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Fredric J. Janzen - One of the best experts on this subject based on the ideXlab platform.
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do covariances between maternal behavior and embryonic physiology drive Sex ratio evolution under Environmental Sex Determination
Journal of Heredity, 2019Co-Authors: Fredric J. Janzen, David M Delaney, Timothy S Mitchell, Daniel A WarnerAbstract:Fisherian Sex-ratio theory predicts Sexual species should have a balanced primary Sex ratio. However, organisms with Environmental Sex Determination (ESD) are particularly vulnerable to experiencing skewed Sex ratios when Environmental conditions vary. Theoretical work has modeled Sex-ratio dynamics for animals with ESD with regard to 2 traits predicted to be responsive to Sex-ratio selection: 1) maternal oviposition behavior and 2) sensitivity of embryonic Sex Determination to Environmental conditions, and much research has since focused on how these traits influence offspring Sex ratios. However, relatively few studies have provided estimates of univariate quantitative genetic parameters for these 2 traits, and the existence of phenotypic or genetic covariances among these traits has not been assessed. Here, we leverage studies on 3 species of reptiles (2 turtle species and a lizard) with temperature-dependent Sex Determination (TSD) to assess phenotypic covariances between measures of maternal oviposition behavior and thermal sensitivity of the Sex-determining pathway. These studies quantified maternal behaviors that relate to nest temperature and Sex ratio of offspring incubated under controlled conditions. A positive covariance between these traits would enhance the efficiency of Sex-ratio selection when primary Sex ratio is unbalanced. However, we detected no such covariance between measures of these categories of traits in the 3 study species. These results suggest that maternal oviposition behavior and thermal sensitivity of Sex Determination in embryos might evolve independently. Such information is critical to understand how animals with TSD will respond to rapidly changing environments that induce Sex-ratio selection.
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Sex specific survival to maturity and the evolution of Environmental Sex Determination
Evolution, 2016Co-Authors: Eric L. Charnov, Lisa E Schwanz, Gerardo A Cordero, Fredric J. JanzenAbstract:Four decades ago, it was proposed that Environmental Sex Determination (ESD) evolves when individual fitness depends on the environment in a Sex-specific fashion--a form of condition-dependent Sex allocation. Many biological processes have been hypothesized to drive this Sex asymmetry, yet a general explanation for the evolution of Sex-determining mechanisms remains elusive. Here, we develop a mathematical model for a novel hypothesis of the evolution of ESD, and provide a first empirical test using data across turtles. ESD is favored when the Sex-determining environment affects annual survival rates equivalently in males and females, and males and females mature at different ages. We compare this hypothesis to alternative hypotheses, and demonstrate how it captures a crucially different process. This maturation process arises naturally from common life histories and applies more broadly to condition-dependent Sex allocation. Therefore, it has widespread implications for animal taxa. Across turtle species, ESD is associated with greater Sex differences in the age at maturity compared to species without ESD, as predicted by our hypothesis. However, the effect is not statistically significant and will require expanded empirical investigation. Given variation among taxa in Sex-specific age at maturity, our survival-to-maturity hypothesis may capture common selective forces on Sex-determining mechanisms.
