Periodic Reporting for period 3 - MuBoEx (Mushroom Body Expansion in Heliconius butterflies)
Período documentado: 2021-03-01 hasta 2022-08-31
How and why do some species evolve bigger brains than others? The major goal of our project is to understand the evolutionary and development mechanisms that control the expansion of brain components, and explore its behavioural consequences. We study the evolution of a pair of structures in the insect brain called the mushroom bodies (MBs). MBs are prominent ‘higher order’ structures that integrate sensory information and store memories of past experience. Insect MBs share a conserved ground plan, but their size and morphology vary extensively across species.
We have established a new study system, Heliconius butterflies, to explore the evolution of MB diversity. Heliconius MBs are among the largest of any insect, and 3-4X larger than typical for Lepidoptera, including genera within Heliconiini, the tribe to which they belong. This dramatic enlargement is hypothesised to form the basis of enhanced spatial memory associated with a dietary innovation, whereby adult Heliconius collect and digest pollen grains to obtain an adult source of amino acids. Heliconius collect pollen from a restricted range of plants that occur at low densities, apparently visiting these plants at the same times, and in the same order, every day. This behaviour, known as trap-lining, suggests that they learn routes between key resources. Several lines of evidence already suggest that this foraging behaviour involves an enhanced capacity for visually orientated memory in Heliconius. This cognitive adaptation is hypothesised to involve “some elaboration of the nervous system”, with the MBs being a key candidate. However, the link between MB expansion in Heliconius, cognitive enhancement and the evolution of pollen feeding remains untested.
Our project aims to answer four key questions that span behaviour, neuroanatomy and development to provide a cohesive understanding of both how and why MBs expanded in Heliconius:
1. How does MB expansion enhance cognitive ability?
We are asking whether an increased rate of MB expansion coincides with the origin of pollen feeding, and conducting a series of behavioural experiments to test whether increases in MB size are associated with improved visual memories, and whether improved learning performance is also observed, or traded off against, other aspects of learning.
2. What aspects of MB structure contribute to its volumetric expansion?
We are using variation in MB volume across Heliconiini species as a framework for detailed anatomical studies. We are asking i) whether neuron number, synaptic density and structure increased predictably with volumetric expansion, or evolve independently, reflecting changes in local connectivity; ii) whether MB expansion involves novel or increased input from different sensory structures; and iii) whether such changes are associated with the evolution of specialised sub-components of the MB. Finally, we aim to link these traits to variation in performance in our learning experiments.
3. What developmental mechanisms control MB expansion?
MB evolution in Heliconius provides a dramatic example of region-specific evolutionary expansion, and a novel system for investigating the developmental mechanisms that control brain size. By adopting a comparative developmental approach we are investigating how increases in MB neuron number are produced, considering four potential mechanisms: i) an increase in progenitor cell number, ii) accelerated cell-cycle rates during neurogenesis, iii) an extension in the overall duration of neurogenesis, iv) reduced or delayed patterns of apoptosis.
4. What is the genetic basis of MB expansion?
Our project is providing a new insect system for studying the molecular basis of increased neural proliferation. By investigating patterns of positive selection across the genome that are coincident with periods of rapid MB expansion, together with assays of gene expression and regulation. we are working towards identifying genes implicated in shaping MB evolution, and investigate the functional effects of selection on these loci. This will provide the first investigation into the genetic basis of MB evolution.
Although we focus on one group of butterflies, our objectives tackle major questions that cut across study systems and apply even to human evolution. For example, does brain expansion necessarily increase computational power? Does it reflect novel functions formed by modified connections between brain regions? How is brain size and structure regulated? How do environmental cues interact with developmental programs? And is behavioural evolution driven by independent changes in the function of specific brain components, or changes in the way brain regions interact? By studying a tractable group of insects, which combine extreme trait differences with the ability to use a range of experimental techniques, we will provide new clues to these big questions.
We have established a new study system, Heliconius butterflies, to explore the evolution of MB diversity. Heliconius MBs are among the largest of any insect, and 3-4X larger than typical for Lepidoptera, including genera within Heliconiini, the tribe to which they belong. This dramatic enlargement is hypothesised to form the basis of enhanced spatial memory associated with a dietary innovation, whereby adult Heliconius collect and digest pollen grains to obtain an adult source of amino acids. Heliconius collect pollen from a restricted range of plants that occur at low densities, apparently visiting these plants at the same times, and in the same order, every day. This behaviour, known as trap-lining, suggests that they learn routes between key resources. Several lines of evidence already suggest that this foraging behaviour involves an enhanced capacity for visually orientated memory in Heliconius. This cognitive adaptation is hypothesised to involve “some elaboration of the nervous system”, with the MBs being a key candidate. However, the link between MB expansion in Heliconius, cognitive enhancement and the evolution of pollen feeding remains untested.
