Biology Reference
In-Depth Information
One reason that relatedness structure could vary across colonies is due to variation
in the number of times that the queen has mated. Our above argument, that the
workers favour an investment ratio biased 3:1 towards sisters, was based on the
assumption that the queen had mated only once, leading to workers being three times
more related to their sisters (
r
0.25). How would the
ESS for workers change if their queens had mated multiply? If a queen has mated
multiply, the workers would still be related to their brothers by
r
=
0.75) than their brothers (
r
=
0.25, via the genes
that they share through their mother (Box 13.1). However, multiple mating reduces
the relatedness of workers to their sisters, because they are less likely to share genes
via the paternal route (Fig. 13.4). In the extreme, if two sisters were fathered by
different males (half-siblings), then they would only be related by
r
=
Workers are
relatively less
related to sisters
when their queen
has mated
multiply
=
0.25, which
represents the sum of the relatedness through their mother (
r
=
0.25) and their fathers
(
r
0) (Box 13.2).
Consider now the consequence of when, in some colonies, the queen has mated singly
but in other colonies the queen has mated multiply. A key assumption is that workers
know how many times their queen has mated -evidence for this is provided later. In the
colonies where the queen has mated singly, we would expect the workers to favour a sex
ratio biased 3:1 towards females. In contrast, in the colonies where the queen has mated
multiply, the extent to which the workers are more closely related to sisters has been
reduced, and so a less biased sex ratio is favoured. However, things get more complicated
because we have to consider the consequences of what is happening at the population
level. The female bias favoured in the singly mated colonies increases the relative value
of males, because each male will on average mate with more females. This selects on the
colonies with multiply mated queens to invest a higher proportion of resources in males.
At the same time, this lower investment in females by multiply mated colonies decreases
the value of producing males, and hence selects on the colonies with singly mated
queens to invest a higher proportion of resources in females. These effects feedback on
each other, leading to the situation where the ESS is for colonies with singly mated
queens to produce only or predominantly females, and colonies with multiply mated
queens to produce only or predominantly males.
To summarize this fairly complicated argument, the key point is that variation across
colonies in number of times that the queen has mated will lead to variation in the extent
to which workers are more related to sisters. Workers could capitalize on this variation
in the 'relatedness asymmetry' by conditionally producing the sex which maximizes
their genetic contribution to the next generation. In colonies where queens mated
singly, the workers are relatively more related to sisters, so they would do best to produce
only or predominantly females. In colonies where queens mated mutiply, the workers
are relatively less related to their sisters, so they would do best to produce only or
predominantly males.
The most amazing thing is that workers of some ants appear to do exactly this! Lotta
Sundström (1994) studied the wood ant
Formica truncorum
on a number of small
islands just off the southwest coast of Finland. She used allozyme markers to determine
how many times each queen had mated and correlated this to the sex ratio of
reproductives produced. As predicted by Boomsma and Grafen, she found that colonies
with singly mated queens produced predominantly females, and colonies with multiply
mated queens produced predominantly males (Fig. 13.11). Overall, Sundström was
able to explain an impressive 66% of the variation in sex ratios across colonies.
=
Workers favour
the production of
females in nests
where the queen
has mated singly,
and males in nests
where the queen
has mated
multiply
Some woods ants
adjust their brood
sex ratio in
response to the
number of times
their queen has
mated