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a novel hypothesis for the adaptive maintenance of Environmental Sex Determination in a turtle
Proceedings of The Royal Society B: Biological Sciences, 2014Co-Authors: Rickyjohn Spencer, Fredric J. JanzenAbstract:Temperature-dependent Sex Determination (TSD) is widespread in reptiles, yet its adaptive significance and mechanisms for its maintenance remain obscure and controversial. Comparative analyses identify an ancient origin of TSD in turtles, crocodiles and tuatara, suggesting that this trait should be advantageous in order to persist. Based on this assumption, researchers primarily, and with minimal success, have employed a model to examine Sex-specific variation in hatchling phenotypes and fitness generated by different incubation conditions. The unwavering focus on different incubation conditions may be misplaced at least in the many turtle species in which hatchlings overwinter in the natal nest. If overwintering temperatures differentially affect fitness of male and female hatchlings, TSD might be maintained adaptively by enabling embryos to develop as the Sex best suited to those overwintering conditions. We test this novel hypothesis using the painted turtle (Chrysemys picta), a species with TSD in which eggs hatch in late summer and hatchlings remain within nests until the following spring. We used a split-clutch design to expose field-incubated hatchlings to warm and cool overwintering (autumn-winter-spring) regimes in the laboratory and measured metabolic rates, energy use, body size and mortality of male and female hatchlings. While overall mortality rates were low, males exposed to warmer overwintering regimes had significantly higher metabolic rates and used more residual yolk than females, whereas the reverse occurred in the cool temperature regime. Hatchlings from mixed-Sex nests exhibited similar Sex-specific trends and, crucially, they were less energy efficient and grew less than same-Sex hatchlings that originated from single-Sex clutches. Such Sex- and incubation-specific physiological adaptation to winter temperatures may enhance fitness and even extend the northern range of many species that overwinter terrestrially.
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population Sex ratios under differing local climates in a reptile with Environmental Sex Determination
Evolutionary Ecology, 2014Co-Authors: Jeanine M Refsnider, Daniel A Warner, Carrie L Milnezelman, Fredric J. JanzenAbstract:Populations that experience different local climates, such as those along a latitudinal gradient, must match life history traits to local Environmental conditions. In species with temperature-dependent Sex Determination, such as many reptiles, population Sex ratio is strongly influenced by local climate, yet local climate differs substantially among populations in geographically-widespread species. We studied the painted turtle at three sites across the species’ geographic range to gain a mechanistic understanding of how Sex ratios are produced under different local climates. We combined data on maternal nest-site choice, nest incubation temperature, and the resultant offspring Sex ratio of populations across a climatic gradient, to demonstrate how geographic variation in behavior and physiology translates into Sex ratios among populations of a widely-distributed species. We found that populations across the species’ geographic range match incubation conditions with local climatic conditions through population-specific adjustment of maternal nest-site choice. Incubation temperatures during the thermosensitive period were cooler and clutches were more male-biased in the south, with populations farther north having warmer incubation temperatures and more female-biased Sex ratios, yet adult Sex ratios were not strongly biased in any population. Most components of maternal nest-site choice varied latitudinally among populations, suggesting that the species may have a considerable repertoire for responding to climate change through adjustment of nest-site choice.
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population Sex ratios under differing local climates in a reptile with Environmental Sex Determination
Evolutionary Ecology, 2014Co-Authors: Jeanine M Refsnider, Daniel A Warner, Carrie L Milnezelman, Fredric J. JanzenAbstract:Populations that experience different local climates, such as those along a latitudinal gradient, must match life history traits to local Environmental conditions. In species with temperature-dependent Sex Determination, such as many reptiles, population Sex ratio is strongly influenced by local climate, yet local climate differs substantially among populations in geographically-widespread species. We studied the painted turtle at three sites across the species' geographic range to gain a mechanistic understanding of how Sex ratios are produced under different local climates. We combined data on maternal nest- site choice, nest incubation temperature, and the resultant offspring Sex ratio of populations across a climatic gradient, to demonstrate how geographic variation in behavior and physiology translates into Sex ratios among populations of a widely-distributed species. We found that populations across the species' geographic range match incubation conditions with local climatic conditions through population-specific adjustment of maternal nest-site choice. Incubation temperatures during the thermosensitive period were cooler and clut- ches were more male-biased in the south, with populations farther north having warmer
Taisen Iguchi - One of the best experts on this subject based on the ideXlab platform.
