Evolutionary biogeography of the reef-building coral genus Galaxea across the Indo-Pacific ocean

Stony corals (Scleractinia) form the basis for some of the most diverse
ecosystems on Earth, but we have much to learn about their evolutionary
history and systematic relationships. In order to improve our
understanding of species in corals, we here investigated phylogenetic
relationships between morphologically defined species and genetic lineages
in the genus Galaxea (Euphyllidae) using a combined phylogenomic and
phylogeographic approach. Previous studies revealed the nominal species G.
fascicularis included three genetically well-differentiated lineages (L, S
& L+) in the western Pacific, but their distribution and
relationship to other species in the genus were unknown. Based on genomic
(RAD-seq) and mitochondrial sequence data (non-coding region between cytb
and ND2) we investigated whether the morphological taxa represent
genetically coherent entities and what is the phylogenetic relationship
and spatial distribution of the three lineages of G. fascicularis
throughout the observed species range. Using the RAD-seq data, we find
that the genus Galaxea is monophyletic and contains three distinct clades:
an Indo-Pacific, a Pacific, and a small clade restricted to the Chagos
Archipelago. The three lineages of G. fascicularis were associated with
different RAD-seq clades, with the ‘L’ lineage showing some morphological
distinction from the other two lineages (larger more asymmetrical polyps).
In addition to these, three more genetic lineages in G. fascicularis may
be distinguished – a Chagossian, an Ogasawaran, and one from the
Indian-Red Sea. Among nominal taxa for which we have multiple samples, G.
horrescens was the only monophyletic species. The mitochondrial non-coding
region is highly conserved apart from the length polymorphism used to
define L, S & L+ lineages and lacks the power to distinguish
morphological and genetic groups resolved with genomic RAD-sequencing. The
polyphyletic nature of most species warrants a careful examination of the
accepted taxonomy of this group with voucher collections and their
comparison to type specimens to resolve species boundaries. Further
insight to the speciation process in corals will require international
cooperation for the sharing of specimens to facilitate scientific
discovery.