Wollesen et al. EvoDevo (2015) 6:41 DOI 10.1186/s13227-015-0037-z EvoDevo RESEARCH Open Access The ParaHox gene Gsx patterns the apical organ and central nervous system but not the foregut in scaphopod and cephalopod mollusks Tim Wollesen1*, Sonia Victoria Rodríguez Monje1, Carmel McDougall2, Bernard M. Degnan2 and Andreas Wanninger1 Abstract Background: It has been hypothesized that the ParaHox gene Gsx patterned the foregut of the last common bilate- rian ancestor. This notion was corroborated by Gsx expression in three out of four lophotrochozoan species, several ecdysozoans, and some deuterostomes. Remarkably, Gsx is also expressed in the bilaterian anterior-most central nervous system (CNS) and the gastropod and annelid apical organ. To infer whether these findings are consistent with other mollusks or even lophotrochozoans, we investigated Gsx expression in developmental stages of representatives of two other molluscan classes, the scaphopod Antalis entalis and the cephalopod Idiosepius notoides. Results: Gsx is not expressed in the developing digestive tract of Antalis entalis and Idiosepius notoides. Instead, it is expressed in cells of the apical organ in the scaphopod trochophore and in two cells adjacent to this organ. Late- stage trochophores express Aen-Gsx in cells of the developing cerebral and pedal ganglia and in cells close to the pavilion, mantle, and foot. In postmetamorphic specimens, Aen-Gsx is expressed in the cerebral and pedal ganglia, the foot, and the nascent captacula. In early squid embryos, Ino-Gsx is expressed in the cerebral, palliovisceral, and optic ganglia. In late-stage embryos, Ino-Gsx is additionally expressed close to the eyes and in the supraesophageal and posterior subesophageal masses and optic lobes. Developmental stages close to hatching express Ino-Gsx only close to the eyes. Conclusions: Our results suggest that Gsx expression in the foregut might not be a plesiomorphic trait of the Lopho- trochozoa as insinuated previously. Since neither ecdysozoans nor deuterostomes express Gsx in their gut, a role in gut formation in the last common bilaterian ancestor appears unlikely. Gsx is consistently expressed in the bilaterian anterior-most CNS and the apical organ of lophotrochozoan larvae, suggesting a recruitment of Gsx into the forma- tion of this organ in the Lophotrochozoa. The cephalopod posterior subesophageal mass and optic ganglia and the scaphopod pedal ganglia also express Gsx. In summary, Gsx expression only appears to be conserved in the anterior- most brain region during evolution. Accordingly, Gsx appears to have been recruited into the formation of other expression domains, e.g., the apical organ or the foregut, in some lophotrochozoans. Keywords: Brain, Cephalopoda, Evolution, Development, Hox, Homeobox genes, Invertebrate, Lophotrochozoa, Mollusca, Ontogeny, Scaphopoda, Lophotrochozoa *Correspondence: [email protected] 1 Department of Integrative Zoology, Faculty of Life Sciences, University of Vienna, 1090 Vienna, Austria Full list of author information is available at the end of the article © 2015 Wollesen et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Wollesen et al. EvoDevo (2015) 6:41 Page 2 of 18 Background apical organ in the gastropod mollusk G. varia and the The Hox and ParaHox gene clusters are considered to be annelid P. dumerilii (Table 1; [12, 13]). In addition, Gsx derived from a hypothetical ProtoHox cluster by duplica- expression was also found in the radula sac, a molluscan tion [1]. Both belong to the homeobox gene family and evolutionary novelty [12]. Recent phylogenomic analyses exhibit highly conserved amino acid sequences in phylo- on mollusks have revived a classical hypothesis placing genetically distantly related animals [1, 2]. In the major- the Aculifera, i.e., the worm-shaped and spicule-bearing ity of bilaterians investigated, it has been shown that aplacophorans and the eight-shelled polyplacophorans, Hox genes are expressed in tempo-spatial collinearity as a sister group to the Conchifera [30–32]. The Conchif- during development, in particular in neuroectodermal era is an anatomically diverse clade comprising scapho- domains [2, 3]. Cephalopod and gastropod mollusks were pods, gastropods, bivalves, monoplacophorans, and among the first examples among bilaterians that appar- cephalopods. Until now, conchiferan interrelationships ently do not exhibit such a collinear mode of Hox gene are unsettled, and attempts to infer the evolution of their expression [4, 