Worldwide genotyping in the planktonic foraminifer Globoconella inflata: implications for life history and paleoceanography.

Raphaël Morard,Frédéric Quillévéré,Gilles Escarguel,Colomban De Vargas,Christophe J Douady,Thibault De Garidel-Thoron

doi:10.1371/journal.pone.0026665

Abstract

The planktonic foraminiferal morpho-species Globoconella inflata is widely used as a stratigraphic and paleoceanographic index. While G. inflata was until now regarded as a single species, we show that it rather constitutes a complex of two pseudo-cryptic species. Our study is based on SSU and ITS rDNA sequence analyses and genotyping of 497 individuals collected at 49 oceanic stations covering the worldwide range of the morpho-species. Phylogenetic analyses unveil the presence of two divergent genotypes. Type I inhabits transitional and subtropical waters of both hemispheres, while Type II is restricted to the Antarctic subpolar waters. The two genetic species exhibit a strictly allopatric distribution on each side of the Antarctic Subpolar Front. On the other hand, sediment data show that G. inflata was restricted to transitional and subtropical environments since the early Pliocene, and expanded its geographic range to southern subpolar waters ∼700 kyrs ago, during marine isotopic stage 17. This datum may correspond to a peripatric speciation event that led to the partition of an ancestral genotype into two distinct evolutionary units. Biometric measurements performed on individual G. inflata from plankton tows north and south of the Antarctic Subpolar Front indicate that Types I and II display slight but significant differences in shell morphology. These morphological differences may allow recognition of the G. inflata pseudo-cryptic species back into the fossil record, which in turn may contribute to monitor past movements of the Antarctic Subpolar Front during the middle and late Pleistocene.

Highlights

Planktonic foraminifera are pelagic protists whose calcareous shells have built up one of the most complete and continuous fossil archive of biodiversity changes over the last 180 Myrs
Together with molecular clock analyses calibrated on the fossil record [9]– [11], these results strongly support the hypothesis that distinct genotypes within the classical morpho-species correspond to cryptic, or rather pseudo-cryptic biological species, i. e., sibling species that can be differentiated based on subtle morphological features [15]
Genetic Variation in Globoconella inflata Small SubUnit (SSU) ribosomal DNA (rDNA) sequences of 21 Globoconella inflata individuals randomly selected from contrasted water masses (Table 1, Figure 1) reveal small but robust molecular differences between specimens collected north and south of the Antarctic Subpolar Front (Table 2, Figure S1)

Summary

Introduction

Planktonic foraminifera are pelagic protists whose calcareous shells have built up one of the most complete and continuous fossil archive of biodiversity changes over the last 180 Myrs. Paleoceanographers derive reconstructions of past climates based on empirical relationships between extant environmental parameters of the surface oceans (e.g., temperature, primary production) and the abundance or chemical composition of shells of individual morpho-species from surface sediment samples [3], [4]. These correlations lie on the working assumption that each species has its own, stable habitat preferences that are transferable back into the past to reconstruct changes of water masses physical properties. We investigate the rDNA genetic diversity and biogeography within the morpho-species Globoconella inflata d’Orbigny 1839

Methods

Results

Conclusion