Biology Reference
In-Depth Information
Selfish sex ratio distorters
Throughout this topic we have assumed that individuals behave as if they are
maximizing their fitness. However, if a gene can increase its own transmission, at a cost
to the individual or other genes in the genome, then it may be selected to do so. Some of
the clearest examples of such selfish genetic elements are
sex ratio distorters
.
In some fly species it has been observed that the offspring of certain males tend to be
predominantly daughters, when a Fisherian 50% sons is expected. This bias is caused by
a sex ratio distorter on one of the sex-determining chromosomes (Jaenike, 2001). Flies
have similar genetic sex determination to mammals, in that the sex chromosome can be
either X or Y, with individuals that have one of each chromosome (XY) developing into
a male, and individuals with two Xs developing into females (YY cannot occur, because
the female always provides one X). The sex ratio distortion is caused by meiotic driving
X genes, which somehow cause Y bearing sperm to die, leading to an non-Mendelian
increase in frequency of the X chromosome in the next generation. Y chromosome drive
has also been discovered, leading to certain males having predominantly sons.
Why are such sex ratio distorters not more common? A major factor that can prevent
the spread of sex ratio distorters is that the rest of the genome will be selected to repress
them. Consider the case of when an X driver spreads, leading to a female-biased
population sex ratio. In this case, due to Fisherian selection for an unbiased sex ratio,
the fitness of any individual who has an X driver will be reduced, because they will
produce the more common sex. Consequently, if a mutant at any other locus is able to
suppress the driver, and restore a more normal sex ratio to the population, then it will be
favoured. Egbert Leigh (1971) pointed out that this leads to the rest of the genome
becoming united as a 'parliament of genes' to suppress sex ratio distorters. A huge
variety of suppressors have been found that return the sex ratio to more equal values,
acting against both X and Y drivers (Burt & Trivers, 2006). Indeed, crosses between fly
species that produce unbiased sex ratios often show that they contain sex ratio distorters,
just that these are hidden because they also contain suppressors.
An amazing example of just how quickly suppressors of sex ratio distorters can spread
is provide by the work of Greg Hurst and colleagues on south-east Asian and Polynesian
populations of the butterfly
Hypolimnas bolina
. Populations of this butterfly from
Polynesia contain a bacteria from the genus
Wolbachia
that causes males to die, and
hence causes a female-biased sex ratio (Charlat
et al
., 2005). This bacteria is only passed
maternally, through eggs, and so males represent a relative dead end for it. Such male
killing is favoured in species such as
H. bolina
, where the larvae develop gregariously,
because the death of males frees up resources for their sisters, who are likely to contain
genetically identical bacteria (Hurst, 1991). Consequently, a gene in the bacteria for
male killing would spread, because male death would increase the transmission of other
copies of that gene. This idea of 'kin selection' is discussed in greater detail in Chapter 11.
In contrast, male killing did not occur in south-east Asian populations of this butterfly,
despite the fact that they also carry
Wolbachia
. Hornett
et al
. (2006) tested whether this
variation across species could be explained by suppressors of the male killer. They did this
by taking butterflies from Polynesia and south-east Asia (Philippines and Thailand), and
then mating them for a number of generations, in combinations that produced genetic
backgrounds from butterflies from each area, with
Wolbachia
from each area. They found
that the genetic background of the host butterfly played the key role -
Wolbachia
from both
Genes can be
selected to distort
the sex ratio
away from that
which is optimal
for the individual,
if it increases their
transmission to
the next
generation
The rest of the
genome is
selected to
suppress the
action of sex ratio
distorters
Maternally
transmitted
bacteria can be
selected to kill
males that carry
them
