Abstract
BackgroundHomoscleromorpha is the fourth major sponge lineage, recently recognized to be distinct from the Demospongiae. It contains <100 described species of exclusively marine sponges that have been traditionally subdivided into 7 genera based on morphological characters. Because some of the morphological features of the homoscleromorphs are shared with eumetazoans and are absent in other sponges, the phylogenetic position of the group has been investigated in several recent studies. However, the phylogenetic relationships within the group remain unexplored by modern methods.Methodology/Principal FindingsHere we describe the first molecular phylogeny of Homoscleromorpha based on nuclear (18S and 28S rDNA) and complete mitochondrial DNA sequence data that focuses on inter-generic relationships. Our results revealed two robust clades within this group, one containing the spiculate species (genera Plakina, Plakortis, Plakinastrella and Corticium) and the other containing aspiculate species (genera Oscarella and Pseudocorticium), thus rejecting a close relationship between Pseudocorticium and Corticium. Among the spiculate species, we found affinities between the Plakortis and Plakinastrella genera, and between the Plakina and Corticium. The validity of these clades is furthermore supported by specific morphological characters, notably the type of spicules. Furthermore, the monophyly of the Corticium genus is supported while the monophyly of Plakina is not.Conclusions/SignificanceAs the result of our study we propose to restore the pre-1995 subdivision of Homoscleromorpha into two families: Plakinidae Schulze, 1880 for spiculate species and Oscarellidae Lendenfeld, 1887 for aspiculate species that had been rejected after the description of the genus Pseudocorticium. We also note that the two families of homoscleromorphs exhibit evolutionary stable, but have drastically distinct mitochondrial genome organizations that differ in gene content and gene order.
Highlights
Sponges are exclusively aquatic and predominantly filter-feeding animals that play an important role in benthic ecosystems
We chose to present the topologies obtained by maximum likelihood (ML) method for each marker, indicating for each node the support found by the different methods (Figs. 2 and 3)
Inside clade A, the monophyly of the genus Oscarella depends on the fluctuating positions of O. microlobata and Pseudocorticium jarrei: Oscarella is monophyletic according to the 28S topologies vs paraphyletic according to the 18S topologies
Summary
Sponges (phylum Porifera) are exclusively aquatic and predominantly filter-feeding animals that play an important role in benthic ecosystems. Homoscleromorphs can be predominant and they seem to be strong competitors for space, overgrowing massive sponges, sea fans and erect bryozoans [13,14,15]. Their fossil record dates back at least to the Early Carboniferous [16], and is documented in the Early and Upper Jurassic [17]. Homoscleromorpha is the fourth major sponge lineage, recently recognized to be distinct from the Demospongiae It contains ,100 described species of exclusively marine sponges that have been traditionally subdivided into 7 genera based on morphological characters. The phylogenetic relationships within the group remain unexplored by modern methods
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