Wednesday 25 April 2012

On the odd life cycle of bumblebees

I stopped by the comfrey patch this morning. The comfrey has now been in blossom for a good month. A Small White butterfly was feeding on comfrey, but I couldn't snap her. A ginger queen Bombus pascuorum fed on the blossom. A queen wasp got comfortable on a leaf and basked in the sun, as did some hoverflies. While I watched a couple of Anthophora plumipes males patrolling and feeding, a tiny worker of the tree bumblebee Bombus hypnorum, the first of the year, turned up (above). As I compared the sizes of these two bees – the bumblebee worker was smaller than A. plumipes - I reflected on the different life cycles of these two bee species. A. plumipes, a solitary bee, has males and females, while bumblebees, in addition of males and females (queens), have a worker caste. In bumblebees there are no males for most of the year: A male has a relatively short - if hectic - life. Upon emergence in the summer, he will find a queen and mate with her. The queen will store his sperm and use it to fertilise her eggs the following spring, when she will emerge from overwintering, build a nest and lay fertilised eggs, which will hatch into worker larvae. She will be busy collecting nectar and pollen from early flowering trees and plants to rear these larvae, and once the workers emerge, they will take over from the queen in collecting more nectar and pollen for subsequent eggs, although they won't reproduce themselves.  In the summer, the last batch of eggs of the queen will produce new queens and males, which the workers will help rear. Once they leave the nest, the founder queen and workers will die and the cycle starts again. Wasps, ants, bees, and bumblebees (hymenopterans) share a weird way to determine the sex of the offspring: fertilised eggs become females and unfertilised eggs become males: a system called haplodiploidy.
  Most sexual organisms have two sets of chromosomes: a set they inherit from their mother and another that comes from their father, but as bumblebees and other hymenopterans do not have a dad they just have a single set of chromosomes, the set coming from their mother.
How strange is that? If you think about it, the system means that bumblebee males don’t have dads, they also cannot have sons, only daughters, although they can have grandsons and have granddads. In addition, this arrangement causes strange relationships between the family members. We are equally related to our parents than to our kids – on average – but due to haplodiploidy, bumblebee sisters are more strongly related than they are to their mums. This is because full sisters have received an identical set of chromosomes from their dad, in addition to the set they receive from their mum, which is a mixture from the sets the queen received from her parents.
 This unbalanced genetic relationship between mother-daughters and sisters is thought to underlay the evolution of the worker caste, which do not reproduce, but help their mother to rear their own sisters.
The solitary bee Anthophora plumipes
A wasp? No, the exquisite wasp mimic hoverfly Myothropa florea
This is the real wasp. A queen common wasp 
Hoverfly Syrphus ribesii 
Another basking fly
A queen Bombus pascuorum


Anonymous said...

It's worth noting that if they get the chance, in certain circumstances workers will attempt to lay eggs, but because they haven't been fertilized only males are produced. It depends on how 'fit' the queen is in controlling the nest. Some great examples documented in Sladen's classic book: 'The Humble-bee: its life-history and how to domesticate it'. First published 1892, re-printed 1989.

Africa Gomez said...

Thank you for your comment norwegica and for the reference. It would take a very long blog post to explain all the intricacies of the bumblebee life cycle and I am bound to have left more stuff unmentioned.