Cementing mussels to oysters in the pteriomorphian tree: a phylogenomic approach

Abstract

Mussels (Mytilida) are a group of bivalves with ancient origins and some of the most important commercial shellfish worldwide. Mytilida consists of approximately 400 species found in various littoral and deep-sea environments, and are part of the higher clade Pteriomorphia, but their exact position within the group has been unstable. The multiple adaptive radiations that occurred within Pteriomorphia have rendered phylogenetic classifications difficult and uncertainty remains regarding the relationships among most families. To address this phylogenetic uncertainty, novel transcriptomic data were generated to include all five orders of Pteriomorphia. Our results, derived from complex analyses of large datasets from 41 transcriptomes and evaluating possible pitfalls affecting phylogenetic reconstruction (matrix occupancy, heterogeneity, evolutionary rates, evolutionary models), consistently recover a well-supported phylogeny of Pteriomorphia, with the only exception of the most complete but smallest data matrix (Matrix 3: 51 genes, 90% gene occupancy). Maximum-likelihood and Bayesian mixture model analyses retrieve strong support for: (i) the monophyly of Pteriomorphia, (ii) Mytilida as a sister group to Ostreida, and (iii) Arcida as sister group to all other pteriomorphians. The basal position of Arcida is congruent with its shell microstructure (solely composed of aragonitic crystals), whereas Mytilida and Ostreida display a combination of a calcitic outer layer with an aragonitic inner layer composed of nacre tablets, the latter being secondarily lost in Ostreoidea.

Form and function of tentacles in pteriomorphian bivalves

“…Although the evolutionary origin and diversification of tentacles during Pteriomorphia radiation are still speculative, our results support the homology of MFT and IFT in Pterioidea and Ostreoidea, considering their larger size and abundance ventrally, the presence of ciliary clusters at the tip, and branched innervation pattern (Table ). Even though information for additional taxa would be necessary to support such a conclusion, our results are in accordance with phylogenetic hypotheses that recover the sister relationship between these superfamilies (Tëmkin, ; Lemer et al, ; Figure ). In contrast, profound anatomical differences, such as tentacle innervation, cilia organization, and size distribution along dorsal to ventral regions (Table ), indicate IFT and MFT are not homologous across Pteriomorphia.…”

“…Schematic representation of the Pteriomorphia phylogeny, including current orders (in bold) and superfamilies. Redrawn after Lemer, González, Bieler, and Giribet ()…”

The purplish bifurcate mussel Mytilisepta virgata gene expression atlas reveals a remarkable tissue functional specialization

“…Our phylogenomics approach, which took into account a set of 247 single-copy orthologous genes detected in all the studied species, is expected to provide a highly reliable estimate of the evolutionary relationship of M. virgata with other mussel species, as recently demonstrated by a series of phylogenomics studies which have helped to discern the phylogenetic relationship of Mollusca and Bivalvia [41, 56–58]. …”

Structure and crystallography of foliated and chalk shell microstructures of the oyster Magallana: the same materials grown under different conditions

“…Although the bulk of both valves is made up of continuous layers of sheet-like foliae with a thin external layer of prisms (which is thicker on the right than left valve), shells are often characterized by discontinuous lense like intercalations of ‘chalk’ 2,3 (termed by Gray 4 ). Whereas foliated calcite is a common (but not universal) development in pteriomorph bivalves and ubiquitous within the order Pectinida and the superfamily Ostreoidea 5 , chalk is unique amongst other molluscan microstructures and is found only in the Ostreidae. It is very distinctive, being composed of a loose array of crystals, made up of long laths (blades and leaflets) placed perpendicular to the growth surface, seemingly with a high degree of disorganization, and which is notably soft and porous (80% according to 6 ).…”