ALBERT

Description: Previous genetic studies of extant planktonic foraminifera have provided evidence that the traditional, strictly morphological definition of species in these organisms underestimates their biodiversity. Here, we report the first case where this pattern is reversed. The modern (sub)tropical species plexus Globigerinoides sacculifer is characterized by large morphological variability, which has led to the proliferation of taxonomic names attributed to morphological end-members within the plexus. In order to clarify the taxonomic status of its morphotypes and to investigate the genetic connectivity among its currently partly disjunct (sub)tropical populations, we carried out a global survey of two ribosomal RNA regions (SSU and ITS-1) in all recent morphotypes of the plexus collected throughout (sub)tropical surface waters of the global ocean. Unexpectedly, we find an extremely reduced genetic variation within the plexus and no correlation between genetic and morphological divergence, suggesting taxonomical overinterpretation. The genetic homogeneity within the morphospecies is unexpected, considering its partly disjunct range in the (sub)tropical Atlantic and Indo-Pacific and its old age (early Miocene). A sequence variant in the rapidly evolving ITS-1 region indicates the existence of an exclusively Atlantic haplotype, which suggests an episode of relatively recent (last glacial) isolation, followed by subsequent resumption of unidirectional gene flow from the Indo-Pacific into the Atlantic. This is the first example in planktonic foraminifera where the morphological variability in a morphospecies exceeds its rDNA genetic variability. Such evidence for inconsistent scaling of morphological and genetic diversity in planktonic foraminifera could complicate the interpretation of evolutionary patterns in their fossil record.

Description: Porosity in planktonic foraminifers (the proportion of the shell surface covered by pores) is a conspicuous quantitative trait, well preserved in fossil shells and implicated as a source of environmental information. Despite its potential, the functional importance of porosity remains poorly understood. It is likely that pores are important in gas exchange, and differences in shell porosity among species or within species may reflect differences in metabolic rates or ambient oxygen concentration. Theoretically, porosity also affects the weight of the shell; and differences in porosity may reflect an adaptation to the specific density of the seawater or differences in allocation of resources to calcification (shell calcification intensity). Finally, there is evidence that porosity may differ between closely related cryptic species. Here we analyzed the potential role of porosity as a regulator of calcification intensity in Orbulina universa by combining biometric measurements based on sediment surface samples from the western Atlantic with a modelling approach. Specimens of O. universa were analyzed concerning their shell size, shell thickness, and shell porosity under light and scanning electron microscopy, and weighed using a microbalance. The resulting empirically derived model shows an effect size of shell thickness that is 7.5 times larger than the effect of shell porosity on the overall shell calcification intensity. This indicates that porosity is unlikely to be used by this species to regulate calcification intensity. By implementing the model on literature data which analyzed calcification intensity in O. universa, we also show that porosity differences among cryptic species in O. universa are unlikely to explain the observed differences in calcification intensity within the species. These findings indicate that functional explanations for differences in porosity in planktonic foraminifers have to be sought outside of calcification or density regulation and, conversely, that the observed differences in calcification intensity are likely driven by shell thickness and their relationship with environmental forcing can be applied without correction for porosity.

Description: Morphological evolution in the Late Cretaceous (Maastrichtian) Contusotruncana lineage of planktonic foraminifera was studied at DSDP Sites 525 (South Atlantic) and 384 (North Atlantic). A multivariable approach was used to separate aspects of form controlled by geographical variation (size, spiral roundness of the test, percentage of kummerform specimens) from those due to changes that occurred simultaneously in geographically distant populations of the lineage (shell conicity, number of chambers in the last whorl).A gradual increase in mean shell conicity was observed over the last 3 million years of the Cretaceous. It arose from the combination of a rapid development of highly conical shells after 68.5 Ma and a long-term trend of progressive disappearance of the ancestral morphotype. Therefore, despite the gradual change in “mean form,” the morphological evolution in the Contusotruncana lineage differs from the classical image of phyletic gradualism. The gradual increase in mean shell conicity in the lineage was accompanied by a remarkable decrease in its absolute abundance (shell accumulation rate), suggesting that the changes in shell morphology might not have been neutral with respect to natural selection. Apparently, gradual change in “mean form” of fossil lineages does not require an equally gradual development of morphological novelties. It may be caused by natural selection operating on a constant range of variation in populations living in environments without geographical barriers.

Description: Previous genetic studies of extant planktonic foraminifera have provided evidence that the traditional, strictly morphological definition of species in these organisms underestimates their biodiversity. Here, we report the first case where this pattern is reversed. The modern (sub)tropical species plexus Globigerinoides sacculifer is characterized by large morphological variability, which has led to the proliferation of taxonomic names attributed to morphological end-members within the plexus. In order to clarify the taxonomic status of its morphotypes and to investigate the genetic connectivity among its currently partly disjunct (sub)tropical populations, we carried out a global survey of two ribosomal RNA regions (SSU and ITS-1) in all recent morphotypes of the plexus collected throughout (sub)tropical surface waters of the global ocean. Unexpectedly, we find an extremely reduced genetic variation within the plexus and no correlation between genetic and morphological divergence, suggesting taxonomical overinterpretation. The genetic homogeneity within the morphospecies is unexpected, considering its partly disjunct range in the (sub)tropical Atlantic and Indo-Pacific and its old age (early Miocene). A sequence variant in the rapidly evolving ITS-1 region indicates the existence of an exclusively Atlantic haplotype, which suggests an episode of relatively recent (last glacial) isolation, followed by subsequent resumption of unidirectional gene flow from the Indo-Pacific into the Atlantic. This is the first example in planktonic foraminifera where the morphological variability in a morphospecies exceeds its rDNA genetic variability. Such evidence for inconsistent scaling of morphological and genetic diversity in planktonic foraminifera could complicate the interpretation of evolutionary patterns in their fossil record.