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Hormones and Animal Social Behavior - Elizabeth Adkins-Regan - Google Livres
Sex steroids mediate the expression of sexually dimorphic or sex-specific traits that are important both for mate choice within species and for behavioral isolation between species.
We investigated divergence in sex steroid signaling between two sympatric species of threespine stickleback Gasterosteus aculeatus: These sympatric forms diverge in both male display traits and female mate choice behaviors, which together contribute to asymmetric behavioral isolation in sympatry.
By contrast, none of the sex steroids examined were significantly different between nesting males of the two forms. However, combining the plasma sex steroid data with testis transcriptome data suggested that the efficiency of the conversion of testosterone into ketotestosterone has likely diverged between forms.
Within forms, plasma testosterone levels in males were significantly correlated with male body size, a trait important for female mate choice in the two sympatric species. These results demonstrate that substantial divergence in sex steroid signaling can occur between incipient sympatric species.
We suggest that investigation of the genetic and ecological mechanisms underlying divergence in hormonal signaling between incipient sympatric species will provide a better understanding of the mechanisms of speciation in animals.
August 3, ; Accepted: November 23, ; Published: This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors have declared that no competing interests exist. Hormones mediate the regulation of diverse phenotypic traits  , .
Therefore, differences in hormonal pathways between sympatric species can underlie divergence in traits important for adaptation  and reproductive isolation.
Thus, hormonal studies are crucial for a better understanding of speciation mechanisms. For example, when sexual selection drives or promotes speciation  ,  , it is particularly important to determine whether divergence in hormonal signaling regulates the expression of sexually dimorphic traits that contribute to reproductive isolation between species. Even when ecological adaptation plays a major role in speciation  ,  , hormonal signaling could modulate the expression of adaptive traits that contribute to reproductive isolation.
Although the ecological and genetic mechanisms of speciation have been extensively investigated during the last few decades  ,  ,  , relatively little is known about the hormonal basis for speciation. Sex steroids mediate the expression of sexually dimorphic or sex-specific traits important for behavioral isolation, such as courtship ornaments and female mate choice behaviors  ,  , .
Because male ornaments and female mate choice behaviors are often sexually antagonistic, plasma levels of sex steroid hormones regulating these traits can also be sexually antagonistic  ,  , .
For example, androgens are important mediators of the expression of male ornaments. Therefore, sexual selection should favor high androgen levels in males. By contrast, high androgen levels might be detrimental in females, because possession of exaggerated ornaments may be energetically costly or attract predators  ,  , or because high androgen levels may suppress immune response  ,  ,  , . Therefore, the optimal values of sex steroid levels are likely to differ between the sexes.
However, significant genetic correlations exist between males and females for sex steroid levels  ,  , which can constrain the evolution of sexual dimorphism in sex steroid levels within species, as well as divergence in the magnitude of sexual dimorphism between closely related species. Understanding how the patterns of sex differences in steroid levels can diverge between closely related species will provide insight into the physiological mechanisms underlying speciation.
The threespine stickleback Gasterosteus aculeatus species complex is a great model for exploring the genetic and ecological mechanisms underlying phenotypic divergence and reproductive isolation between closely related species  ,  ,  , .
Tremendous diversification of threespine stickleback has occurred during the last few million years and resulted in the evolution of multiple phenotypically and ecologically divergent forms, which are often reproductively isolated in sympatry  ,  , .
Divergence in male display traits often contributes to reproductive isolation between sympatric forms  ,  ,  , . In sticklebacks, sex steroids not only regulate expression of sexually dimorphic traits  ,  ,  ,  ,  ,  ,  ,  , but also suppress immune response . These empirical data suggest that sex steroid levels may be sexually antagonistic in sticklebacks.
Although sex differences in plasma levels of several sex steroids have been found in a European stickleback population  , little is known about variation in sex steroid levels or in the magnitude of the sex differences between divergent stickleback forms. In the present study, we investigated plasma sex steroid levels of nesting males and spawning females in a sympatric pair of Japanese threespine stickleback, comprising the Pacific Ocean and Japan Sea forms  , .
These two forms diverged 1. After the glacial recession, these two forms were brought into secondary contact. Although the two forms are currently sympatric in coastal regions of eastern Hokkaido, Japan, they are reproductively isolated, in part due to asymmetric behavioral isolation  ,  , . Asymmetric behavioral isolation is one of the isolating barriers between them. Pacific Ocean females virtually always mate with Pacific Ocean males, while Japan Sea females mate with males of both forms at similar frequencies  ,  , .
Pacific Ocean female mate choice is based on divergence in male body size and male dorsal pricking behavior . Pacific Ocean males and females are larger than Japan Sea males and females, and Pacific Ocean females prefer to mate with larger males. In addition, Pacific Ocean females also do not like the aggressive dorsal pricking of Japan Sea males. Previous genetic mapping in the Japanese sympatric pair revealed that divergence in sexually dimorphic traits mapped to the sex chromosomes .
This result is consistent with the theoretical prediction that sexual antagonism can be resolved by sex-linkage  ,  , . However, sex-linkage is not the only mechanism by which sexual antagonism can be resolved, but sex-specific transcriptional regulation is another important mechanism .
Sex steroid hormones are important mediators of sex-specific transcriptional regulation in vertebrates . Sex steroid hormones are mainly secreted from the gonads Fig.
