Effective heritability of targets of sex-ratio selection under environmental sex determination

Selection is expected to maintain primary sex ratios at an evolutionary equilibrium. In organisms with temperature-dependent sex determination (TSD), targets of sex-ratio selection include the thermal sensitivity of the sex-determining pathway (hereafter, sex determination threshold) and nest-site choice. However, offspring sex may be canalized for nests located in thermally extreme environments; thus, genetic variance for the sex determination threshold is not expressed and is invisible to direct selection. The concept of 'effective heritability' accounts for this dependence and provides a more realistic prediction of the expected evolutionary response to selection in the wild. Past estimates of effective heritability of the sex determination threshold, which were derived from laboratory data, suggested that the potential for the sex determination threshold to evolve in the wild was extremely low. We re-evaluated original estimates of this parameter by analysing field-collected measures of nest temperatures, vegetation cover and clutch sex ratios from nests in a population of painted turtles (Chrysemys picta). We coupled these data with measurements of broad-sense heritability of the sex determination threshold in C. picta, using an experiment that splits clutches of eggs between a constant temperature (i.e. typical laboratory incubation) and a daily fluctuating temperature (i.e. similar to natural nests) with the same mean. We found that (i) the effective heritability of the sex determination threshold appears to have been historically underestimated and the effective heritability of nest-site choice has been overestimated and (ii) significant family-by-incubation treatment interaction exists for sex for C. picta between constant- and fluctuating-temperature regimes. Our results suggest that the thermal sensitivity of the sex-determining pathway may play a larger, more complex role in the microevolution of TSD than traditionally thought.     

Segregating variation for temperature-dependent sex determination in a lizard

Temperature-dependent sex determination (TSD) was first reported in 1966 in an African lizard. It has since been shown that TSD occurs in some fish, several lizards, tuataras, numerous turtles and all crocodilians. Extreme temperatures can also cause sex reversal in several amphibians and lizards with genotypic sex determination. Research in TSD species indicates that estrogen signaling is important for ovary development and that orthologs of mammalian genes have a function in gonad differentiation. Nevertheless, the mechanism that actually transduces temperature into a biological signal for ovary versus testis development is not known in any species. Classical genetics could be used to identify the loci underlying TSD, but only if there is segregating variation for TSD. Here, we use the ‘animal model'' to analyze inheritance of sexual phenotype in a 13-generation pedigree of captive leopard geckos, Eublepharis macularius, a TSD reptile. We directly show genetic variance and genotype-by-temperature interactions for sex determination. Additive genetic variation was significant at a temperature that produces a female-biased sex ratio (30 °C), but not at a temperature that produces a male-biased sex ratio (32.5 °C). Conversely, dominance variance was significant at the male-biased temperature (32.5 °C), but not at the female-biased temperature (30 °C). Non-genetic maternal effects on sex determination were negligible in comparison with additive genetic variance, dominance variance and the primary effect of temperature. These data show for the first time that there is segregating variation for TSD in a reptile and consequently that a quantitative trait locus analysis would be practicable for identifying the genes underlying TSD.     

Quantitative Genetics of Sex Ratio Traits in the Parasitic Wasp, Nasonia Vitripennis

We detected significant parent-offspring regressions for the first sex ratio (the sex ratio produced by a female in a fresh host) and the second sex ratio (the sex ratio produced by a female in a previously parasitized host) in the parasitic wasp, Nasonia vitripennis. For both traits, estimates of the narrow-sense heritability range from &0.05 to &0.15 (depending on how the data are analyzed). The study population was derived from isofemale strains created from wasps captured in a single bird nest. The same population exhibited no significant parent-offspring regression for the brood sizes associated with the first and second sex ratios. There may be a significant negative parent-offspring regression for diapause proportion in the first sex ratio broods. The estimates of the genetic correlations between first and second sex ratios are positive although almost all are not significantly different from 0.0. To our knowledge, this study is the first ``fine-scale' analysis of genetic variation for sex ratio traits in any species of insect. Such studies are an essential part of the assessment of the validity of claims that sex ratio traits are locally optimal.     

Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene

A test for heritability of the sex ratio in human genealogical data is reported here, with the finding that there is significant heritability of the parental sex ratio by male, but not female offspring. A population genetic model was used to examine the hypothesis that this is the result of an autosomal gene with polymorphic alleles, which affects the sex ratio of offspring through the male reproductive system. The model simulations show that an equilibrium sex ratio may be maintained by frequency dependent selection acting on the heritable variation provided by the gene. It is also shown that increased mortality of pre-reproductive males causes an increase in male births in following generations, which explains why increases in the sex ratio have been seen after wars, also why higher infant and juvenile mortality of males may be the cause of the male-bias typically seen in the human primary sex ratio. It is concluded that various trends seen in population sex ratios are the result of changes in the relative frequencies of the polymorphic alleles of the proposed gene. It is argued that this occurs by common inheritance and that parental resource expenditure per sex of offspring is not a factor in the heritability of sex ratio variation.     

Sex Determination and Sex Ratios in Crocodylus palustris

Incubation temperature determines sex in the mugger crocodile,Crocodylus palustris. Exclusively females are produced at constanttemperatures of 28.0°C through 31°C. At 32.5°C,only males are produced. Both sexes are produced in varyingproportions at 31.5, 32.0, and 33.0°C. Embryo survival isnot affected within this range, but developmental rate and totalincubation time are strongly temperature dependent. In naturalnests laid in breeding enclosures, cool incubation temperaturesproduced only females whereas males were produced only in warmnests. Clutch sex ratios were female or male biased. Yearlysex ratios (=percent male) varied from 0.05 to 0.58; overallsex ratio during six nesting seasons was 0.24 (1 male: 3 females).Sex ratio and incubation time vary with nest location and temperaturein a manner consistent with the constant temperature results.Incubation time decreases with increasing incubation temperature,and is an accurate predictor of sex ratio in the field and laboratory. To date, temperature-dependent sex determination (TSD) has beenreported in five species of Crocodylus and in three speciesof Alligatorinae; but the TSD patterns in these groups differ.The TSD pattern of C. palustris is similar to that of C. porosus.Nesting in C. palustris is synchronized with the seasonal availabilityof thermal regimes suitable for incubation. Resultant sex ratiosare a consequence of when and where eggs are laid. Early nestsare located in warm, sunny sites; in contrast, late season nestsare located in the shade. An egg transplant experiment demonstratedthat sex ratios could be altered by simple manipulations ofnest temperatures in the field. The adaptive significance ofTSD in crocodilians may relate to the influence of incubationtemperature on various hatchling attributes, particularly growth.     

Narrow-sense heritability of body size and its response to different developmental temperatures in Culex quinquefasciatus (Say 1923)

Body size is an important trait involved in overall fitness through its effects on mating success, fecundity, resource acquisition and mortality, and desiccation resistance. In this study, we raised inbred Culex quinquefasciatus mosquito cohorts at different developmental temperatures of 20°, 23°, and 27° C. As an indicator of the amount of genetic variation in body size, we estimated the narrow-sense heritability of body sizes defined as wing aspect ratios. Our results show that narrow-sense heritability of the body size increased as the developmental temperature increased. We also detected the presence of strong genotype-by-environment (G × E) interaction from low cross-environmental correlations. The body size of each temperature regime followed the general rule that higher temperatures produce smaller individuals. We suggest that the increase in genetic variation with increasing temperature might be due to an unleashing of the cryptic genetic variation of the putative genes affecting body size. We conclude that this increase in genetic variation tracking the environmental (developmental temperature) change could have considerable implications for the distribution and range expansion of Cx. quinquefasciatus, especially in warmer environments.     

