Plasma testosterone correlates with morph type across breeding substages in male white-throated Sparrows

Supergenes on steroids

At the birth of supergenes, the genomic landscape is dramatically re-organized leading to pronounced differences in phenotypes and increased intrasexual diversity. Two of the best-studied supergenes in vertebrates are arguably the inversion polymorphisms on chromosomes 2 and 11 in the white-throated sparrow (Zonotrichia albicollis) and the ruff (Calidris pugnax), respectively. In both species, regions of suppressed recombination determine plumage coloration and social behavioural phenotypes. Despite the apparent lack of gene overlap between these two supergenes, in both cases the alternative phenotypes seem to be driven largely by alterations in steroid hormone pathways. Here, we explore the interplay between genomic architecture and steroid-related genes. Due to the highly pleiotropic effects of steroid-related genes and their universal involvement in social behaviour and transcriptomic regulation, processes favouring their linkage are likely to have substantial effects on the evolution of behavioural phenotypes, individual fitness, and life-history strategies. We propose that inversion-related differentiation and regulatory changes in steroid-related genes lie at the core of phenotypic differentiation in both of these interesting species. This article is part of the theme issue 'Genetic basis of adaptation and speciation: from loci to causative mutations'.

Inside the supergene of the bird with four sexes

The white-throated sparrow (Zonotrichia albicollis) offers unique opportunities to understand the adaptive value of supergenes, particularly their role in alternative phenotypes. In this species, alternative plumage morphs segregate with a nonrecombining segment of chromosome 2, which has been called a 'supergene'. The species mates disassortatively with respect to the supergene; that is, each breeding pair consists of one individual with it and one without it. This species has therefore been called the "bird with four sexes". The supergene segregates with a behavioral phenotype; birds with it are more aggressive and less parental than birds without it. Here, we review our efforts to identify the genes inside the supergene that are responsible for the behavioral polymorphism. The gene ESR1, which encodes estrogen receptor α, differs between the morphs and predicts both territorial and parental behavior. Variation in the regulatory regions of ESR1 causes an imbalance in expression of the two alleles, and the degree to which this imbalance favors the supergene allele predicts territorial singing. In heterozygotes, knockdown of ESR1 causes a phenotypic switch, from more aggressive to less aggressive. We recently showed that another gene important for social behavior, vasoactive intestinal peptide (VIP), is differentially expressed between the morphs and predicts territorial singing. We hypothesize that ESR1 and VIP contribute to behavior in a coordinated way and could represent co-adapted alleles. Because the supergene contains more than 1000 individual genes, this species provides rich possibilities for discovering alleles that work together to mediate life-history trade-offs and maximize the fitness of alternative complex phenotypes.

Parent and offspring genotypes influence gene expression in early life

Parents can have profound effects on offspring fitness. Little, however, is known about the mechanisms through which parental genetic variation influences offspring physiology in natural systems. White-throated sparrows (Zonotrichia albicollis, WTSP) exist in two genetic morphs, tan and white, controlled by a large polymorphic supergene. Morphs mate disassortatively, resulting in two pair types: tan male × white female (T × W) pairs, which provide biparental care and white male × tan female (W × T) pairs, which provide female-biased care. To investigate how parental composition impacts offspring, we performed RNA-seq on whole blood of WTSP nestlings sampled from nests of both pair types. Parental pair type had a large effect on nestling gene expression, with 881 genes differentially expressed (DE) and seven correlated gene coexpression modules. The DE genes and modules expressed at higher levels in W × T nests with female-biased parental care function in metabolism and stress-related pathways resulting from the overrepresentation of proteolysis and stress-response genes (e.g., SOD2, NR3C1). These results show that parental genotypes and/or associated behaviours influence nestling physiology, and highlight avenues of further research investigating the ultimate implications for the maintenance of this polymorphism. Nestlings also exhibited morph-specific gene expression, with 92 differentially expressed genes, comprising immunity genes and genes encompassed by the supergene. Remarkably, we identified the same regulatory hub genes in these blood-derived expression networks as were previously identified in adult WTSP brains (EPM2A, BPNT1, TAF5L). These hub genes were located within the supergene, highlighting the importance of this gene complex in structuring regulatory networks across diverse tissues.

