By Hannah Corsini, Kingfisher Writer

Published April 5th, 2024

Population genomics may be the key to understanding the current insect apocalypse. While historically, in the United Kingdom insects have been rich in abundance, insects such as butterflies have declined at least 70% in the past four decades. A look back into ‘The Great Yellow Bumblebee’ exemplifies a story of the decline of British rare species.

Population genomics for conservation has previously focused largely on vertebrate species, but this is short-sighted. There are around 11 million animal species living on this planet, and approximately 5.5 million of those are insects. Insects are integral to vibrant and essential ecosystems around the globe.

Insect declines have been reported worldwide, with the vast majority of insect populations diminishing, whilst a minority of pest species become increasingly common. Approximately, 70% of butterfly species in the United Kingdom have declined over the last 40 years, and five species are now extinct. Similarly, Germany has lost 75% of its flying insect biomass over the last 27 years.

Conservation genomics can be a powerful tool to investigate the interaction between the risks that climate change and biodiversity loss can pose to insect populations and their genetic traits. Scientists can use genetic techniques to assess the overall health of small insect populations, which is particularly useful when the population is endangered.

‘Insect declines have been reported worldwide.’

For example, in his book ‘Bee Quest,’ Dave Goulson describes his voyage to the Scottish Hebrides to search for the Great Yellow Bumblebee after decades of its population declining—a species often thought to be the rarest bumblebee in the UK.

The Great Yellow Bumblebee now only exists in the UK as sparse populations in remote Scotland. When these populations were genetically sampled, something incredibly rare was found—the male individuals were diploid.

A ‘diploid’ cell contains two sets of chromosomes. The vast majority of all human cells are diploid, with the only exception being our gametes, or sex cells: namely, sperm and eggs. Sex cells contain one set of chromosomes, and in the case of reproduction, the sex cells will fuse: creating a cell which contains two sets.

Sex determination is a little bit different in bees, however. Normally, male bees are haploid—possessing just one set of chromosomes—and their sisters, the worker bees, are diploid. The males are born from unfertilised eggs, meaning that whilst they have a mother, they have no father. Diploid female bees always have two different sex chromosomes, whilst male bees will normally only inherit one chromosome, from their mother.

But in the Scottish Great Yellow Bumblebee species, fertilised eggs were producing males, and these males had two sets of chromosomes. The scientists ascribed this peculiarity to the effects of inbreeding within the small bee populations. Normally, when a queen mates with a drone, the sex chromosomes are different enough that she will only create a diploid female bee. However, if the two sex chromosomes are the same, e.g. because of relatedness she will create a diploid male bee.

‘The Great Yellow Bumblebee now only exists in the UK as sparse populations in remote Scotland.’

In inbred bee populations, the chances of producing a male are a lot higher. If a female bee mates with a drone with one of the same sex chromosomes as her, she has two possibilities: creating a diploid male or diploid female—so the odds for each are one in two. For example, if we say the queen bee has sex chromosomes AB (a queen will always have two different chromosomes) and the drone has the sex chromosome B, she can create either a bee with sex chromosomes AB (a diploid female) or AA (a diploid male).

This creates issues for the population because male bees are functionally useless; their entire purpose in life is to attempt to reproduce, which leaves all of the work to the female bees. Diploid males are especially useless as they are sterile so they cannot even mate.

Why did this arise? If vast areas of land are unsuitable for the species, this means there will be a wider distance between the existing populations. If the populations are far apart, they cannot interbreed. This means that when the queen bee takes her virgin flight (the moment at which a queen bee finally mates) she will have fewer mates to choose from, and may select incompatible or closely related male bees. This is a phenomenon known as habitat fragmentation and can lead to effects like the diploidy shown in the male Great Yellow Bumblebees of the Scottish Hebrides.

Population genomics can also reveal beneficial traits—particularly traits that may help species survive in the face of climate change. For example, genetic techniques enabled scientists to determine the genes which help the yellow-faced bumblebee, Bombus vosnesenskii (a bee common to North America), survive fluctuating temperature conditions.

