Evolution of Male Mating Success During Local Adaptation in Seed Beetles
Bahareh Zaferani
Degree project inbiology, Bachelor ofscience, 2011 Examensarbete ibiologi 45 hp tillkandidatexamen, 2011
Biology Education Centre and Department ofAnimal Ecology, Uppsala University
Table of Contents
Abstract ... 2
Introduction ... 3
Materials and Methods ... 6
Study Animals ... 6
Experimental lines ... 6
Experimental Procedure ... 7
Mating trials ... 7
Evolution of body size ... 9
Statistical Analyzes ... 10
Results ... 11
General results ... 11
Results of the first part of the experiment (30°C) ... 13
Results of the second part of the experiment (36°C) ... 15
Discussion ... 17
Acknowledgements ... 20
References ... 20
Abstract
Natural selection and sexual selection interact during adaptation to a novel or changing environment. Theoretically, sexual selection can reinforce natural selection if male mating success is condition-dependent and males that are better adapted to the local environment are in a better condition. However, the empirical data to date are scare and the results are controversial. We used replicate
experimental lines of the seed beetle Callosobruchus maculatus adapted to different temperature regimes (30°C, Ancestral or 36°C, Thermal) to test the hypothesis that locally adapted males will have higher reproductive success than males from non- locally adapted populations. Thermal lines had been adapting to a novel
environment for >30 generations and had previously shown to evolve genetic divergence in life-history traits after 16 generations of selection. Experimental males were allowed to compete against “tester” males of standardized genetic background for the access to “tester” females. We recorded male mating success, mating activity, time to mating, copulation duration and lifetime reproductive success (LRS) measured as a total number of offspring produced in a competitive setting through life. This measure of LRS included both pre-copulatory (mating success) and post-copulatory (sperm competition) reproductive success of males from experimental lines across the two temperature regimes. We further estimated genetic divergence in male body size, a putative sexually selected trait, between populations adapted to different temperatures. Despite pronounced difference in male body size (males from Thermal treatment were 43% larger than their
counterparts from Ancestral treatment in a common garden experiment), we found no evidence that locally adapted males had higher success in any of the traits we measured. These results support some of the previous work in this field and call into question the role of sexual selection in adaptation to novel environments.
Furthermore, this study does not support the hypothesis that male body size is sexually selected in C. maculatus, which has important implications for future studies in this popular model organism.
Introduction
The theory of natural selection by Charles Darwin states that organisms better adapted to their environment have higher rates of survival than those less matched to survive (Darwin 1859). Following the rise of the modern synthesis that combined Darwinism with the genetic theory of natural selection, the evolution is narrowly defined as differential non-random reproduction of alleles; alleles which produce traits that are more favorable in a particular environment will be more abundant in the next generation. However, Darwin also documented that there are some
examples of clearly non-adaptive, sexual traits that evidently would not be beneficial for the survival of their owners. He suggested that such traits might evolve if they are sexually selected, that is if they increase the individuals’
reproductive success, even if those traits are too costly for their survival (Darwin 1871). This form of selection depends not on a struggle for existence but on the battle between the individuals of one sex, typically males, for access to mates from other sex, typically females. The result is the evolution of traits that enhance success in mating (usually male-male competition) and sperm competition (Andersson 1994). Darwin also recognized another mechanism of sexual selection, which is called mate choice, where members of one sex, generally females, preferentially mate with the members of the opposite sex. It should be noted that recent studies emphasized the widespread occurrence and evolutionary significance of male mate choice and female-female competition (Bonduriansky 2001, Clutton-Brock 2007).
Thus, sexual selection is a form of natural selection that acts via reproductive success and most commonly recognized forms of sexual selection are mate choice and intra-sexual competition (Andersson 1994).
Natural and sexual selection are interacting as a population adapts to a new environment (Blows 2002). It is not clear, however, how sexual selection as a powerful evolutionary force affects the evolution of traits unrelated to mating. The net effect of sexual selection on nonsexual fitness is controversial. On one side, it is costly to have elaborated display traits and preferences for them; it may decrease the nonsexual fitness of individuals possessing them, as well as their offspring (Andersson 1994, Arnqvist and Rowe 2005). On the other hand, sexual selection may reinforce nonsexual fitness if an individual's attractiveness and quality are genetically correlated. Hence, sexual selection has the potential to constrain or reinforce the evolution of life history-traits and, therefore, local adaptation (Candolin and Heuschele 2008, Correia et al. 2010). Some former studies have argued that if there is a positive correlation between male condition (nonsexual fitness) and sexual performance, sexual selection is more likely to accelerate the rate of adaptation (Lorch et al. 2003). Sexual selection can thus promote adaptation
by fixing beneficial alleles; assuming locally adapted males will be in better
condition (Lorch et al. 2003, Proulx 1999, 2001, 2002, Whitlock 2000, Dolgin et al.
