March 21, 2025 • 15 mins
The Director of the Milner Centre for Evolution, Professor Turi King, talks to Dr Leslie Turner about her research on our understanding how speciation takes place.
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Episode Transcript
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(00:02):
Hello and welcome. You are listening to a podcast by the Milner Centre for Evolution at
the University of Bath. I'm Professor Turi King, your host, and today I'm talking to Leslie Turner,
whose research focuses on understanding how speciation takes place. Leslie,
you're looking at the evolution of species and how that happens. So, for those people who don't know
(00:26):
what is the definition of a species and what are the mechanisms where new species evolve?
So, there's many different definitions for species, but one that's broadly used,
and the one that I use is the biological species concept, which defines species as
groups of organisms which are reproductively isolated from each other. And that focus
(00:47):
on reproductive isolation is really useful because it focuses on that mechanism of what
is stopping genes from going back and forth between the groups. So, stopping gene flow.
So basically, they can't reproduce with one another, they're kind of reproductively separate,
that's the definition of a species?Yes. So, either they won't mate with
each other or when they do mate, the hybrids that they produce are either not born or have
(01:13):
something wrong with them. Or it could be that the hybrids themselves are unable to reproduce, or
they're just not very well fit to the environment because they're a mix between the two species,
so their traits may not be a good fit.So how does species evolve then? Because
presumably you have organisms that are kind of next to each other and somethings stopping them
from interbreeding. So how do new species evolve?Well, there's different types of reproductive
(01:37):
isolation barriers. So, the sort of simplest one is what we call allopatric speciation. So,
where the groups of organisms are separated by some sort of geographic barrier. So,
then the groups are literally separated from each other and they're unable to exchange genes.
But then there's other kinds of ways that barriers can happen even in really close together places
(01:58):
you can have separation based on what's available in the environment. So, one of the most rapid
cases of speciation is the cichlid fish in crater lakes in Africa and in South America.
And there one species gets into the lake and then diverges into a huge variety based on
what they eat and on where they are in the lake.And so, the way they maintain separation is by
(02:23):
that sort of microallopatry. So, staying in one area, but also, they'll do things
like use colour for mate choice. And if the colour of the fish changes, then that will
keep them from mating and stop the gene flow.So, you're looking at one of the foundations of
evolution that new species evolve over time. So, what drew you to this area of research?
(02:44):
So, I've always been interested in animals, so I loved going to the zoo when I was a kid,
I knew all the different animal species names, and I was just interested in watching them. And
interested in how the huge diversity we see in form and function has arisen on the planet. Yeah.
And then I think people don't understand as much that speciation is still not very
(03:06):
well understood. So even though Darwin's most famous work was called On the Origin of Species,
Darwin talked more about adaptation or change in species through time and didn't directly address
the speciation or splitting process. So, we're still trying to figure out how that happens now.
So, what's the theories on that?I mean, some of it is, you know,
(03:28):
depending on this geography and what's going on. And as sequencing technology has gotten
more available and we've been able to look in wild populations, we've been able to get a better sense
of how much there is speciation with gene flow or without gene flow. So, we're kind of measuring how
much that is and how you can have the gene flow reduce over time. So, what are those reproductive
(03:51):
isolation barriers. We can actually go after them in nature and look at how they're preventing gene
flow, versus before where we, if we wanted to understand those processes, we really were
limited to looking at species that you could cross in the lab and saying, okay, so if we cross these,
did they have reduced fertility or do we see, you know, hybrids with problems, that kind of thing.
(04:12):
So, species is it essentially where there's so many genetic differences between these
species that they just basically can't interbreed and have kids?
Yeah. So, it can be there's different kinds. So that would be intrinsic isolation which
is where there's a problem with the organism itself. So, I study hybrid male sterility is
one of the main things I study. So that's where the hybrid males of mice they can
(04:35):
be produced but they have lower reproductive ability. So, problems with spermatogenesis.
