Unexpected co-linearity of Hox gene expression in an aculiferan mollusk

Abstract

BackgroundMollusca is an extremely diverse animal phylum that includes the aculiferans (worm-like aplacophorans and eight-shelled polyplacophorans) and their sister group, the conchiferans, comprising monoplacophorans, bivalves (clams, mussels), gastropods (snails, slugs), scaphopods (tusk shells) and cephalopods (squids, octopuses). Studies on mollusks have revealed an overall number of 11 Hox genes in seven out of eight molluscan “class”-level taxa, but expression data of key developmental regulators such as homeotic genes are only available for three gastropod and two cephalopod species. These show that Hox genes are involved in the formation of specific features including shell, foot, funnel or tentacles and not in antero-posterior body plan patterning as in most other bilaterian animals. The role of Hox genes in non-conchiferan (i.e., aculiferan) mollusks remains entirely unknown.ResultsHere we present the first data on the expression of seven Hox genes in apolyplacophoran mollusk, Acanthochitona crinita. In A. crinita the Hox genes Acr-Hox1-5, Hox7 and Post2 are expressed in a co-linear pattern along the antero-posterior axis, but not in molluscan-specific features such as the shell or the foot. The expression pattern is restricted to the post-trochal region and the transcripts are present in ecto-, endo- and mesodermal cell layers. Contrary to the situation in gastropods and cephalopods, we did neither find Hox gene expression in distinct neural subsets of A. crinita, nor in its developing shell plates.ConclusionsOur analysis and comparison with other lophotrochozoans indicate that the basal role of Hox genes is in antero-posterior axis patterning in mollusks, similar to the vast majority of bilaterian animals, and that this role has been conserved in polyplacophorans, while co-option into patterning of evolutionary novelties emerged either at the base of Conchifera or independently in gastropods and cephalopods. These morphological innovations most likely contributed to the evolutionary success of its representatives, as exemplified by, e.g., the wide ecological range and species richness of gastropods.

Clustered brachiopod Hox genes are not expressed collinearly and are associated with lophotrochozoan novelties

“…In the other investigated spiralians, there is only either genomic information (e.g., the mollusks L. gigantea and C. gigas) or expression analysis (e.g., the mollusks Gibbula varia and Haliotis asinina) (16,44,49,54). Most of these previous gene expression studies have demonstrated coordinated spatial or temporal expression of Hox genes along the animal's anteroposterior axis (45,46,61) or in organ systems, such the nervous system (49,54). However, the absence of studies revealing a correlation between Hox gene expression and their genomic organization in these animals hinders reconstruction of the putative mechanisms that preserve Hox clusters in Lophotrochozoa, and thus preclude generalizations about possible scenarios of Hox cluster evolution across all animals.…”

“…Although Hox gene expression is known for a handful of spiralian species (26,41,43,(45)(46)(47)(48)(49)(50)(51)(52)(53)(54), the relationship between genomic organization and expression domains is known for only three of these species, namely the annelids C. teleta and H. robusta and the planarian S. mediterranea. Consistent with their disintegrated Hox clusters, H. robusta and S. mediterranea show no temporal collinearity and only remnants of spatial collinearity (41,51,52).…”

Comparative transcriptomics enlarges the toolkit of known developmental genes in mollusks

“…In the light of these results, the characterisation of the Hox and ParaHox gene sets described herein, which includes four aculiferan species, provides an important prerequisite for gene expression studies, and thus research into assessing the putative functional plasticity of these genes across Mollusca. As a matter of fact, expression patterns of ten Hox (all representatives except Post-1 ) and one ParaHox gene ( Cdx ), based on the transcriptome of the polyplacophoran Acanthochitona crinita analysed herein, have recently become available from our group [41, 44]. These studies show that the Hox genes in polyplacophorans are expressed in a conserved anterior-posterior pattern along the primary (i.e., longitudinal) body axis.…”

“…However, given the high morphological disparity, the complex life cycles, and the striking variation during the ontogeny among molluscan taxa, there is a considerable lack of molecular studies dealing with the expression of key developmental genes in this phylum. As such, only a few gene expression studies have been published, including Hox genes [35–41] and ParaHox genes [42–44]. These studies suggest a high plasticity and recruitment into novel functions of these genes at least in cephalopods and gastropods.…”

Cell Proliferation Pattern and Twist Expression in an Aplacophoran Mollusk Argue Against Segmented Ancestry of Mollusca

“…Confocal scans were conducted using a Leica TCS SP5 II confocal laser scanning microscope equipped with LAS AF, Version 2.6.0–2.6.3. A 405 nm laser was used for autofluorescence signal of the larvae in fluorescence mode and a 633 nm laser was used to scan the gene expression signal using the reflection mode (compare Jékely and Arendt, 2007; Fritsch et al., 2015). The obtained image data were further analyzed and processed with LAS AF Lite, Version 3.3.0.…”