These genes are involved in heat and cold tolerance and usually control metabolic processes. One of them is protein folding, a process which occurs in the vast majority of cells and is essential to life. If proteins cannot fold properly, then cell reactions cannot occur and the cell can die. If protein folding can occur at higher temperatures, then that organism is more likely to survive at those temperatures, which could come in clutch during the ongoing climate crisis.

Scientists have also examined the genes which enabled the diamondback moth to become such a successful pest in variable climates around the globe. Not only can this give some genetic perspective on why other species are less successful, but can also help scientists to work towards pest management of the moth.

‘Habitat fragmentation can lead to effects like the diploidy shown in the male Great Yellow Bumblebees of the Scottish Hebrides.’

Genomics has also helped scientists break away from species-specific conservation models. Unlike what we are mostly taught at school, the boundaries between what constitutes different species are often fluid, which is evident when we look at DNA samples. Genome-wide analyses allow scientists to determine traits which are either important or harmful for the ‘fitness’ of a species, i.e. its ability to survive and reproduce.

The question of ‘what makes something distinct enough to be a distinct species and not just another variation within a species?’ is one that was asked by Darwin himself, but that still plagues the ecological community today. Organisms that are classified together as ‘species’ may exhibit a vast range of traits varying by lineage and region. Insect population genomics can allow us to ensure that this within-species diversity is maintained and that each ‘species’ does not become completely homozygous— carrying one form of a gene alone—and therefore genetically identical.

‘Genome-wide analyses allow scientists to determine traits which are either important or harmful for the ‘fitness’ of a species.’

Instead of focusing on a species as a whole, we can now zero in on isolated subspecies populations, which may have diverged significantly from the rest of their species. Examples include the Dewy Ringlet butterfly population in the Apennines Mountains.

The Apennines are a mountain range in Southern Europe, which hosts a small population of Dewy Ringlets far away from the rest of their species, scattered across Europe but previously thought to have disappeared from the area. This population is at risk of extinction because climate change is warming their alpine habitat, but they cannot migrate any higher to remain in suitable temperatures.

If scientists had used a species-specific conservation model in this case, they may have ignored the Dewy Ringlet since it already exists throughout Europe. But if we approach this from a more holistic point of view, we can see that the butterfly has important local value, including region-specific genetic traits, making it a high priority for conservation.

One of the challenges faced by insect population genomics is that insects have a shorter generation time. Because insects reproduce so quickly and in such high quantities, their populations are more susceptible to genetic mutations. Consequently, they experience evolution more quickly than other animals.

This has the benefit of creating greater genetic diversity as seen in the Dewy Ringlets, but it also makes conservation plans a lot harder as the species are rapidly changing and there is no one-size-fits-all approach. Instead, actions need to be taken on a local scale to protect the diversity of insect life, something the entirety of natural ecosystems are dependent on.

Population genomics and conservation

Genetics & genomics are especially useful in the context of conservation. Even before the advancement of sequencing technology, biologists were aware of the concept of ‘genetic drift.’ This is the idea that in small populations, randomness has more of an impact on the genes that are present. Chance events (e.g. fires) can mean that some individuals die before reproducing and their ‘alleles’ versions of a particular gene) are not passed on. In a small population, the gene pool is also small, making it less likely for there to be other individuals with that allele. This leads to-the allele dying out with the individual.

‘...the butterfly has important local value, including region-specific genetic traits, making it a high priority for conservation.’

Conversely, the individuals that survive the fire will be able to reproduce and pass on their alleles, leading to those alleles increasing in frequency, potentially becoming the only form of the gene present.

The decline of butterflies and bumblebees illustrates the insights that genomics can offer into the effects of inbreeding and population dynamics. The genetic insights provided by genomics not only shed light on the underlying causes of population declines but also offer potential solutions for fostering resilience in the face of environmental challenges.

Featured Image: Europeana / Unsplash

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