Abstract
The coccolithophore family Noëlaerhabdaceae contains a number of taxa that are very abundant in modern oceans, including the cosmopolitan bloom-forming Emiliania huxleyi. Introgressive hybridization has been suggested to account for incongruences between nuclear, mitochondrial and plastidial phylogenies of morphospecies within this lineage, but the number of species cultured to date remains rather limited. Here, we present the characterization of 5 new Noëlaerhabdaceae culture strains isolated from samples collected in the south-east Pacific Ocean. These were analyzed morphologically using scanning electron microscopy and phylogenetically by sequencing 5 marker genes (nuclear 18S and 28S rDNA, plastidial tufA, and mitochondrial cox1 and cox3 genes). Morphologically, one of these strains corresponded to Gephyrocapsa ericsonii and the four others to Reticulofenestra parvula. Ribosomal gene sequences were near identical between these new strains, but divergent from G. oceanica, G. muellerae, and E. huxleyi. In contrast to the clear distinction in ribosomal phylogenies, sequences from other genomic compartments clustered with those of E. huxleyi strains with which they share an ecological range (i.e., warm temperate to tropical waters). These data provide strong support for the hypothesis of past (and potentially ongoing) introgressive hybridization within this ecologically important lineage and for the transfer of R. parvula to Gephyrocapsa. These results have important implications for understanding the role of hybridization in speciation in vast ocean meta-populations of phytoplankton.
Highlights
Members of the coccolithophore family Noëlaerhabdaceae have numerically dominated coccolithophore communities over the last 20 million years and continue to do so in present day oceans (Raffi et al, 2006)
The 3 noëlaerhabdaceaen genera with extant representatives, Reticulofenestra, Gephyrocapsa, and Emiliania, are distinguished according to details of coccolith morphology: the elements in the shields of Reticulofenestra coccoliths are relatively well calcified such that there are no slits between them, Gephyrocapsa coccoliths typically have the same degree of shield calcification as Reticulofenestra and possess a conjunct bridge formed from extended inner tube elements spanning the central area, while Emiliania coccoliths do not possess a bridge and have less well calcified shield-elements such that slits exist between them (Young et al, 2003; Bendif and Young, 2014)
Gephyrocapsa is thought to have evolved from Reticulofenestra, and Emiliania, which first appeared in the fossil record only 291 ka (Raffi et al, 2006) is thought to have evolved from one of the Gephyrocapsa species
Summary
Members of the coccolithophore family Noëlaerhabdaceae have numerically dominated coccolithophore communities over the last 20 million years and continue to do so in present day oceans (Raffi et al, 2006). The Noëlaerhabdaceae dominated most Neogene nannofossil assemblages with the dominant genera being successively Cyclicargolithus Bukry (NN1-6), Reticulofenestra Hay, Mohler and Wade (NN6-16), Pseudoemiliania Gartner (NN16-19), Gephyrocapsa Kamptner (NN19-20), and Emiliania Hay and Mohler (NN21) (Bown, 1998). Gephyrocapsa is thought to have evolved from Reticulofenestra (evolution of the bridge), and Emiliania, which first appeared in the fossil record only 291 ka (Raffi et al, 2006) is thought to have evolved from one of the Gephyrocapsa species (loss of bridge, reduced calcification of shield elements)
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