Browsing by Author "Ketterson, Ellen D."

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Adaptation, Exaptation, and Constraint: A Hormonal Perspective
(American Society of Naturalists, 1999-07) Ketterson, Ellen D.; Nolan, Val Jr
We approach conceptual issues in evolutionary biology from an endocrinological perspective, noting that single hormones typically act on several target tissues and thereby mediate suites of correlated phenotypic traits. When several components of such a suite are beneficial, an important evolutionary question is whether all are adaptations or some are exaptations. The answer may depend on whether the traits arose in response to selection on variation in systemic levels of the hormone on variation in responsiveness of target tissues to invariant levels of the hormone. If the former, selection probably acted directly on fewer than all traits; beneficial traits arising indirectly would be exaptations. In contrast, multiple beneficial traits that arose out of independent changes in target‐tissue sensitivity to invariant hormone levels could all be adaptations. Knowledge of specific hormonal mechanisms as well as of historical selective regimes will be necessary to draw such distinctions. Endocrine constraints on evolution can be studied experimentally by applying hormones systemically and measuring interdependent responses of beneficial and detrimental traits to selection (phenotypic engineering with hormones). Supposing that alteration of one trait in isolation would enhance fitness, cases in which the net effect of endocrine alteration of multiple traits is to depress fitness provide evidence for constraints. We briefly report results of recent studies employing hormonal manipulations, stressing our own work on the dark‐eyed junco (Junco hyemalis: Emberizidae).
Deciphering Information Encoded in Birdsong: Male Songbirds with Fertile Mates Respond Most Strongly to Complex, Low-Amplitude Songs Used in Courtship
(The American Naturalist, 2011-08-15) Reichard, Dustin G.; Rice, Rebecca J.; Vanderbilt, Carla C.; Ketterson, Ellen D.
Research on the function of acoustic signals has focused on high-amplitude long-range songs (LRS) and largely ignored low-amplitude songs produced by many species during close-proximity, conspecific interactions. Low-amplitude songs can be structurally identical to LRS (soft LRS), or they can be widely divergent, sharing few spectral and temporal attributes with LRS (short-range song [SRS]). SRS is often more complex than LRS and is frequently sung by males during courtship. To assess function, we performed two playback experiments on males of a socially monogamous songbird. We compared responses of males whose mates were fertile or nonfertile with differences in song structure (SRS vs. LRS and soft LRS), amplitude (SRS and soft LRS vs. LRS), and tempo (slow SRS vs. fast SRS). Males responded more strongly to SRS than to LRS or soft LRS, indicating that song structure had a greater effect on response than song amplitude. SRS tempo did not detectably affect male response. Importantly, males responded more strongly to SRS when their mates were fertile, presumably because hearing SRS can indicate that a male’s mate is being courted by an intruding male and a strong response can deter extrapair competitors. We conclude that low-amplitude songs can function in both inter- and intrasexual communication and should receive greater attention in future studies of mate choice and male-male competition.
Hormones and Life Histories: An Integrative Approach
(American Society of Naturalists, 1992-11) Ketterson, Ellen D.; Nolan, Val Jr
Because of their role in mediating life‐history trade‐offs, hormones are expected to be strongly associated with components of fitness; however, few studies have examined how natural selection acts on hormonal variation in the wild. In a songbird, the dark‐eyed junco (Junco hyemalis), field experiments have shown that exogenous testosterone alters individuals’ resolution of the survival‐reproduction trade‐off, enhancing reproduction at the expense of survival. Here we used standardized injections of gonadotropin‐releasing hormone (GnRH) to assay variation in the testosterone production of males. Using measurements of annual survival and reproduction, we found evidence of strong natural selection acting on GnRH‐induced increases in testosterone. Opposite to what would be predicted from the survival‐reproduction trade‐off, patterns of selection via survival and reproduction were remarkably similar. Males with GnRH‐induced testosterone production levels that were slightly above the population mean were more likely to survive and also produced more offspring, leading to strong stabilizing selection. Partitioning reproduction into separate components revealed positive directional selection via within‐pair siring success and stabilizing selection via extrapair mating success. Our data represent the most complete demonstration of natural selection on hormones via multiple fitness components, and they complement previous experiments to illuminate testosterone’s role in the evolution of life‐history trade‐offs.
How Parasites Affect, and are Affected by, Host Physiology, Behavior, and Breeding System
([Bloomington, Ind.] : Indiana University, 2017-08) Slowinski, Samuel P; Ketterson, Ellen D.; Lively, Curtis M.