But there's also other kinds of isolation that can happen that aren't due to the organism itself,
but rather how it fits with the environment. So, my friend Kira Delmore, studies the genetics of
migration in different bird species. So, one is Swainson’s thrush. So, she's interested in
(04:58):
understanding how their migratory path differs depending on which subspecies they are. And so,
the hybrids they produce, they are perfectly fit it seems like. But if they go in this
intermediate migratory route, it's over mountains and they have a lower survival probability. So,
it's just that they're intermediate and they don't fit well to the environment.
(05:19):
So, you're really interested in reproductive barriers. Talk me through that.
So, the one that I've studied the most is hybrid sterility. And I work on house mice which are the
wild relatives of lab mice. House mice have been spread by humans, but they started off in northern
Asia and India kind of area and then went into Europe through two different pathways. And by
(05:44):
the time they got back together again, after going these two different pathways, they had had enough
genetic changes happening so that where they meet, they hybridize, they make hybrids. You know,
all the mice that are in the hybrid zone in that region will be hybrids. But the hybrids have,
you know, different kinds of problems. And one of those is that both males and females have reduced
fertility. But it's been studied more in males.And most of the mice that we catch are not fully
(06:10):
sterile. So, we brought back all these hybrids to the lab, and most of them were able to produce
offspring, but about 30% of them had testis weight and or sperm count that was lower than what you
see in the parentals. And so, they have somewhat reduced fertility. There's been some evidence of
higher parasite loads in some hybrid populations. I've also studied the gut microbiome of hybrid
(06:33):
mice, and their gut microbiome is altered. So, they have all of these different kind
of sub fertility, sub fitness kind of effects.And so, while the hybrid zone is there and is
stable and all these hybrids are produced, they can't escape the
hybrid zone and get out and compete with the pure populations that have higher fitness.
And then we are not directly measuring fitness. We can estimate it from looking at, you know, how
(06:59):
allele frequencies change over space. So, the two subspecies I work on are Mus musculus domesticus
and Mus musculus musculus. So domesticus and musculus for short. And so, we can for example
look at a particular gene, how quickly as you move across space do you go from domesticus genotypes
to musculus genotypes, across the hybrid zone.And we compare that to the genome wide average.
(07:24):
So, if it's a gene that we think is contributing to reproductive isolation,
then you'll have a sharper transition there, or a Klein. So, it's called Klein analysis. And so
that will be much more steep for a gene that is involved in reproductive isolation than for
the genome wide average. And conversely, you can also look at what we call adaptive introgression.
(07:45):
So, introgression is just another word for gene flow except for that it's between species. So,
we can look at that pattern of introgression, and we can also look to see if there's adaptive
introgression. So, if you had something that was beneficial say, you know, disease resistance or
something from domesticus, then it might have a shallower Klein, or shallower transition
(08:07):
because it would be adaptive in musculus.So even though we're looking mostly at these
reproductive barriers, or trying to find the problems in hybrids,
this introgression or gene flow between species can also be beneficial in some cases.
So, you mentioned you've been looking at microbiome in mice. Tell me about that.
Yeah. So, we are looking in the same hybrid mice where I was studying hybrid
(08:29):
sterility for a long time. Then we were interested in seeing if the gut microbiome of these mice is
altered, because we know that gut microbiome has really important effects on fitness for
things that are kind of more obvious, like digestion and nutrition processing,
but also things like mental health in humans have been linked. So, there's this gut brain axis. So,
(08:54):
the gut microbiome is really important.And in a previous paper looking in wild-caught
hybrids, we showed that the gut microbiome of wild-caught hybrids is different from
both parental species. So, we call this dysbiosis, so, it's altered. And it was also associated with
inflammation of the gut. So, we were interested in studying that in more detail. And so,
(09:17):
we looked at mice that were from the hybrid zone. We created little stocks of these mice
and then crossed them in the laboratory.And the important thing in our study was
that the gut microbiome was derived from the wild. So, they had come from the wild,
and they had never been what's called re derived, which is what has happened to all
(09:38):
the laboratory strains of mice, which is at some point they went through in vitro fertilization,
you know, so embryos were put into mothers of a different line and that resets the microbiome.