Thus, to gain further insight into the mechanisms underlying divergence in reproductive traits that are important for behavioral isolation between the Japanese sympatric sticklebacks, we compared sex steroid signaling among nesting males and spawning females of the Japanese sympatric pair. Because steroidogenic pathways in sticklebacks have not been elucidated, steroidogenic pathways in rainbow trout are shown here  ,  ,  , .
Steroidogenic enzymes are shown in red. HSD, hydroxysteroid dehydrogenase; P, cytochrome P Plasma ketotestosterone levels were significantly higher in males than in females Fig. Plasma concentrations of testosterone exhibited different patterns of sexual dimorphism between forms Fig.
The sample size is shown above each column. Females differed in the levels of all steroids examined, with Japan Sea females having significantly higher plasma concentrations than Pacific Ocean females Fig. Solid lines and dotted lines indicate regression lines for the Japan Sea form and the Pacific Ocean form, respectively.
Closed circles, Japan Sea form; open circles, Pacific Ocean form. Because female sex steroid levels have diverged between the Japanese stickleback species, we also investigated whether there is divergence in the upstream signaling pathways for sex steroid production. The synthesis and secretion of sex steroid hormones from gonads is stimulated by pituitary glycoprotein hormones, follicle-stimulating hormone FSH and luteinizing hormone LH .
Thus, substantial divergence in reproductive hormone signaling exists between spawning females of these two sympatric stickleback species. There were no significant differences between nesting males of the two forms in the plasma concentrations of any steroids examined Fig. Interestingly, significant correlations were found between plasma testosterone levels and body length. Within both forms, males with higher testosterone levels were larger in standard length in both forms Fig.
Solid lines and dotted lines indicate regression lines for Japan Sea form and Pacific Ocean form, respectively. The slopes of the two regression lines were not significantly different see the text. Because testosterone is a precursor of ketotestosterone Fig. These data further suggest that additional differences in the steroidogenic pathway are likely to exist between forms. We used microarrays to compare the transcriptome of testis, a main sex-steroidogenic organ, between nesting males of the two forms.
Although there is great variation between individuals, cluster analysis of samples using transcript levels of mRNA encoding proteins involved in steroidogenic pathways  revealed two distinct clusters. One cluster corresponds to the Pacific Ocean form and the other to the Japan Sea form see the clusters below the heat map in Fig.
These results suggest that there is divergence in the steroidogenic pathways between nesting males of the two forms. A Microarray analysis of testis. Heat map and clustering analysis of the transcripts of genes encoding steroidogenic pathways are shown. Different lines represent different probes. For most genes, multiple independent probes were designed for the same gene. Cluster analysis of probes shown on the left side of the heat map indicates that signals of different probes representing the same gene product gave rise to similar signals.
In the heat map, red colors indicate high fluorescence signals, while green colors indicate low fluorescence signals. Overall, transcript levels of these genes are higher in the Japan Sea males than in the Pacific Ocean males Fig. Taken together, these data suggest that the conversion efficiency of testosterone into ketotestosterone may be higher in the Japan Sea males than in the Pacific Ocean males.
Taken together, these results suggest that there is divergence in steroidogenic pathways between nesting males of the two forms. Our results demonstrate that two sympatric stickleback species have diverged in sex steroid hormone signaling pathways.
First, the patterns of sexual dimorphism in plasma sex steroid levels differ between the sympatric forms. In the Pacific Ocean form, males have higher levels of plasma testosterone than females, whereas levels of plasma testosterone did not show a sex difference in the Japan Sea form. Interestingly, a previous study on a European threespine stickleback population revealed that spawning females had higher concentrations of plasma testosterone concentrations than nesting males  ,  ,  , which is contrary to what we have found in the Pacific Ocean form.
These data demonstrate that there is substantial variation in sexual dimorphism in plasma testosterone levels among threespine stickleback populations. Divergence in the sex steroid signaling pathways may play an important role in the divergence in female reproductive traits between the two forms. Sex steroids can also regulate oogenesis and oocyte maturation in fishes . Because the Japan Sea females and Pacific Ocean females diverge in both female mate choice behaviors  ,  and fecundity traits  , further studies including hormonal manipulation experiments  ,  should be conducted to investigate the functional roles of sex steroids in female sticklebacks.
By using the genomic tools available for sticklebacks  ,  , it will be possible to use a similar approach to further investigate the genetic basis of divergence in sex steroid signaling pathways in the Japanese species pair. Third, male testosterone levels were significantly correlated with male body size within forms, although the plasma testosterone levels were not significantly different between forms.
Male body size is important for female mate choice and male-male aggression in the threespine stickleback  ,  , . Furthermore, body size divergence is involved in behavioral isolation between sympatric species, including the Japanese sympatric pairs  ,  , . Although testosterone is known to regulate skeletal development in many animals  ,  ,  , we currently do not know whether variation in testosterone level is the cause or the consequence of variation in body size.
It is also possible that differential expression patterns of androgen receptors rather than plasma levels of testosterone contribute to phenotypic divergence . Therefore, further studies on the link between testosterone signaling and male body size are required for a better understanding of the contribution of hormonal regulation to traits involved in ecological adaptation and reproductive isolation.
Intra-specific variation in sex steroids is known to regulate reproductive polymorphisms across diverse taxa  ,  ,  ,  ,  , . By contrast, little is known about the role of divergence in sex steroid signaling in speciation. Our results demonstrate that divergence in sex steroid signaling pathways can occur between incipient species breeding in sympatry.
Sex steroids not only regulate the expression of reproductive traits, but also of other physiological traits, such as salinity tolerance  , migratory behaviors  , and feeding behaviors  ,  , .