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methyl farnesoate synthesis is necessary for the Environmental Sex Determination in the water flea daphnia pulex
Journal of Insect Physiology, 2015Co-Authors: Kenji Toyota, Hitoshi Miyakawa, Shinichi Miyagawa, Chizue Hiruta, Kenjiro Furuta, Yukiko Ogino, Tetsuro Shinoda, Norihisa Tatarazako, Joseph R Shaw, Taisen IguchiAbstract:Sex-Determination systems can be divided into two groups: genotypic Sex Determination (GSD) and Environmental Sex Determination (ESD). ESD is an adaptive life-history strategy that allows control of Sex in response to Environmental cues in order to optimize fitness. However, the molecular basis of ESD remains largely unknown. The micro crustacean Daphnia pulex exhibits ESD in response to various external stimuli. Although methyl farnesoate (MF: putative juvenile hormone, JH, in daphnids) has been reported to induce male production in daphnids, the role of MF as a Sex-determining factor remains elusive due to the lack of a suitable model system for its study. Here, we establish such a system for ESD studies in D. pulex. The WTN6 strain switches from producing females to producing males in response to the shortened day condition, while the MFP strain only produces females, irrespective of day-length. To clarify whether MF has a novel physiological role as a Sex-determining factor in D. pulex, we demonstrate that a MF/JH biosynthesis inhibitor suppressed male production in WTN6 strain reared under the male-inducible condition, shortened day-length. Moreover, we show that juvenile hormone acid O-methyltransferase (JHAMT), a critical enzyme of MF/JH biosynthesis, displays MF-generating activity by catalyzing farnesoic acid. Expression of the JHAMT gene increased significantly just before the MF-sensitive period for male production in the WTN6 strain, but not in the MFP strain, when maintained under male-inducible conditions. These results suggest that MF synthesis regulated by JHAMT is necessary for male offspring production in D. pulex. Our findings provide novel insights into the genetic underpinnings of ESD and they begin to shed light on the physiological function of MF as a male-fate determiner in D. pulex.
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gonadal differentiation in reptiles exhibiting Environmental Sex Determination
Sexual Development, 2014Co-Authors: Satomi Kohno, Shinichi Miyagawa, Taisen Iguchi, Benjamin B Parrott, Ryohei Yatsu, Brandon C Moore, Louis J GuilletteAbstract:As temperature-dependent Sex Determination (TSD) and homozygote or heterozygote genetic Sex Determination (GSD) exist in multiple reptilian taxa, they represent Sex Determination systems that have emerged de novo. Current investigations have revealed that the genetic mechanisms used by various reptilian species are similar to those used by other vertebrates. However, the recent completion or near completion of various reptilian genome projects suggests that new studies examining related species with and without TSD could begin to provide additional insight into the evolution of TSD and GSD in vertebrate ancestors. Major questions still remain concerning germ cell migration and specification, the differentiation of gonadal accessory cells, such as the Sertoli cells and granulosa cells of the developing testis and ovary, respectively, and the mechanisms by which gene expression is regulated during TSD events. Further, reptilian sentinels and their mechanisms of gonadogenesis will likely remain important indicator species for Environmental health. Thus, ongoing and new investigations need to tie molecular information to gonadal morphogenesis and function in reptiles. Such data will not only provide important information for an understanding of the evolution of these phenomena in vertebrates, but could also provide an important understanding of the health of the environment around us.
