Intraspecific heterogeneity, multiple allelism and morphological divergence between morphotypes of Thalassiosira allenii (Bacillariophyta) from the Sea of Japan

ABSTRACT Thalassiosira allenii is reported for the first time from marine phytoplankton of the Russian far East. Five strains representing three morphological variants of T. allenii, from three locations in the Sea of Japan, were investigated. The D8–D10 domain of LSU rDNA and rbcL of T. allenii were sequenced for the first time. Three morphological forms of T. allenii showed completely identical rbcL sequences and distinctly different nuclear rDNA sequences. The genetic differences between morphotypes were determined by the number of allelic variants of LSU rDNA, one of which was common to all studied strains. Two different types of the D1–D3 domain and three different types of the D8–D10 divergent domain were detected in the cultured strains of T. allenii. The three morphotypes are described in detail and compared. One allele of D1–D3 and one allele of D8–D10 were identified in the strains of T. allenii from two stations in Ussuri Bay. Two and three alleles of the D8–D10 domain were identified in the strains with an ambiguous morphotype and in T. allenii var. striata, respectively, from one station in Amur Bay. Subdivision of T. allenii into morphotypes based on morphology matched that based on molecular-genetic analyses using D8–D10 LSU rDNA. Caution is recommended in using LSU rDNA sequences and rbcL divergence as the principal criterion for delimiting and metabarcoding T. allenii varieties/morphotypes. The lack of genetic differentiation in rbcL, and the presence of clear intra-specific genetic differentiation among morphotypes in two regions of LSU rDNA suggest a link between morphological differences and the nuclear genome. The D8–D10 domain of LSU rDNA appears more suitable for discriminating morphotypes within T. allenii. The rbcL is an effective marker for recognizing T. allenii at species level, but not at infraspecific level.