Take one for the team

You know it makes sense

… would have been a much better title for an interesting recent paper on the impact of Varroa on honey bee colonies. More specifically, the snappily titled “Social apoptosis in honey bee super organisms” (Page et al., 2016 Scientific Reports 6: 27210 doi:10.1038/srep27210) attempts to answer how and why the natural host of Varroa, the Eastern honey bee (Apis cerana), copes with mite infestation whereas ‘our’ bees (Apis mellifera), the Western honey bee, succumbs within 2-3 years without mite-control? The paper is Open Access so you don’t need to pay to read it and you can find it here.

Only the good damaged die young ^†

The authors demonstrate that A. cerana mite-associated pupae die before they emerge, whereas those of A. mellifera do not. As a consequence of this the mite levels are unable to build up to damaging levels in the colony. Essentially the pupae on which the mites feed die very quickly, meaning the mite also dies. They determined this by uncapping and examining age-matched pupae one day before natural emergence (see below) in Varroa-infested or uninfested colonies. Varroa-associated pupae (upper row in the image below) had all died during pupation.

Infested (above) and control (below) A. cerana pupae

In an extension to this study the authors showed that puncturing pupae with a sterile glass needle and then re-sealing the cell (you can do this with gelatin) also results in the pupae dying. The needle used had the same diameter as the chelicerae of the Varroa mite, so this treatment recapitulated the physical damage caused by the mite. Since the needle was sterile it was unlikely that the pupae were dying from exposure to the viruses (or other pathogens) transmitted by the Varroa mite. Instead, it seems that the Eastern honey bee has evolved mechanisms of “self-sacrifice” in response to wounding that result in the death of damaged pupae before the infesting mite has had a chance to multiply. Clever.

Social apoptosis

Apoptosis is the term used by cell biologists to describe a series of events that are also called programmed cell death seen, for example, in virus-infected cells. If a cell detects that it is virus infected, a cascade of signalling events result in it undergoing apoptosis (it dies), so preventing the infecting virus from replicating properly and spreading to neighbouring cells in the organism. Social apoptosis is a similar process, the death of an infected – or infested – member of the superorganism, the honey bee colony.

Immunity is a term meaning ‘having resistance to’, for example immunity to measles due to prior vaccination or infection. Generally, immunity is a reflection of strength of the recipient or exposed to the ‘abuse’ caused by the infectious agent. In contrast, the mechanism described for A. cerana is the opposite of this, instead being a form of immunity through weakness or susceptibility.

A. cerana has additional resistance mechanisms that help it combat Varroa infestation including enhanced grooming, removal of mites from unsealed brood, entombing multiply mite-associated drone brood (it’s not clear to me whether this is the same mechanism as the social apoptosis reported here), increased hygienic behaviour and shorter developmental cycles. These will have evolved over the millennia that the mite and bee have associated.

Any chance A. mellifera will evolve a similar mechanism?

Possibly, but I’m not holding my breath. There are already hygienic strains of A. mellifera – for example, VSH bees developed by the USDA group at Baton Rouge. These typically uncap and discard Varroa-associated pupae. This isn’t the same process as the social apoptosis reported here in A. cerana. The latter pupae die prematurely, thereby preventing mite reproduction. While we’re on the subject of Varroa and genetic resistance – do VSH A. mellifera strains open and discard mite-associated pupae … a) early enough to prevent significant levels of mite replication, and b) without releasing progeny mites from the cells they were raised in? I’m aware of the rates at which they clear out Varroa infested cells, but not either the timing of these events or the fate of any Varroa released at the same time.

It’s difficult to imagine a practical strategy to select for A. mellifera honey bee pupae that are more sensitive to Varroa infestation … our bees are currently too robust.

^† Billy Joel wrote Only the good die young which appeared on his 1977 album The Stranger. “[Not] so much anti-Catholic as pro-lust” Joel explained when it was censored, inevitably ensuring its chart success. The song has more to do with the birds and the bees … 😉