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Environmental Sex Determination in the branchiopod crustacean daphnia magna deep conservation of a doubleSex gene in the Sex determining pathway
PLOS Genetics, 2011Co-Authors: Yasuhiko Kato, Hajime Watanabe, Kaoru Kobayashi, Taisen IguchiAbstract:Sex-determining mechanisms are diverse among animal lineages and can be broadly divided into two major categories: genetic and Environmental. In contrast to genetic Sex Determination (GSD), little is known about the molecular mechanisms underlying Environmental Sex Determination (ESD). The DoubleSex (Dsx) genes play an important role in controlling Sexual dimorphism in genetic Sex-determining organisms such as nematodes, insects, and vertebrates. Here we report the identification of two Dsx genes from Daphnia magna, a freshwater branchiopod crustacean that parthenogenetically produces males in response to Environmental cues. One of these genes, designated DapmaDsx1, is responsible for the male trait development when expressed during Environmental Sex Determination. The domain organization of DapmaDsx1 was similar to that of Dsx from insects, which are thought to be the sister group of branchiopod crustaceans. Intriguingly, the molecular basis for Sexually dimorphic expression of DapmaDsx1 is different from that of insects. Rather than being regulated Sex-specifically at the level of pre–mRNA splicing in the coding region, DapmaDsx1 exhibits Sexually dimorphic differences in the abundance of its transcripts. During embryogenesis, expression of DapmaDsx1 was increased only in males and its transcripts were primarily detected in male-specific structures. Knock-down of DapmaDsx1 in male embryos resulted in the production of female traits including ovarian maturation, whereas ectopic expression of DapmaDsx1 in female embryos resulted in the development of male-like phenotypes. Expression patterns of another D. magna Dsx gene, DapmaDsx2, were similar to those of DapmaDsx1, but silencing and overexpression of this gene did not induce any clear phenotypic changes. These results establish DapmaDsx1 as a key regulator of the male phenotype. Our findings reveal how ESD is implemented by selective expression of a fundamental genetic component that is functionally conserved in animals using GSD. We infer that there is an ancient, previously unidentified link between genetic and Environmental Sex Determination.
Kenji Toyota - One of the best experts on this subject based on the ideXlab platform.
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methyl farnesoate synthesis is necessary for the Environmental Sex Determination in the water flea daphnia pulex
Journal of Insect Physiology, 2015Co-Authors: Kenji Toyota, Hitoshi Miyakawa, Shinichi Miyagawa, Chizue Hiruta, Kenjiro Furuta, Yukiko Ogino, Tetsuro Shinoda, Norihisa Tatarazako, Joseph R Shaw, Taisen IguchiAbstract:Sex-Determination systems can be divided into two groups: genotypic Sex Determination (GSD) and Environmental Sex Determination (ESD). ESD is an adaptive life-history strategy that allows control of Sex in response to Environmental cues in order to optimize fitness. However, the molecular basis of ESD remains largely unknown. The micro crustacean Daphnia pulex exhibits ESD in response to various external stimuli. Although methyl farnesoate (MF: putative juvenile hormone, JH, in daphnids) has been reported to induce male production in daphnids, the role of MF as a Sex-determining factor remains elusive due to the lack of a suitable model system for its study. Here, we establish such a system for ESD studies in D. pulex. The WTN6 strain switches from producing females to producing males in response to the shortened day condition, while the MFP strain only produces females, irrespective of day-length. To clarify whether MF has a novel physiological role as a Sex-determining factor in D. pulex, we demonstrate that a MF/JH biosynthesis inhibitor suppressed male production in WTN6 strain reared under the male-inducible condition, shortened day-length. Moreover, we show that juvenile hormone acid O-methyltransferase (JHAMT), a critical enzyme of MF/JH biosynthesis, displays MF-generating activity by catalyzing farnesoic acid. Expression of the JHAMT gene increased significantly just before the MF-sensitive period for male production in the WTN6 strain, but not in the MFP strain, when maintained under male-inducible conditions. These results suggest that MF synthesis regulated by JHAMT is necessary for male offspring production in D. pulex. Our findings provide novel insights into the genetic underpinnings of ESD and they begin to shed light on the physiological function of MF as a male-fate determiner in D. pulex.