Extrapair mating and the strength of sexual selection: insights from a polymorphic species

Extrapair mating could drive sexual selection in socially monogamous species, but support for this hypothesis remains equivocal. We used lifetime fitness data and a unique model species, the dimorphic white-throated sparrow (Zonotrichia albicollis), to examine how extrapair mating affects the potential for sexual selection. In this species, the morphs employ distinct reproductive strategies, with white males pursuing extrapair mating at higher rates than tan counterparts. Social and extrapair mating is disassortative by morph, with paternity exchange occurring primarily between pairs composed of white males and tan females. Bateman gradients and Jones indexes indicated stronger sexual selection via mate numbers in white males than in females and tan males, and generally did not differ between females as compared with tan males. Extrapair mating contributed more to the Bateman gradient for white than tan males, and white males also had higher variance in annual reproductive success. However, variance in lifetime reproductive success did not differ between morphs or sexes. Moreover, extrapair mating did not increase variance in male reproductive success relative to apparent patterns, and within-pair success accounted for much more variance than extrapair success. Thus, extrapair mating by white males increases Bateman gradients and the potential for sexual selection via mate numbers. However, our latter results support previous research suggesting that extrapair mating may play a limited role in driving the overall potential for sexual selection.

Actuarial senescence in a dimorphic bird: different rates of ageing in morphs with discrete reproductive strategies

It is often hypothesized that intra-sexual competition accelerates actuarial senescence, or the increase in mortality rates with age. However, an alternative hypothesis is that parental investment is more important to determining senescence rates. We used a unique model system, the white-throated sparrow (Zonotrichia albicollis), to study variation in actuarial senescence. In this species, genetically determined morphs display discrete mating strategies and disassortative pairing, providing an excellent opportunity to test the predictions of the above hypotheses. Compared to tan-striped males, white-striped males are more polygynous and aggressive, and less parental. Tan-striped females receive less parental support, and invest more into parental care than white-striped females, which are also more aggressive. Thus, higher senescence rates in males and white-striped birds would support the intra-sexual competition hypothesis, whereas higher senescence rates in females and tan-striped birds would support the parental investment hypothesis. White-striped males showed the lowest rate of actuarial senescence. Tan-striped females had the highest senescence rate, and tan-striped males and white-striped females showed intermediate, relatively equal rates. Thus, results were inconsistent with sexual selection and competitive strategies increasing senescence rates, and instead indicate that senescence may be accelerated by female-biased parental care, and lessened by sharing of parental duties.

Morph-Specific Patterns of Reproductive Senescence: Connections to Discrete Reproductive Strategies

How reproductive strategies contribute to patterns of senescence in natural populations remains contentious. We studied reproductive senescence in the dimorphic white-throated sparrow, an excellent species for exploring this issue. Within both sexes the morphs use distinct reproductive strategies, and disassortative pairing by morph results in pair types with distinct parental systems. White morph birds are more colorful and aggressive than tan counterparts, and white males compete for extrapair matings, whereas tan males are more parental. Tan males and white females share parental care equally, whereas white males provide little parental support to tan females. We found morph-specific patterns of reproductive senescence in both sexes. White males exhibited greater reproductive senescence than tan males. This result likely reflects the difficulty of sustaining a highly competitive reproductive strategy as aging progresses rather than high physiological costs of competitiveness, since white males were also long-lived. Moreover, morph was not consistently related to reproductive senescence across the sexes, arguing against especially high costs of the traits associated with white morph identity. Rather, tan females exhibited earlier reproductive senescence than white females and were short-lived, perhaps reflecting the challenges of unsupported motherhood. Results underscore the importance of social dynamics in determining patterns of reproductive senescence.

Circulating testosterone and feather-gene expression of receptors and metabolic enzymes in relation to melanin-based colouration in the barn owl

Knowledge of how and why secondary sexual characters are associated with sex hormones is important to understand their signalling function. Such a link can occur if i) testosterone participates in the elaboration of sex-traits, ii) the display of an ornament triggers behavioural response in conspecifics that induce a rise in testosterone, or iii) genes implicated in the elaboration of a sex-trait pleiotropically regulate testosterone physiology. To evaluate the origin of the co-variation between melanism and testosterone, we measured this hormone and the expression of enzymes involved in its metabolism in feathers of barn owl (Tyto alba) nestlings at the time of melanogenesis and in adults outside the period of melanogenesis. Male nestlings displaying smaller black feather spots had higher levels of circulating testosterone, potentially suggesting that testosterone could block the production of eumelanin pigments, or that genes involved in the production of small spots pleiotropically regulate testosterone production. In contrast, the enzyme 5α-reductase, that metabolizes testosterone to DHT, was more expressed in feathers of reddish-brown than light-reddish nestlings. This is consistent with the hypothesis that testosterone might be involved in the expression of reddish-brown pheomelanic pigments. In breeding adults, male barn owls displaying smaller black spots had higher levels of circulating testosterone, whereas in females the opposite result was detected during the rearing period, but not during incubation. The observed sex- and age-specific co-variations between black spottiness and testosterone in nestling and adult barn owls may not result from testosterone-dependent melanogenesis, but from melanogenic genes pleiotropically regulating testosterone, or from colour-specific life history strategies that influence testosterone levels.

