ECOS 38 (4)

Winner - ECOS Student Article 2017 Competition, supported by Conservation Careers and Green&Blue

Quantifying genetic diversity can be expensive and time consuming. So in a field which is already fighting for funding why should we bother applying genetic study to active conservation?

Planning for resistance and resilience in plant populations

It is impossible to know which combinations of these pressures will be exerted on plants in coming years and so increasing genetic diversity as a whole represents the best chance of ensuring their survival. There are also more specific situations in which genetic data can be used to inform active conservation decisions and measures. Here I discuss some examples with a specific focus on plants but the underlying points are applicable to genetic conservation as a whole. Hopefully in the future there will be greater collaboration between conservation groups and academic research on the topic of genetic conservation.  Environmental pressures faced by Britain’s protected flora means that without these conservation approaches much of their adaptive potential is already being lost.In recent years the process of conserving a plant species has become a multi-faceted exercise. It is no longer possible to ensure long-term conservation through only protecting a geographical area. By not being aware of the genetic value and vulnerabilities present within protected plant populations we leave the door open to disaster in the face of mounting environmental instability. Fragmentation of wild areas combined with climate change and the increasing globalisation of pests means we should be trying to increase the potential for resistance and adaptability within threatened and protected populations as much as possible. If both individuals and populations are more genetically diverse they represent a larger adaptive range which may contain genes linked to resistance to a new disease, greater flexibility in growth temperature and the like. In more restricted or uniform  populations this pool of options becomes significantly smaller and the plants are less likely to be able to weather any new selective pressures that come their way.

Which areas to protect?

The decision between which areas to grant protection is influenced by a number of factors including presence of rare species, rarity of habitats present and history of the area. Presence of rare genetic variants or a higher genetic variation within keystone plant species is, however, not often on the list. The genetic ‘strength’ of key species can have important impacts on whether the population represents a good investment for long term viability; if individuals are highly inbred this may translate into greater disease susceptibility or lower reproductive success in future years. If a set of individuals are fully asexual then this can represent a highly vulnerable population which may require out-crossing by hand to increase its genetic portfolio. Conversely, rare cytotypes and cryptic subspecies may be revealed through studying genetic relatedness. These can be locally adapted cytotypes that may show traits such as increased salt or heavy metal tolerance. Polyploidy in plants is also not always visible and can sometimes be a precursor to the formation of a new species. Some of these features may be of national significance if an area possesses unique ecological conditions. Areas with a history of geological or environmental change can also cause hybridisation or speciation leading to pockets of diversity. Therefore, understanding the value of an area in this way has the possibility to greatly influence choices for protection. It can also lend support to existing applications for protection as higher genetic variation may often correlate with features that conservation bodies intuitively wish to preserve such as older, natural and larger areas.

A large reduction in population size, e.g. through human influence or disease, will often leave a population of reduced diversity. Therefore, even if the population recovers it may be genetically vulnerable.

Artificial intervention

There are a limited number of strategies which plants can adopt in deteriorating conditions: migrate, slowly adapt, or be flexible. If we can better understand the coping strategies of an individual species it can inform how we try and aid its survival. A common intervention to bolster dwindling populations is to artificially increased population size. However, without genetic context, this can lead to numerous, often invisible, consequences. Loss of local cytotypes, through genetic swamping or outbreeding depression, is where differently adapted populations mix to form a population adapted to neither. Identifying compatible populations, for instance with ecological niche modelling, to identify populations that have been under similar ecological pressures, prior to introduction can inform choices for such interventions. This can also be useful in the reverse, when a species must be moved in order to ensure its survival. Combined genetic and environmental data can be used to calculate seed transfer zones which describe the areas between which relocation would have the smallest consequences.

Habitat fragmentation is an increasing threat for British wildlife and thus for many plants migration is no longer an option. A side effect of fragmentation is reduced gene flow between populations which can be accompanied by changes in environmental conditions, especially those now found to be on an edge. Reduced population size and reduced outbreeding options will normally lead to inbreeding depression which will have differing impacts dependent on species. Isolation thresholds represent the maximum distance a population can be isolated before it begins to lose genetic variation as genetic material is no longer transferred to other populations. This can help inform which populations are most in need of pre-emptive action such as the provision of wildlife corridors.

Ex-situ seed conservation may act as a lifeboat for many species but it does not allow for slow, stochastic adaptation in response to the environment. This translates into a population that when re-introduced into the world may be unable to cope with the apparent sudden change thrust upon it. This underlines the importance of conserving genetic diversity in-situ, where adaptation can still take place. Genetic screening can help us understand how best to preserve the widest range of diversity currently available. This may mean sampling particular populations or individuals in order to maximise the chances of some being correctly adapted, or plastic enough to be, when germinated in the future.

Identifying vulnerability and resistance

Genetic plasticity is becoming an incredibly sought after trait in plants as globalisation of pests and therefore disease will have profound impacts on these effectively sessile organisms. I have already acknowledged the definition of ecologically important traits is likely to change with time, but one sure to be of relevance is disease resistance. Ash dieback is a very timely example of this. Work at Queen Mary University of London is currently using genetic screens of European ash trees in order to identify genes linked to resistance, reporting success in predicting decreased susceptibility through genetics.

When an area has been through a reduction in population size, through fragmentation and disease,  it can leave the population vulnerable. When sex is free and frequent across a small population the outcome can be homogenous population; sharing the same susceptibilities to disease and preferences in environment. Should ecological conditions then change it is more likely that a wider proportion of the population will suffer. Populations such as this can appear to be maintaining well in the absence of such change but repeated sampling can alert to decrease of diversity over time, allowing an early-warning for populations that have increasing vulnerability. These erosions in ecologically dominant plant species can have knock-on effects for the ecosystem generally, including reducing its resilience.

Ash dieback is a disease having a profound impact on native ash trees. Work is currently underway to use genetic and computational techniques to identify genes that may be linked to resistance.

Applying conservation genetics

Whilst a large proportion of conservation is aimed at damage control and the race to save the most endangered, conservation genetics is focussed on the protection of dynamic units capable of adaptation into the future rather than individuals. Conservation genetics acts to anticipate the future pressures a species may face and create resilient and robust populations. Plants suffer most from the consequences of low genetic diversity. Genetic study will therefore be of increasing importance for conservation decision making and response efforts. There are still many challenges in increasing the speed and accessibility of genetic technologies for informing conservation, with the hefty time commitment required providing a large portion of the reason behind their scarcity. However, I hope that this article has still managed to stress the importance of such work and what can be lost if we do not apply it where most needed.

References

  • Harper, A.L., McKinney, L.V., Nielsen, L.R., Havlickova, L., Li, Y., Trick, M., Fraser, F., Wang, L., Fellgett, A., Sollars, E.S. and Janacek, S.H., 2016. Molecular markers for tolerance of European ash (Fraxinus excelsior) to dieback disease identified using Associative Transcriptomics. Scientific reports, 6.
  • Kramer, A.T. and Havens, K., 2009. Plant conservation genetics in a changing world.Trends in plant science14(11), pp.599-607.
  • Young, A., Boyle, T. and Brown, T., 1996. The population genetic consequences of habitat fragmentation for plants.Trends in Ecology & Evolution11(10), pp.413-418.

ELLEN BAKER

The author is an MSc student at RBG Kew.

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Keywords

Polyploidy – Duplication of the genome to produce individuals with increased chromosome counts

Cytotypes – Different ploidy levels  within a species

Inbreeding depression – Negative phenotypic effects caused by breeding with individuals similar to oneself

 

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