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nmda receptor activation upstream of methyl farnesoate signaling for short day induced male offspring production in the water flea daphnia pulex
BMC Genomics, 2015Co-Authors: Kenji Toyota, Hitoshi Miyakawa, Yukiko Ogino, Norihisa Tatarazako, Katsushi Yamaguchi, Shuji Shigenobu, Shinichi MiyagawaAbstract:Background The cladoceran crustacean Daphnia pulex produces female offspring by parthenogenesis under favorable conditions, but in response to various unfavorable external stimuli, it produces male offspring (Environmental Sex Determination: ESD). We recently established an innovative system for ESD studies using D. pulex WTN6 strain, in which the Sex of the offspring can be controlled simply by changes in the photoperiod: the long-day and short-day conditions can induce female and male offspring, respectively. Taking advantage of this system, we demonstrated that de novo methyl farnesoate (MF) synthesis is necessary for male offspring production. These results indicate the key role of innate MF signaling as a conductor between external Environmental stimuli and the endogenous male developmental pathway. Despite these findings, the molecular mechanisms underlying up- and downstream signaling of MF have not yet been well elucidated in D. pulex.
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molecular cloning of doubleSex genes of four cladocera water flea species
BMC Genomics, 2013Co-Authors: Kenji Toyota, Hitoshi Miyakawa, Yasuhiko Kato, Hajime Watanabe, Masaru Sato, Naomi Sugiura, Shinichi MiyagawaAbstract:The gene doubleSex (dsx) is known as a key factor regulating genetic Sex Determination in many organisms. We previously identified two dsx genes (DapmaDsx1 and DapmaDsx2) from a freshwater branchiopod crustacean, Daphnia magna, which are expressed in males but not in females. D. magna produces males by parthenogenesis in response to Environmental cues (Environmental Sex Determination) and we showed that DapmaDsx1 expression during embryonic stages is responsible for the male trait development. The D. magna dsx genes are thought to have arisen by a cladoceran-specific duplication; therefore, to investigate evolutionary conservation of Sex specific expression of dsx genes and to further assess their functions in the Environmental Sex Determination, we searched for dsx homologs in four closely related cladoceran species. We identified homologs of both dsx genes from, D. pulex, D. galeata, and Ceriodaphnia dubia, yet only a single dsx gene was found from Moina macrocopa. The deduced amino acid sequences of all 9 dsx homologs contained the DM and oligomerization domains, which are characteristic for all arthropod DSX family members. Molecular phylogenetic analysis suggested that the dsx gene duplication likely occurred prior to the divergence of these cladoceran species, because that of the giant tiger prawn Penaeus monodon is rooted ancestrally to both DSX1 and DSX2 of cladocerans. Therefore, this result also suggested that M. macrocopa lost dsx2 gene secondarily. Furthermore, all dsx genes identified in this study showed male-biased expression levels, yet only half of the putative 5’ upstream regulatory elements are preserved in D. magna and D. pulex. The all dsx genes of five cladoceran species examined had similar amino acid structure containing highly conserved DM and oligomerization domains, and exhibited Sexually dimorphic expression patterns, suggesting that these genes may have similar functions for Environmental Sex Determination in cladocerans.
Shinichi Miyagawa - One of the best experts on this subject based on the ideXlab platform.
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methyl farnesoate synthesis is necessary for the Environmental Sex Determination in the water flea daphnia pulex
Journal of Insect Physiology, 2015Co-Authors: Kenji Toyota, Hitoshi Miyakawa, Shinichi Miyagawa, Chizue Hiruta, Kenjiro Furuta, Yukiko Ogino, Tetsuro Shinoda, Norihisa Tatarazako, Joseph R Shaw, Taisen IguchiAbstract:Sex-Determination systems can be divided into two groups: genotypic Sex Determination (GSD) and Environmental Sex Determination (ESD). ESD is an adaptive life-history strategy that allows control of Sex in response to Environmental cues in order to optimize fitness. However, the molecular basis of ESD remains largely unknown. The micro crustacean Daphnia pulex exhibits ESD in response to various external stimuli. Although methyl farnesoate (MF: putative juvenile hormone, JH, in daphnids) has been reported to induce male production in daphnids, the role of MF as a Sex-determining factor remains elusive due to the lack of a suitable model system for its study. Here, we establish such a system for ESD studies in D. pulex. The WTN6 strain switches from producing females to producing males in response to the shortened day condition, while the MFP strain only produces females, irrespective of day-length. To clarify whether MF has a novel physiological role as a Sex-determining factor in D. pulex, we demonstrate that a MF/JH biosynthesis inhibitor suppressed male production in WTN6 strain reared under the male-inducible condition, shortened day-length. Moreover, we show that juvenile hormone acid O-methyltransferase (JHAMT), a critical enzyme of MF/JH biosynthesis, displays MF-generating activity by catalyzing farnesoic acid. Expression of the JHAMT gene increased significantly just before the MF-sensitive period for male production in the WTN6 strain, but not in the MFP strain, when maintained under male-inducible conditions. These results suggest that MF synthesis regulated by JHAMT is necessary for male offspring production in D. pulex. Our findings provide novel insights into the genetic underpinnings of ESD and they begin to shed light on the physiological function of MF as a male-fate determiner in D. pulex.
