Low variation in partial cytochrome oxidase subunit I (COI) mitochondrial sequences in the coralline demosponge Astrosclera willeyana across the Indo-Pacific

Partial sequences of the mitochondrial DNA (mtDNA) gene cytochrome oxidase subunit 1 (COI) were analysed from individuals of the coralline demosponge Astrosclera willeyana sensu lato out of ten Indo-Pacific populations from the Red Sea to the central Pacific. This taxon is widely distributed in cryptic coral reef habitats of the Indo-Pacific and is regarded as a modern representative of long-extinct, formerly reef-building stromatoporoid-type sponges. The aims were to clarify phylogeographic and taxonomic relationships in this living fossil and to explore mitochondrial DNA sequence variation over a wide geographic range. Very low variability was observed across the Indo-Pacific, as only three COI haplotypes were identified, with a maximum p-distance of 0.418% and low nucleotide diversity (pi=0.00049). Very low genetic structure was revealed among populations: Haplotype 1 was found in all specimens from nine Pacific populations (N=45), separated by distances of more than 7,000 km; haplotype 2 was restricted to the Red Sea population (N=4); haplotype 3 was only found in the Tuamoto specimens (N=7). COI data presented here do not support the hypothesis of at least two sibling species belonging to genus Astrosclera in the Pacific. Considering the maximum geographic distance of more than 20,000 km between sampled populations, mtDNA COI sequence variation observed here is among the lowest reported to date for a diploblastic taxon and adds to the growing evidence of a general mtDNA conservation in sponges. It is argued that this low mtDNA variation in A. willeyana s.l. is due to a low rate of mtDNA evolution caused by a combination of long generation time and low metabolic rate.