(Phys.org) —It’s not always easy to tell if a fish is male or female:
they look more or less the same.
But there are exceptions, such as guppies and, as with humans,
guppy genitalia varies in size across the species.
But while male genitalia are necessary to transfer sperm,
why should they be so different in shape and length among species?
This question has puzzled evolutionary biologists for a long time, and still does.
If their job is simply to transfer sperm,
why should male genitalia diverge in size or shape?
In a study to understand factors involved in genital evolution,
published in Proceedings of the Royal Society B: Biological Sciences on Wednesday,
my colleagues and I found guppy genital length
is genetic – for females as well as males.
Guppies breed using internal fertilisation (copulation)
and as a result are anatomically quite different
from the thousands of other fish species that have external fertilisation.
Male guppies have external structures that allow them
to transfer sperm into the female’s body – notably,
a penis-like structure called the gonopodium.
On close inspection, it isn’t difficult to spot the “penis” of males,
which is used to transfer sperm into the female.
We used guppies as a model system to answer questions
on the evolution of male genitalia.
Sexual selection is thought to be generally important for genital evolution,
and empirical support for this is growing.
But the exact mechanisms by which selection acts on male genitalia remain unclear.
Evolutionary biologists have identified a number of factors that may be involved.
A certain genital shape might improve a male’s ability to win fertilisation
when in competition with the sperm of other males,
or females might show a preference for males with large genitalia
or genitalia of a given shape.
But there can also be an arms-race between female and male genitalia,
in which female genitalia evolve to minimise direct
or indirect costs imposed by male genitalia.
Disentangling these processes is challenging,
but it is especially useful to know whether male and female traits
are inherited and how correlated they are,
as this helps us to understand the potentiality for evolution
to affect genital evolution.
Our research at the Centre for Evolutionary Biology
focused on answering two questions:
1.Are male and female genital size and shape
inherited from one generation to the next?
2.Are genital traits correlated between males and females?
To tackle those questions my colleagues and I used a quantitative genetic
approach that allowed us to explore both aspects simultaneously.
But, you might rightly ask, what is a “quantitative genetic approach”?
Despite the fancy definition, understanding how this method works
is not difficult and consists of creating families
of full- and half-siblings—full-sibs share the same mum and dad,
and half-sibs share the same dad but a different mum—and then measuring
the length and shape of male and female genitalia.
These data (more than 700 individuals measured) were analysed to determine
the extent to which individuals resemble each other due to shared genotypes.
In other words, the degree to which genital traits can be inherited.
Our data show that both male and female genital traits can be inherited,
and moreover, the length of the male “penis” is genetically correlated
with the length of the female genital duct.
This means – in the case of the guppy – males with a longer “penis”
have sons with a longer “penis” and daughters with a longer genital duct.
What do our results mean?
To answer this question we need to take a small step back,
to our previous work in which we showed how males with a longer “penis”
are better able to inseminate females
when they attempt to copulate without their consent.
Indeed, when forced copulations occur
females try to escape from harassing males,
so that a longer “penis” evolves to make it easier
to inseminate unwilling females.
Females, for their part, may evolve a longer gonoduct as it possibly allows them
to retain some control over fertilisation during forced copulations,
making difficult for males to deposit sperm closer to the fertilisation site
(the ovary), in a sort of “resistance”.
Future aims will be to specifically study how, and to what extent,
the female genital duct’s length can influence the fertilisation outcome
when sperm are transferred during forced copulations.
Our findings are in agreement with the idea that the different interests
in reproduction between the sexes (what is called sexual conflict)
has fuelled the evolution of male genitalia,
leading us to think that the corresponding evolution of female genitalia
is likely to be a counter-adaptation to the evolution of male genitalia.
That is, the evolution of male genitalia is far away
to be simply driven by the aim of transferring sperm,
and females have something to say in this respect.
June 3, 2013 by Clelia Gasparini / The Conversation – All rights reserved.