Effects of spontaneous mutation accumulation on sex ratio traits in a parasitoid wasp

Sex allocation theory has proved extremely successful at predicting when individuals should adjust the sex of their offspring in response to environmental conditions. However, we know rather little about the underlying genetics of sex ratio or how genetic architecture might constrain adaptive sex-ratio behavior. We examined how mutation influenced genetic variation in the sex ratios produced by the parasitoid wasp Nasonia vitripennis. In a mutation accumulation experiment, we determined the mutability of sex ratio, and compared this with the amount of genetic variation observed in natural populations. We found that the mutability (h(2)(m)) ranges from 0.001 to 0.002, similar to estimates for life-history traits in other organisms. These estimates suggest one mutation every 5-60 generations, which shift the sex ratio by approximately 0.01 (proportion males). In this and other studies, the genetic variation in N. vitripennis sex ratio ranged from 0.02 to 0.17 (broad-sense heritability, H(2)). If sex ratio is maintained by mutation-selection balance, a higher genetic variance would be expected given our mutational parameters. Instead, the observed genetic variance perhaps suggests additional selection against sex-ratio mutations with deleterious effects on other fitness traits as well as sex ratio (i.e., pleiotropy), as has been argued to be the case more generally.     

Sex differences in the genetic architecture of aggressiveness in a sexually dimorphic spider

Sex differences in the genetic architecture of behavioral traits can offer critical insight into the processes of sex‐specific selection and sexual conflict dynamics. Here, we assess genetic variances and cross‐sex genetic correlations of two personality traits, aggression and activity, in a sexually size‐dimorphic spider, Nuctenea umbratica. Using a quantitative genetic approach, we show that both traits are heritable. Males have higher heritability estimates for aggressiveness compared to females, whereas the coefficient of additive genetic variation and evolvability did not differ between the sexes. Furthermore, we found sex differences in the coefficient of residual variance in aggressiveness with females exhibiting higher estimates. In contrast, the quantitative genetic estimates for activity suggest no significant differentiation between males and females. We interpret these results with caution as the estimates of additive genetic variances may be inflated by nonadditive genetic effects. The mean cross‐sex genetic correlations for aggression and activity were 0.5 and 0.6, respectively. Nonetheless, credible intervals of both estimates were broad, implying high uncertainty for these estimates. Future work using larger sample sizes would be needed to draw firmer conclusions on how sexual selection shapes sex differences in the genetic architecture of behavioral traits.     

Genetic Control of Glycolysis in Human Erythrocytes

We have studied heritability of the concentration of each glycolytic intermediate and adenine nucleotide in the cytosol of human erythrocytes obtained from a random sample of apparently healthy young individuals. Preliminary to analysis of heritability, each trait was statistically described and the effects attributable to variation in measured concomitants were removed by regression. Heritability was estimated using the family-set method. This method removes covariances between the index case, sibling and first cousin, due to those environmental determinants of the phenotypic values that are shared with a matched, unrelated control member of the family set. It also removes covariances due to environments that are shared by siblings and first cousins. Heritability was estimated by employing the fact that the variance of differences between first cousins minus the variance of differences between full siblings estimates three-fourths of the additive genetic variance. The heritability estimates for G6P†, F6P, ATP and some other metabolite concentrations are high and significantly greater than zero. The heritabilities of G6P and F6P are likely attributable to genetic variation in the in vivo activity of HK and/or PFK, because the concentrations of these metabolites are tightly controlled by the two regulatory enzymes. Statistically significant heritability estimates for HK and PFK mass action ratios strongly suggest genes are responsible for a portion of the quantitative variation in these enzyme activities. Since HK and PFK regulate glycolysis and the production of ATP, genetic variation in their activities might be causally related to the heritability of ATP concentration.     

