Phylogenomics, Origin, and Diversification of Anthozoans (Phylum Cnidaria)

Abstract

Anthozoan cnidarians (corals and sea anemones) include some of the world's most important foundation species, capable of building massive reef complexes that support entire ecosystems. Although previous molecular phylogenetic analyses have revealed widespread homoplasy of the morphological characters traditionally used to define orders and families of anthozoans, analyses using mitochondrial genes or rDNA have failed to resolve many key nodes in the phylogeny. With a fully resolved, time-calibrated phylogeny for 234 species constructed from hundreds of ultraconserved elements and exon loci, we explore the evolutionary origins of the major clades of Anthozoa and some of their salient morphological features. The phylogeny supports reciprocally monophyletic Hexacorallia and Octocorallia, with Ceriantharia as the earliest diverging hexacorals; two reciprocally monophyletic clades of Octocorallia; and monophyly of all hexacoral orders with the exception of the enigmatic sea anemone Relicanthus daphneae. Divergence dating analyses place Anthozoa in the Cryogenian to Tonian periods (648–894 Ma), older than has been suggested by previous studies. Ancestral state reconstructions indicate that the ancestral anthozoan was a solitary polyp that had bilateral symmetry and lacked a skeleton. Colonial growth forms and the ability to precipitate calcium carbonate evolved in the Ediacaran (578 Ma) and Cambrian (503 Ma) respectively; these hallmarks of reef-building species have subsequently arisen multiple times independently in different orders. Anthozoans formed associations with photosymbionts by the Devonian (383 Ma), and photosymbioses have been gained and lost repeatedly in all orders. Together, these results have profound implications for interpretation of the Precambrian environment and the early evolution of metazoans.

Knockout of a singleSoxgene resurrects an ancestral cell type in the sea anemoneNematostella vectensis

“…Importantly, the mutant spirocytes also retain the ability to discharge, confirming this mutation is functionally relevant and is, therefore, a bona fide homeotic transformation ( 7 ). This is the first evidence of homeotic control of cell fate in a cnidarian, suggesting that cellular homeosis is an ancient mechanism that contributed to the expansion of biodiversity that emanated from the last common ancestor of cnidarians and bilaterians, over 800MYA ( 35 ). Specifically, our ability to resurrect an ancestral cell type by manipulating a single gene demonstrates how homeosis links developmental flexibility with environmental selection pressures to drive the emergence of novel cell types.…”

Early development and coloniality in Oligophylloides from the Devonian of Morocco—Are Heterocorallia Palaeozoic octocorals?

“…Fernández-Martínez et al 2019 [ 60 ] described unequivocal octocoral sclerites associated with a tabulate coral. Octocorals as a group are monophyletic [ 39 , 61 – 63 ] and their higher-level molecular phylogeny is relatively well known. Some smaller taxonomic groups may, however, be poly- or paraphyletic, as is in the case of Keratoisidinae [ 64 ].…”

Corallicolids: The elusive coral-infecting apicomplexans

“…The first molecular data from corallicolid plastid rRNA suggested that they were deep-branching sisters to apicomplexans [ 14 ]. This was an eye-catching possibility, since corals are also an ancient group of animals [ 23 ], and the closest known photosynthetic relatives of apicomplexans ( Chromera and Vitrella ) are also associated with corals [ 24 , 25 ] ( Fig 2 ). Soon after the discovery of plastids in Plasmodium and Toxoplasma , it was proposed that apicomplexan parasitism might have arisen through an intermediate stage of photosymbiosis [ 26 ].…”