Evaluating summary statistics used to test for incomplete lineage sorting: mito-nuclear discordance in the reef sponge Callyspongia vaginalis.

Conflicting patterns of population differentiation between the mitochondrial and nuclear genomes (mito-nuclear discordance) have become increasingly evident as multilocus data sets have become easier to generate. Incomplete lineage sorting (ILS) of nucDNA is often implicated as the cause of such discordance, stemming from the large effective population size of nucDNA relative to mtDNA. However, selection, sex-biased dispersal and historical demography can also lead to mito-nuclear discordance. Here, we compare patterns of genetic diversity and subdivision for six nuclear protein-coding gene regions to those for mtDNA in a common Caribbean coral reef sponge, Callyspongia vaginalis, along the Florida reef tract. We also evaluated a suite of summary statistics to determine which are effective metrics for comparing empirical and simulated data when testing drivers of mito-nuclear discordance in a statistical framework. While earlier work revealed three divergent and geographically subdivided mtDNACOI haplotypes separated by 2.4% sequence divergence, nuclear alleles were admixed with respect to mitochondrial clade and geography. Bayesian analysis showed that substitution rates for the nuclear loci were up to 7 times faster than for mitochondrial COI. Coalescent simulations and neutrality tests suggested that mito-nuclear discordance in C. vaginalis is not the result of ILS in the nucDNA or selection on the mtDNA but is more likely caused by changes in population size. Sperm-mediated gene flow may also influence patterns of population subdivision in the nucDNA.