So, laboratory mice have gut microbiomes that are much less diverse than wild mice. And it's
been shown, for example, that if you take the gut microbiome from a wild mouse and put it
(10:01):
into a lab mouse, then that really changes how they respond to things like cancer, you know,
colon cancer, but also how they are infected with influenza. So, it has huge effects.
So, what we were interested in in our study was looking at this natural microbiome and
figuring out what genes in the mice affect the composition of their microbiome. So,
(10:25):
we looked, you know, all throughout the genome with these snips and looked for associations
with abundances of all the bacterial taxa. And identified about 430 regions of the
genome that are associated with bacterial taxa.And interestingly, even though there has been
this re derivation in lab mice and so the taxa that were associated were somewhat different,
(10:47):
we did find significant overlap in the regions of the genome we found, that affect the gut
microbiome with what had been founded lab mice.So even though the gut microbiome composition
changes, it seems like there's certain regions of the genome that consistently affect that
composition. And by looking at this more complex microbiome, we also found a lot
(11:09):
of novel loci that weren't previously known.So that's really interesting because mice are
model organisms, is the eventual kind of goal of this to kind of go, well, can we transfer
that knowledge to what we know in humans?Yeah. So, one of the things that we do in
the paper is compare the genes that we found to genes that are known to affect
(11:29):
conditions like IBD in humans. And we also found a lot of those genes that are good candidates.
So, one of our initial candidate genes was a gene, sirtuin 5, which we find evidence that
it changes with circadian rhythm. And so, the expression of the gene goes up and down through
the day. And the level of the associated bacterium goes up and down through the day.
(11:53):
So that involved our graduate student having to stay up all night and collect mouse poop
all through the night. And she was able to do this in a normal strain of mice and a
strain where we had knocked out this gene. And so, then we can, you know, really get
at how the gene is affecting the gut microbiome.So where would you like your research to go next?
(12:16):
So, one thing would be trying to functionally characterize our candidate genes. So, showing
exactly what they're doing. And then another would be to look more in natural populations
at our candidate genes and look for this evidence of fitness effects. So, you know,
how have the allele frequencies changed through the hybrid zone. Yeah, and then compare what we
(12:36):
found to other species. Is there a kind of a common set of genes or a common set of genetic
mechanisms that cause reproductive isolation.And so, so far, we found just associations
with differences in the sequence of the DNA. But in other taxa, there's been an importance
of inversions. So that's a larger scale mutation where you get a bit of chromosome
(12:59):
that gets turned around and when you have those inversions that prevents recombination or exchange
of genes between chromosomes during meiosis.So, what that can do is keep a set of genes
together, so they don't mix up between the two species. So that
can really contribute to speciation because you get these blocks of genes that have the
(13:21):
identity of one species or the other.So, we have not found evidence that
inversions play a big role in hybrid sterility in the hybrid zone. But we haven't looked very
carefully at that because the sequence technology has just, you know, has been improving over time.
But it's still hard to look at those larger scale mutations. So, trying to figure out
(13:41):
if those play a role in some of these traits and then comparing them between species, yeah.
So, what gets you up in the morning? What is it that really excites you
about the research that you're doing?I have always been interested in speciation
throughout my career, so I guess that that's really what keeps me going, that diversity.
So, since I finished my PhD, I've been working on this hybrid sterility and
(14:04):
other traits in house mice. So, post zygotic isolation, so what happens after fertilization.
For my PhD, I was working in Peromyscus, which are North American mice. And there I was looking
at evolution of egg and sperm proteins. And so that's a different form of isolation called
gametic isolation, or post mating prezygotic isolation. And so that's where proteins on the
(14:29):
surface of the sperm and egg are changing. And it's a kind of lock and key sort of mechanism. And
those are some of the most rapidly evolving genes.But I'm also really broadly interested in lots
of different areas of evolutionary biology. When teaching, my favourite thing to teach
is actually behavioural ecology. So, I think mostly keeping an open mind and
seeing where the research goes is my M.O.Leslie, thank you so much for talking with
(14:54):
me. This was a podcast by the Milner Centre for Evolution at the University of Bath. I'm
Turi King and thank you for listening.If you have any thoughts or comments
on this or any other episodes, please contact us through social media. For
more information about the Milner Centre for Evolution, you can visit our website.