Testosterone production in response to exogenous gonadotropin releasing hormone (GnRH challenge) depends on social environment and color polymorphism

Testosterone is an important mediator of behavior, morphology and physiology. A cascade of signals regulates the amount of testosterone (T) circulating in the plasma; in response to stimulus the hypothalamus releases gonadotropin-releasing hormone (GnRH), which triggers secretion of gonadotropins from the pituitary, stimulating the synthesis and release of T from the gonads. Previous work has shown that changes to the social environment can alter circulating T-levels, which may have important fitness consequences, but it is currently unclear whether these changes are due to alterations in the signal from the brain, or changes in the ability of the pituitary and gonads to respond to this signal. Further, the strength and direction of response to a changing environment may differ according to life-history strategy. Species with genetically determined alternative strategies offer a pathway for examining these differences. Here we use a finch with a genetically determined polymorphism, the Gouldian finch (Erythrura gouldiae), to determine whether T-levels change in response to social environment. We also use injections of GnRH to determine whether these changes are due to alterations in the ability of the pituitary and gonads to respond to this signal. We found that social environment (presence of females) had a rapid effect on male circulating T-levels, and that this difference was reflected in responsiveness to GnRH. We observed no overall morph differences in T-levels, but we did observe morph differences in the pattern of T secretion across environments, and morph differences in the repeatability of T-levels across time and environment.

Testosterone production ability predicts breeding success and tracks breeding stage in male finches

Testosterone (T) is an important mediator of reproductive behaviours and potential target for selection. However, there are few data relating natural variation in T to fitness estimates. Here, we used the GnRH challenge (an injection of gonadotropin-releasing hormone which stimulates maximal T release), to examine how individual differences in T relate to reproductive success and how T changes across date and breeding stage. We measured pre- and post-challenge T, in captive male Gouldian finches (Erythrura gouldiae), before and after introducing females, and across breeding stage. Post-challenge T before introducing females positively predicted breeding success. Post-challenge T levels were unrelated to date, but strongly related to stage; T production ability was strongly attenuated in incubating males. Prechallenge T levels related only to date. Our results suggest that T production ability is an important target for selection and that when males invest heavily in parental care they reduce their sensitivity to GnRH.

Estrogen Receptor Alpha as a Mediator of Life-History Trade-offs

Trade-offs between competitive and parental strategies often are mediated by sex steroids. The mechanisms underlying steroid signaling and metabolism may therefore serve as targets of disruptive selection that leads to alternative behavioral phenotypes. White-throated sparrows exhibit two color morphs that differ in both competitive and parental behavior; white-striped (WS) birds engage in more territorial singing, whereas tan-striped (TS) birds provision nestlings more often. Although WS birds have higher levels of plasma testosterone (T) and estradiol than do TS birds, experimental equalization of these hormones does not abolish morph differences in singing. Neural sensitivity to sex steroids may differ between the morphs because the gene for estrogen receptor alpha (ERα) has been captured by a chromosomal rearrangement found only in the WS birds. We recently showed that expression of this gene differs between the morphs and may drive the behavioral polymorphism. First, the ERα promoter region contains fixed polymorphisms that affect transcription efficiency in vitro. Second, in a free-living population, local expression of ERα depends strongly on morph and predicts both territorial singing and parental provisioning. Differential ERα expression is particularly striking in the medial amygdala; WS birds have three times more ERα mRNA than do TS birds. This difference persists during the non-breeding season and is unaffected by exogenous T treatment. Finally, preliminary data generated by RNA-seq confirm that ERα expression in MeA is both differentially expressed and correlated with territorial singing. Together, these results suggest that ERα may be a target of disruptive selection that leads to alternative behavioral strategies. Our future directions include a more detailed analysis of the ERα promoter regions to determine the molecular basis of differential expression as well as gene network analyses to identify genes connected to ERα.

