Travel Precision of Migratory Birds: The Role of Genes
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Transcript: English(auto-generated)
00:07
My research is really motivated by my major interest in avian biology and in particular the phenomenon of bird migration. With our work we want to understand how this behavior is modulated and formed.
00:21
So what are the genes that are driving this phenotype or this behavior? By migration I mean coordinated, oriented and periodically repeating mass movements of all or a fraction of individuals of one species or a population. In birds this behavior ranges from spectacular mass movement of flocking starlings to the almost invisible movement of tiny little passerines.
00:46
And these are particularly fascinating for me. They migrate at night and especially young birds on their first migration they do this alone so they don't have anyone to follow. And they do this over thousands of kilometers often to a different continent and with amazing precision. So the central question of our research is really how do they do this?
01:04
How do genes allow birds to find their way with such accuracy? And we do know that birds do possess a couple of adaptations to cope with this challenge of migration. So birds are equipped with an inherited initial direction and a time schedule that tells them when to leave and also when to stop.
01:23
So birds integrate this information into a spatial temporal orientation program which tells them when to leave, which direction to fly to and also when to stop. If you want to understand this behavior on the molecular level to really identify the genes that are shaping this variation you want to work with a species that exhibits variation in that behavior.
01:44
And this is why we work on black caps because black cap populations across their distribution range exhibit the entire spectrum of migratory phenotypes. We have long distance, medium distance, short distance migrants and also resident populations of the same species that do no longer migrate.
02:01
In addition to this difference in the propensity and distance traveled, they also exhibit variation in their orientation phenotype. So some birds go southeast in autumn whereas other populations migrate southwest. There is an area where neighboring populations with these distinctly different orientation programs meet and that's called a migratory divide.
02:22
In addition to the southeast and southwest orientation there is a more recently established migratory route that takes birds up north to winter in the British islands. So they migrate northwest and that is especially exciting because it allows us to really study evolution in action.
02:41
Black caps in general because of this huge variability in the migratory phenotype are an excellent study species to understand what genes modulate this variation. The migration behavior is something that happens in mid-air and it covers thousands of kilometers and often stretches over different continents.
03:04
It's actually possible to study this behavior in the lab and that is what has traditionally been used. So migratory birds that are kept in cages exhibit so-called migratory restlessness behavior once the migratory season comes in. That means they are so eager to migrate that even in a cage they exhibit migration behavior.
03:23
So they flap their wings and they hop around and they do this in a very coordinated manner. So they hop in a direction that they would normally migrate to. And we can use the characterization of this orientation and also of the timing as a proxy for migration.
03:40
Most of our understanding so far really is based on this assessment in a caged migrant. But that's also the limitation of previous research because everything is based on these indirect measures. But if you want to understand and really identify the genes that are driving this variation, you want to characterize this focal phenotype,
04:02
so the migration behavior, as precisely as possible and in its natural habitat. And this is only recently becoming accessible to do on small songbirds and that's exactly what we do. So we use geolocator technology to first characterize orientation preference of blackcaps across the migratory divide
04:20
and also use this tracking technology to see on where birds that are spending their winters in the UK actually breed. Because this is something we haven't understood at all so far or we haven't been able to study so far. So a geolocator is a light level archival tag that has been miniaturized to be used on small songbirds.
04:43
So it weighs less than half a gram or so. You can mount it as kind of a backpack on the bird and then the bird flies to its wintering area and comes back. And the data is stored on this geolocator so you have to recapture this bird in order to access and download the data. And then the data is basically a time series of light intensity data linked to date and time.
05:07
And then you can interpret this time series and infer day lengths as an estimate of latitude and also the time of midday as an estimate of longitude and then use this data to reconstruct the position of the bird along its migratory journey.
