An investigation of genetic variation within northwest Atlantic Porphyra (Bangiales, Rhodophyta) with specific phylogeographic analysis of the common, rocky intertidal species, Porphyra umbilicalis
To investigate the phylogeography of the rocky intertidal red alga, Porphyra umbilicalis Kutzing, a restriction fragment polymorphism assay (RFLP) of the ribulose bisphosphate carboxylase large subunit ( rbcL) was developed to accurately distinguish P. umbilicalis from the other morphologically similar species in the North Atlantic. Initial screening of &sim;800 Porphyra specimens resulted in the additional discovery of a cryptic Porphyra taxon. The presence and variability of group-I introns of the ribosomal small subunit (SSU) were screened in North Atlantic species of Porphyra in order to assess whether they could be biogeographically informative. In an initial screening for the helix 50 intron, using flanking primers with the Polymerase Chain Reaction, the intron was detected in some, but not all, individuals within populations and across species. The amplified intron also exhibited variable sizes between and within species. Sequence analysis of the helix 50 introns revealed conserved blocks of nucleotides between introns of different species and highly variable regions that were species-specific. Additional screenings of the ribosomal small subunit (SSU) from a collection of Northwest Atlantic Porphyra were conducted for the presence of the helices 21 and 50 introns. However, instead of using two flanking primers, the second screening used an internal primer (located within either the helix 50 or helix 21 intron) and a nearby flanking primer in the SSU. Using these primers the frequency of detecting the intron in individual algal samples increased significantly (>90%). Although phylogenetic analysis of the helix 50 intron in select Northwest Atlantic Porphyra are generally similar to previously reported SSU phylogenies, some differences in topology suggest that horizontal transmission of the intron between species may have occurred. In contrast to previous studies in which the helix 50 intron was detected only in fraction of the accessions, an intraspecific survey using combined external and internal primers detected the helix 50 intron in all 28 samples of Porphyra umbilicalis collected across the geographic range of the species. A survey of P. umbilicalis also revealed that the helix 50 introns were present in two different sizes (710 bp and 1188 bp). The sequence of the larger version of the helix 50 intron encodes a His-Cys open reading frame that has been associated with mobility of group-I introns in other organisms. (Abstract shortened by UMI.).
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