Phenotypic plasticity is a term familiar to evolutionary biologists. It refers to the ability of an organism to respond to a changing environment by changing its physical properties – its phenotype. For example, metabolic rate changes with temperature and resource availability.

We usually assume that such changes are adaptive, that is, changes are in the same direction as selection and so will increase the fitness (reproductive output) of the organism in that environment. But, importantly, we don’t usually test for the adaptive significance of phenotypic plasticity because we don’t typically estimate selection in different environments when we assess plasticity.

Lukas Schuster and his supervisors, Craig White and Dustin Marshall, used the model species Bugula neritina to investigate whether changes in metabolic rates in response to different field environments are an example of adaptive phenotypic plasticity. To their surprise they found that, while Bugula exhibited plasticity in metabolic rate, it was not adaptive.

Bugula is a small filter feeding colonial bryozoan that is often found on the undersides of piers. It is also found on vertical surfaces such as pier pilings, although the increased UV radiation and sedimentation experienced on vertical surfaces combine to make this a more stressful living environment.

Lukas collected mature colonies of Bugula from the field and then spawned them in the laboratory and settled the larvae onto small acetate sheets. This allowed Lukas to deploy the Bugula on vertically or horizontally suspended panels (corresponding to harsh and benign environments respectively) and to return colonies to the laboratory to measure metabolic rates. They did two experimental runs to test the consistency of the results.

As a first step, Lukas and his supervisors had to determine how selection on metabolic rate varies across harsh and benign environments. In other words, they needed to establish the relationship between metabolic rate and reproductive output (fitness) in each environment.

They deployed newly settled Bugula to a common, benign environment for three weeks before returning these colonies to the laboratory to measure metabolic rates. Half of the colonies were then deployed into the harsh environment and half was kept in the benign environment. Growth, survival and lifetime reproductive output were then tracked for each colony; this allowed the team to determine whether there was any fitness advantage associated with particular metabolic rates in each environment.

Surprisingly, they found no differences in selection on metabolic rates in the two environments. Instead, in one experimental run, they found evidence that smaller individuals with lower metabolic rates and larger individuals with higher metabolic rates went on to produce more offspring in both environments. This suggests that metabolic rate is unlikely to evolve independently of other traits.

To measure plasticity Lukas returned all colonies to the laboratory to measure metabolic rates for a second time. Colonies from the harsh environment had overall lower metabolic rates compared to colonies from the benign environment.

Given the strong and consistent metabolic response to the different environments that the team observed, it would have been tempting to infer that such a response increases fitness. While this seems intuitive, it is not consistent with what they know about selection on metabolic rate in the different environments. There was no difference in the relationship between metabolic rates and reproductive outputs in the two environments and so, although the changes they saw in metabolic rate with environment show that metabolic rate is plastic, their results show that such plasticity is not always adaptive.

Lukas and his supervisors emphasise the importance of assessing selection on a trait in the different environments before assuming that ‘plastic’ responses to different environments are necessarily adaptive. Instead, metabolic plasticity may merely represent a passive response due to correlations with other traits or there may be limits to physiological plasticity due to biochemical constraints. Nonetheless, further studies are needed in order to understand the drivers and consequences of metabolic plasticity in the field.

This research was published in the journal Oikos.

May has been a busy month for the postgraduate students within the Centre for Geometric Biology. Not least with Amanda Pettersen graduating just before flying out to Sweden to begin a postdoc position.

Alex Gangur, who arrived from the UK in February of this year is immersed in pilot studies to help him design an experiment that will be central to his PhD research.  

Alex is interested in how natural selection will play out in areas that differ in productivity. He is planning a large, long term laboratory experiment where he will manipulate food densities of a marine, harpacticoid copepod (Tisbe sp.) to provide environments that are able to support different numbers of copepod, that is, have different carrying capacities. Alex will then be able to track numerous life history characteristics, such as size, reproductive effort, age at reproduction etc, in the copepods over multiple generations.  

In order to be sure that his proposed food densities create environments where population growth is limited by food and not some other factor, Alex is testing his experimental food densities in a pilot study. If populations stop increasing as food is increased then the population size is limited by something other than food and Alex will need to use a lower food concentration.

