Regulatory gene expression during the patterning of molluscan shells has only recently drawn the attention of scientists. We show that several Hox genes are expressed in association with the shell gland and the mantle in the marine vetigastropod Gibbula varia (L.). The expression of Gva-Hox1, Gva-Post2, and Gva-Post1 is initially detected in the trochophore larval stage in the area of the shell field during formation of embryonic shell. Later, during development, these genes are expressed in the mantle demonstrating their continuous role in larval shell formation and differentiation of mantle edge that secretes the adult shell. Gva-Hox4 is expressed only late during the development of the veliger-like larva and may also be involved in the adult shell morphogenesis. Additionally, this gene also seems to be associated with secretion of another extracellular structure, the operculum. Our data provide further support for association of Hox genes with shell formation which suggest that the molecular mechanisms underlying shell synthesis may consist of numerous conserved pattern-formation genes. In cephalopods, the only other molluscan class in which Hox gene expression has been studied, no involvement of Hox genes in shell formation has been reported. Thus, our results suggest that Hox genes are coopted to various functions in molluscs.

BackgroundTranscription factors that determine retinal development seem to be conserved in different phyla throughout the animal kingdom. In most representatives, however, only a few of the involved transcription factors have been sampled and many animal groups remain understudied. In order to fill in the gaps for the chelicerate group of arthropods, we tested the expression pattern of the candidate genes involved in the eye development in the embryo of the wandering spider Cupiennius salei. One main objective was to profile the molecular development of the eyes and to search for possible variation among eye subtype differentiation. A second aim was to form a basis for comparative studies in order to elucidate evolutionary pathways in eye development.ResultsWe screened the spider embryonic transcriptome for retina determination gene candidates and discovered that all except one of the retinal determination genes have been duplicated. Gene expression analysis shows that the two orthologs of all the genes have different expression patterns. The genes are mainly expressed in the developing optic neuropiles of the eyes (lateral furrow, mushroom body, arcuate body) in earlier stages of development (160 to 220 h after egg laying). Later in development (180 to 280 h after egg laying), there is differential expression of the genes in disparate eye vesicles; for example, Cs-otxa is expressed only in posterior-lateral eye vesicles, Cs-otxb, Cs-six1a, and Cs-six3b in all three secondary eye vesicles, Cs-pax6a only in principal eye vesicles, Cs-six1b in posterior-median, and posterior-lateral eye vesicles, and Cs-six3a in lateral and principal eye vesicles.ConclusionsPrinciple eye development shows pax6a (ey) expression, suggesting pax6 dependence, although secondary eyes develop independently of pax6 genes and show differential expression of several retinal determination genes. Comparing this with the other arthropods suggests that pax6-dependent median eye development is a ground pattern of eye development in this group and that the ocelli of insects, the median eyes of chelicerates, and nauplius eyes can be homologised. The expression pattern of the investigated genes makes it possible to distinguish between secondary eyes and principal eyes. Differences of gene expression among the different lateral eyes indicate disparate function combined with genetic drift.Electronic supplementary materialThe online version of this article (doi:10.1186/s13227-015-0010-x) contains supplementary material, which is available to authorized users.

Hox transcription factors, a subfamily of homeobox genes, are expressed in distinct, often overlapping domains along the anterior-posterior body axis of animal embryos. Here, we report the sequence and expression pattern of Hox2, Hox3, Hox4, Hox5, Lox5, Hox7, Lox4, and Lox2 in different larval stages during the encapsulated development of the marine gastropod Gibbula varia. Our results show that all Gva-Hox genes are expressed in ectoderm-derived cells. Hox2, Hox3, Hox4, Hox5, and Hox7 are expressed in overlapping patterns in the pedal, pleural, oesophageal, and visceral ganglia, supporting the ancestral role of Hox genes in the neurogenesis processes in bilaterians. Gva-Hox1, Gva-Post2, and Gva-Post1 genes are involved in shell morphogenesis and have apparently lost their role in neurogangliogenesis. Lox5, Lox4, and Lox2 are expressed in different cells of the apical organ during the earlier larval stage (trochophore) and the cerebral ganglia during later larval stages (veliger). These results support the hypothesis that apical organ neurosensory cells contribute to the formation of cerebral ganglia commissures during metamorphosis. Gva-Hox7 and Gva-Lox4 are additionally expressed in the prototroch of the trochophore and in the velar area of the veliger larvae. This contradicts with the expression of these genes in the annelids, where most of Hox genes are expressed in the posttrochal area and are involved in segmental determination. Therefore, expression of Hox genes may serve as an example of co-option and plasticity of gene function during evolution of gastropods.

