Shared ancestry of algal symbiosis and chloroplast sequestration in foraminifera

Abstract

Foraminifera are unicellular organisms that established the most diverse algal symbioses in the marine realm. Endosymbiosis repeatedly evolved in several lineages, while some engaged in the sequestration of chloroplasts, known as kleptoplasty. So far, kleptoplasty has been documented exclusively in the rotaliid clade. Here, we report the discovery of kleptoplasty in the species Hauerina diversa that belongs to the miliolid clade. The existence of kleptoplasty in the two main clades suggests that it is more widespread than previously documented. We observed chloroplasts in clustered structures within the foraminiferal cytoplasm and confirmed their functionality. Phylogenetic analysis of 18S ribosomal RNA gene sequences showed that H. diversa branches next to symbiont-bearing Alveolinidae. This finding represents evidence of of a relationship between kleptoplastic and symbiotic foraminifera.. Analysis of ribosomal genes and metagenomics revealed that alveolinid symbionts and kleptoplasts belong to the same clade, which suggests a common ancestry.