Hello and welcome. You are listening to a podcast by the Milner Centre for Evolution at
the University of Bath. I'm Professor Turi King, your host, and today I'm talking to Leslie Turner,
whose research focuses on understanding how speciation takes place. Leslie,
you're looking at the evolution of species and how that happens. So, for those people who don't know
(00:26):
what is the definition of a species and what are the mechanisms where new species evolve?
So, there's many different definitions for species, but one that's broadly used,
and the one that I use is the biological species concept, which defines species as
groups of organisms which are reproductively isolated from each other. And that focus
(00:47):
on reproductive isolation is really useful because it focuses on that mechanism of what
is stopping genes from going back and forth between the groups. So, stopping gene flow.
So basically, they can't reproduce with one another, they're kind of reproductively separate,
that's the definition of a species?Yes. So, either they won't mate with
each other or when they do mate, the hybrids that they produce are either not born or have
(01:13):
something wrong with them. Or it could be that the hybrids themselves are unable to reproduce, or
they're just not very well fit to the environment because they're a mix between the two species,
so their traits may not be a good fit.So how does species evolve then? Because
presumably you have organisms that are kind of next to each other and somethings stopping them
from interbreeding. So how do new species evolve?Well, there's different types of reproductive
(01:37):
isolation barriers. So, the sort of simplest one is what we call allopatric speciation. So,
where the groups of organisms are separated by some sort of geographic barrier. So,
then the groups are literally separated from each other and they're unable to exchange genes.
But then there's other kinds of ways that barriers can happen even in really close together places
(01:58):
you can have separation based on what's available in the environment. So, one of the most rapid
cases of speciation is the cichlid fish in crater lakes in Africa and in South America.
And there one species gets into the lake and then diverges into a huge variety based on
what they eat and on where they are in the lake.And so, the way they maintain separation is by
(02:23):
that sort of microallopatry. So, staying in one area, but also, they'll do things
like use colour for mate choice. And if the colour of the fish changes, then that will
keep them from mating and stop the gene flow.So, you're looking at one of the foundations of
evolution that new species evolve over time. So, what drew you to this area of research?
(02:44):
So, I've always been interested in animals, so I loved going to the zoo when I was a kid,
I knew all the different animal species names, and I was just interested in watching them. And
interested in how the huge diversity we see in form and function has arisen on the planet. Yeah.
And then I think people don't understand as much that speciation is still not very
(03:06):
well understood. So even though Darwin's most famous work was called On the Origin of Species,
Darwin talked more about adaptation or change in species through time and didn't directly address
the speciation or splitting process. So, we're still trying to figure out how that happens now.
So, what's the theories on that?I mean, some of it is, you know,
(03:28):
depending on this geography and what's going on. And as sequencing technology has gotten
more available and we've been able to look in wild populations, we've been able to get a better sense
of how much there is speciation with gene flow or without gene flow. So, we're kind of measuring how
much that is and how you can have the gene flow reduce over time. So, what are those reproductive
(03:51):
isolation barriers. We can actually go after them in nature and look at how they're preventing gene
flow, versus before where we, if we wanted to understand those processes, we really were
limited to looking at species that you could cross in the lab and saying, okay, so if we cross these,
did they have reduced fertility or do we see, you know, hybrids with problems, that kind of thing.
(04:12):
So, species is it essentially where there's so many genetic differences between these
species that they just basically can't interbreed and have kids?
Yeah. So, it can be there's different kinds. So that would be intrinsic isolation which
is where there's a problem with the organism itself. So, I study hybrid male sterility is
one of the main things I study. So that's where the hybrid males of mice they can
(04:35):
be produced but they have lower reproductive ability. So, problems with spermatogenesis.
But there's also other kinds of isolation that can happen that aren't due to the organism itself,
but rather how it fits with the environment. So, my friend Kira Delmore, studies the genetics of
migration in different bird species. So, one is Swainson’s thrush. So, she's interested in
(04:58):
understanding how their migratory path differs depending on which subspecies they are. And so,
the hybrids they produce, they are perfectly fit it seems like. But if they go in this
intermediate migratory route, it's over mountains and they have a lower survival probability. So,
it's just that they're intermediate and they don't fit well to the environment.