2006, Correia et al. 2010). “ Sexual selection can enhance the purging of genetic load, thereby providing a considerable advantage to sexual reproduction (Agrawal 2001, Siller 2001), improving population mean fitness. Each of these conclusions, however, is based on “good genes” scenario that assumes male display to be an honest indicator of male condition (i.e. display is condition-dependent or display intensity and condition show positive genetic covariance).” (Lorch et al. 2003)
Contrary to these hypotheses, sexual conflict theory proposes that sexual selection is likely to decrease population mean fitness and constrain adaptation. Due to inter-locus sexual conflict males reduce females fitness by damaging the females directly (Parker 1979, Rice 1996, Arnqvist and Rowe 2005). Additionally, ‘gender load’ in a new environment could also hinder adaptation due to sexually
antagonistic coevolution (Gavrilets et al. 2001, Kokko and Brooks 2003, Fricke et al.
2010). Despite the fact that how sexual selection affects the rate of adaptation in a novel environment is still unclear to evolutionary biologists, there are few empirical studies have been done to investigate this question. In fact, to my knowledge only four experimental evolution studies could be mentioned for that matter, however, the outcomes with respect to the effect of sexual selection on the rate of local adaptation are in contrast with each other - negative (Rundle et al. 2006, Maklakov et al. 2009), null (Holland 2002), and positive (Frick and Arnqvist 2007) and therefore support different hypotheses.
There is robust evidence that many sexually selected traits are indeed condition dependent (Hollis et al. 2009). According to this and other studies, males in better condition are often more successful at attracting mates (Jennions et al.
2001, Correia et al. 2010). Sexual selection operates not only on sexually selected traits and behaviors but also any traits that can affect mating success, including general condition (Dolgin et al. 2006). Then we might think of a simple assumption here, that male mating success is likely to depend on the match between his
phenotype and the local environment (Correia et al. 2010, Van Doorn et al. 2009).
Therefore males evolved in a particular environment should have higher mating success compared to competitors evolved in an alternative environment (Dolgin et al. 2006, Van Doorn et al. 2009).
Despite the fact that the role of sexual selection in adaptation has been studied for a long time, whether it reinforces or constrains adaptation to new environments, the evolution of male mating success during such adaptation is still quite undetermined (Candolin and Heuschele 2008). Here we investigated whether sexual selection is promoting or obstructing local adaptation. We predicted that in case of reinforcement, locally adapted males should have a higher mating success
compared to non-locally adapted males. In case of constrain locally adapted males should have a lower mating success than non-locally adapted males.
In order to test this hypothesis we decided to use replicate lines of the seed beetle Callosobruchus maculatus that has been adapted to 30°C (the control
treatment, C) and another set of replicated lines adapted to 36°C (the thermal treatment, T) for many generations. We estimated the mating success of males from C and T treatments in competition with standardized background males for the access to standardized background females by conducting mating trials for both lines under both 30°C and 36°C.
Materials and Methods Study Animals
Seed beetles, Callosobruchus maculatus (Coleoptera: Bruchidae), are agricultural pest insects originally from tropical parts of the world, however today they spread out to almost all the world including Central America, Africa, Asia, Australia and southern Europe. This species is one of the most manageable, flexible and vital among laboratory animal systems. C. maculatus is excessively easy to handle, sustain and has a very quick life cycle indeed (Messina 1993, Fox et al. 2004). The newly laid eggs are glued to the bean surface (seeds in Family Fabaceae) by females. Eggs are located singly and after oviposition, it takes 8-10 days for a beetle larva (maggot) to tunnel directly from the egg into the bean. 25-35 days after the deposition of an egg at 25°-30°C, a 2mm “window” appears at the place where the beetle is pupating then the adult emergence takes place through the seed coat from the bean. Adults are fully grown 24-36 hours after emergence and they can live without food and water.