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nmda receptor activation upstream of methyl farnesoate signaling for short day induced male offspring production in the water flea daphnia pulex
BMC Genomics, 2015Co-Authors: Kenji Toyota, Hitoshi Miyakawa, Yukiko Ogino, Norihisa Tatarazako, Katsushi Yamaguchi, Shuji Shigenobu, Shinichi MiyagawaAbstract:Background The cladoceran crustacean Daphnia pulex produces female offspring by parthenogenesis under favorable conditions, but in response to various unfavorable external stimuli, it produces male offspring (Environmental Sex Determination: ESD). We recently established an innovative system for ESD studies using D. pulex WTN6 strain, in which the Sex of the offspring can be controlled simply by changes in the photoperiod: the long-day and short-day conditions can induce female and male offspring, respectively. Taking advantage of this system, we demonstrated that de novo methyl farnesoate (MF) synthesis is necessary for male offspring production. These results indicate the key role of innate MF signaling as a conductor between external Environmental stimuli and the endogenous male developmental pathway. Despite these findings, the molecular mechanisms underlying up- and downstream signaling of MF have not yet been well elucidated in D. pulex.
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gonadal differentiation in reptiles exhibiting Environmental Sex Determination
Sexual Development, 2014Co-Authors: Satomi Kohno, Shinichi Miyagawa, Taisen Iguchi, Benjamin B Parrott, Ryohei Yatsu, Brandon C Moore, Louis J GuilletteAbstract:As temperature-dependent Sex Determination (TSD) and homozygote or heterozygote genetic Sex Determination (GSD) exist in multiple reptilian taxa, they represent Sex Determination systems that have emerged de novo. Current investigations have revealed that the genetic mechanisms used by various reptilian species are similar to those used by other vertebrates. However, the recent completion or near completion of various reptilian genome projects suggests that new studies examining related species with and without TSD could begin to provide additional insight into the evolution of TSD and GSD in vertebrate ancestors. Major questions still remain concerning germ cell migration and specification, the differentiation of gonadal accessory cells, such as the Sertoli cells and granulosa cells of the developing testis and ovary, respectively, and the mechanisms by which gene expression is regulated during TSD events. Further, reptilian sentinels and their mechanisms of gonadogenesis will likely remain important indicator species for Environmental health. Thus, ongoing and new investigations need to tie molecular information to gonadal morphogenesis and function in reptiles. Such data will not only provide important information for an understanding of the evolution of these phenomena in vertebrates, but could also provide an important understanding of the health of the environment around us.
