Mammalian Sexual Dimorphism is Associated with Changes in Sizes of Brain Development Gene Families

The Director of the Milner Centre for Evolution, Professor Turi King, talks to Dr Benjamin Padilla-Morales about the research paper he is first author on, and that has just been published in Nature Communications, titled: Sexual size dimorphism in mammals is associated with changes in the size of gene families related to brain development.

Episode Transcript

(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 Dr Benjamin Padilla-Morales, who is a bioinformatician. He's first author on the paper

in Nature Communications (00:17):
Sexual size dimorphism  in mammals is associated with changes in the size
of gene families related to brain development.So, Ben before we talk about the paper, let's get
a couple of the basics out of the way for people who don't know. So, what is sexual dimorphism?
Sexual dimorphism is like the difference between males and females in different traits.

(00:43):
For example, you can find birds that they have a difference in their coloration. In very classic
example, Peacocks. Males are very flashy, they have this long tail with different colours, and
they move right, so, they can attract the female.And if you see the female, they have a more like,

(01:05):
brownish colours, right. And it's because they are not actually trying to advertise
themselves to the male, in this case, the male is trying to advertise for the female.
Another example, it's the behaviours, right. How male and female interact like a display, a dance.

(01:27):
Also, like, bringing gifts for the female to show that they are fit for the role of a parent,
that they have the ability to get resources.But in this case, for this study, this paper,
we actually studied the difference in their mass, in their size, in mammals.

(01:48):
So, what was the question that you were looking at with this paper? What did you want to find
out when you first started it?Actually, the question will be,
how sexual selection, and we are using sexual size dimorphism as a link to sexual selection,
is effected by genome evolution? How the foundation of all of us, has actually

(02:11):
been affected by the difference in sizes.So how does sexual size dimorphism evolve?
Do we know kind of why it's evolved that way?Well, as many things in biology, it's not a black
and white answer, it's like a multifactorial origin. It could be related to, for example,

(02:32):
something that is called sexual selection, which is like, there are some traits that are selected
by the species, in this case females and males, and different pressures that have selected these
traits, to the point that these differences, in this case in size, have been marked. But
also naturally selected reasons that are more related to the environment where the

(02:56):
animals live and how they live their lives.So how did you go about doing the study?
We collected the genomes from already established databases. We found around
120 genes at that time. We compared genomes between each other, and then we actually found

(03:17):
how similar are they between the species.And then we make those groups, those gene
families, and then with those gene families, then we compare that data with the collected
data linked to sexual size dimorphism, by doing an old, kind of, statistical matching.
So, you're trying to understand how sexual size dimorphism, kind of, relates to the evolution of

(03:41):
gene families? So, gene families can expand, and they can contract, and you're trying to
understand what's going on here with those, in relation to this sexual size dimorphism.
Yes. We found in this case that sexual size dimorphism is actually making an expansion on

(04:01):
genes linked to olfactory functions, right.Meanwhile, at the same time, we have found
that there is contraction on genes related to brain development, which is something that,
in some kind of way, astonish us when we found that. It was something very interesting.
So presumably if gene families are expanding, it's because they're being expressed more,

(04:25):
they're needed more. So, if gene families are contracting, is it because they're not
being expressed as much so they're not needed, so you can lose genes and it doesn't matter?
It's more related to like, for the species that have a sexual size dimorphism, they are investing
more, in this case in olfactory functions.Apparently, the selection pressures that

(04:48):
are happening in their environment, linked to the same species, from males,
from females, even from the environment, are pushing their genes to olfactory receptors.
So, what's the thinking as to why that is?Well, olfactory receptors are important for sexual
selected functions, such as identifying mates, and also between other males, for example, to identify

(05:16):
each other. It's like territory and also defence of the territory. For example, for some species of
small mammals, they actually use the olfactory receptors to identify their offspring. So, it's
an important function, and therefore because it's also very resource consuming, the development of a

(05:38):
brain, for mammals, and mammals are really brain focused, they are like, lagging on that stage.
Is that suggesting that brain development is being sacrificed in exchange for growth in
body size, or what's going on there?Yeah, well, we could call it like,
a trade-off. It's like, okay, the species are having a greater sexual size dimorphism,

(06:02):
are investing more in those gene families linked to olfactory receptors, than the ones
related to brain development, and such.So, were you looking at brain size in
relation to mammal size? Sort of like the ratio of brain size to body size.
Well, in one part of the paper we analysed the size of the brain related to the body,

(06:28):
but that's like in parallel, it's not inside of the same analysis.
But what we found is that apparently some of the gene families that are linked with sexual size
dimorphism, are also linked to this brain size. Because we found a very similar link between the

(06:49):
set of genes. Well, we just decided to keep going with the set related to sexual size dimorphism.
So, what's the benefit of having a big brain?Well, the benefit of having a big brain,
there are some examples, like in birds and mammals, that the more complex behaviour

(07:09):
they develop, also they develop a more monogamous relationship between their mates. And this also,
they are pushed to have biparental care. Both the mum and the dad invest resources and time
to take care of their offspring, right.And that actually requires big brains

(07:31):
for many reasons, related to interact, or also to gather, and to recognize.
So, Ben what's next for you? What do you want to work on next?
Well, for example, related to this paper in the specific, to make it more interesting
is to include another trait that is linked to sexual selection, to put a seal of okay,

(07:54):
we are actually looking at something related to sexual selection, which in
this case could be testis size between males.In some rodents, the testis size related to the
body, it's quite big compared with bigger mammals. So, in those kind of situations,
it's been linked that there is also another sexual selection in competition between the

(08:17):
sperm quality, which is not as physical, to correlate that as sexual size dimorphism,
but it's also linked to sexual selection.And if we mix these two ideas,
it's like getting to the same point from a different angle. And therefore,
we could actually find something very cool. Maybe we can find that, maybe in this case testis size

(08:41):
is more linked to, maybe not olfactory receptors, but maybe to immunological genes perhaps, right,
it's just a hunch, and that could be cool.Ben, thank you so much for talking with me.
This was a podcast by the Milner Centre for Evolution at the University of Bath.

(09:02):
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 via our X channel @MilnerCentre.
For more information about the Milner Centre for Evolution, you can visit our website.