Under these conditions, adults spend their confined lifespan 12 to 14 days mating and laying eggs (Fox et al. 2003, Howe and Currie 1964). There are some
morphological differences between adult females and males in seed beetles that can be distinguished by simply observing them with the naked eye. The most practical point about these animals is that, the laboratory environment is very much like the environment that these beetles would experience in nature. They readily mate under laboratory conditions as well as they do in their natural habitats. It is easy to isolate virgin beetles by providing females with a large amount of beans, in which case, some of the strains lay only one egg per bean. Temperature and relative humidity (RH) are the most important variables influencing generation times in seed beetles (Howe and Currie 1964, Schoof 1941). We kept beetles in temperature- controlled chambers with a glass of water, which results in RH of 45%±10%.
Experimental lines
Our experimental lines were taken from a population called Nigeria mix (N.mix).
The temperature was slowly increased from 30°C to 36°C over a period of 16 generations, after which the beetles were kept under 30°C for another ~20 generations (Hallsson 2011). We established three groups of beetles which were adapted to constant 30°C (the base line or ancestral, N.mix), 30°C (the control treatment, C) and 36°C (the thermal treatment, T). There were 4 populations of C (C1,C2,C3,C4) and 4 populations of T (T1,T2,T3,T4). Additionally we mixed the 4 populations of T to make a new base line for T, which was adapted to 36°C (T.mix).
Experimental Procedure
It was a two-part experiment. The exact same procedure was done for both parts except for the temperature. The first part was conducted under 30°C and the second part under 36°C.
Developing F0 generation:
First we prepared 50 Petri dishes with 25 g of black-eyed beans per each population (4 populations of C or T + Base population N.mix = 250 Petris in total). Then we isolated beans in four 48-well virgin chambers per population (under original temperature), one egg per bean. After the hatching, we put one virgin male and one virgin female into a 90mm Petri dish and labeled the Petris, and put them under 30°C (the 1st part), under 36°C (the 2nd part).
Developing F1 generation:
The same as first step for F0 was done for F1. Then we isolated 12 beans from each Petri into virgin chambers (one egg per bean). In the last step we took one virgin male and a virgin female only from each Petri (from 12 beans of the virgin chambers (from step 2)) and used them to prepare 50 new Petris for each population
considering avoiding pairing brothers with sisters! 250 Petris were kept under 30°C (the 1st part), under 36°C (the 2nd part).
It should be noted that in the first part, we kept 4 populations of T under 30°C, and in the second part we kept 4 populations of C under 36°C for two generations without selection, in order to remove the environmental effects. Additionally we isolated one egg per bean to avoid larvae competition.
Mating trials
Mating trials were conducted in the climate room with required temperature (30°C or 36°C) and humidity (45%±10%) in small Petri dishes (35mm). We used a thermo hygrometer to monitoring temperature and relative humidity. We had 25 mating trials per populations (in total for both treatments 200 trials). Virgin males from each four populations of both treatments competed with the marked, irradiated and virgin Nigeria mix-male (in the first part under 30°C) and T.mix-male (in the second part under 36°C) for the access to virgin Nigeria mix-females (Figure 1) and virgin T.mix-females (Figure 2). We decided to use irradiated beetles as competitors for the mating trials, since they were able to mate and behave like normal beetles and they were also sterile so we could count the un-hatched eggs subsequently. Each mating trial was run for 20 minutes. For the first 10 minutes it was observed
constantly and for the last 10 minutes the mountings were counted. A stopwatch was used for recording the time of mating and copulation duration. Copulation generally begins within 5-10 minutes, and the duration varies between 2-10 minutes. For better understanding I drew a simple design of the mating trials (Figure 1).
Figure 1. The first part of the experiment, mating trials under 30°C. 25 mating trials per population were observed. Virgin males from each of the four populations from both treatments competed with the marked and irradiated virgin N. mix males for the access to virgin N. mix females.
30°c
25
N.m C1 C2 C3 C4
ir
25
N.m T1 T2 T3 T4
ir mating trials per
population
mating trials per population
Figure 2. The second part of the experiment, mating trials under 36°C. 25 mating trials per population were observed. Virgin males from each of the four populations from both treatments competed with the marked and irradiated virgin N. mix males for the access to virgin T. mix females.
After observing each trial, the three beetles were removed and put into a 90mm Petri dish with 25 g of beans.
