First evaluation of mitochondrial DNA as a marker for phylogeographic studies of Calcarea: a case study from Leucetta chagosensis

In most animals mitochondrial DNA (mtDNA) evolves much faster than nuclear DNA. Therefore, and because of its shorter coalescent time, mitochondrial (mt) markers provide better resolution to trace more recent evolutionary events compared to nuclear DNA. But in contrast to most other Metazoa, previous studies suggested that in sponges mitochondrial sequence evolution is much slower, making mtDNA less suitable for studies at the intraspecific level. However, these observations were made in the class Demospongiae and so far no data exist for calcareous sponges (Class Calcarea). We here provide the first study that evaluates intraspecific mt sequence variation in Calcarea. We focus on arguably the best-studied species Leucetta chagosensis, for which three nuclear DNA marker datasets existed previously. We here sequenced the partial mitochondrial cytochrome oxidase subunit III gene (cox3). Our analyses reveal an unexpected variability of up to 8.5% in this mitochondrial marker. In contrast to other sponges where this marker evolves considerable slower than the nuclear internal transcribed spacer region (ITS), we found that cox3 in L. chagosensis evolves about five times as fast as ITS. The variability is similar to that of nuclear intron data of the species. The phylogeny inferred with cox3 is congruent with other markers, but separates earlier reported genetic groups much more distinctively than nuclear DNA. This provides further evidence for cryptic speciation in L. chagosensis. All these features make calcarean mtDNA exceptional among sponges and show its suitability for phylogeographic studies and potential as a species-specific (DNA barcoding) marker to distinguish morphologically identical cryptic species.