of neural ectodermal-related genes and the neural architecture of various larval apical organs remains elusive, one plausible hypothesis is that evolutionary modifications to the combinatorial expression domains of these genes have contributed to cellular diversity of larval apical organs.
Evaluating alternative viewpoints regarding either the wide-scale homology or independent origin of larval apical organs requires more developmental data focused on the molecular specification of various neurogenic tissue domains from additional bilaterian animals with structurally diverse larval nervous systems. Recent reports on the development and structure of the sensory cells and larval nervous system in brachiopods [39, 46] suggests that the broad specification of the neural ectoderm, simple ciliary photoreceptors, and wide-spread usage of conserved neurotransmitters within their basiepithelial nervous systems may yield key insights into the evolution of larval traits. Although the larval nervous systems of brachiopods have distinct features from what is observed in the larval forms of both spiralians and deuterostomes, all of these larval types share some morphologically similar peptidergic neuronal cell types [35, 39]. How these cell types are deployed within anterior neural tissues may yield key insights into the origin and diversification of bilaterian larval nervous systems. The putative homology of neural structures within the larval nervous systems of brachiopods and spiralians (for example, apical organ, cerebral ganglion, and ventral nerve cord) is also not understood. Furthermore, because the spatial expression of particular neural-related genes (for example, NK2.1; ) differs in ambulacralian deuterostomes and spiralians (gastropods and polychaetes), the larval apical organs of protostomes and deuterostomes are generally considered not to be homologous structures (but see ). However, this conclusion does not fully take into account the different neural ectodermal domains that comprise adult, bilaterian anterior nervous systems, and how they are sometimes integrated with or separate from the larval nervous system. For these reasons we have investigated the development of the larval nervous system in the rhynchonelliform brachiopod, Terebratalia transversa, by analyzing the expression patterns of eight genes known to have roles in specification and differentiation of anterior neural tissues in other bilaterian animals.