The first 100 Petri dishes were kept in an incubator under 30°C for at least 33 days and the other 100 dishes were kept under 36°C also for at least 33 days. Within these days the hatched and un-hatched eggs were counted. Moreover, after that period in order to count the offspring we put the dishes into the freezer room for a day. Then the offspring were also counted.
Evolution of body size
Evolution of body size was measured as dry body mass. Since male body size is a sexually selected trait in seed beetles we decided to weigh 10 males and 10 females per population from both treatments (C and T) and the same number of males and females from the Base line (Nigeria mix).
36°c
25
T.
mix
C1 C2 C3 C4
ir
25
T.
mix T1 T2 T3 T4
ir mating trials per
population
mating trials per population
Statistical Analyzes
We used general linear mixed models to test for the differences in different dependent variables between the treatments, where treatments were modeled as fixed factors and populations were modeled as random factor. We used general mixed linear models on arcsine-square-root-transformed proportions and mixed model with binomial error distribution and logit link function in Bayesian
framework in mcmcGLMM package in R for binomially distributed data such as ratios.
Results
General results:
1. Evolution of body size (measured as dry body mass) in response to selection:
high-temperature (T) populations
‘base’ line, while there was no difference between ‘base’ l populations (Figure 3 and 4).
selected trait in seed beetles in general and
then we looked at only male body size and we found that
than base-males (Figure 6). There was also a significant difference of body mass between the females and males (Figure 5).
Figure 3. Evolution of body size mea
selection lines (Base, Control and Thermal population) (both sexes) are significantly bigger than
difference between ‘base’ line and control (C) populations.
(<0.05).
0 0.0005 0.001 0.0015 0.002 0.0025 0.003
Base
Body Mass(g)
(measured as dry body mass) in response to selection:
temperature (T) populations evolved to be bigger (both sexes) than ancestral
‘base’ line, while there was no difference between ‘base’ line and control (C)
and 4). This is important because male body size is a sexually selected trait in seed beetles in general and C. maculatus in particular. Therefore then we looked at only male body size and we found that T-males were 43% larger
males (Figure 6). There was also a significant difference of body mass the females and males (Figure 5).
Figure 3. Evolution of body size measured as body mass (grams) in response to the selection lines (Base, Control and Thermal population). Thermal (T) populations
are significantly bigger than ancestral ‘base’ line, while there is ine and control (C) populations. F2, 4.89 = 74.89,
C T
Selection Lines
(measured as dry body mass) in response to selection:
evolved to be bigger (both sexes) than ancestral e and control (C)
This is important because male body size is a sexually Therefore were 43% larger males (Figure 6). There was also a significant difference of body mass
) in response to the . Thermal (T) populations ral ‘base’ line, while there is no
= 74.89, P = 0.0002
Figure 4. Evolution of body size measured as populations of each selection lines
populations (T1, T2, T3 and T4 than (both sexes) than ancest
‘base’ line and control (C) populations.
Figure 5. There is a significant difference of body mass F1, 4.28 = 33.26, P = 0.0036 (<0.05)
0 0.0005 0.001 0.0015 0.002 0.0025 0.003
Base C1
Body Mass(g)
0 0.0005 0.001 0.0015 0.002 0.0025
Body Mass(g)
dy size measured as body mass (grams) in respon
populations of each selection lines (Base, Control (C1, C2, C3 and C4) and Thermal s (T1, T2, T3 and T4)). Thermal (T) populations are significantly bigger (both sexes) than ancestral ‘base’ line, while there is no difference between
ine and control (C) populations. F2, 4.89 = 74.89, P = 0.0002 (<0.05).
There is a significant difference of body mass (grams) between the sexes (<0.05)
C2 C3 C4 T1 T2 T3
Populations
Female Male
Sexes
) in response to the and Thermal . Thermal (T) populations are significantly bigger
no difference between (<0.05).
between the sexes.
T4
Figure 6. A significant difference of body mass (g
three selection lines (Base, Control (C) and Thermal (T) population). Males of Thermal (T) line are bigger than males of both other lines.
2. There is no significant difference in mating success linear models on arcsine-square
model with binomial error distribution and logit link function in Bayesian framework in mcmcGLMM package in R). The data offer no support for the idea t adaptation to the local environment affect mating success.
Results of the first part of the experiment
3. There was no difference in relative mounting frequency
populations to the Base line (mean ± SE: T = 0.80 ± 0.03; C = 0.74 ± 0.03;
P = 0.2070).
