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Sex with Strangers

bees

Credit: Antagain/iStockphoto

By Emily Remnant & Ben Oldroyd

An invasive honey bee species is mating with local honey bees in Far North Queensland. What are the consequences for the Australian honey bee industry?

We often define a species as a group of animals or plants that can mate and produce fertile offspring. But when a species becomes divided by a physical barrier, such as an emerging desert, the populations on either side of the barrier can diverge by natural selection and inbreeding until there are two new species.

What happens when two such related species are reunited? Will the males and females recognise each other and mate, and if they do, what happens to the offspring? An example of this kind of reunification of species has just occurred in Cairns, and the answers to these questions are instructive.

The western honey bee, Apis mellifera, is native to Africa and Europe. It has been domesticated for more than 6000 years and is now used throughout the world for honey production and crop pollination.

Its closest evolutionary relative is the eastern hive bee Apis cerana. It has also been domesticated for millennia, but until recently has been confined to its native range in Asia.

DNA evidence suggests that the last common ancestor of A. cerana and A. mellifera lived six million years ago – at about the same time as the last common ancestor of chimpanzees and humans was running around in Africa. A. cerana and A. mellifera probably became separated by an ancient period of desertification in the Middle East, confining what became A. cerana to Asia and what became A. mellifera to Africa.

A. mellifera was introduced to Australia by European settlers in 1822, and now forms the basis of the honey and pollination industry. Today there are about 500,000 A. mellifera colonies kept by 2000 beekeepers across Australia. In addition there are millions of feral colonies. Recent estimates suggest that there are two or three feral colonies per square kilometre across the continent. Without these feral and domestic honey bees we’d lose about one-third of our food production, so the health of our bees is really important.

A. cerana has been spreading too. In the 1980s, missionaries moved A. cerana from Java in Indonesia to Papua New Guinea. The bees became invasive, spreading to the Solomon Islands in 2003 and to Cairns in 2007. It’s thought that the Cairns population, which now covers 10,000 km2, is derived from a single swarm that arrived on a shipping container from PNG or the Solomons. Fortunately the swarm did not carry the Varroa mite, a deadly bee parasite, so Australian beekeepers have dodged that bullet – for now.

Virgin honey bee queens of all species mate during a brief period early in life. To mate, a virgin queen leaves her hive and seeks out a nearby mating area, usually an open field lined by trees. Suburban football fields are favourite locations. A mating area at number 1 oval at Sydney University has been in regular use since 1977!

Bee-mating areas are aptly named “drone congregation areas”. Each day, thousands of drones gather in the air for several hours, waiting for the arrival of a virgin queen. Drones detect the arrival of a queen by the smell of her sex pheromones, which she secretes from a gland near her mouth. Drones pursue the queen, and mate with her in flight one at a time until she has mated with between six and ten males. A queen often flies the next day, and mates with another ten drones, but after that she never mates again.

Queens must mate with multiple drones to ensure the best possible genetic composition of her offspring and a healthy colony. The queen stores the sperm obtained during her mating in a sperm storage organ called the spermatheca. When drones mate they turn inside out to evert their endophallus, a strange penis (Fig. 1). Thus mating is fatal to males.

All honey bee species have common mating behaviours and similar sex pheromones. Because mating opportunities are so limited and the competition to mate is so furious, honey bee males will try to copulate with any fast-moving object that comes into the drone congregation area. For example, they will try to mate with a pebble tossed in the air, and we’ve seen them chasing dragonflies. This made us wonder if there would be interspecific matings happening in Cairns and, if there are, what would the consequences be for the spread of the invasive species and for the beekeeping industry?

We started our investigation by determining the time of day when the drones leave their hives to mate. In Asia there are up to four honey bee species in most areas (some species have only a limited distribution). The bees avoid interspecific matings by having their own time to fly – kind of like landing slots at an airport – and their own mating places. For instance, some species mate above the tree canopy while others mate beside the canopy. Our observations showed that in Cairns there is significant overlap in the A. mellifera and A. cerana drone departure times, indicating that it is likely that both species meet during their mating flights.

Next we looked for A. mellifera and A. cerana drone congregation areas in suburban Cairns. We raised artificial queen pheromone aloft using a helium balloon in likely spots in ovals and parks (Fig. 2). We located two A. cerana drone congregation areas when we observed drones trying to copulate with our lures. Both sites were also suitable as congregation areas for A. mellifera drones. This strongly suggested that there were opportunities in both time and place for inter-specific matings.

We then used DNA fingerprinting to see if we could find evidence of inter­specific matings. We collected A. mellifera and A. cerana queens from Cairns, and detected A. cerana sperm in the spermatheca of one-third of the A. mellifera queens.

In contrast, we did not detect A. mellifera sperm in the spermatheca of A. cerana queens. We think that when the larger A. mellifera drones mate with the smaller A. cerana queens they may injure the queens, which then die. Alternatively, it may be that the A. cerana queens refuse to mate with the A. mellifera drones, perhaps by holding their abdomen in the wrong position as they fly.

Next we determined the significance of the interspecific matings. Do A. mellifera queens inseminated with A. cerana sperm produce hybrid eggs that fail to hatch, or unfertilised eggs that develop into male drones? To do this we used the laboratories of the Australian Quarantine and Inspection Service at Cairns Airport to artificially inseminate ten A. mellifera virgin queens with sperm from either A. cerana drones or with saline (Fig. 3). The results were highly variable, but one queen inseminated with A. cerana semen produced hybrid eggs.

But do these hybrid eggs hatch? To answer this question we sampled hundreds of eggs from hives headed by interspecifically mated A. mellifera queens in the Cairns area. We again used DNA fingerprinting to determine how many queens were fertilised by sperm from the incorrect A. cerana drone fathers. We failed to detect any hybrid eggs in our samples, suggesting that if any of the eggs had been incorrectly fertilised, they were in very low numbers or had been cleaned from the hive by nurse bees prior to our sampling.

In summary, we showed that interspecific mating occurs in one-third of A. mellifera queens in the Cairns region where A. cerana is now present. A. mellifera queens that mate in areas where A. cerana colonies are present are very likely to encounter A. cerana males and mate with them.

Depending on the proportion of A. cerana males that mate with the queen, the fertility of the queen will be reduced. This will reduce the productivity of colonies headed by A. mellifera queens that mate in A. cerana areas.

Colonies with a miss-mated queen are likely to be less effective honey producers and pollinators, so it may be better for beekeepers to source queens from areas where the invasive A. cerana is not present. If A. cerana spreads, we may see a retreat of the A. mellifera queen breeding industry further south.

Interestingly, if interspecific mating is lethal to A. cerana queens, mating with A. mellifera drones may help curtail the spread of A. cerana in Australia. On the other hand, if A. cerana queens are just clever at avoiding A. mellifera males, then there will be little impediment to A. cerana spreading across all tropical areas of Australia – and perhaps to subtropical areas as well.

Mating interference is an under-appreciated aspect of invasive species, and is just one more reason to be vigilant about the introduction of new species to Australia.

Ben Oldroyd and Emily Remnant work in the Behaviour and Genetics of Social Insects Laboratory in the University of Sydney’s School of Biological Sciences.