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molecular cloning of doubleSex genes of four cladocera water flea species
BMC Genomics, 2013Co-Authors: Kenji Toyota, Hitoshi Miyakawa, Yasuhiko Kato, Hajime Watanabe, Masaru Sato, Naomi Sugiura, Shinichi MiyagawaAbstract:The gene doubleSex (dsx) is known as a key factor regulating genetic Sex Determination in many organisms. We previously identified two dsx genes (DapmaDsx1 and DapmaDsx2) from a freshwater branchiopod crustacean, Daphnia magna, which are expressed in males but not in females. D. magna produces males by parthenogenesis in response to Environmental cues (Environmental Sex Determination) and we showed that DapmaDsx1 expression during embryonic stages is responsible for the male trait development. The D. magna dsx genes are thought to have arisen by a cladoceran-specific duplication; therefore, to investigate evolutionary conservation of Sex specific expression of dsx genes and to further assess their functions in the Environmental Sex Determination, we searched for dsx homologs in four closely related cladoceran species. We identified homologs of both dsx genes from, D. pulex, D. galeata, and Ceriodaphnia dubia, yet only a single dsx gene was found from Moina macrocopa. The deduced amino acid sequences of all 9 dsx homologs contained the DM and oligomerization domains, which are characteristic for all arthropod DSX family members. Molecular phylogenetic analysis suggested that the dsx gene duplication likely occurred prior to the divergence of these cladoceran species, because that of the giant tiger prawn Penaeus monodon is rooted ancestrally to both DSX1 and DSX2 of cladocerans. Therefore, this result also suggested that M. macrocopa lost dsx2 gene secondarily. Furthermore, all dsx genes identified in this study showed male-biased expression levels, yet only half of the putative 5’ upstream regulatory elements are preserved in D. magna and D. pulex. The all dsx genes of five cladoceran species examined had similar amino acid structure containing highly conserved DM and oligomerization domains, and exhibited Sexually dimorphic expression patterns, suggesting that these genes may have similar functions for Environmental Sex Determination in cladocerans.
Steven Freedberg - One of the best experts on this subject based on the ideXlab platform.
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population structure leads to male biased population Sex ratios under Environmental Sex Determination
Evolution, 2018Co-Authors: Emily G Butka, Steven FreedbergAbstract:Spatial structure has been shown to favor female-biased Sex allocation, but current theory fails to explain male biases seen in many taxa, particularly those with Environmental Sex Determination (ESD). We present a theory and accompanying individual-based simulation model that demonstrates how population structure leads to male-biased population Sex ratios under ESD. Our simulations agree with earlier work showing that the high productivity of female-producing habitats creates a net influx of Sex-determining alleles into male-producing habitats, causing larger Sex ratio biases, and lower productivity in male-producing environments (Harts et al. 2014). In contrast to previous findings, we show that male-biasing habitats disproportionately impact the global Sex ratio, resulting in stable male-biased population Sex ratios under ESD. The failure to detect a male bias in earlier work can be attributed to small subpopulation sizes leading to local mate competition, a condition unlikely to be met in most ESD systems. Simulations revealed that consistent male biases are expected over a wide range of population structures, Environmental conditions, and genetic architectures of Sex Determination, with male excesses as large as 30 percent under some conditions. Given the ubiquity of genetic structure in natural populations, we predict that modest, enduring male biased allocation should be common in ESD species, a pattern consistent with reviews of ESD Sex ratios.
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weakened purifying selection leads to elevated mutation load under Environmental Sex Determination
Journal of Evolutionary Biology, 2014Co-Authors: Steven Freedberg, Spencer J DebenportAbstract:In many gonochoristic taxa, Sex is influenced by developmental environment, a system that can lead to temporal fluctuations in offspring Sex ratio. Demographic models suggest that only short-lived species with Environmental Sex Determination (ESD) are negatively impacted by Sex-ratio fluctuations, yet these models fail to account for the potential mutation load associated with reductions in genetically effective population sizes. In this study, we developed a series of individual-based simulation models that explore the fixation rates of mildly deleterious alleles under different Sex-determining systems and examine the impacts of variation in lifespan and offspring Sex ratio. Populations with ESD exhibited increases in fixation rates in both short- and long-lived populations, but substantial increases were limited to populations characterized by a combination of high Sex-ratio variation and short lifespan. Fixation rates were negatively associated with effective population size, indicating that purifying selection operates less efficiently under ESD relative to genotypic Sex Determination. Reductions in effective population size could be attributed to both intragenerational forces (unequal Sex ratio) and intergenerational forces (variable census population sizes). Levels of temporal Sex-ratio variation calculated from wild populations of ESD species were capable of yielding large increases in fixation rates, although this relationship was strongly mediated by lifespan. Our results may help to explain the limited phylogenetic distribution of ESD in short-lived taxa.