4. No difference in time to mating 10.85; F1, 6.58 = 1.39, P = 0.2784).
5. No difference in copulation duration 280.59 ± 13.82; F1, 5.15 = 2.38,
A
0.0000 0.0005 0.0010 0.0015 0.0020 0.0025
Base
Body Mass (g)
cant difference of body mass (grams) between the males from Base, Control (C) and Thermal (T) population). Males of are bigger than males of both other lines.
There is no significant difference in mating success (both using general mixed square-root-transformed proportions and using mixed model with binomial error distribution and logit link function in Bayesian framework in mcmcGLMM package in R). The data offer no support for the idea t adaptation to the local environment affect mating success.
of the experiment (30°C):
There was no difference in relative mounting frequency between T and C populations to the Base line (mean ± SE: T = 0.80 ± 0.03; C = 0.74 ± 0.03;
No difference in time to mating (mean ± SE: T = 86.29 ± 12.69; C = 106.03 ±
= 0.2784).
in copulation duration (mean ± SE: T = 312.26 ± 15.15; C =
= 2.38, P = 0.1815).
A
B
Base C T
Selection Lines
) between the males from Base, Control (C) and Thermal (T) population). Males of
(both using general mixed transformed proportions and using mixed model with binomial error distribution and logit link function in Bayesian
framework in mcmcGLMM package in R). The data offer no support for the idea that
between T and C populations to the Base line (mean ± SE: T = 0.80 ± 0.03; C = 0.74 ± 0.03; F1, 5.96 = 2.0,
(mean ± SE: T = 86.29 ± 12.69; C = 106.03 ±
(mean ± SE: T = 312.26 ± 15.15; C =
B
T
6. No difference in net male reproductive success
eggs fertilized or adult offspring produced in a competition against steri line male (Figure 7, 8 and 9)
Figure 7. The difference of the proportion of hatched eggs in response to selection lines (Control (C) and Thermal (T)
measured as proportion of eggs fertilized in a competition against sterilized ‘base’
line male in 30°C.
Figure 8. The difference of the proportion of hatched eggs in response to the populations of each selection lines (Base, Control (C1, C2, C3 and C4
populations (T1, T2, T3 and T4)
measured as proportion of eggs fertilized in a competition against sterilized ‘base’
line male in 30°C.
0.46 0.47 0.48 0.49 0.5 0.51 0.52 0.53 0.54
Proportion of Hatched Eggs
0.42 0.44 0.46 0.48 0.5 0.52 0.54 0.56 0.58
Proportion of Hatched Eggs
No difference in net male reproductive success measured as proportion of eggs fertilized or adult offspring produced in a competition against sterilized ‘base’
).
Figure 7. The difference of the proportion of hatched eggs in response to selection es (Control (C) and Thermal (T). No difference in net male reproductive success measured as proportion of eggs fertilized in a competition against sterilized ‘base’
Figure 8. The difference of the proportion of hatched eggs in response to the populations of each selection lines (Base, Control (C1, C2, C3 and C4) and Thermal
opulations (T1, T2, T3 and T4). No difference in net male reproductive success measured as proportion of eggs fertilized in a competition against sterilized ‘base’
C T
Selection Lines
measured as proportion of lized ‘base’
Figure 7. The difference of the proportion of hatched eggs in response to selection No difference in net male reproductive success measured as proportion of eggs fertilized in a competition against sterilized ‘base’
Figure 8. The difference of the proportion of hatched eggs in response to the ) and Thermal No difference in net male reproductive success measured as proportion of eggs fertilized in a competition against sterilized ‘base’
Figure 9. The difference of the proportion of offsprin (Control (C) and Thermal (T)).
measured as proportion of adult offspring produced in a competition against sterilized ‘base’ line male 30
Results of the second part of the experiment
7. There was no difference in relative mounting frequency
populations to the Base line (mean ± SE: T = 0.46 ± 0.02; C = 0.52 ± 0.02;
2.47, P = 0.1671).
8. No difference in time to mating F1, 6 = 2.72, P = 0.1501).
9. No difference in copulation duration
± 9.00; F1, 6 = 2.58, P = 0.1593).