New insights into the hormonal and behavioural correlates of polymorphism in white-throated sparrows, Zonotrichia albicollis

The white-throated sparrow is a promising model for behavioural neuroendocrinology and genetics because behaviour and endocrine function may be linked to a chromosomal rearrangement that determines plumage colour. The notion that the two colour morphs, tan-striped (TS) and white-striped (WS), differ predictably in aggression and parenting has been widely accepted, despite conflicting evidence. It is also hypothesized that morph-typic behaviour is hormone mediated, yet no field study has measured sex steroids and behaviour in the same birds. Here, we re-evaluate the TS and WS phenotypes, describe the conditions under which they differ and investigate relationships between sex steroids and behaviour. We report that (1) during territorial intrusions, WS males were more aggressive than TS birds, but this difference was restricted to singing; WS males sang more than TS males but showed identical levels of physical aggression. WS females sang more than TS females and were also more physically aggressive. (2) TS males provisioned young more frequently than did WS males, but only during first broods. The parental strategy of WS males was flexible, and during replacement broods, WS and TS males provisioned at equal rates. (3) Consistent with previous studies, we detected no morph difference in female provisioning. (4) Plasma testosterone and dihydrotestosterone were higher in WS males than in TS males during periods of peak territorial defence and during first broods; within breeding stages, male androgen levels were positively correlated with singing and negatively correlated with provisioning. Plasma oestradiol levels were higher in WS females than in TS females and higher during peak territorial defence; oestradiol levels tended to be positively correlated with singing. Overall, our results refine the TS and WS phenotypes, show that behavioural differences between them are restricted to periods with relatively high mating opportunity, and demonstrate an association between sex steroids and morph-typic behaviour. These results will inform future studies of this promising model.

Hormonal regulation of vasotocin receptor mRNA in a seasonally breeding songbird

Behaviors associated with breeding are seasonally modulated in a variety of species. These changes in behavior are mediated by sex steroids, levels of which likewise vary with season. The effects of androgens on behaviors associated with breeding may in turn be partly mediated by the nonapeptides vasopressin (VP) and oxytocin (OT) in mammals, and vasotocin (VT) in birds. The effects of testosterone (T) on production of these neuropeptides have been well-studied; however, the regulation of VT receptors by T is not well understood. In this study, we investigated steroid-dependent regulation of VT receptor (VTR) mRNA in a seasonally breeding songbird, the white-throated sparrow (Zonotrichia albicollis). We focused on VTR subtypes that have been most strongly implicated in social behavior: V1a and oxytocin-like receptor (OTR). Using in situ hybridization, we show that T-treatment of non-breeding males altered V1a and OTR mRNA expression in several regions associated with seasonal reproductive behaviors. For example, T-treatment increased V1a mRNA expression in the medial preoptic area, bed nucleus of the stria terminalis, and ventromedial hypothalamus. T-treatment also affected both V1a and OTR mRNA expression in nuclei of the song system; some of these effects depended on the presence or absence of a chromosomal rearrangement that affects singing behavior, plasma T, and VT immunolabeling in this species. Overall, our results strengthen evidence that VT helps mediate the behavioral effects of T in songbirds, and suggest that the chromosomal rearrangement in this species may affect the sensitivity of the VT system to seasonal changes in T.

Estrogen receptor α polymorphism in a species with alternative behavioral phenotypes

The evolution of behavior relies on changes at the level of the genome; yet the ability to attribute a behavioral change to a specific, naturally occurring genetic change is rare in vertebrates. In the white-throated sparrow (Zonotrichia albicollis), a chromosomal polymorphism (ZAL2/2(m)) is known to segregate with a behavioral phenotype. Individuals with the ZAL2(m) haplotype engage in more territorial aggression and less parental behavior than individuals without it. These behaviors are thought to be mediated by sensitivity to sex steroids, and the chromosomal rearrangement underlying the polymorphism has captured a prime candidate gene: estrogen receptor 1 (ESR1), which encodes estrogen receptor α (ERα). We therefore hypothesized that the behavioral effects of the ZAL2(m) rearrangement are mediated by polymorphism in ESR1. We report here that (i) the ESR1 promoter region contains fixed polymorphisms distinguishing the ZAL2(m) and ZAL2 alleles; (ii); those polymorphisms regulate transcription efficiency in vitro and therefore potentially do the same in vivo (iii); the local expression of ERα in the brain depends strongly on genotype in a free-living population; and (iv) ERα expression in the medial amygdala and medial preoptic area may fully mediate the effects of genotype on territorial aggression and parenting, respectively. Thus, our study provides a rare glimpse of how a chromosomal polymorphism has affected the brain and social behavior in a vertebrate. Our results suggest that in this species, differentiation of ESR1 has played a causal role in the evolution of phenotypes with alternative life-history strategies.

