1. University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, Maryland 21613, USA
2. Institute of Human Virology, University of Maryland, School of Medicine, Baltimore, Maryland 21201, USA
3. Cell Biology and Molecular Genetics, University of Maryland – College Park, College Park, Maryland 20742, USA
4. Present address: Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, USA.

Correspondence to: Matthew D. Johnson^1,4 Correspondence and requests for materials should be addressed to M.D.J. (Email: johnson@marine.rutgers.edu).

Abstract

It is well documented that organelles can be retained and used by predatory organisms, but in most cases such sequestrations are limited to plastids of algal prey¹. Furthermore, sequestrations of prey organelles are typically highly ephemeral² as a result of the inability of the organelle to remain functional in the absence of numerous nuclear-encoded genes involved in its regulation, division and function³. The marine photosynthetic ciliate Myrionecta rubra (Lohmann 1908) Jankowski 1976 (the same as Mesodinium rubrum)⁴ is known to possess organelles of cryptophyte origin^{5, 6, 7, 8, 9}, which has led to debate concerning their status as permanent symbiotic or temporary sequestered fixtures^{5, 6, 7, 8, 9, 10, 11, 12, 13}. Recently, M. rubra has been shown to steal plastids (that is, chloroplasts) from the cryptomonad, Geminigera cryophila, and prey nuclei were observed to accumulate after feeding¹⁰. Here we show that cryptophyte nuclei in M. rubra are retained for up to 30 days, are transcriptionally active and service plastids derived from multiple cryptophyte cells. Expression of a cryptophyte nuclear-encoded gene involved in plastid function declined in M. rubra as the sequestered nuclei disappeared from the population. Cytokinesis, plastid performance and their replication are dependent on recurrent stealing of cryptophyte nuclei. Karyoklepty (from Greek karydi, kernel; kleftis, thief) represents a previously unknown evolutionary strategy for acquiring biochemical potential.

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