Parasitism, Parthenogenesis and Population Collapse
The South African Cape honey bee, Apis mellifera capensis is a social parasite of the African (“Savannah”) honey bee Apis mellifera scutellata. A.m. capensis workers can activate their ovaries within queenright colonies and start laying diploid eggs, maturing into other laying workers (a process termed “thelytokous parthenogenesis”). These bees are genetically identical clones of their mothers, and within parasitised colonies, continue laying eggs until the host colony’s falls from a lack of working workers.
Severe colony losses were reported when migratory beekeepers moved A.m. capensis into a region with A.m. scutellata colonies. Observations on the parasitic interactions between the two subspecies provided further understanding into honey bee reproduction and colony behaviours, and this article presents an overview of the research.
Parthenogenesis in Honey Bees
Parthenogenesis is a form of asexual reproduction in which females birth offspring without fertilization by a male. This is no surprise to the beekeeper, as both queens and workers can lay unfertilized eggs that hatch into male drones. Arrhenotokous parthenogenesis is the term given to male offspring, whilst Cape honey bee workers perform “thelytokous parthenogenesis”1, ultimately producing female offspring (Oldroyd et al., 2008).
When queen bees lay unfertilized eggs, this is arrhenotokous parthenogenesis in action. Unmated queens and the typical worker bees lay unfertilized eggs which hatch into drones2. Being able to control the sex of colony members is advantageous to the colony’s function throughout the seasons. Long, long ago, any queens that indiscriminately laid drones and workers eggs would have quickly found herself with a Darwin award and an escort out of the gene pool; hives do not need drones in dearth or winter.
A (very) Brief Overview of Parthenogenesis
Non-technical readers are welcome to skip this subsection
There are three ways to to generate a diploid egg at the cellular level:
- Maintain a diploid state through retention of the second polar body (what A.m. capensis does),
- Endoreduplication (replication of nuclear DNA without cell division) or,
- Mitotic division without cytokinesis (mitosis proceeds as normal, but the cell does not split in two).
How does the egg get activated without fertilization? Activation can be achieved due to environmental factors (temperature) or physiological factors (i.e. pheromones, as in the case for A.m. capensis). Once activated, the egg undergoes embryogenesis using DNA from the maternal genome – this produces offspring that are a genetic clone of their mother, at the cost of reduced genetic diversity.
Arrhenotokous (“Male-Only”) Parthenogenesis
In short, the queen lays an unfertilized egg in a cell, whose larvae grows into a male, drone bee. If a worker bee has her ovarioles activated, then she too will lay unfertilized eggs that develop into drones (office workers do not mate) .
Following mating flights, queens store sperm in the spermatheca. As she feels for the cell with her forelegs, she decides whether to fertilize the egg prior to laying. When Her Majesty lays unfertilized eggs, arrhenotokous parthenogenesis takes place as it develops into a drone. Drones are haploid, and are identical to their mother queen – hence the saying “male bees have no fathers, only grandfathers” – The queen was produced from a fertilized egg.
Workers on the other hand are non-reproductive females in the colony, and when beekeepers spot worker-laid eggs, it is a sign of queenlessness, or queen failure. Worker-laid eggs are haphazardly placed inside the cell, and multiple eggs are found in a single cell. Should the eggs hatch, they will become drones. For the beekeeper, the general solution to laying workers is in requeening their hive.
How Cape Honey Bees Produce Worker Bees: Thelytokous Parthenogenesis
In a modification of sex cell division (meiosis), diploidy is restored by fusion of components from the first polar body and the egg pronucleus, forming the zygote nucleus (“diploid egg”)3 (Verma & Ruttner 1983). Cape workers can reactivate their ovaries and lay eggs when a queen is absent from their hive (or even in her present, but far less frequently). As the eggs are laid diploid, they develop into female worker bees, clones of their mother, but not their queen (who would remain their genetic “mother”).
The advantage of thelytokous worker reproduction means that if a colony becomes queenless, they could requeen themselves from a diploid worker-laid egg. One explanation for the evolution of this ability could be South Africa’s Western Cape climate: prone to sudden shifts and high winds – making it difficult for the (now-heavy) queen to return home from her mating flights.
