4:15 PM - 4:30 PM
[BPT04-04] Morphological differences and ontogenetic modification in genetic types of planktic foraminifer Pulleniatina obliquiloculata
Keywords:Morphometrics, planktic foraminifers, genetic type, cryptic species, test size, chamber formation
Planktic foraminifers are one of the most diverged plankton in the pelagic oceans. Their fossilized calcite tests have been widely used to examine the evolution throughout the geological history and to reconstruct the paleo-environmental changes in geosciences. In the last two decades, molecular phylogeographic studies discovered high number of biological species (=genetic types) in morphospecies over the world oceans. Many genetic types show the specific geographic distribution implying that they have ecological differences from one another. Although these genetic types potentially possess ecological traits that could be novel environmental indicators in high resolution, their genetic identification is not applicable to fossil specimens, which lack DNA information. We now need to reassess morphological characters of foraminiferal tests at biological species level. However, many planktic foraminiferal tests have complicate structures, in particular in chamber coiling (i.e., trochospiral coiling). The latest chamber covers over previous chambers along with growth. Such complicate chamber formation requires us to develop new analytical methods on planktic foraminifers. Here, we challenged to identify morphological differences between genetic types based on both 2 dimensions (2D) and 3 dimensions (3D) morphometric analyses.
The planktonic foraminifer Pulleniatina obliquiloculata is widely distributed in the tropical oceans, and the phylogeography of three genetic types (Types I, IIa, and IIb) is well established. The accurate divergence time estimation demonstrated that these genetic types diverged according with the development of the tropical oceans in the Pliocene-Pleistocene (Ujiié and Ishitani, 2016). These genetic types could be key indicators for understanding the evolutionary process with the development of the tropical ocean environment. In the present study, we used 102 specimens of P. obliquiloculata collected from the central Pacific. They were genetically identified as Type I (34 specimens) and Type II (68 specimens). We measured 11 parameters at the aperture side of each specimen by using digital microscope, because the shape of aperture is distinctive between juvenile (semicircle-shape) and adult (crescent-shape). We also analyzed the 3D tomography reconstructed via micro-focus X-ray computed tomography scanning. In the 2D morphometric analyses, the ratio of long to short axes of test showed different shapes: oval and round shapes, in congruent with juvenile and adult stages. The 3D tomographic analyses indicated that the adult specimens added only 2 or 3 chambers to the juvenile forms. Thus, this species rapidly increases the size and changes the shape by covering of the last globular chamber in the latest growth stage. Between genetic types (Types I and II), the test size obtained in the 2D analysis is significantly different: Type II is bigger than Type I. As the size of their juveniles is almost same, Type II grows bigger than Type I to be matured. Comparing to the accumulation rate of these last few chambers, the increase rate of the whole test size is small. This is congruent with the differences of the curvature, which was calculated based on the orbits between the barycenter of the chambers next to each other, between genetic types. The curvature of Type II showed smaller angle rather than that of Type I, indicating tight coiling. In addition, even in the juvenile specimens, the opening angle of the aperture is significantly different between genetic types: Type II is larger than Type I. Thus, our morphometric analyses succeeded to show the morphological differences between two genetic types. Significant modification of the test shape along with growth suggests that the growth rate is different between genetic types probably due to ecological control such as nutrient uptake.
Reference
Ujiié Y, Ishitani Y. 2016. Evolution of a Planktonic Foraminifer during Environmental Changes in the Tropical Oceans. PLoS ONE 11(2): e0148847. doi:10.1371/journal.pone.0148847
The planktonic foraminifer Pulleniatina obliquiloculata is widely distributed in the tropical oceans, and the phylogeography of three genetic types (Types I, IIa, and IIb) is well established. The accurate divergence time estimation demonstrated that these genetic types diverged according with the development of the tropical oceans in the Pliocene-Pleistocene (Ujiié and Ishitani, 2016). These genetic types could be key indicators for understanding the evolutionary process with the development of the tropical ocean environment. In the present study, we used 102 specimens of P. obliquiloculata collected from the central Pacific. They were genetically identified as Type I (34 specimens) and Type II (68 specimens). We measured 11 parameters at the aperture side of each specimen by using digital microscope, because the shape of aperture is distinctive between juvenile (semicircle-shape) and adult (crescent-shape). We also analyzed the 3D tomography reconstructed via micro-focus X-ray computed tomography scanning. In the 2D morphometric analyses, the ratio of long to short axes of test showed different shapes: oval and round shapes, in congruent with juvenile and adult stages. The 3D tomographic analyses indicated that the adult specimens added only 2 or 3 chambers to the juvenile forms. Thus, this species rapidly increases the size and changes the shape by covering of the last globular chamber in the latest growth stage. Between genetic types (Types I and II), the test size obtained in the 2D analysis is significantly different: Type II is bigger than Type I. As the size of their juveniles is almost same, Type II grows bigger than Type I to be matured. Comparing to the accumulation rate of these last few chambers, the increase rate of the whole test size is small. This is congruent with the differences of the curvature, which was calculated based on the orbits between the barycenter of the chambers next to each other, between genetic types. The curvature of Type II showed smaller angle rather than that of Type I, indicating tight coiling. In addition, even in the juvenile specimens, the opening angle of the aperture is significantly different between genetic types: Type II is larger than Type I. Thus, our morphometric analyses succeeded to show the morphological differences between two genetic types. Significant modification of the test shape along with growth suggests that the growth rate is different between genetic types probably due to ecological control such as nutrient uptake.
Reference
Ujiié Y, Ishitani Y. 2016. Evolution of a Planktonic Foraminifer during Environmental Changes in the Tropical Oceans. PLoS ONE 11(2): e0148847. doi:10.1371/journal.pone.0148847