Our project aims to answer four key questions that span behaviour, neuroanatomy and development to provide a cohesive understanding of both how and why MBs expanded in Heliconius:
1. How does MB expansion enhance cognitive ability?
We are asking whether an increased rate of MB expansion coincides with the origin of pollen feeding, and conducting a series of behavioural experiments to test whether increases in MB size are associated with improved visual memories, and whether improved learning performance is also observed, or traded off against, other aspects of learning.
2. What aspects of MB structure contribute to its volumetric expansion?
We are using variation in MB volume across Heliconiini species as a framework for detailed anatomical studies. We are asking i) whether neuron number, synaptic density and structure increased predictably with volumetric expansion, or evolve independently, reflecting changes in local connectivity; ii) whether MB expansion involves novel or increased input from different sensory structures; and iii) whether such changes are associated with the evolution of specialised sub-components of the MB. Finally, we aim to link these traits to variation in performance in our learning experiments.
3. What developmental mechanisms control MB expansion?
MB evolution in Heliconius provides a dramatic example of region-specific evolutionary expansion, and a novel system for investigating the developmental mechanisms that control brain size. By adopting a comparative developmental approach we are investigating how increases in MB neuron number are produced, considering four potential mechanisms: i) an increase in progenitor cell number, ii) accelerated cell-cycle rates during neurogenesis, iii) an extension in the overall duration of neurogenesis, iv) reduced or delayed patterns of apoptosis.
4. What is the genetic basis of MB expansion?
Our project is providing a new insect system for studying the molecular basis of increased neural proliferation. By investigating patterns of positive selection across the genome that are coincident with periods of rapid MB expansion, together with assays of gene expression and regulation. we are working towards identifying genes implicated in shaping MB evolution, and investigate the functional effects of selection on these loci. This will provide the first investigation into the genetic basis of MB evolution.
Although we focus on one group of butterflies, our objectives tackle major questions that cut across study systems and apply even to human evolution. For example, does brain expansion necessarily increase computational power? Does it reflect novel functions formed by modified connections between brain regions? How is brain size and structure regulated? How do environmental cues interact with developmental programs? And is behavioural evolution driven by independent changes in the function of specific brain components, or changes in the way brain regions interact? By studying a tractable group of insects, which combine extreme trait differences with the ability to use a range of experimental techniques, we will provide new clues to these big questions.
We are pursuing our four objectives in tandem, with dedicated, specialist team members working on each problem. In the first half of the project we have made significant progress on each front.
1. How does MB expansion enhance cognitive ability?
To understand the evolutionary history and ecological associations driving MB expansion we collected, stained and imaged in excess of 400 individual butterflies, representing ~45 species spanning the Heliconiini tribe. We are collecting volumetric data from these images to reconstruct the evolution of MB expansion and test whether it has a single origin that co-occurred with the origin of pollen feeding. We have also developed several cognitive assays to understand how Heliconius use information from their environment, and how this differs from other Heliconiini. These include tests of traits that must contribute to trap-line foraging, including spatial learning, landmark learning, circadian memory, reversal learning and long-term memory retention.
2. What aspects of MB structure contribute to its volumetric expansion?
To go beyond volumetric comparisons we are investigating what cellular changes cause differences in MB size. We have traced projection pathways, which carry sensory information to the MB, to test whether MB expansion is associated with visual specialisation. We have also estimated how many neurons comprise MBs of different sizes, and whether the density of connections between these neurons is conserved across species.
3. What developmental mechanisms control MB expansion?
To understand how changes in MB size are produced by deviation in brain development, we have sampled 12 developmental stages that span the larval period, metamorphosis during pupation, and emergence of the adult butterfly. We are comparing patterns of development and neural proliferation in two Heliconius and two related Heliconiini with small MBs. We have identified key stages where MB development diverges across species are now investigating how this is brought about by changes in gene regulation.
4. What is the genetic basis of MB expansion?
Finally, to complement our developmental work, we have compiled a genomic dataset including 58 species from across the Heliconiini. This includes new genomes sequenced by our team, to have representatives of all Heliconiini genera, and re-assembled and improved genomes for many other species. We are now beginning to analyse this dataset to identify protein coding genes and regulatory elements whose patterns of molecular evolution suggest a role in MB evolution.
1. How does MB expansion enhance cognitive ability?
To understand the evolutionary history and ecological associations driving MB expansion we collected, stained and imaged in excess of 400 individual butterflies, representing ~45 species spanning the Heliconiini tribe. We are collecting volumetric data from these images to reconstruct the evolution of MB expansion and test whether it has a single origin that co-occurred with the origin of pollen feeding. We have also developed several cognitive assays to understand how Heliconius use information from their environment, and how this differs from other Heliconiini. These include tests of traits that must contribute to trap-line foraging, including spatial learning, landmark learning, circadian memory, reversal learning and long-term memory retention.
2. What aspects of MB structure contribute to its volumetric expansion?
To go beyond volumetric comparisons we are investigating what cellular changes cause differences in MB size. We have traced projection pathways, which carry sensory information to the MB, to test whether MB expansion is associated with visual specialisation. We have also estimated how many neurons comprise MBs of different sizes, and whether the density of connections between these neurons is conserved across species.
