Thursday, 23 July 2015

Helanterä and Uller: Superorganisms as model systems

For the first guest talk of the meeting, we heard from Dr Heikki Helanterä, who is a biologist from the University of Helsinki, working on eusocial insects. Heikki is beginning a new project in which he tests the idea that eusocial insect colonies can be compared with organisms. He considers the best candidates to be those colonies in which workers are sterile - terminally differentiated - because this is when 'the cool stuff happens'. For example, the queens can mate multiply, bringing all kinds of genetic diversity into the colony, because the workers aren't in a position to do anything about it.

Helanterä is interested in establishing whether sufficient heritable variation exists at the level of whole insect colonies to support a between-colony selection process, in which colonies act as units of selection in their own right.
We know that there is variance at this level. For example, colonies exhibit different 'personalities', such as for levels of aggression, which covaries with kin structure. Unfortunately, it seems like data about colony-level heritability and colony-level fitness are  hard to come by, what with colonies having extremely long lives and insects having the habit of flying of into the distance when its time to mate.

However, on theoretical grounds we have good reason to doubt that there is much heritability of whole-colony traits. This is because whole-colony traits - such as foraging organisation, hygiene and communication -are determined by all of the members of the colony. The members of the colony will represent a particular array of genotypes, depending on the number of males with whom the queen mates during the lifetime of the colony. But a new queen who founds a new colony can represent only a fraction of that diversity - she only carries her own father's genes. So much of the genetic diversity that underpins a colony's traits is lost when a new colony is formed.

There is evidence that genetic diversity is fitness affecting - for example, colonies in which the queen has mated a larger number of times are more disease-resistant. But benefits like this cannot be passed on unless there is some heritable mechanism fixing the number of times that the queen mates, for example. And the state of the art, so far, is that we just don't know how many of a queen's properties, or how many of a colony's properties, are stably passed down colony lineages.

Helanterä's new project will see him taking claims for colony superorganismality seriously and scrutinising them - can we really think of workers insects as somatic material and males as sperm?
He will test the extent to which eusocial colonies can be compared with paradigm organisms by searching for homologous life history traits such as anisogamy and iteroparity.

Dr Tobias Uller, is Doctor of Zoology at the Universities of Oxford and Lund. In response to Helanterä's talk he agreed that the notion of eusocial insect colonies acting as units of selection in their own right is threatened by consideration of the details of colony formation. A colony's character is emergent...but the genetic variation that, according to a standard view, underpins this emergent character, is lost in the bottleneck through which every daughter colony must pass. But Uller suggested that we might need to search wider for the 'missing heritability' in colony-level selection. For example, we know that packets of fungus are transported across generations of leafcutter ant colonies. Perhaps there are pheromone signals or odour profiles passed on also.

Uller says we need to get over thinking that colony phenotypes must be genetically programmed..and start thinking of them as constructed during each cycle instead. Many non-genetic systems may provide scaffolding, enabling heredity at the colony level.

You can listen to the whole talk and response here. Uller's commentary begins at 52 mins 38.

Next post: Merlin and Clark - Extending inheritance

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