BackgroundPresence of all three ParaHox genes has been described in deuterostomes and lophotrochozoans, but to date one of these three genes, Xlox has not been reported from any ecdysozoan taxa and both Xlox and Gsx are absent in nematodes. There is evidence that the ParaHox genes were ancestrally a single chromosomal cluster. Colinear expression of the ParaHox genes in anterior, middle, and posterior tissues of several species studied so far suggest that these genes may be responsible for axial patterning of the digestive tract. So far, there are no data on expression of these genes in molluscs.ResultsWe isolated the complete coding sequences of the three Gibbula varia ParaHox genes, and then tested their expression in larval and postlarval development. In Gibbula varia, the ParaHox genes participate in patterning of the digestive tract and are expressed in some cells of the neuroectoderm. The expression of these genes coincides with the gradual formation of the gut in the larva. Gva-Gsx patterns potential neural precursors of cerebral ganglia as well as of the apical sensory organ. During larval development this gene is involved in the formation of the mouth and during postlarval development it is expressed in the precursor cells involved in secretion of the radula, the odontoblasts. Gva-Xolx and Gva-Cdx are involved in gut patterning in the middle and posterior parts of digestive tract, respectively. Both genes are expressed in some ventral neuroectodermal cells; however the expression of Gva-Cdx fades in later larval stages while the expression of Gva-Xolx in these cells persists.ConclusionsIn Gibbula varia the ParaHox genes are expressed during anterior-posterior patterning of the digestive system. This colinearity is not easy to spot during early larval stages because the differentiated endothelial cells within the yolk permanently migrate to their destinations in the gut. After torsion, Gsx patterns the mouth and foregut, Xlox the midgut gland or digestive gland, and Cdx the hindgut. ParaHox genes of Gibbula are also expressed during specification of cerebral and ventral neuroectodermal cells. Our results provide additional support for the ancestral complexity of Gsx expression and its ancestral role in mouth patterning in protostomes, which was secondarily lost or simplified in some species.

The genes otd/otx, six3, pax6 and engrailed are involved in eye patterning in many animals. Here we describe the expression pattern of the homologs to otd/otx, six3, pax6 and engrailed in the developing Euperipatoides kanangrensis embryos. Special reference is given to the expression in the protocerebral/ocular region. E.kanangrensis otd is expressed in the posterior part of the protocerebral/ocular segment before, during, and after eye invagination. E.kanangrensis otd is also expressed segmentally in the developing ventral nerve cord. The E.kanangrensis six3 is located at the extreme anterior part of the protocerebral/ocular segment and not at the location of the developing eyes. Pax6 is expressed in a broad zone at the posterior part of the protocerebral/ocular segment but only week expression can be seen at early onset of eye invagination. In late stages of development, the expression in the eye is upregulated. Pax6 is also expressed in the invaginating hypocerebral organs, thus supporting earlier suggestions that the hypocerebral organs in onychophorans are glands. Pax6 transcripts are also present in the developing ventral nerve cord. The segment polarity gene engrailed is expressed at the dorsal side of the developing eye including only a subset of the cells of the invaginating eye vesicle. We show that engrailed is not expressed in the neuroectoderm of the protocerebral/ocular segment as in the other segments. In addition, we discuss other aspect of otd, six3 and pax6 expression that are relevant to our understanding of evolutionary changes in morphology and function in arthropods.