10. No difference in net male reproductive success
eggs fertilized or adult offspring produced in a competition against sterilized ‘base’
line male (Figure 10 and 11
0 5 10 15 20 25 30 35 40 45 50
C
Proportion of Offspring
Figure 9. The difference of the proportion of offspring in response to selection lines (Control (C) and Thermal (T)). No difference in net male reproductive success
adult offspring produced in a competition against 30°C.
of the experiment (36°C):
There was no difference in relative mounting frequency between T and C populations to the Base line (mean ± SE: T = 0.46 ± 0.02; C = 0.52 ± 0.02;
No difference in time to mating (mean ± SE: T = 79.01 ± 7.38; C = 96.25 ± 7.38;
No difference in copulation duration (mean ± SE: T = 239.39 ± 9.00; C = 259.85
= 0.1593).
No difference in net male reproductive success measured as proportion of eggs fertilized or adult offspring produced in a competition against sterilized ‘base’
(Figure 10 and 11).
C T
Selection Lines
g in response to selection lines No difference in net male reproductive success adult offspring produced in a competition against
between T and C populations to the Base line (mean ± SE: T = 0.46 ± 0.02; C = 0.52 ± 0.02; F1, 5.98 =
= 79.01 ± 7.38; C = 96.25 ± 7.38;
(mean ± SE: T = 239.39 ± 9.00; C = 259.85
measured as proportion of eggs fertilized or adult offspring produced in a competition against sterilized ‘base’
Figure 10. The difference of the proportion of hatched eggs in response to selection lines (Control (C) and Thermal (T)).
measured as proportion of eggs fertilized in a competition against sterilized ‘base’
line male in 36°C.
Figure 11. The difference of the proportion of offspring in response to selection lines (Control (C) and Thermal (T)).
measured as proportion of adult offspring produced in a competition against sterilized ‘base’ line male 36
0.41 0.42 0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.5
Proportion of Hatched Eggs
24 25 26 27 28 29 30 31 32 33
C
Proportion of Offspring
Figure 10. The difference of the proportion of hatched eggs in response to selection hermal (T)). No difference in net male reproductive success measured as proportion of eggs fertilized in a competition against sterilized ‘base’
11. The difference of the proportion of offspring in response to selection lines (Control (C) and Thermal (T)). No difference in net male reproductive success
adult offspring produced in a competition against 36°C.
C T
Selection Lines
C T
Selection Lines
Figure 10. The difference of the proportion of hatched eggs in response to selection No difference in net male reproductive success measured as proportion of eggs fertilized in a competition against sterilized ‘base’
11. The difference of the proportion of offspring in response to selection lines No difference in net male reproductive success
adult offspring produced in a competition against
Discussion
According to previous studies large males in seed beetles are more successful at mating with females (Savalli and Fox 1999). Therefore, the body size is likely to be a good indicator of male mating success in seed beetles. In our experiment we found a significant difference in body size between males from the Thermal lines (T) and the males from the Base line and Control lines (C). In fact, T-males were 43% larger than C-males, however, we did not find any significant difference in mating success between T-males and C-males.
We considered several different aspects of male mating success, such as mounting frequency, time to mating and copulation duration, as well as who succeeds in mating with the female first, but there were not any significant
differences between local and non-local males neither under 30 nor under 36 . Because of the relatively large sample sizes and an examination of many different reproductive traits combined with the observed effect sizes in our experiment, we can suggest that there is no significant difference in reproductive success between the local and non-local males in both environments (Base or ancestral, 30 , and Thermal, 36 ). Therefore, unlike some previous studies that supported the idea of well adapted to a particular environment male to be more successful at mating, in our study we found that the non-local males were as good as local males at mating with the females (Frick and Arnqvist 2007, Dolgin 2006).
The experiment was conducted under both temperatures to test if there is any difference between the local and non-local males in mating success and post- copulatory sexual selection due to correlated evolution of male reproductive performance in response to thermal adaptation. Moreover, we kept the Thermal populations of T under 30 , and in the second part of the experiment we kept Control populations under 36 for two generations without selection, in order to minimize the environmental effects. For instance, if we had done the experiment only in the ancestral environment (30°C), it was possible that increased body size of males from Thermal lines combined with their potentially reduced performance in the novel environment would effectively cancel each other out (cf. Holland 2002), thereby resulting in no difference between the selection regimes. However, since we showed that control males are as good as males from Thermal lines in mating under 36°C, we can rule out that explanation.
