title: Transcriptomic evidence that longevity of acquired plastids in the photosynthetic slugs Elysia timida and Plakobranchus ocellatus does not entail lateral transfer of algal nuclear genes.
abstract: Sacoglossan sea slugs are unique in the animal kingdom in that they sequester and maintain active plastids that they acquire from the siphonaceous algae upon which they feed, making the animals photosynthetic. Although most sacoglossan species digest their freshly ingested plastids within hours, four species from the family Plakobranchidae retain their stolen plastids (kleptoplasts) in a photosynthetically active state on timescales of weeks to months. The molecular basis of plastid maintenance within the cytosol of digestive gland cells in these photosynthetic metazoans is yet unknown but is widely thought to involve gene transfer from the algal food source to the slugs based upon previous investigations of single genes. Indeed, normal plastid development requires hundreds of nuclear-encoded proteins, with protein turnover in photosystem II in particular known to be rapid under various conditions. Moreover, only algal plastids, not the algal nuclei, are sequestered by the animals during feeding. If algal nuclear genes are transferred to the animal either during feeding or in the germ line, and if they are expressed, then they should be readily detectable with deep-sequencing methods. We have sequenced expressed mRNAs from actively photosynthesizing, starved individuals of two photosynthetic sea slug species, Plakobranchus ocellatus Van Hasselt, 1824 and Elysia timida Risso, 1818. We find that nuclear-encoded, algal-derived genes specific to photosynthetic function are expressed neither in P. ocellatus nor in E. timida. Despite their dramatic plastid longevity, these photosynthetic sacoglossan slugs do not express genes acquired from algal nuclei in order to maintain plastid function.
authors.affiliation: Zoologisches Forschungsmuseum Alexander Koenig, Bonn, Germany. hwaegele@evolution.uni-bonn.de
authors.forename: Heike
authors.initials: H
authors.lastname: Wägele
authors.forename: Oliver
authors.initials: O
authors.lastname: Deusch
authors.forename: Katharina
authors.initials: K
authors.lastname: Händeler
authors.forename: Rainer
authors.initials: R
authors.lastname: Martin
authors.forename: Valerie
authors.initials: V
authors.lastname: Schmitt
authors.forename: Gregor
authors.initials: G
authors.lastname: Christa
authors.forename: Britta
authors.initials: B
authors.lastname: Pinzger
authors.forename: Sven B
authors.initials: SB
authors.lastname: Gould
authors.forename: Tal
authors.initials: T
authors.lastname: Dagan
authors.forename: Annette
authors.initials: A
authors.lastname: Klussmann-Kolb
authors.forename: William
authors.initials: W
authors.lastname: Martin
mesh_terms: D000818:Animals; D001483:Base Sequence; D002467:Cell Nucleus / Q000235:genetics*; D000460:Chlorophyta / Q000166:cytology / Q000235:genetics*; D020224:Expressed Sequence Tags; D049851:Gastropoda / Q000145:classification / Q000235:genetics* / Q000502:physiology* / Q000648:ultrastructure; D020869:Gene Expression Profiling; D022761:Gene Transfer, Horizontal; D008969:Molecular Sequence Data; D010788:Photosynthesis / Q000235:genetics*; D010802:Phylogeny; D018087:Plastids / Q000235:genetics* / Q000648:ultrastructure