A new case of kleptoplasty in animals: Marine flatworms steal functional plastids from diatoms

Abstract

To date, sea slugs have been considered the only animals known to sequester functional algal plastids into their own cells, via a process called "kleptoplasty." We report here, however, that endosymbionts in the marine flatworms Baicalellia solaris and Pogaina paranygulgus are isolated plastids stolen from diatoms. Ultrastructural data show that kleptoplasts are located within flatworm cells, while algal nuclei and other organelles are absent. Transcriptomic analysis and rbcL amplicons confirm the absence of algal nuclear mRNA and reveal that the plastids originate from different species of diatoms. Laboratory experiments demonstrated photosynthetic activity and short-term retention of kleptoplasts in starved worms. This lineage of flatworms represents the first known case of functional kleptoplasty involving diatoms and only the second known case of kleptoplasty across the entire tree of animals.

Fig. 1. Light, fluorescence, and transmission electron micrographs of B. solaris and its kleptoplasts.

(A) Live specimen with eyes (arrow), pharynx, and golden-brown kleptoplasts; darker plastids are being degraded. (B) Kleptoplasts and empty frustules of pennate diatoms (large black arrowheads). (C) Autofluorescence of kleptoplasts densely packed under the epidermis and within the mesenchyme. (D) Heterokont kleptoplasts in host cells. (E) Cell junction (small black arrowheads) between two host cells, each containing a heterokont kleptoplast surrounded by at least two membranes (small white arrowheads). l, lipid vesicle; m, mitochondria; n, nucleus; p, plastid; ph, pharynx; py, pyrenoid. Scale bars, 50 (A), 20 (B), 50 (C), and 1 μm (D) and 200 nm (E).

Fig. 2. Light, confocal, and transmission electron micrographs of P. paranygulgus and its kleptoplasts.

(A) Live specimen with eyes (arrow), pharynx, and golden-brown kleptoplasts. (B) Intact (golden brown) and degraded (double black arrowheads) kleptoplasts. (C) Autofluorescence of kleptoplasts in a juvenile specimen; degraded plastids (double white arrowheads) show no autofluorescence. (D and E) Heterokont kleptoplasts adjacent to flatworm mitochondria, lipid droplets and cell junction (arrowheads). Scale bars, 50 (A), 20 (B), 50 (C), 2 (D), and 1 μm (E).

Fig. 3. A phylogeny of five partial plastid genes based on transcripts from the transcriptome of B. solaris demonstrates that its kleptoplasts are of diatom origin.

A concatenated 2956–base pair alignment of psaB, psbA, psbB, atpA, and rbcL was run in MrBayes v3.2.6. Support was assessed with Bayesian posterior probabilities (pp, above branches) and maximum likelihood bootstrap replicates (bs, below branches) from a RAxML analysis. Unsupported branches are collapsed (pp < 0.95) or indicated with a “/” when not supported in RAxML. Scale bar, substitutions/site.

Fig. 4. Functional kleptoplasty and short-term retention of plastids in B. solaris is shown by photosynthetic activity and the loss of plastids in starved specimens over time.

(A) Photosynthetic activity in B. solaris specimens starved for 7 days (n = 25) compared with the chlorophytic alga T. tetrathele (~500,000 cells/ml). Gross photosynthesis was calculated by summing the net photosynthesis and respiration rates. Data represent means ± SE. (B) Plastid retention (proportion of kleptoplastic individuals) and survival rate (proportion of surviving individuals) in filtered seawater (n = 20) and under differential treatments with the photosynthesis inhibitor monolinuron (n = 20 in each treatment). FSW, filtered seawater.