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

Abstract

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.

Species composition, diversity, and distribution of the genus Ulva along the coast of Jeju Island, Korea based on molecular phylogenetic analysis

“…reticulata (which belongs to the U . ohnoi complex) through postzygotic isolation has been documented [33]. Although we could not determine distinct species precisely for the 47 specimens in the ITS phylogeny (Table 1), U .…”

Taxonomy of <italic>Ulva</italic> causing blooms from Jeju Island, Korea with new species, <italic>U. pseudo-ohnoi</italic> sp. nov. (Ulvales, Chlorophyta)

“…U. pseudo-ohnoi was closely related to U. spinulosa and U. reticulata Forsskål (Matsumoto and Shimada 2015). By Monotilla et al (2018), U. reticulata was already not conspecific with U. spinulosa and U. ohnoi based on ITS analysis. In the U. reticulata clade of Monotilla et al (2018), the specimens of U. reticulata was collected from Philippiness (Monitilla et al 2018).…”

“…By Monotilla et al (2018), U. reticulata was already not conspecific with U. spinulosa and U. ohnoi based on ITS analysis. In the U. reticulata clade of Monotilla et al (2018), the specimens of U. reticulata was collected from Philippiness (Monitilla et al 2018). Monotilla et al (2018) did not analyze the type materials from the type locality where is Saudi Arabia and Yemen (Silva et al 1996).…”

“…In the U. reticulata clade of Monotilla et al (2018), the specimens of U. reticulata was collected from Philippiness (Monitilla et al 2018). Monotilla et al (2018) did not analyze the type materials from the type locality where is Saudi Arabia and Yemen (Silva et al 1996). Therefore, it is hard to identify these specimens as the true U. reticulata taxonomically.…”

Exhaustive reanalysis of barcode sequences from public repositories highlights ongoing misidentifications and impacts taxa diversity and distribution: a case study of the Sea Lettuce.

“…Ulva pseudorotundata and U. gigantea ITS1 sequences are only described in the present study and (Fort et al, 2020). The U. ohnoi clade contains three sequences from this study, as well as 23 U. ohnoi vouchers [described in (Hiraoka et al, 2004;Lawton, Mata, de Nys, & Paul, 2013;Monotilla et al, 2018)]. Three erroneous U. fasciata sequences were found, all from unpublished sources.…”