Parasites comprise a striking diversity of lifeforms, and probably evolved from free-living organisms many times. It has even been argued that most species on earth are parasites. Parasites can have profound effects on their hosts, and the biology of hosts, in turn, affects the evolution and spread of parasites. My dissertation research examined the effects of parasites on the physiology, behavior, and breeding systems of their hosts, as well as the effects of host physiology and behavior on parasitism. In chapters one and two, I focused on the disease ecology of a natural system: haemosporidian (blood) parasites and an avian host, the dark-eyed junco. In chapter one, I measured the association between junco long-distance migration behavior and infection with haemosporidian parasites. I found that a migrant population of juncos maintains a significantly lower prevalence of haemosporidian parasite infections relative to a closely related and seasonally sympatric sedentary junco population, suggesting that longdistance host migration may be associated with reduced parasitism. In chapter two, I showed that experimental elevation of circulating testosterone levels in hosts does not affect the prevalence of haemosporidian parasite infections in a wild population of juncos, and that haemosporidian infections do not affect host telomere degradation. I also showed that the prevalence of haemosporidian parasite infections increases with host age. In chapters three and four, I used an experimental host-parasite system in the lab to assess how coevolving parasites affect the breeding system of their hosts. In chapter three, I showed that coevolving bacterial parasites (Serratia marcescens) can constrain the spread of self-fertilization into obligately outcrossing populations of nematode (Caenorhabditis elegans) hosts. This result supports the Red Queen hypothesis and contributes to a large body of evidence that antagonistic coevolution between hosts and parasites can maintain biparental sex. Finally, in chapter four, I showed that the presence of parasites (S. marcescens) in the environment does not induce plastic changes in the propensity to outcross in hosts (C. elegans) capable of both outcrossing and self-fertilization, and that coevolutionary interactions with S. marcescens parasites does not cause C. elegans hermaphrodites to evolve a higher propensity to outcross.
Natural Selection on Testosterone Production in a Wild Songbird Population
(The American Naturalist, 2010-04-15) McGlothlin, Joel W.; Whittaker, Danielle J.; Schrock, Sara E.; Gerlach, Nicole M.; Jawor, Jodie M.; Snajdr, Eric A.; Ketterson, Ellen D.
Because of their role in mediating life‐history trade‐offs, hormones are expected to be strongly associated with components of fitness; however, few studies have examined how natural selection acts on hormonal variation in the wild. In a songbird, the dark‐eyed junco (Junco hyemalis), field experiments have shown that exogenous testosterone alters individuals’ resolution of the survival‐reproduction trade‐off, enhancing reproduction at the expense of survival. Here we used standardized injections of gonadotropin‐releasing hormone (GnRH) to assay variation in the testosterone production of males. Using measurements of annual survival and reproduction, we found evidence of strong natural selection acting on GnRH‐induced increases in testosterone. Opposite to what would be predicted from the survival‐reproduction trade‐off, patterns of selection via survival and reproduction were remarkably similar. Males with GnRH‐induced testosterone production levels that were slightly above the population mean were more likely to survive and also produced more offspring, leading to strong stabilizing selection. Partitioning reproduction into separate components revealed positive directional selection via within‐pair siring success and stabilizing selection via extrapair mating success. Our data represent the most complete demonstration of natural selection on hormones via multiple fitness components, and they complement previous experiments to illuminate testosterone’s role in the evolution of life‐history trade‐offs.
De novo transcriptome sequencing in a songbird, the dark-eyed junco (Junco hyemalis): Genomic tools for an ecological model system
(BMC Genomics, 2012-07-09) Peterson, Mark P.; Whittaker, Danielle J.; Ambreth, Shruthi; Sureschandra, Suhas; Buechlein, Aaron; Podicheti, Ram; Choi, Jeong-Hyeon; Lai, Zhao; Mockatis, Keithanne; Colbourne, John; Tang, Haixu; Ketterson, Ellen D.
Though genomic-level data are becoming widely available, many of the metazoan species sequenced are laboratory systems whose natural history is not well documented. In contrast, the wide array of species with very well-characterized natural history have, until recently, lacked genomics tools. It is now possible to address significant evolutionary genomics questions by applying high-throughput sequencing to discover the majority of genes for ecologically tractable species, and by subsequently developing microarray platforms from which to investigate gene regulatory networks that function in natural systems. We used GS-FLX Titanium Sequencing (Roche/454-Sequencing) of two normalized libraries of pooled RNA samples to characterize a transcriptome of the dark-eyed junco (Junco hyemalis), a North American sparrow that is a classically studied species in the fields of photoperiodism, speciation, and hormone-mediated behavior.
Phenotypic Integration of Sexually Selected Traits in a Songbird
([Bloomington, Ind.] : Indiana University, 2010-06-01) McGlothlin, Joel W.; Ketterson, Ellen D.
