Kleptocytosis: A Novel Parasitic Strategy for Accelerated Reproduction via Host Protein Stealing in Varroa destructor

Abstract

Varroa destructor must produce mature offspring within the inflexible nine-day window framing the pupal development of their honey bee host. Missing this deadline renders the foundress mites fitness zero, establishing evolutionary pressure to accelerate reproduction and development. Through micro-computed tomography and modeling Varroas energy budget, we found each egg grows to constitute [~]18% of the foundresss body volume in 30 hours, yet accounts for less than 10% of her energy usage. We hypothesized that this small energy investment is a successful strategy because of Varroas long-ignored ability to traffic intact host proteins. Through gel electrophoresis, concomitant immunodetection, and MS/MS, we detected several intact, bee-derived proteins in mite eggs, including metamorphic proteins (hexamerins) and egg-yolk precursors (vitellogenin and apolipophorin) which likely reduce the parasites direct reproductive investment. We then expressed recombinant Halotag labelled Apis vitellogenin to determine the route conveying exogenous proteins into the mites oocyte. We detected fluorescent vitellogenin in the lyrate organ and observed a tube-like extension of the lyrate organ connecting to the ovum, likely providing an avenue for intact host proteins. Finally, we tested the hypothesis that exogenous proteins maintain functionality in the parasite. Varroa metamorphose despite their inability to produce hexamerins, critical metamorphic proteins. Through label-free quantification of proteins in metamorphosing mites, we observed a hexamerin depletion pattern consistent with usage as a metamorphic amino acid reservoir. We describe this process as "kleptocytosis," denoting movement of stolen intact macromolecules from host to a parasite cell. Given their fixed developmental timeline, this pathway presents a promising target for novel Varroa management strategies. Significance StatementGlobal honey bee health is threatened by the parasitic mite Varroa destructor; its success due in part to its rapid reproduction and accelerated development. By combining biological modeling, micro-computed tomography, fluorescence imaging, and quantitative proteomics, we show that Varroa siphon intact, ostensibly functional host proteins conveying them directly to their oocyte. This avoids the energetically inefficient process of digesting and reconstructing ingested proteins and affords the capacity to utilize proteins that it does not produce de novo facilitating rapid reproduction and accelerated development. We call this process "kleptocytosis," and identify anatomical adaptations which apparently facilitate protein movement. This work exposes a new target in Varroas physiology, providing a promising direction for more effective management strategies.