Genetic 'Battle of the Sexes' More Important to Evolution Than Thought, Beetle Study Suggests
ScienceDaily (Nov. 7, 2010) — A new study of beetles shows a genetic 'battle of the sexes' could be much harder to resolve and even more important to evolution than previously thought.
These are two male broad-horned flour beetles locked in battle. (Credit: Dr. Kensuke Okada)
This battle, observed across many species and known as intralocus sexual conflict, happens when the genes for a trait which is good for the breeding success of one sex are bad for the other -- sparking an 'evolutionary tug-o-war' between the sexes.
It has previously been thought these issues were only resolved when the trait in question evolves to become sex-specific in its development -- meaning the trait only develops in the gender it benefits and stops affecting the other. An example of this is male peacocks' tails, used for mating displays, which are not present in females.
However, a new study by the universities of Exeter (UK), Okayama and Kyushu (both Japan) published Nov. 4 in Current Biology shows this doesn't always bring an end to conflict -- as even when the trait becomes sex-specific, knock-on effects can still disadvantage the other sex.
Professor Dave Hosken, from the Centre for Ecology & Conservation (Cornwall) at the University of Exeter, said: "This kind of genetic tussle is everywhere in biology. For example, in humans, male hips are optimised for physical activity, whereas female hips also need to allow child bearing. That's the sort of evolutionary conflict we're talking about, and these conflicts were previously thought to be resolved by sex-specific trait development.
"What we're seeing in this study is that this isn't always the end of the sexual conflict. This means it's no longer clear how or when, if ever, these conflicts get fully resolved and this means it could be more important to the evolutionary process than has generally been thought."
In this study, the researchers looked at broad-horned flour beetles, where males have massively enlarged mandibles used to fight other males for mating supremacy. The enlarged mandibles aren't present in the females at all -- meaning this is a sex-specific trait.
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Current Biology
doi:10.1016/j.cub.2010.10.023
Intralocus Sexual Conflict Unresolved by Sex-Limited Trait Expression
Tomohiro Harano1, Kensuke Okada2, 3, , , Satoshi Nakayama2, Takahisa Miyatake2 and David J. Hosken3, ,
1 Laboratory of Ecological Science, Department of Biology, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan
2 Laboratory of Evolutionary Ecology, Graduate School of Environmental Science, Okayama University, Okayama 700-8530, Japan
3 Centre for Ecology and Conservation, Biosciences, University of Exeter, Cornwall Campus, Penryn TR10 9EZ, UK
Received 19 August 2010;
revised 23 September 2010;
accepted 11 October 2010.
Published online: November 4, 2010.
Available online 4 November 2010.
Summary
Sexually antagonistic selection generates intralocus sexual conflict, an evolutionary tug-of-war between males and females over optimal trait values [[1], [2], [3] and [4]]. Although the potential for this conflict is universal, the evolutionary importance of intralocus conflict is controversial because conflicts are typically thought to be resolvable through the evolution of sex-specific trait development [[1], [2], [3], [4], [5], [6], [7] and [8]]. However, whether sex-specific trait expression always resolves intralocus conflict has not been established. We assessed this with beetle populations subjected to bidirectional selection on an exaggerated sexually selected trait, the mandible. Mandibles are only ever developed in males for use in male-male combat, and larger mandibles increase male fitness (fighting [[9] and [10]] and mating success, as we show here). We find that females from populations selected for larger male mandibles have lower fitness, whereas females in small-mandible populations have highest fitness, even though females never develop exaggerated mandibles. This is because mandible development changes genetically correlated characters, resulting in a negative intersexual fitness correlation across these populations, which is the unmistakable signature of intralocus sexual conflict [1]. Our results show that sex-limited trait development need not resolve intralocus sexual conflict, because traits are rarely, if ever, genetically independent of other characters [11]. Hence, intralocus conflict resolution is not as easy as currently thought.
Highlights
► Evolutionary tug-of-wars can occur between males and females over optimal trait size
► Sex-limited trait development is thought to diffuse these sexual conflicts
► We use experimental evolution to show that this is not always true
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