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Sexual conflict over the maintenance of sex: effects of sexually antagonistic coevolution for reproductive isolation of parthenogenesis.

Kawatsu K - PLoS ONE (2013)

Bottom Line: First, the model based on adaptive-dynamics theory demonstrates that the resultant antagonistic coevolution between male coercion and a female barrier fundamentally ends in either the prevalence of sex or the co-occurrence of two reproductive modes.Therefore, as shown by the individual-based model, the establishment of obligate parthenogenesis in the population requires the simultaneous evolution of strong reproductive isolation between males and parthenogens.These findings should shed light on the interspecific diversity of reproductive modes as well as help to explain the prevalence of sexual reproduction.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Insect Ecology, Graduate School of Agriculture, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto, Japan. kazutakawatsu@gmail.com

ABSTRACT
Sexual reproduction involves many costs. Therefore, females acquiring a capacity for parthenogenetic (or asexual) reproduction will gain a reproductive advantage over obligately sexual females. In contrast, for males, any trait coercing parthenogens into sexual reproduction (male coercion) increases their fitness and should be under positive selection because parthenogenesis deprives them of their genetic contribution to future generations. Surprisingly, although such sexual conflict is a possible outcome whenever reproductive isolation is incomplete between parthenogens and the sexual ancestors, it has not been given much attention in the studies of the maintenance of sex. Using two mathematical models, I show here that the evolution of male coercion substantially favours the maintenance of sex even though a female barrier against the coercion can evolve. First, the model based on adaptive-dynamics theory demonstrates that the resultant antagonistic coevolution between male coercion and a female barrier fundamentally ends in either the prevalence of sex or the co-occurrence of two reproductive modes. This is because the coevolution between the two traits additionally involves sex-ratio selection, that is, an increase in parthenogenetic reproduction leads to a female-biased population sex ratio, which will enhance reproductive success of more coercive males and directly promotes the evolution of the coercion among males. Therefore, as shown by the individual-based model, the establishment of obligate parthenogenesis in the population requires the simultaneous evolution of strong reproductive isolation between males and parthenogens. These findings should shed light on the interspecific diversity of reproductive modes as well as help to explain the prevalence of sexual reproduction.

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Population densities of the two sexes as a function of the degree of reproductive isolation.Solid and dashed lines indicate the densities of males and females, respectively. Parameters used here are r  = 1.25, h  = 0.01, α  = 0.01, μ = 1.0, and βm  =  βf  = 0.001.
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pone-0058141-g001: Population densities of the two sexes as a function of the degree of reproductive isolation.Solid and dashed lines indicate the densities of males and females, respectively. Parameters used here are r  = 1.25, h  = 0.01, α  = 0.01, μ = 1.0, and βm  =  βf  = 0.001.

Mentions: Fertilised females sexually produce sons and daughters with equal probability, and unfertilised females produce only daughters parthenogenetically. For viability selection, I assume that investing in male coercion or a female barrier imposes a mortality cost on males and females, respectively. The phenotype-dependent mortalities are determined by the following functions: cm(y)  = 1 – exp[−βmy2] for males and cf(x)  = 1 – exp[−βfx2] for females (also note that cm(y) and cf(x) are mortalities, not probability densities of the distribution of x and y). That is, individuals incur harsher mortality costs with increased investment in the trait. The parameter βm and βf scales a selection coefficient of mortality for male coercion and a female barrier, respectively. The non-negative equilibrium densities of males and females (M* and F*) in this system is obtained as(1)where the parameter r and h indicates the intrinsic birth rate and the density-dependent mortality, respectively (see Methods). These equations indicate that equilibrium densities of males and females depend on the degree of isolation which is determined by the values of x and y in the population. Therefore, the population sex ratio becomes F*/M*  = 1–2(cf(x) – cm(y))/r if male coercion can evolve to eliminate the reproductive isolation between males and females (i.e., x  =  y), but otherwise the male population will go extinct (Fig. 1).