Natural selection favors traits that fit not only the external environment, but also the internal environment of the organism. As a consequence, traits often show a pattern of correlation, or phenotypic integration. In this dissertation, I examined both the evolutionary processes and the physiological mechanisms that generate phenotypic integration. I studied a natural population of a songbird, the dark-eyed junco (Junco hyemalis), focusing on the male "mating phenotype," the suite of morphology, physiology, and behavior used to attract and compete for mates. In Chapter 1, I review literature suggesting that correlational selection, which occurs when traits interact in their effects on fitness, may have effects on the physiological mechanisms that underlie integrated suites of traits. In Chapter 2, I found that correlational sexual selection favored an association between body size and a white patch on the tail feathers ("tail white"), an ornament used both in courtship and male-male competition. I also found that body size and tail white were genetically correlated. These results suggest that correlational selection may maintain the integration of the two traits. In Chapters 3-5, I focus on the role of the steroid hormone testosterone in the mating phenotype. In Chapter 3, I measured natural variation in testosterone levels and found that more attractive males had higher androgen responsiveness. That is, males with more tail white produced more testosterone in response to an injection of GnRH, a hypothalamic hormone. This suggests that investment in mating behavior (which seems to be controlled by testosterone) may covary with attractiveness. Indeed, in Chapter 4, I found that androgen responsiveness naturally covaries with both mating and parental behavior. Males that produced more testosterone defended their territories more vigorously and fed their offspring less often. Finally, in Chapter 5, I examined how selection acts on androgen responsiveness, and found that males with very high or very low responsiveness were less likely to survive. Combined, these studies suggest that testosterone, on a physiological level, and correlational selection, on an evolutionary level, act as integrators of the male mating phenotype.
Testosterone Affects Neural Gene Expression Differently in Male and Female Juncos: A Role for Hormones in Mediating Sexual Dimorphism and Conflict
(PLOS One, 2013-04-16) Peterson, Mark P.; Rosvall, Kimberly A.; Choi, Jeong-Hyeon; Ziegenfus, Charles; Tang, Haixu; Colbourne, John K.; Ketterson, Ellen D.
Despite sharing much of their genomes, males and females are often highly dimorphic, reflecting at least in part the resolution of sexual conflict in response to sexually antagonistic selection. Sexual dimorphism arises owing to sex differences in gene expression, and steroid hormones are often invoked as a proximate cause of sexual dimorphism. Experimental elevation of androgens can modify behavior, physiology, and gene expression, but knowledge of the role of hormones remains incomplete, including how the sexes differ in gene expression in response to hormones. We addressed these questions in a bird species with a long history of behavioral endocrinological and ecological study, the dark-eyed junco (Junco hyemalis), using a custom microarray. Focusing on two brain regions involved in sexually dimorphic behavior and regulation of hormone secretion, we identified 651 genes that differed in expression by sex in medial amygdala and 611 in hypothalamus. Additionally, we treated individuals of each sex with testosterone implants and identified many genes that may be related to previously identified phenotypic effects of testosterone treatment. Some of these genes relate to previously identified effects of testosterone-treatment and suggest that the multiple effects of testosterone may be mediated by modifying the expression of a small number of genes. Notably, testosterone-treatment tended to alter expression of different genes in each sex: only 4 of the 527 genes identified as significant in one sex or the other were significantly differentially expressed in both sexes. Hormonally regulated gene expression is a key mechanism underlying sexual dimorphism, and our study identifies specific genes that may mediate some of these processes.
Testosterone and Avian Life Histories: Effects of Experimentally Elevated Testosterone on Behavior and Correlates of Fitness in the Dark-Eyed Junco (Junco hyemalis)
(American Society of Naturalists, 1992-12) Ketterson, Ellen D.; Nolan, Val Jr; Wolf, Licia; Ziegenfus, Charles
Hormones influence many aspects of organismal behavior, physiology, and morphol- ogy, and thus hormones may lie at the root of many life-history trade-offs. By manipulating hormones we can create novel phenotypes (i.e., perform phenotypic engineering) and attempt to relate phenotypic variation to fitness. We report the effect of testosterone treatment on parental behavior and vocal behavior of adult male dark-eyed juncos. Testosterone partially suppressed paternal behavior and increased the frequency of song. When we compared treated males and controls for nine potential correlates of fitness (offspring growth and survival to the age of 10 d, condition of females, length of the interval between consecutive nestings, size of subsequent clutches and broods, mate retention within and between breeding seasons, and survival rate), we found no statistical differences. In some measures treated males outperformed controls, but in most the reverse was true. The power of some of our tests was not great enough to detect small differences. At this stage of our investigation, three interpretations of our results seem almost equally probable: (1) a broad range of behavioral phenotypes is selectively neutral in the junco, (2) male parental behavior is beneficial to males only in some years or habitats, or (3) we have yet to measure the correlates of fitness that are most strongly affected by the behavioral changes induced by elevated testosterone