5]. Tempo-spatial collinear expression of body plans are scarce (c.f. [31, 32]; but see [33, 34]). the three ParaHox genes has also been proposed for the The present study deals with two conchiferans, the last common bilaterian ancestor [1]. It has been hypoth- scaphopod Antalis entalis Jeffreys 1869 and the cephalo- esized that Gsx was expressed in the foregut, Xlox in pod squid Idiosepius notoides Berry, 1921 (Fig. 1). Adult the midgut, and Cdx in the hindgut in the last common scaphopods and cephalopods exhibit a pronounced bilaterian ancestor [1, 6]. While Xlox expression in the dorso-ventral body axis as opposed to the majority of midgut and Cdx expression in the hindgut was found in bilaterians that exhibit a pronounced antero-posterior various bilaterians, no Gsx expression has been reported body axis (Fig. 1). In adult scaphopods, the mouth and in the foregut of any deuterostome representative to date foot are located ventrally, while the pavilion (i.e., the [1, 6]. This was explained by the fact that the blastopore mantle cavity opening on the opposite side) marks the does not develop into the prospective mouth in deuter- dorsal pole (Fig. 1b). In adult cephalopods, the funnel ostomes. Deuterostomes instead evolved a new mouth and (parts of) the circumoral brachial crown are con- and hence Gsx might have lost its role in patterning the sidered to be homologous to the foot of other mollusks anterior-most region of the digestive tract. Interestingly, [35] (Fig. 1c). The brachial crown and the funnel define the deuterostome hemichordate Ptychodera flava, for the ventral side, while the mantle apex is located dor- example, does express Gsx around the blastopore, how- sally (Fig. 1c). Thus, the dorso-ventral axis constitutes ever, apparently not in the digestive tract of subsequent the major body axis in these animals. In both clades, the developmental stages [7]. Holland anticipated that proto- cerebral ganglia are located anteriorly (labeled blue in stome invertebrates may show Gsx expression in the fore- Fig. 1), while the statocysts are located at the posterior gut since their blastopore usually does become the future pole (dashed circles in Fig. 1b, c). mouth [1, 6]. Data on the ecdysozoan and lophotrochozoan condi- Ontogeny of the scaphopod Antalis entalis and the tion show, however, an ambiguous picture. While all cephalopod Idiosepius notoides ecdysozoans investigated so far do not appear to express In the scaphopod, A. entalis gastrulation occurs at 12 h Gsx in their digestive tract, the situation in lophotro- after fertilization (hpf) at 21–23 °C (Fig. 2a). At 14 hpf, chozoans is less clear (Table 1). The annelids Platynereis a trochophore larva develops that exhibits an episphere dumerilii and Nereis virens and the gastropod Gibbula with an apical organ and tuft (red dashed circles in varia express Gsx in their foregut [12–14], while the Fig. 2). The episphere is divided from the hyposphere annelid Capitella teleta does not [15]. Comparisons with by a prototroch (Fig. 2b; see also [36–38]). The gastro- the condition in the Cnidaria, the putative bilaterian sis- pod trochophore resembles the latter, but while the api- ter group, do not appear to contribute to inferring the cal region develops into the prospective anterior region ancestral state of Gsx expression in the Bilateria since the in gastropods, it develops into the prospective ventral different germ layers cannot be homologized convinc- region in scaphopods (see scaphopod condition in Fig. 2; ingly among the Cnidaria and the Bilateria. In addition, [12, 36–38]). The blastopore of the gastrula develops into Gsx expression patterns are not consistent among cnidar- the mouth in A. entalis and lecithotrophic early-stage ians. In the planula larvae of Nematostella vectensis, Cly- trochophore larvae already possess a through-gut with tia hemisphaerica, and Podocoryne carnea [9–11], Gsx is mouth and anus (Fig. 2a, b). The apical organ exhibits expressed in the endoderm, while it is expressed in the two serotonin-like immunoreactive cell somata (labeled ectoderm of the planula of Acropora millepora [8]. red in Fig. 2b), and the nascent shell field is located in Gsx is also involved in the development of the CNS the anterior region of the hyposphere (Fig. 2b; [39]). The in bilaterians, and it is expressed in distinct cells of the apical organ of mid-stage trochophore larvae (21 hpf) Wollesen et al. EvoDevo (2015) 6:41 Page 3 of 18 Table 1 Gsx gene expression domains in metazoan developmental