Another is that having worker clones of the queen during reproductive swarming is more advantageous to Her Majesty, as she and her clones will swarm and reproduce without the genetic material of her mating partners (in the form of drone sperm from traditionally fertilized eggs; confusing I know, Oldroyd et al., 2008).
As mentioned above, Cape worker-laid eggs scattered throughout the comb, but are later distributed into single cells as the colony rears them like any other brood. As a result, A.m. capensis colonies consists of workers from different mothers, both queen and her daughter workers. It is when Cape workers lay eggs in other colonies that issues arise, as you’ll read below.
Sidenote: Other Honey Bee Worker Subspecies can Perform Thelytokous Parthenogenesis
Both Mackensen (1943) and Tucker (1958) report thelytokous parthenogenesis in other honey bee subspecies. The former author noted that both the Punic (Tunsian Honey bee; possibly A.m. lamarckii or A.m. intermissa) and the Syrian bee, A.m. syriaca can occasionally produce workers from unfertilized eggs. Tucker references further studies by Mackensen that highlight how A.m. ligustica and A.m. carinca, our beloved Italian and Austrian bees – also show limited levels of thelytokous parthenogenesis.
So parthenogenic male and females are possible in most honey bee subspecies, with Cape honey bees being the most extreme cases – does this mean they have a different reproductive system?
The Reproductive System of Cape Honey Bees
In general, worker bees have less ovarioles than the queen (<5 ovarioles per ovary versus 180 ovarioles per ovary for the queen). In Cape honey bees, the number of tubular structures within a worker is associated with the likelihood of her becoming a laying worker, with the average being between 10-20 ovarioles per ovary (Allsopp et al., 2003). This large variation within A.m. capensis is affected by nutrition quality and quantity, as well as the genetic makeup of the larva and her nurse feeders.
In a sample of the worker populations within in an A.m. capensis colony, around 10% of the workers were parthenogenic offspring of other workers, or clones of their sisters (Beekman et al., 2009). If you’re wondering whether these worker-laid eggs could turn into a queen, then the answer is yes – the eggs are relocated into a queen cell cup, and the colony proceeds to raise a queen (Goudie et al., 2012). The success of these worker-produced queens drives the selection for thelytokous workers, as the colony can continue rearing brood and producing honey.
Unmated Cape Honey Bee Queens Lay Both Male and Female Eggs
A study by Oldroyd and colleagues showed that when A.m. capensis queens were denied mating, they were still able to lay female eggs via thelytoky, and still lay drones as per usual using arrhenotokous parthenogenesis (Oldroyd et al., 2008). They showed that queens have some control over the level of cell division in their eggs, and can determine whether an egg becomes diploid or remains haploid prior to an egg to become diploid prior to laying. They do make mistakes, with drones being found in worker cells and vice versa.
The authors discount the possibility that the queen is laying eggs at random and the workers rear those which are placed correctly. Previous studies indicate that workers rear larvae in both drone and worker comb, no matter their caste type (Calderone and Kuenen, 2001).
Normally, a queen lays worker eggs by releasing a small amount of sperm with the unfertilized egg inside the cell. For parthenogenic birth, the queen would need to manipulate meiosis after the egg is laid (and has left her body) to restore diplody. If the next cell was for a drone, she must again alter the meiotic processes maintain to haploidy; continually switching back and forth between the two genotypes depending on the cell structure she encounters (Oldroyd et al., 2008).
Confirmation of Thelytoky: No Sperm in Worker Spermatheca
The spermatheca diameter of A.m. capensis workers is around a fifth as large as their queen. When compared to European honey bees, the sperm storage vessels of Cape honey bee workers is three times as large, despite both European and Cape queens having a spermathecae around the same size. Examinations by Hepburn & Crewe (1991), found no sperm in worker spermathecae. Since a large number of eggs were diploid despite the sperm-free tubules, they concluded that the female bees were produced by thelytokous workers.
