Photosynthesis from stolen chloroplasts can support sea slug reproductive fitness

Abstract

Some sea slugs are able to steal functional chloroplasts (kleptoplasts) from their algal food sources, but the role and relevance of photosynthesis to the animal host remain controversial. While some researchers claim that kleptoplasts are slowly digestible 'snacks', others advocate that they enhance the overall fitness of sea slugs much more profoundly. Our analysis shows light-dependent incorporation of ¹³C and ¹⁵N in the albumen gland and gonadal follicles of the sea slug Elysia timida, representing translocation of photosynthates to kleptoplast-free reproductive organs. Long-chain polyunsaturated fatty acids with reported roles in reproduction were produced in the sea slug cells using labelled precursors translocated from the kleptoplasts. Finally, we report reduced fecundity of E. timida by limiting kleptoplast photosynthesis. The present study indicates that photosynthesis enhances the reproductive fitness of kleptoplast-bearing sea slugs, confirming the biological relevance of this remarkable association between a metazoan and an algal-derived organelle.

Keywords: Sacoglossa; fatty acid; kleptoplast; reproduction.

Figure 1.

¹³C and ¹⁵N incorporation in the digestive tubules of Elysia timida. Light microscopy picture and corresponding δ¹³C and δ¹⁵N NanoSIMS images of E. timida incubated in artificial seawater enriched with 2 mM NaH¹³CO₃ and 20 µM¹⁵NH₄Cl, for 6 h in the presence of light. Digestive tubules (dt) appear green on the light microscopy micrographs due to the presence of numerous chloroplasts. After 6 h of incubation strong ¹³C and ¹⁵N enrichment is observed in these structures. (Online version in colour.)

Figure 2.

¹³C and ¹⁵N incorporation in the albumen gland of Elysia timida. Light microscopy pictures and corresponding δ¹³C and δ¹⁵N NanoSIMS images of E. timida in an isotopic dual labelling pulse-chase experiment incubated in artificial seawater enriched with 2 mM NaH¹³CO₃ and 20 µM ¹⁵NH₄Cl, in the presence of light for pulse (12 and 36 h) and chase (3 h), and in the dark for 36 h. ag, albumen gland; dt, digestive tubules. (Online version in colour.)

Figure 3.

¹³C and ¹⁵N incorporation in the gonadal follicles of Elysia timida. Light microscopy pictures and corresponding δ¹³C and δ¹⁵N NanoSIMS images of E. timida in an isotopic dual labelling pulse-chase experiment incubated in artificial seawater enriched with 2 mM NaH¹³CO₃ and 20 µM ¹⁵NH₄Cl, in the presence of light for pulse (12 and 36 h) and chase (3 h), and in the dark for 36 h. fo, gonadal follicles; dt, digestive tubules; ag, albumen gland. (Online version in colour.)

Figure 4.

¹³C incorporation in the main fatty acids of Elysia timida.¹³C (‰) in most abundant fatty acids of E. timida as a function of time (h) in an isotopic labelling pulse-chase experiment in artificial seawater enriched with 2 mM NaH¹³CO₃ in the presence of light (open circles) or in dark-incubated specimens for 36 h (closed circles). Mean ± s.e., n = 3. (Online version in colour.)

Figure 5.

Fecundity of Elysia timida. Number of eggs spawned by E. timida exposed to a 14 : 10 h light/dark photoperiod and a scalar irradiance of 40–160 µmol photons m⁻² s⁻¹ (regular light) or 5 µmol photons m⁻² s⁻¹ (reduced light) for 28 days. The line is the median, the x represents the mean, top and bottom of the box are the 75% and 25% percentile, and the whiskers represent the maximum and minimum values. Animals were fed continuously with Acetabularia acetabulum. Differences between treatments were significant at p = 0.007.