Jakobida

Abstract

Jakobida is a small group (<20 described species) that is related to Heterolobosea and Euglenozoa. Jakobids are free-living heterotrophs with two flagella. They primarily eat prokaryotes that are captured by suspension feeding, using a current produced by the posterior flagellum (which has a dorsal vane), and an "excavate"-type feeding groove. Most are marine or freshwater aerobes, although the Stygiellidae (Stygiella, Velundella) are marine and brackish water anaerobes. Most jakobids are free-swimming cells, some of which temporarily attach to surfaces, while Histionidae (e.g., Histiona, Reclinomonas) are freshwater sessile forms that sit within conical or wineglass-shaped organic loricas. Jakobids have rarely been identified as major components of microbial ecosystems, except in some anoxic marine waters. They are of special evolutionary importance, how- ever, because their mitochondrial genomes retain more ancestral bacterial-like features than those of other eukaryotes. The mitochondrial genomes of aerobic jakobids encode more genes than those of any other eukaryote group; around 100 genes in total, including up to 69 protein-coding genes, ~10 of which occur in no other mitochondrial genome examined to date. In particular, they encode (subunits of) a bacterial-type RNA polymerase, while the mitochondrial RNA polymerase in other eukaryotes is a nucleus-encoded single-subunit enzyme with viral affinities. This retention by jakobids of the inferred-to-be-original mitochon- drial RNA polymerase is an important datum for inferring the evolutionary history of eukaryotic cells, including the mitochondrial symbiosis. Malawimonads are a small group of heterotrophic flagellates that superficially resemble jakobids, but are of uncertain evolutionary position within eukaryotes and thus also of particular evolutionary importance.