Our results are in contrast with those of Dolgin (2006) that have shown the positive effect of local adaptation on male mating success in Drosophila
melanogaster. This contrast is interesting, however, since in their study they did not find the significant difference for all the sets. Our findings are in accordance with some previous studies such as Correia et al. (2010) that have offered no support for the idea that adaptation to the local environment affect mating success. They also
did not find any significant differences in relative mating success of warm-adapted males in warm or cold assay environment.
Species arise due to evolution by natural selection in broad sense (Darwin 1859). However, the question is how does selection drive speciation? And what are the mechanisms? What are the relative roles of a narrow-sense natural selection and sexual selection in this process? According to the review article by Schluter (2009), there are many ways for arising new species by selection and they can be categorized into two wide groups: ecological speciation and mutation-order speciation. Ecological speciation corresponds to the evolution of reproductive isolation between populations or subpopulations and is the result of an adaptation to different environments or different ecological niches (Schluter 2000,
2001,Rundle and Nosil 2005). Natural selection operates differently between
different environments that can lead to fixation of different alleles that each of them is beneficial in only one environment and not in the other. Mutation-order
speciation refers to the evolution of reproductive isolation, which occurs by the fixation of different beneficial mutations between disconnected populations that would be favorable in different environments (Mani and Clarke 1990, Schluter 2009). “Speciation by sexual selection is ecological speciation if ecologically based divergent selection drives divergence of mating preferences” (Schluter 2009), therefore according to this the setup of our experiment was suitable to test for the occurrence of the initial steps of ecological speciation by sexual selection by evaluating the presence of reproductive isolation between the populations. In our experiment there were two lines adapted to different environments and as
mentioned above it is the perfect setting for reproductive isolation therefore ecological speciation to occur. However, we did not find any evidence of increased reproductive isolation in lines from Thermal regime compared to the Control lines.
Although our experimental lines have shown significant adaptation to their local environments (Halsson 2011), there was no increased reproductive isolation in lines adapted to stressful novel environment.
There are some potential explanations for why we did not find any significant differences between the males, despite the fact that we found the significant genetic divergence in male body size between our experimental lines.
Due to individual behavioral differences in animals such as beetles, the males could be acting differently from one another and since females should choose between them it may not be only about the male body size but also other traits that were not measured in this study. Our result that the non-local males were as good as local males at mating with the females in both treatments (30°C and 36°C) regardless of the significant difference in their body size suggests that mating success might be affected by traits such as condition or general activity levels, rather than body size.
Taken at face value, these data does not support the idea that most alleles favored
by natural selection are also favored by sexual selection (Hughes 1995, Whitlock and Bourguet 2000, Radwan 2004). It is possible that genetic variation in male sexual performance (e.g., through courtship and sperm competition) is largely unrelated to the genetic variation for the remainder of fitness (i.e., juvenile survival, development rate, and female fecundity). Alternatively, sexual selection may
reinforce some components of nonsexual selection but oppose others, yielding, on average, no relationship. The power of this study was limited by the number of mating trials could be observed by one person during a day. Since the beetles would get old, we had to observe them during maximum four days.
There is a serious situation in science requiring great effort regarding publication bias. How the data to the purpose of the benefits of sexual selection are appraised could be the potential for publication bias. It is not difficult to interpret, support and publish positive evidence in well-known journals. Although we like to announce the positive correlation between sexual selection and adaptation it has not been found yet. Our result together with recent theories and experiments, indicates that the effect of nonsexual fitness on the adaptive mate choice is not the main force behind sexual selection, however it might be because of the benefits to those genes that are responsible for enhancing the mating and fertilization rates of the males expressing them (Holland 2002). After all, the mechanism of local
adaptation may have been too complex to identify mating success differences. Our experimental lines have shown significant adaptation to their local environments but local males did not have higher reproductive success in their environment compared to males from a different environment. This suggests that the correlation between male mating success and local adaptation is not so clear and strong that adaptation always brings strong effects on mating success. Contrary to some recent theoretical models (Van Doorn et al 2010), these data suggest that sexual selection is not necessarily a main driving force behind local adaptation (Candolin and Heuschele 2008, Maklakov et al. 2009). However, only a handful of such studies have been conducted on a limited number of organisms and more date are required to test these hypotheses in different taxa before any general conclusions could be reached.
Acknowledgements
This project could not have been done without Dr. Alexei Maklakov, who not only served as my supervisor but also encouraged and challenged me through my academic program. He never accepted less than my best efforts. Thank you.
I would also like to thank my family and friends for their everyday support and love.
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