Examination of prezygotic and postzygotic isolating barriers in tropical Ulva (Ulvophyceae, Chlorophyta): evidence for ongoing speciation

Phylogenetic clades based on DNA sequences such as the chloroplast rbcL gene and the nuclear ITS region are frequently used to delimit algal species. However, these molecular markers cannot accurately delimit boundaries among some Ulva species. Although Ulva reticulata and Ulva ohnoi occasionally bloom in tropical to warm-temperate regions and are clearly distinguishable by their reticulate or plain blade morphology, they have few or no sequence divergences in these molecular markers and form a monophyletic clade. In this study, to clarify the speciation and species delimitation in the U.reticulata-ohnoi complex clade, reproductive relationships among several sexual strains from the Philippines and Japan including offspring that originated from the type specimen of U.ohnoi were examined by culturing and hybridization in addition to the ITS-based analysis. As a result, both prezygotic and postzygotic reproductive isolation were revealed to occur between genetically perforated U.reticulata and imperforate U.ohnoi. They were also separated on the basis of sequence analysis of the ITS region. That strongly supports that the two taxa are independent biological species. Although no prezygotic barrier among the Philippine and Japanese strains of U.reticulata was observed, unexpectedly zoospores produced by hybrid sporophytes in some of their combinations mostly failed to develop, indicating partial formation of a postzygotic barrier despite a 0.2% divergence in the ITS sequence. These findings suggest speciation is still ongoing in U.reticulata.