3. What developmental mechanisms control MB expansion?
To understand how changes in MB size are produced by deviation in brain development, we have sampled 12 developmental stages that span the larval period, metamorphosis during pupation, and emergence of the adult butterfly. We are comparing patterns of development and neural proliferation in two Heliconius and two related Heliconiini with small MBs. We have identified key stages where MB development diverges across species are now investigating how this is brought about by changes in gene regulation.
4. What is the genetic basis of MB expansion?
Finally, to complement our developmental work, we have compiled a genomic dataset including 58 species from across the Heliconiini. This includes new genomes sequenced by our team, to have representatives of all Heliconiini genera, and re-assembled and improved genomes for many other species. We are now beginning to analyse this dataset to identify protein coding genes and regulatory elements whose patterns of molecular evolution suggest a role in MB evolution.
Our work to date has laid the foundation for Heliconius to become a major study system for evolutionary neurobiology. In the second half of the project we will continue to develop and extend this work:
1. How does MB expansion enhance cognitive ability?
We will complete our comparative analysis of MB evolution. Alongside our broad range of behavioural experiments we will demonstrate key behavioural innovations associated with MB expansion across species, and how these behaviours affect, and are affected by, MB development within species. We are also extending this project to investigate the effects of age on cognitive ability. MB expansion in Heliconius is associated with a major increase in longevity. We will explore how they have negated deleterious effects of aging during this transition.
2. What aspects of MB structure contribute to its volumetric expansion?
We are continuing to unravel the cellular changes associated with expanded MBs. Our next aims are to determine how the diversity of MB cell types and internal architecture of the MB lobes have changed. Some evidence suggest MB lobes are essential for memory retrieval, and also send projections to other brain regions to influence behavioural action. In addition, we are beginning to explore the association between the MB and another brain region critical for sensory integration and motion control, the central complex. This structure is implicated in spatial memory in fruit flies, but little is known about how the MB and central complex co-evolve.
3. What developmental mechanisms control MB expansion?
We will complete our neurodevelopmental studies, combining data on MB growth with cellular assays on cell proliferation and cell death to identify determine how increases in MB neuron number are produced by changes in cell proliferation during neurogenesis. We will complete our investigations into the regulatory control of these processes, identifying genes that are linked to divergent developmental trajectories in small and large MBs.
4. What is the genetic basis of MB expansion?
We are now at a point where we are beginning to analyse our genomic dataset. We aim to integrate our genome analyses with our morphometric dataset to identify protein coding genes and regulatory elements with accelerated rates of evolution, duplications or deletions that coincide or co-evolve with MB expansion. We will integrate this with our gene regulation studies, to test whether any of the genes associated with neurodevelopmental divergence has histories of adaptive evolution that are consistent with playing a role in MB expansion. This will reveal the first candidate genes associated with MB evolution.
Our final aim is to integrate these approaches, to test the behavioural, developmental and neuroanatomical significance of our candidate genes.
1. How does MB expansion enhance cognitive ability?
We will complete our comparative analysis of MB evolution. Alongside our broad range of behavioural experiments we will demonstrate key behavioural innovations associated with MB expansion across species, and how these behaviours affect, and are affected by, MB development within species. We are also extending this project to investigate the effects of age on cognitive ability. MB expansion in Heliconius is associated with a major increase in longevity. We will explore how they have negated deleterious effects of aging during this transition.
2. What aspects of MB structure contribute to its volumetric expansion?
We are continuing to unravel the cellular changes associated with expanded MBs. Our next aims are to determine how the diversity of MB cell types and internal architecture of the MB lobes have changed. Some evidence suggest MB lobes are essential for memory retrieval, and also send projections to other brain regions to influence behavioural action. In addition, we are beginning to explore the association between the MB and another brain region critical for sensory integration and motion control, the central complex. This structure is implicated in spatial memory in fruit flies, but little is known about how the MB and central complex co-evolve.
3. What developmental mechanisms control MB expansion?
We will complete our neurodevelopmental studies, combining data on MB growth with cellular assays on cell proliferation and cell death to identify determine how increases in MB neuron number are produced by changes in cell proliferation during neurogenesis. We will complete our investigations into the regulatory control of these processes, identifying genes that are linked to divergent developmental trajectories in small and large MBs.
4. What is the genetic basis of MB expansion?
We are now at a point where we are beginning to analyse our genomic dataset. We aim to integrate our genome analyses with our morphometric dataset to identify protein coding genes and regulatory elements with accelerated rates of evolution, duplications or deletions that coincide or co-evolve with MB expansion. We will integrate this with our gene regulation studies, to test whether any of the genes associated with neurodevelopmental divergence has histories of adaptive evolution that are consistent with playing a role in MB expansion. This will reveal the first candidate genes associated with MB evolution.
Our final aim is to integrate these approaches, to test the behavioural, developmental and neuroanatomical significance of our candidate genes.