Individual variation in testosterone and parental care in a female songbird; the dark-eyed junco (Junco hyemalis)

When competition for sex-specific resources overlaps in time with offspring production and care, trade-offs can occur. Steroid hormones, particularly testosterone (T), play a crucial role in mediating such trade-offs in males, often increasing competitive behaviors while decreasing paternal behavior. Recent research has shown that females also face such trade-offs; however, we know little about the role of T in mediating female phenotypes in general, and the role of T in mediating trade-offs in females in particular. Here we examine the relationship between individual variation in maternal effort and endogenous T in the dark-eyed junco, a common songbird. Specifically, we measure circulating T before and after a physiological challenge (injection of gonadotropin releasing hormone, GnRH), and determine whether either measure is related to provisioning, brooding, or the amount of T sequestered in egg yolk. We found that females producing more T in response to a challenge spent less time brooding nestlings, but provisioned nestlings more frequently, and deposited more T in their eggs. These findings suggest that, while T is likely important in mediating maternal phenotypes and female life history tradeoffs, the direction of the relationships between T and phenotype may differ from what is generally observed in males, and that high levels of endogenous T are not necessarily as costly as previous work might suggest.

Sex and species differences in plasma testosterone and in counts of androgen receptor-positive cells in key brain regions of Sceloporus lizard species that differ in aggression

We studied neuroendocrine correlates of aggression differences in adults of two Sceloporus lizard species. These species differ in the degree of sex difference in aggressive color signals (belly patches) and in aggression: Sceloporus undulatus (males blue, high aggression; females white, low aggression) and Sceloporus virgatus (both sexes white, lower aggression). We measured plasma testosterone and counted cells expressing androgen receptor-like immunoreactivity to the affinity-purified polyclonal AR antibody, PG-21, in three brain regions of breeding season adults. Male S. undulatus had the highest mean plasma testosterone and differed significantly from conspecific females. In contrast, there was no sex difference in plasma testosterone concentrations in S. virgatus. Male S. undulatus also had the highest mean number of AR-positive cells in the preoptic area: the sexes differed in S. undulatus but not in S. virgatus, and females of the two species did not differ. In the ventral medial hypothalamus, S. undulatus males had higher mean AR cell counts compared to females, but again there was no sex difference in S. virgatus. In the habenula, a control brain region, the sexes did not differ, and although the sex by species interaction significant was not significant, there was a trend (p=0.050) for S. virgatus to have higher mean AR cell counts than S. undulatus. Thus hypothalamic AR cell counts paralleled sex and species differences in aggression, as did mean plasma testosterone levels in these breeding-season animals.

Neurogenomic mechanisms of aggression in songbirds

Our understanding of the biological basis of aggression in all vertebrates, including humans, has been built largely upon discoveries first made in birds. A voluminous literature now indicates that hormonal mechanisms are shared between humans and a number of avian species. Research on genetics mechanisms in birds has lagged behind the more typical laboratory species because the necessary tools have been lacking until recently. Over the past 30 years, three major technical advances have propelled forward our understanding of the hormonal, neural, and genetic bases of aggression in birds: (1) the development of assays to measure plasma levels of hormones in free-living individuals, or "field endocrinology"; (2) the immunohistochemical labeling of immediate early gene products to map neural responses to social stimuli; and (3) the sequencing of the zebra finch genome, which makes available a tremendous set of genomic tools for studying gene sequences, expression, and chromosomal structure in species for which we already have large datasets on aggressive behavior. This combination of hormonal, neuroendocrine, and genetic tools has established songbirds as powerful models for understanding the neural basis and evolution of aggression in vertebrates. In this chapter, we discuss the contributions of field endocrinology toward a theoretical framework linking aggression with sex steroids, explore evidence that the neural substrates of aggression are conserved across vertebrate species, and describe a promising new songbird model for studying the molecular genetic mechanisms underlying aggression.