Pseudo-cryptic speciation in coccolithophores

Abstract

Coccolithophores are a group of calcifying unicellular algae that constitute a major fraction of oceanic primary productivity, play an important role in the global carbon cycle, and are key biostratigraphic marker fossils. Their taxonomy is primarily based on the morphology of the minute calcite plates, or coccoliths, covering the cell. These are diverse and include widespread fine scale variation, of which the biological/taxonomic significance is unknown. Do they represent phenotypic plasticity, genetic polymorphisms, or species-specific characters? Our research on five commonly occurring coccolithophores supports the hypothesis that such variation represents pseudocryptic speciation events, occurring between 0.3 and 12.9 million years ago from a molecular clock estimation. This finding suggests strong stabilizing selection acting on coccolithophorid phenotypes. Our results also provide strong support for the use of fine scale morphological characters of coccoliths in the fossil record to improve biostratigraphic resolution and paleoceanographic data retrieval.

Fig. 2.

Linearized tufA tree from which age estimations in Ma were obtained. The node used to calibrate this tree (i.e., relating genetic branch length to geological age) is 23 Ma old (25, 26). Nodes older than 25 Ma should not be dated from this tree, because from approximately that age unrecoverable multiple nucleotide substitutions were observed (data not shown).

Fig. 1.

(A) Mean coccolith diameter in C. leptoporus (from refs. and , and our culture collection) with the strains that were genetically characterized [note that strains of Large (L) and Ambiguous Large (L^*) fall into the Intermediate (I) morphotype size range]. The image is of C. leptoporus L^* with a single coccolith indicated. (B) Most parsimonious tufA tree with branch lengths (circles) and coccolith morphologies. Notice the ring-like structure in the central area of morphotypes L and L^*, which is absent from type I.