05:24
So that gives us the accurate phenotype for each bird and we also take a blood sample of each of these birds to then use the DNA that is in those nucleated blood cells to sequence the entire genome of each individual. And that allows us to compare individual variation of each bird on the behavior level
05:47
with individual variation of each bird on the level of the genome. As key findings I want to highlight what we find on the behavioral level by tracking wild migrants.
06:01
And then I also want to give you a first insight into what we find on the level of the genetic control. And here we focus on a population-based average of the behavior. So when we use geolocated technology to characterize orientation across the migratory divide, we actually found and could confirm the migratory divide hypothesis.
06:23
And that means that we could find birds that follow all different orientation angles between the pure southeast and southwest orientation. And this so-called hybrid swarm now allows us to link this individual variation in this phenotype of interest to the individual variation of the genomes of these birds
06:44
that we are currently sequencing. We also were wondering how this new phenotype of birds overwintering in the UK actually came about. So we don't really know where the breeding origin of these birds are. And it was assumed that these birds come primarily from the area of the hybrid zone
07:03
or this migratory divide. And our tracks actually confirm that this is not true, but these birds breed all over Europe. So there isn't one particular population on the continent that then migrates to the UK. But the birds that are overwintering in the UK actually come from breeding grounds
07:20
as diverse as Poland or Italy or France. It's not one distinct population. And that was totally surprising. And it's very exciting to follow up on this. So to match phenotype to genotype, we laid the groundwork by assembling a reference genome for the black cap. We are currently focusing on a population-based average of all the different phenotypes
07:44
that the black cap exhibits with respect to migratory behavior. We find that variation on the genome scale is actually very low between the very different behavioral phenotypes. So whatever differs between different migratory behavior on the genome level
08:00
very likely actually relates to this behavior because the populations are otherwise very similar. And the clearest difference is between migratory and non-migratory populations. The differences in the genomes of migrants and residents localize on very few regions on the genome. They are very short also.
08:22
And the genes that are under these regions relate to functions that do make sense in the context of adaptation to residency. So they encode, for example, learning or memory formation, for energy expenditure, or appetite control. And these are all processes that are very relevant in an adaptation to a resident phenotype.
08:46
In these population-based average analyses, we are considering all different traits together. So the propensity, the distance, and the orientation of a migrant. We see that most differences relate to residency. But to understand what is the difference between the different migratory phenotypes,
09:04
we can now use the individually phenotyped birds or the behavioral data for these birds and link it to their individual genomes. But that is only possible on the individual level, and that is what we are going to do in the next step.
09:21
Our work on understanding the genetics of behavior will not only increase our understanding of this behavior itself and its huge variability, its evolutionary potential, but it will also have important implications for related areas such as sensory biology. For example, when we understand why a bird migrates southwest as opposed to southeast,
09:44
we can then look into how, for example, the magnetic compass that helps the bird to orient on this direction functions on a molecular level. Possibly most importantly, understanding what migration genes actually are and what they are doing could help us to forecast adaptations of population
10:03
to changing ecological parameters such as the climate. So if we understand how populations adjust to changing ecological conditions, this could help us to also use the information we gain on a very common species such as a black cap to feed into conservation management plans for more threatened migratory species.
10:29
Genotypic variation on this migration behavior can be altered on many different regulatory levels, and so far we have only looked at the sequence level. But subtle changes in the DNA sequence can lead to huge differences in gene expression profiles.
10:45
So as one key complementary approach, we are also looking at these gene expression differences, and here we are additionally making use of the fact that the migratory phenotype is only expressed during migration. And so we can not only contrast differences in migratory phenotype,
11:02
but also compare gene expression differences during migration with off-season controls to really identify the signaling cascades and the regulatory mechanisms that are driving variation in this behavior. Ultimately, providing proof of principle basically will be imperative,
11:21
and it will be challenging to manipulate identified candidate structures in vivo, but this will be the only way to really prove the effect of disruption of these genes on the migratory behavior in a migratory bird. With the increased accessibility of also functional genomic tools to black caps, for example,
11:42
this will be possible in the future.