(05:19):
So, you're really interested in reproductive barriers. Talk me through that.
So, the one that I've studied the most is hybrid sterility. And I work on house mice which are the
wild relatives of lab mice. House mice have been spread by humans, but they started off in northern
Asia and India kind of area and then went into Europe through two different pathways. And by
(05:44):
the time they got back together again, after going these two different pathways, they had had enough
genetic changes happening so that where they meet, they hybridize, they make hybrids. You know,
all the mice that are in the hybrid zone in that region will be hybrids. But the hybrids have,
you know, different kinds of problems. And one of those is that both males and females have reduced
fertility. But it's been studied more in males.And most of the mice that we catch are not fully
(06:10):
sterile. So, we brought back all these hybrids to the lab, and most of them were able to produce
offspring, but about 30% of them had testis weight and or sperm count that was lower than what you
see in the parentals. And so, they have somewhat reduced fertility. There's been some evidence of
higher parasite loads in some hybrid populations. I've also studied the gut microbiome of hybrid
(06:33):
mice, and their gut microbiome is altered. So, they have all of these different kind
of sub fertility, sub fitness kind of effects.And so, while the hybrid zone is there and is
stable and all these hybrids are produced, they can't escape the
hybrid zone and get out and compete with the pure populations that have higher fitness.
And then we are not directly measuring fitness. We can estimate it from looking at, you know, how
(06:59):
allele frequencies change over space. So, the two subspecies I work on are Mus musculus domesticus
and Mus musculus musculus. So domesticus and musculus for short. And so, we can for example
look at a particular gene, how quickly as you move across space do you go from domesticus genotypes
to musculus genotypes, across the hybrid zone.And we compare that to the genome wide average.
(07:24):
So, if it's a gene that we think is contributing to reproductive isolation,
then you'll have a sharper transition there, or a Klein. So, it's called Klein analysis. And so
that will be much more steep for a gene that is involved in reproductive isolation than for
the genome wide average. And conversely, you can also look at what we call adaptive introgression.
(07:45):
So, introgression is just another word for gene flow except for that it's between species. So,
we can look at that pattern of introgression, and we can also look to see if there's adaptive
introgression. So, if you had something that was beneficial say, you know, disease resistance or
something from domesticus, then it might have a shallower Klein, or shallower transition
(08:07):
because it would be adaptive in musculus.So even though we're looking mostly at these
reproductive barriers, or trying to find the problems in hybrids,
this introgression or gene flow between species can also be beneficial in some cases.
So, you mentioned you've been looking at microbiome in mice. Tell me about that.
Yeah. So, we are looking in the same hybrid mice where I was studying hybrid
(08:29):
sterility for a long time. Then we were interested in seeing if the gut microbiome of these mice is
altered, because we know that gut microbiome has really important effects on fitness for
things that are kind of more obvious, like digestion and nutrition processing,
but also things like mental health in humans have been linked. So, there's this gut brain axis. So,
(08:54):
the gut microbiome is really important.And in a previous paper looking in wild-caught
hybrids, we showed that the gut microbiome of wild-caught hybrids is different from
both parental species. So, we call this dysbiosis, so, it's altered. And it was also associated with
inflammation of the gut. So, we were interested in studying that in more detail. And so,
(09:17):
we looked at mice that were from the hybrid zone. We created little stocks of these mice
and then crossed them in the laboratory.And the important thing in our study was
that the gut microbiome was derived from the wild. So, they had come from the wild,
and they had never been what's called re derived, which is what has happened to all
(09:38):
the laboratory strains of mice, which is at some point they went through in vitro fertilization,
you know, so embryos were put into mothers of a different line and that resets the microbiome.
So, laboratory mice have gut microbiomes that are much less diverse than wild mice. And it's
been shown, for example, that if you take the gut microbiome from a wild mouse and put it
(10:01):
into a lab mouse, then that really changes how they respond to things like cancer, you know,
colon cancer, but also how they are infected with influenza. So, it has huge effects.
