Phylogeographic assessment of panmictic Monostroma species from Kuroshio Coast, Japan, reveals sympatric speciation

Abstract

The marine algae genus Monostroma (Thuret) consists of some of the common intertidal green seaweeds in the world. However, reports on its interbreeding potential between geographical radiations or its phylogeography are nonexistent. Here, we show that warmwater Monostroma in SW Japan, previously thought to be composed of two morphological species, viz., Monostroma latissimum and Monostroma nitidum, is indeed a single biological species, as revealed by mating tests. Using morphometry and DNA barcoding by primary and secondary structure analyses of highly variable first internal transcribed spacer (nrDNA ITS1) and more evolutionarily conserved gene encoding small subunit ribosome (nrDNA 18S), we conclude that isolates of Monostroma latissimum-nitidum complex represent an emergence of sympatric speciation. Our molecular phylogenetic analyses suggest that this alga originated in Ise Bay and was deliberately introduced elsewhere in Japan for cultivation purposes. Our phylogenetic analyses also suggest that the character state of the ability to interbreed is more evolutionarily conserved than either ontological or morphological differences in this alga. This is the first report of such an extensive intraspecific sequence divergence in any of the panmictic eukaryotes.