Brachiopod and mollusc biomineralisation is a conserved process that was lost in the phoronid-bryozoan stem lineage

Abstract

Background: Brachiopods and molluscs are lophotrochozoans with hard external shells which are often believed to have evolved convergently. While palaeontological data indicate that both groups are descended from biomineralising Cambrian ancestors, the closest relatives of brachiopods, phoronids and bryozoans, are mineralised to a much lower extent and are comparatively poorly represented in the Palaeozoic fossil record. Although brachiopod and mollusc shells are structurally analogous, genomic and proteomic evidence indicates that their formation involves a complement of conserved, orthologous genes. Here, we study a set of genes comprised of 3 homeodomain transcription factors, one signalling molecule and 6 structural proteins which are implicated in mollusc and brachiopod shell formation, search for their orthologs in transcriptomes or genomes of brachiopods, phoronids and bryozoans, and present expression patterns of 8 of the genes in postmetamorphic juveniles of the rhynchonelliform brachiopod T. transversa.

Results: Transcriptome and genome searches for the 10 target genes in the brachiopods Terebratalia transversa, Lingula anatina, Novocrania anomala, the bryozoans Bugula neritina and Membranipora membranacea, and the phoronids Phoronis australis and Phoronopsis harmeri resulted in the recovery of orthologs of the majority of the genes in all taxa. While the full complement of genes was present in all brachiopods with a single exception in L. anatina, a bloc of four genes could consistently not be retrieved from bryozoans and phoronids. The genes engrailed, distal-less, ferritin, perlucin, sp1 and sp2 were shown to be expressed in the biomineralising mantle margin of T. transversa juveniles.

Conclusions: The gene expression patterns we recovered indicate that while mineralised shells in brachiopods and molluscs are structurally analogous, their formation builds on a homologous process that involves a conserved complement of orthologous genes. Losses of some of the genes related to biomineralisation in bryozoans and phoronids indicate that loss of the capacity to form mineralised structures occurred already in the phoronid-bryozoan stem group and supports the idea that mineralised skeletons evolved secondarily in some of the bryozoan subclades.

Fig. 1

A Cladogram depicting lophotrochozoan interrelationships [23, 43, 44] (in green) with Rouphozoa [45] as outgroup (in black) with an overview of mineralisation capacities. Taxa marked with an obelisk are Cambrian stem groups of their respective sister clades. B Detailed morphology of the mantle margin which excretes the shell of the brachial valve. Lobate cells (pink) secrete the organic portion of the periostracum, while vesicular cells (green) produce the primary layer of the inorganic shell and the outer epithelial cells of the mantle (purple) secrete the secondary layer of the inorganic shell. EC outer mantle epithelial cell, LC lobate cell, PO periostracum, PS periostracal slot, SF fibre of the secondary shell layer, VC vesicular cell. Drawing after Stricker and Reed 1985 [40]

Fig. 2

Cladogram depicting lophophorate relationships and presence–absence matrix of genes for each taxon. Filled dots indicate that an ortholog was found in the available genome/transcriptome, hollow dots indicate the opposite. Yellow dots indicate that multiple orthologs were identified

Fig. 3

A Morphological features of a T. transversa juvenile at 2 dpm, interpreted after Stricker & Reed [40] and Gąsiorowski & Hejnol [46]. B–O Expression patterns of targeted genes. Gene names are indicated in the white bars above the images. For each gene, the left picture depicts a juvenile at 2 dpm, and the right picture a juvenile at 6 dpm. Data could not be retrieved for en and ferritin at 2 dpm. The insert in panel L provides a magnified view of expression within the outer mantle epithelium cells. Black arrowheads indicate expression in the mantle margin, yellow arrowheads in the mantle epithelium, red arrowheads expression in the lophophore, white arrowheads expression in the hinges and blue arrowheads expression in the pedicle opening. Scale bars represent 50 μm (insert in panel L 20 μm). CS chaetal sac, LO lophophore rudiment, MC mantle cavity, MM mantle margin, PE pedicle, PR protegulum, HI future hinge

Fig. 4

Schematic interpretation of expression patterns of genes implicated in mollusc biomineralisation in 2- and 6-dpm juveniles of Terebratalia transversa

Fig. 5

Evolution of biomineralisation in lophophorates. Black parts of the tree indicate biomineralising lineages, while light blue indicates soft-bodied taxa. The ancestral lophophorate has been assumed to be biomineralising due to the similarity between brachiopod and mollusc biomineralisation-related genes. Bryozoan topology after Schwaha et al. [33], Xia et al. [95] and Zhang et al. [94]