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Agricultural practices alter Sex ratios in a reptile with Environmental Sex Determination
Biological Conservation, 2011Co-Authors: Steven Freedberg, Chee Lee, Michael J. PappasAbstract:Anthropogenic changes to the environment have the opportunity to impact natural systems, particularly in organisms that exhibit phenotypic plasticity. Species with Environmental Sex Determination (ESD) are uniquely susceptible to changes in the stimuli that affect Sexual development, potentially leading to maladaptive Sex ratios. We studied the factors affecting Sex Determination and Sex ratios in the common snapping turtle, Chelydra serpentina, in an area heavily impacted by agricultural practices. We investigated the effects of soybean, corn, and sunflower planting on incubation temperatures, Sex ratios, and depredation in naturally laid nests. We also identified and analyzed a novel mitochondrial microsatellite in order to examine the presence of natal homing and determine the likelihood that nest sites impacted by agricultural practices could be transmitted across generations. Females frequently chose to nest in agricultural fields over sand prairie sites, and offspring Sex ratios and depredation rates were significantly influenced by crop planting. Despite detecting considerable genetic variation in our population, we found no relationship between relatedness and nesting location, suggesting that females are not transmitting nesting sites across generations. Our results suggest that agricultural practices can directly impact populations of animals with ESD, and will need to be considered in management decisions.
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Incubation Environment Affects Immune System Development in a Turtle with Environmental Sex Determination.
Journal of herpetology, 2008Co-Authors: Steven Freedberg, Timothy J. Greives, Michael A. Ewert, Gregory E. Demas, Nancy Beecher, Craig E. NelsonAbstract:The developmental environment can have lasting effects on posthatching phenotype in oviparous animals. Innate immune response is one important component of fitness in vertebrates because it provides a generalized defense against infection. In addition, because male vertebrates are at a higher risk of infection than females, males may benefit more from increased innate immunity than females. We determined the effects of incubation temperature on the innate immune response of hatchling map turtles (Graptemys) by incubating eggs at a range of male and female producing-temperatures and assessing plasma complement activity in the resulting hatchlings. We found a significant effect of incubation environment on circulating complement in hatchling Graptemys ouachitensis, with male-producing temperatures yielding the highest innate immune response. Most important, these results demonstrate that immune response is affected by developmental environment in a species with Environmental Sex Determination, potentially resulting in Sex differences in the ability to fend off pathogens.
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Sex ratio variance and the maintenance of Environmental Sex Determination
Journal of Evolutionary Biology, 2007Co-Authors: Steven Freedberg, Douglas R TaylorAbstract:Although variation in population Sex ratios is predicted to increase the extinction rate of clades with Environmental Sex Determination (ESD), ESD is still seen in a wide array of natural systems. It is unclear how this common Sex-determining system has persisted despite this inherent disadvantage associated with ESD. We use simulation modelling to examine the effect of the Sex ratio variance caused by ESD on population colonization and establishment. We find that an accelerating function of establishment success on initial population Sex ratio favours a system that produces variance in Sex ratios over one that consistently produces even Sex ratios. This Sex ratio variance causes ESD to be favoured over genetic Sex Determination, even when the mean global Sex ratio under both Sex-determining systems is the same. Data from ESD populations suggest that the increase in population establishment can more than offset the increased risk of extinction associated with temporal fluctuations in the Sex ratio. These findings demonstrate that selection in natural systems can favour increased variance in a trait, irrespective of the mean trait value. Our results indicate that Sex ratio variation may provide an advantage to species with ESD, and may help explain the widespread existence of this Sex-determining system.