So, what we were interested in in our study was looking at this natural microbiome and
figuring out what genes in the mice affect the composition of their microbiome. So,
(10:25):
we looked, you know, all throughout the genome with these snips and looked for associations
with abundances of all the bacterial taxa. And identified about 430 regions of the
genome that are associated with bacterial taxa.And interestingly, even though there has been
this re derivation in lab mice and so the taxa that were associated were somewhat different,
(10:47):
we did find significant overlap in the regions of the genome we found, that affect the gut
microbiome with what had been founded lab mice.So even though the gut microbiome composition
changes, it seems like there's certain regions of the genome that consistently affect that
composition. And by looking at this more complex microbiome, we also found a lot
(11:09):
of novel loci that weren't previously known.So that's really interesting because mice are
model organisms, is the eventual kind of goal of this to kind of go, well, can we transfer
that knowledge to what we know in humans?Yeah. So, one of the things that we do in
the paper is compare the genes that we found to genes that are known to affect
(11:29):
conditions like IBD in humans. And we also found a lot of those genes that are good candidates.
So, one of our initial candidate genes was a gene, sirtuin 5, which we find evidence that
it changes with circadian rhythm. And so, the expression of the gene goes up and down through
the day. And the level of the associated bacterium goes up and down through the day.
(11:53):
So that involved our graduate student having to stay up all night and collect mouse poop
all through the night. And she was able to do this in a normal strain of mice and a
strain where we had knocked out this gene. And so, then we can, you know, really get
at how the gene is affecting the gut microbiome.So where would you like your research to go next?
(12:16):
So, one thing would be trying to functionally characterize our candidate genes. So, showing
exactly what they're doing. And then another would be to look more in natural populations
at our candidate genes and look for this evidence of fitness effects. So, you know,
how have the allele frequencies changed through the hybrid zone. Yeah, and then compare what we
(12:36):
found to other species. Is there a kind of a common set of genes or a common set of genetic
mechanisms that cause reproductive isolation.And so, so far, we found just associations
with differences in the sequence of the DNA. But in other taxa, there's been an importance
of inversions. So that's a larger scale mutation where you get a bit of chromosome
(12:59):
that gets turned around and when you have those inversions that prevents recombination or exchange
of genes between chromosomes during meiosis.So, what that can do is keep a set of genes
together, so they don't mix up between the two species. So that
can really contribute to speciation because you get these blocks of genes that have the
(13:21):
identity of one species or the other.So, we have not found evidence that
inversions play a big role in hybrid sterility in the hybrid zone. But we haven't looked very
carefully at that because the sequence technology has just, you know, has been improving over time.
But it's still hard to look at those larger scale mutations. So, trying to figure out
(13:41):
if those play a role in some of these traits and then comparing them between species, yeah.
So, what gets you up in the morning? What is it that really excites you
about the research that you're doing?I have always been interested in speciation
throughout my career, so I guess that that's really what keeps me going, that diversity.
So, since I finished my PhD, I've been working on this hybrid sterility and
(14:04):
other traits in house mice. So, post zygotic isolation, so what happens after fertilization.
For my PhD, I was working in Peromyscus, which are North American mice. And there I was looking
at evolution of egg and sperm proteins. And so that's a different form of isolation called
gametic isolation, or post mating prezygotic isolation. And so that's where proteins on the
(14:29):
surface of the sperm and egg are changing. And it's a kind of lock and key sort of mechanism. And
those are some of the most rapidly evolving genes.But I'm also really broadly interested in lots
of different areas of evolutionary biology. When teaching, my favourite thing to teach
is actually behavioural ecology. So, I think mostly keeping an open mind and
seeing where the research goes is my M.O.Leslie, thank you so much for talking with
(14:54):
me. This was a podcast by the Milner Centre for Evolution at the University of Bath. I'm
Turi King and thank you for listening.If you have any thoughts or comments
on this or any other episodes, please contact us through social media. For
more information about the Milner Centre for Evolution, you can visit our website.
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