Foraminiferal evolution as revealed by β‐tubulin data

Abstract

Foraminiferal taxonomy is traditionally based on morphological characteristics, particularly the structure and composition of the test (shell). Recently, a comprehensive SSU‐based molecular phylogeny confirmed some multi‐chambered foraminiferal groups while enhancing our understanding of the single‐chambered, or the so‐called “allogromiid”, taxa. However, some relationships, such as the origin of the foraminiferal order Miliolida, remain unresolved, suggesting an approach involving multiple data sources may improve understanding. Here, we discuss the evolutionary implications of β‐tubulin genes from several species of foraminifera. Foraminiferal β‐tubulin is highly divergent, possibly indicating a period of accelerated evolution of this gene at the base of the foraminiferal lineage, but it is well‐conserved within the group. We show its utility in helping to resolve the phylogenetic position of Miliammina fusca, a foraminiferan whose proper classification has been debated for 70 years. M. fusca is a morphological “chimera”, and we show that its placement in SSU trees is not particularly robust. However, β‐tubulin data assigns Miliammina to the Miliolida, some of whose members show a similar chamber coiling pattern. This placement agrees with an earlier observation that M. fusca's actin isoforms more closely resemble those of traditional milolids. This finding implies that test formation can revert from calcareous to agglutinated, suggesting a reexamination of some relationships inferred from the fossil record. Supported by NSF OPP0003639.