Time-calibrated molecular phylogeny of pteropods

Abstract

Pteropods are a widespread group of holoplanktonic gastropod molluscs and are uniquely suitable for study of long-term evolutionary processes in the open ocean because they are the only living metazoan plankton with a good fossil record. Pteropods have been proposed as bioindicators to monitor the impacts of ocean acidification and in consequence have attracted considerable research interest, however, a robust evolutionary framework for the group is still lacking. Here we reconstruct their phylogenetic relationships and examine the evolutionary history of pteropods based on combined analyses of Cytochrome Oxidase I, 28S, and 18S ribosomal rRNA sequences and a molecular clock calibrated using fossils and the estimated timing of the formation of the Isthmus of Panama. Euthecosomes with uncoiled shells were monophyletic with Creseis as the earliest diverging lineage, estimated at 41-38 million years ago (mya). The coiled euthecosomes (Limacina, Heliconoides, Thielea) were not monophyletic contrary to the accepted morphology-based taxonomy; however, due to their high rate heterogeneity no firm conclusions can be drawn. We found strong support for monophyly of most euthecosome genera, but Clio appeared as a polyphyletic group, and Diacavolinia grouped within Cavolinia, making the latter genus paraphyletic. The highest evolutionary rates were observed in Heliconoides inflatus and Limacina bulimoides for both 28S and 18S partitions. Using a fossil-calibrated phylogeny that sets the first occurrence of coiled euthecosomes at 79-66 mya, we estimate that uncoiled euthecosomes evolved 51-42 mya and that most extant uncoiled genera originated 40-15 mya. These findings are congruent with a molecular clock analysis using the Isthmus of Panama formation as an independent calibration. Although not all phylogenetic relationships could be resolved based on three molecular markers, this study provides a useful resource to study pteropod diversity and provides general insight into the processes that generate and maintain their diversity in the open ocean.

Fig 1. Overview of sampling locations.

Black squares indicate sampling locations of new specimens in this study and white squares indicate all known locations of specimens used from other studies. Sampling locations from [33], [35], and [37] are not indicated because no exact localities were given (see S1 Table).

Fig 2. Maximum Likelihood phylogeny of pteropods based on the concatenated dataset of Cytochrome Oxidase I (amino acid alignment) and ribosomal genes 28S and 18S (nucleotide alignments) excluding the long-branch taxa Heliconoides inflatus and Limacina bulimoides.

Black squares represent a bootstrap support of ≥80%, with small, medium and large black squares representing support within genera, of genera, and above genus level, respectively. Supported euthecosome genera are highlighted in coloured boxes, and representative shell shapes of species are drawn on the right, with all scale bars representing 1 mm. Maximum Likelihood phylogeny including the long-branch taxa Heliconoides inflatus and Limacina bulimoides is shown in S5 Fig. Abbreviations ATL, PAC, and IND denote Atlantic, Pacific, and Indian Ocean origins, respectively, including their sectors in the Southern Ocean.

Fig 3. Fossil-calibrated phylogeny of pteropods (46 taxa, maximum one sequence per taxon per ocean) following Method 1 with crown calibrations based on the oldest known fossils of Hyalocylis, Diacria, Cavolinia, Cuvierinia, Creseis, Limacina, and euthecosomes (see Table 1).

Drawings of fossils used for calibration are shown on the left, with all scale bars representing 1 mm, and calibrations are indicated with letters A to G. Error bars (95%) are shown only for clades with posterior probabilities ≥0.95 in green for uncoiled euthecosomes, purple for coiled euthecosomes, red for pseudothecosomes, and blue for gymnosomes. Abbreviations ATL, PAC, and IND denote Atlantic, Pacific, and Indian Ocean origins, respectively, including their sectors in the Southern Ocean.