Enzymes of the α-carbonic anhydrase gene family (CAs) are essential for the deposition of calcium carbonate biominerals. In calcareous sponges (phylum Porifera, class Calcarea), specific CAs are involved in the formation of calcite spicules, a unique trait and synapomorphy of this class. However, detailed studies on the CA repertoire of calcareous sponges exist for only two species of one of the two Calcarea subclasses, the Calcaronea. The CA repertoire of the second subclass, the Calcinea, has not been investigated so far, leaving a considerable gap in our knowledge about this gene family in Calcarea. Here, using transcriptomic analysis, phylogenetics, and in situ hybridization, we study the CA repertoire of four additional species of calcareous sponges, including three from the previously unsampled subclass Calcinea. Our data indicate that the last common ancestor of Calcarea had four ancestral CAs with defined subcellular localizations and functions (mitochondrial/cytosolic, membrane-bound, and secreted non-catalytic). The evolution of membrane-bound and secreted CAs involved gene duplications and losses, whereas mitochondrial/cytosolic and non-catalytic CAs are evidently orthologous genes. Mitochondrial/cytosolic CAs are biomineralization-specific genes recruited for biomineralization in the last common ancestor of calcareous sponges. The spatial-temporal expression of these CAs differs between species, which may reflect differences between subclasses or be related to the secondary thickening of spicules during biomineralization that does not occur in all species. With this study, we extend the understanding of the role and the evolution of a key biomineralization gene in calcareous sponges.
The diversity and phylogenetic relationships of calcareous sponges are still not completely understood. Recent integrative approaches combined analyses of DNA and morphological observations. Such studies resulted in severe taxonomic revisions within the subclass Calcinea and provided the foundation for a phylogenetically meaningful classification. However, several genera are missing from DNA phylogenies and their relationship to other Calcinea remain uncertain. One of these genera is Leuclathrina (family Leucaltidae). We here describe a new species from the Maldives, Leuclathrina translucida sp. nov., which is only the second species of the genus. Like the type species Leuclathrina asconoides, the new species has a leuconoid aquiferous system and lacks a specialized choanoskeleton. Phylogenetic analyses of the partial 28S ribosomal RNA gene revealed that L. translucida sp. nov. is most closely related to a clade containing the exclusively asconoid genera Ascandra, Levinella and Soleneiscus, and to a clade of the likewise asconoid genus Ernstia. No close relationship exists to other members of the polyphyletic family Leucaltidae, or to any other leuconoid Calcinea. Our results suggest that the leuconoid aquiferous system of Leuclathrina evolved independently from that of other calcineans and that the family assignment of the genus has to be reconsidered. Because the latter requires a more comprehensive family level revision among many genera of Calcinea, we propose to formally retain the genus in Leucaltidae for the time being.