To further fine-tune his experimental protocols, Alex wants to know if he can use chemostats to grow up his copepod populations rather than glass bottles.  

The chemostats work by bubbling air up through the bottom of the chamber which, while an advantage in keeping waste build-up to a minimum, can have a downside as airflow can kill copepods if it gets in under the carapace.  Alex will monitor the growth of a population split between a glass bottle and a chemostat to see if they are the same and, if they are, he can go ahead and use the chemostat. 

Lukas Schuster is approximately half way through his PhD candidature. He is currently running two experiments using one of the lab’s favourite animals; the colonial bryozoan Bugula neritina. Lukas is interested in how metabolic rate (a measure of energy use) co-varies with measures of survival, growth and reproductive rate. By directly linking metabolic rate to measures of fitness Lukas can measure how selection acts on metabolic rate.

In his first experiment for this field season Lukas is deploying newly settled Bugula on panels that are either hung horizontally – a benign environment for Bugula – or vertically; a harsher environment due to increased exposure to UV and sediments. 

We know that the harsher environment tends to reduce growth rates, reproductive output and survival in this species, but what we don’t know is if, or how, selection on metabolic rates differs between these two environments.  Every 2 weeks Lukas collects his panels from the two environments and brings them back to the lab where he measures colony size, growth rate, survival, and reproductive output (number of ovicells) as well as metabolic rate for each of approximately 400 individual colonies. 

Lukas has another 1,000 colonies in the field that he is monitoring fortnightly for a separate experiment. In this one, Lukas has taken a field populations of Bugula and measured metabolic rate for every individual colony. He then selected colonies with low, high or intermediate metabolic rates so that he has effectively created new populations that have different mean metabolic rates. At the same time, he is manipulating densities so that he will be able to tell if it is population density or metabolic rate that is having an effect on the growth rate, survival and reproductive output of the Bugula colonies.

Hayley Cameron is in her final year of her PhD and is following up on earlier experiments. Hayley is also using Bugula neritina as a model species to test some fundamental theoretical ideas. Hayley is interested in the outcomes of maternal investment. She is spawning Bugula in the lab and then choosing big, small or intermediate larvae to settle and create populations of siblings that differ in mean size. Hayley wants to know if it is better to produce a few, larger, offspring, or more, smaller, offspring when they have to compete with their siblings (which many marine invertebrates do).

Hayley has returned her populations of different sizes to the field and every week measures size, number of ovicells and survival for each of her 300 individuals. She has been doing this for 9 weeks so far, but will continue to track each individual’s performance across their entire life span (probably a matter of months). In addition Hayley is spawning her colonies in the lab when they are reproductive because she wants to know what size offspring they make.  Will the bigger offspring do better, even when in competition with their siblings and, in turn, make bigger offspring themselves? Watch this space.

Amanda Petterson with PhD supervisor Prof Dustin Marshall.

Alex will use a chemostat to grow his copepod populations.

Alex is growing algae to feed copepods.

One of Lukas’ Bugula plates

Hayley has created populations of siblings that differ in mean size. Will many small siblings do better than fewer big ones?

Today postdocs Martino Malerba and Giulia Ghedini spent the day investigating if the size of algal cells will affect the grazing rates and total amount ingested for the filter feeding bryozoan Bugula neritina.

To do this they used large and small cells that had been generated through artificial selection of the microalga Dunaliella tertiolecta  and added either equal volumes or cell numbers of each size class to vials containing Bugula. These vials were kept on a roller system to ensure that algal cells didn’t sink and become unavailable for feeding.

Giulia took samples every half an hour for two hours from each of the 60 vials, with the help of volunteer Blake Chan, who then had to run the samples over to Martino who was waiting in a separate building.

Once Martino had the samples he was able to put them through a high-tech flow cytometer and get accurate measurements of the number of algal cells for more than 300 samples.

From the data the team will be able to work out whether the Bugula have consumed more cells of a certain size and whether this equates to a greater overall amount of algal biomass consumed.

This work forms part of a larger program that is investigating whether (and in what way) evolution in size of one species can effect another species.

If you would like more information about this research please contact Martino or Giulia.