Sexual conflict over the maintenance of sex: effects of sexually antagonistic coevolution for reproductive isolation of parthenogenesis.

Kawatsu K - PLoS ONE (2013)

Population densities of the two sexes as a function of the degree of reproductive isolation.Solid and dashed lines indicate the densities of males and females, respectively. Parameters used here are r  = 1.25, h  = 0.01, α  = 0.01, μ = 1.0, and βm  =  βf  = 0.001.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3585248&req=5

pone-0058141-g001: Population densities of the two sexes as a function of the degree of reproductive isolation.Solid and dashed lines indicate the densities of males and females, respectively. Parameters used here are r  = 1.25, h  = 0.01, α  = 0.01, μ = 1.0, and βm  =  βf  = 0.001.
Mentions: Fertilised females sexually produce sons and daughters with equal probability, and unfertilised females produce only daughters parthenogenetically. For viability selection, I assume that investing in male coercion or a female barrier imposes a mortality cost on males and females, respectively. The phenotype-dependent mortalities are determined by the following functions: cm(y)  = 1 – exp[−βmy2] for males and cf(x)  = 1 – exp[−βfx2] for females (also note that cm(y) and cf(x) are mortalities, not probability densities of the distribution of x and y). That is, individuals incur harsher mortality costs with increased investment in the trait. The parameter βm and βf scales a selection coefficient of mortality for male coercion and a female barrier, respectively. The non-negative equilibrium densities of males and females (M* and F*) in this system is obtained as(1)where the parameter r and h indicates the intrinsic birth rate and the density-dependent mortality, respectively (see Methods). These equations indicate that equilibrium densities of males and females depend on the degree of isolation which is determined by the values of x and y in the population. Therefore, the population sex ratio becomes F*/M*  = 1–2(cf(x) – cm(y))/r if male coercion can evolve to eliminate the reproductive isolation between males and females (i.e., x  =  y), but otherwise the male population will go extinct (Fig. 1).

Bottom Line: First, the model based on adaptive-dynamics theory demonstrates that the resultant antagonistic coevolution between male coercion and a female barrier fundamentally ends in either the prevalence of sex or the co-occurrence of two reproductive modes.Therefore, as shown by the individual-based model, the establishment of obligate parthenogenesis in the population requires the simultaneous evolution of strong reproductive isolation between males and parthenogens.These findings should shed light on the interspecific diversity of reproductive modes as well as help to explain the prevalence of sexual reproduction.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Insect Ecology, Graduate School of Agriculture, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto, Japan. kazutakawatsu@gmail.com

ABSTRACT
Sexual reproduction involves many costs. Therefore, females acquiring a capacity for parthenogenetic (or asexual) reproduction will gain a reproductive advantage over obligately sexual females. In contrast, for males, any trait coercing parthenogens into sexual reproduction (male coercion) increases their fitness and should be under positive selection because parthenogenesis deprives them of their genetic contribution to future generations. Surprisingly, although such sexual conflict is a possible outcome whenever reproductive isolation is incomplete between parthenogens and the sexual ancestors, it has not been given much attention in the studies of the maintenance of sex. Using two mathematical models, I show here that the evolution of male coercion substantially favours the maintenance of sex even though a female barrier against the coercion can evolve. First, the model based on adaptive-dynamics theory demonstrates that the resultant antagonistic coevolution between male coercion and a female barrier fundamentally ends in either the prevalence of sex or the co-occurrence of two reproductive modes. This is because the coevolution between the two traits additionally involves sex-ratio selection, that is, an increase in parthenogenetic reproduction leads to a female-biased population sex ratio, which will enhance reproductive success of more coercive males and directly promotes the evolution of the coercion among males. Therefore, as shown by the individual-based model, the establishment of obligate parthenogenesis in the population requires the simultaneous evolution of strong reproductive isolation between males and parthenogens. These findings should shed light on the interspecific diversity of reproductive modes as well as help to explain the prevalence of sexual reproduction.

Show MeSH