Cape Honey Bee Brood Receive More Food Than Host Larvae
One way social parasitism arises in Cape bees is due to increased feeding of their larvae by host colonies. Allsopp & colleagues showed that in an African bee colony (A.m scutellata), Cape honey bee brood received nearly twice as much food as the African honey bee larvae (Allsopp et al., 2003). The increased feeding resulted in larger Cape workers with queen-like characteristics of enlarged spermathecae (sperm storage), increased weight, faster development and reduced pollen baskets (corbiculae).
Could increased feeding be due to a stronger brood hunger pheromone by the Cape larvae? Possibly, considering that when African larvae is placed in a Cape honey bee colony, they received less food from A.m. capensis workers, unlike in their host A.m capensis colonies. In the honey bee life cycle, this brood hunger pheromone signals how much feeding a larva requires (and also suppresses worker ovaries… interesting). This differential feeding is twofold:
- Cape bee larvae produce more hunger pheromones, and
- Cape workers are responsive to higher, or larger, brood pheromone levels than A.m. scutellata.
Less Brood Pheromone Leads to Higher Rates of Laying Workers
Brood pheromone production in A.m. scutellata is lower than A.m. capensis, as described by Wossler (2002). More laying Cape workers are observed in host A.m. scutellata colonies, compared to laying Cape workers in their natural A.m. capensis colonies; this is linked to African colonies having a brood pheromone level below A.m. capensis’ normal thresholds. The overfed Cape larvae develop queen-like characteristics, resulting in improved reproductive capacity, and perhaps being treated in a more queen-like fashion (retinues and feeders).
In adult bees, the availability of rich food sources influences their reproductive potential, promoting ovariole development and egg production in worker bees (Hoover et al., 2006). Combining the increased feeding frequency with a high quality diet (i.e. nectar flows) and a single nucleotide mutation on one gene within Cape honey bees brings us to the problem of social parasitism.
Colony Collapse Following A.m. capensis Parasitism
Genomic analysis determined that a single nucleotide polymorphism in the aptly-named “thelytoky locus” [gene] caused A.m. capensis workers to lay eggs in hives of other subspecies (Aumer et al., 2019). As these eggs are subjected to thelytokous parthenogenesis, they hatch into clones of the Cape worker, and the social parasites are reared into adulthood by the host hive.
Thus begins the host colony’s death spiral; the Cape adults become laying workers, resulting in a non-productive colony. The A.m. capensis pseudo-queens do not forage or clean- like mother, like daughter. Eventually, the Cape clones exceed the colony’s sustainability threshold, and the insufficient number of non-laying workers cause the colony to dwindle and die out.
What Happens to the Host Queens during Cape Honey Bee Parasitism?
As far as I can tell, the host queen is unaffected. Why is that? We know that the queen both controls and signals her presence to the hive with pheromones, and since the intruding Cape bee is a worker, the colony is unalarmed. Worker drift happens often, and the wandering workers are not always killed by the recipient hive, unlike in raids by robber bees.
The queen continues business as usual with her retinue, and I doubt she has the ability to keep track of how many eggs she’s laid in a day (after all, it is the worker bees who guide her to the target comb, not the other way around). She too will fall with her colony as the number of parasitic Cape workers outnumber her worker daughters.
Worker Policing is Absent During A.m.capensis Parasitism
Brood cells containing diploid Cape honey bee eggs are ignored by the host colony (Martin et al., 2002). This is unlike in other subspecies, where worker-laid eggs are promptly removed by other workers via “worker policing“. In the study by Martin and colleagues, they found that when A.m. capensis workers laid eggs in A.m. scutellata colonies, the eggs were allowed to hatch and develop into workers. In contrast, when an A.m. scutellata worker laid eggs in her colony, they were removed by other workers within 24 hours.
The authors suggests that Cape bees could produce a chemical mimic to queen-laid eggs, hence sparing their own. Is it possible that both haploid and diploid eggs possess a scent, and when the full chromosomal count is present (diploid), the resulting scent is enhanced to what bees recognise as a “normal egg”? Questions for Mendel.
