Abstract
BackgroundMarine phytoplankton drift passively with currents, have high dispersal potentials and can be comprised of morphologically cryptic species. To examine molecular subdivision in the marine diatom Thalassiosira rotula, variations in rDNA sequence, genome size, and growth rate were examined among isolates collected from the Atlantic and Pacific Ocean basins. Analyses of rDNA included T. gravida because morphological studies have argued that T. rotula and T. gravida are conspecific.ResultsCulture collection isolates of T. gravida and T. rotula diverged by 7.0 ± 0.3% at the ITS1 and by 0.8 ± 0.03% at the 28S. Within T. rotula, field and culture collection isolates were subdivided into three lineages that diverged by 0.6 ± 0.3% at the ITS1 and 0% at the 28S. The predicted ITS1 secondary structure revealed no compensatory base pair changes among lineages. Differences in genome size were observed among isolates, but were not correlated with ITS1 lineages. Maximum acclimated growth rates of isolates revealed genotype by environment effects, but these were also not correlated with ITS1 lineages. In contrast, intra-individual variation in the multi-copy ITS1 revealed no evidence of recombination amongst lineages, and molecular clock estimates indicated that lineages diverged 0.68 Mya. The three lineages exhibited different geographic distributions and, with one exception, each field sample was dominated by a single lineage.ConclusionsThe degree of inter- and intra-specific divergence between T. gravida and T. rotula suggests they should continue to be treated as separate species. The phylogenetic distinction of the three closely-related T. rotula lineages was unclear. On the one hand, the lineages showed no physiological differences, no consistent genome size differences and no significant changes in the ITS1 secondary structure, suggesting there are no barriers to interbreeding among lineages. In contrast, analysis of intra-individual variation in the multicopy ITS1 as well as molecular clock estimates of divergence suggest these lineages have not interbred for significant periods of time. Given the current data, these lineages should be considered a single species. Furthermore, these T. rotula lineages may be ecologically relevant, given their differential abundance over large spatial scales.
Highlights
Marine phytoplankton drift passively with currents, have high dispersal potentials and can be comprised of morphologically cryptic species
We focused on identifying genetic subdivision in the diatom morphospecies Thalassiosira rotula by simultaneously examining variation in ribosomal DNA (rDNA) sequences, physiology, and genome size from isolates collected from around the globe
Of the 97 field isolates analyzed, 10 had 28S and ITS1 sequences that were identical (100%) to T. gravida culture collection isolates identified by taxonomists (Table 1b) and were designated as T. gravida. rDNA sequences of the remaining field isolates were identical (100%) at the 28S and 99-100% similar at the ITS1 to T. rotula culture collection isolates identified by taxonomists (Table 1a) and were designated as T. rotula
Summary
Marine phytoplankton drift passively with currents, have high dispersal potentials and can be comprised of morphologically cryptic species. Diatoms are a important class of algae These commonly-occurring organisms generate over 20% of global primary production, and play a key role in driving global biogeochemical cycles [10]. They are found in almost all aquatic habitats, are comprised of an estimated 200,000 species [11] and yet only arose in the early Mesozoic (~ 250 mya) [12]. Diatom species have long been identified based on their ornate siliceous frustule, or cell covering, and recently, based on DNA sequence variation. No clear barcoding gene, such as Cox, has been identified for classifying diatom species [21,22]
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