Capensis Calamity: Colony Losses from Cape Honey Bees
Allsopp (1993) summarises the capensis calamity as the death of 50,000-100,000 honey bee colonies over a two year period in several regions of South Africa. Migratory beekeepers moved A.m capensis colonies into an area with native A.m. scutellata, and the subsequent invasion and usurpation of African colonies by the Cape bees led to extensive losses.
The story is familiar: In these migratory colonies, the darker A.m. capensis workers invaded healthy hives, and laid their unfertilized diploid eggs, producing reproductively-active workers. These newly-emerged workers mature and seek other A.m. scutellata colonies, eventually collapsing their newly- infested hosts.
Do Cape Honey Bees Produce Honey?
Yes, Cape bees produce honey inside Cape honey bee hives (but not in the hives of other subspecies). The amount of honey produced varies based on the season, and I would wager that other subspecies like the Italian A.m. linguistica are better producers. After all, their workers are not continually competing with each other to lay eggs. Allsopp’s 2020 review describes beekeeping with A.m. capensis to be:
- More gentle than other African bees.
- As prone to swarming and absconding as other African bees.
- Highly tolerant to pests and disease like Varroa mitesand American foulbrood, and can thrive without treatments.
- Well adapted to the windy fynbos shrubland of the Cape region.
Honey Production in South Africa
The South African Honey Bee industry states that between 2000-2002, the country produced 1,533 tonnes of honey. A hive in a plantation of Eucalyptus trees can produce up to 60 kg of honey per hive per year. They do not state which subspecies are used, and I would wager it is A.m. scutellata, on account of it being more widespread. It is likely that the Cape honey bee colonies belong to the 20-25kg production group.
Bonus: African Honey Bees and Small Hive Beetle Control
Adapted from “Beetles in a Beehive“
A 2003 study by Ellis & colleagues showed that Cape bees perform hygienic behaviour in response to the presence of small hive beetle eggs. As both species are native to the region, they have developed a sort of evolutionary arms race on one another, with the beetles confined to empty honey combs learning to pierce the adjacent cell and lay eggs. While clever, this triggers a cleaning response in the infested brood combs, resulting in removal of said eggs.
Additionally, the Cape bees construct “prisons” using propolis to control small hive beetles (Neumann et al., 2001). Guard bees maintain a constant numbers of beetles inside the cells, and increase the number of propolis prisons when beetle density increases, rather than increasing the numbers of beetles per existing cell.
The authors suggest that this defensive behaviour evolved as a way to manage beetle infestations and limit their proliferation within the colony. However this imposes a selection pressure in which the beetles most resistant to starvation survive. Some 20-years later, research from from Papach and colleagues (2020) showed that the small hive beetle populations within their native African geographies had a higher tolerance to starvation (females survived up to two months without food) than beetles in their invasive North American range.
Is the Cape Honey Bee Found Outside South Africa?
So far, the Cape Honey Bee is restricted to its native range in South Africa. As you’ve read above, mass destruction would follow if this subspecies is introduced to other beekeeping regions. In the Capensis Calamity, migratory commercial beekeepers suffered the most from A.m. capensis parasitism. However hobbyists beekeepers in the area would have also lost their hives, but went unreported. Many countries have imposed biosecurity procedures to restrict the entrance of the Cape honey bees, and so long as those measures remains in place, it is one less pest for global beekeepers to face.
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Footnotes
- Some definitions: Partheno-: is Greek for “virgin”, -genesis is Greek for “creation”. Combined they mean “virgin creation” or “virgin birth”.
Thely-: is Greek for “female” and -tokous is “birth”. Combined it means “female bearing”.
Arrheno-: is Greek for male, -tokous as above, “birth” or “bearing.” Combined we get “male bearing”. ↩︎ - The laying of diploid, or fertilized-egg drones by queens will be addressed in another article. ↩︎
- By contrast in normal queens, it is the egg pronucleus that moves deeper into the ovum where it awaits fertilization by sperm to form the zygote. ↩︎