The Mechanism of Temperature Dependent Sex Determination in Crocodilians: A Hypothesis

Incubation of alligator eggs at 30°C produces 100% females,at 33°C 100% males; temperatures in between produce varyingsex ratios. Wild nests of Alligator mississippiensis show similareffects and the populations are biased towards females. Theincidence and patterns of temperature dependent sex determination(TSD) in other crocodilians are reviewed. Temperature also affectshatchling size and pigmentation patterns, post-hatching growthrates and thermoregulation by juvenile crocodilians. The significanceof temperature sensitive periods defined by temperature shiftexperiments is questioned in relation to a hypothesis to explainthe mechanism of TSD in crocodilians. It is postulated thatthere is an initial sex differentiation mechanism which involvesa quantum period of time and a threshold for a dose of a maledetermining factor. The conditions for induction of males areprecise but exhibit variation between individuals within thepopulation. Females develop by default. The hypothalamus mayhave an important role in a later sex differentiation mechanism.The hypothesis is used to explain the late temperature sensitiveperiods defined by high to low temperature shift experiments,why cooler temperatures are more effective at determining sex,how intermediate temperatures can produce both sexes, the differencesin the pattern between turtles and crocodilians and geographicalsimilarities in the pattern of TSD within crocodilians despitediffering climates. The phylogenetic advantages of TSD in crocodiliansare concerned with the overall reproductive strategy of theanimals. Those crocodilians which are incubated and grow tomaturity under optimal environmental conditions will be bothlarge and male. Larger males are more likely to produce moreoffspring. A review of the effects of the environment on sexdetermination in amphibians and fish suggests that there isa general relationship between size and sex in vertebrates     

Temperature-dependent sex determination in fish revisited: prevalence, a single sex ratio response pattern, and possible effects of climate change

Background

In gonochoristic vertebrates, sex determination mechanisms can be classified as genotypic (GSD) or temperature-dependent (TSD). Some cases of TSD in fish have been questioned, but the prevalent view is that TSD is very common in this group of animals, with three different response patterns to temperature.

Methodology/Principal Findings

We analyzed field and laboratory data for the 59 fish species where TSD has been explicitly or implicitly claimed so far. For each species, we compiled data on the presence or absence of sex chromosomes and determined if the sex ratio response was obtained within temperatures that the species experiences in the wild. If so, we studied whether this response was statistically significant. We found evidence that many cases of observed sex ratio shifts in response to temperature reveal thermal alterations of an otherwise predominately GSD mechanism rather than the presence of TSD. We also show that in those fish species that actually have TSD, sex ratio response to increasing temperatures invariably results in highly male-biased sex ratios, and that even small changes of just 1–2°C can significantly alter the sex ratio from 1∶1 (males∶females) up to 3∶1 in both freshwater and marine species.

Conclusions/Significance

We demonstrate that TSD in fish is far less widespread than currently believed, suggesting that TSD is clearly the exception in fish sex determination. Further, species with TSD exhibit only one general sex ratio response pattern to temperature. However, the viability of some fish populations with TSD can be compromised through alterations in their sex ratios as a response to temperature fluctuations of the magnitude predicted by climate change.     

Environmental sex determination in a splash pool copepod

The sex-determining mechanism has important demographic and genetic consequences by virtue of its effect on the population sex ratio. Here we investigate the effect of temperature dependent sex determination (TSD) on the primary sex ratio of the harpacticoid copepod, Tigriopus californicus . At the two experimental temperatures (15° and 22°C) used in this study, the primary sex ratio is almost always biased in favour of males. Higher temperatures induce masculinization and the change in sex ratio is not caused by differential mortality of the sexes. The mean level of TSD in the population is small (proportion of males increases by ~5% between 15° and 22°C) because only one-third of the families actually exhibit a significant sex-ratio response while the rest of the population is insensitive to temperature. A comparison of the primary sex ratio and the level of TSD between two locations reveals few differences among populations. Finally, individuals still exhibited TSD after having been maintained under constant temperature conditions in the lab for several generations. In addition the proportion of temperature-sensitive individuals remained unchanged. This suggests that the observed level of TSD is not an artefact of testing field-captured individuals in a novel laboratory environment. At this point the adaptive significance of temperature-dependent sex determination in T. californicus remains unknown.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 511–520.     

Environmental sex determination in a reptile varies seasonally and with yolk hormones

Most hypotheses that have been put forward in order to explain the persistence of environmental sex determination (ESD) in reptiles assume a relatively fixed association of sex with temperature-induced phenotype and no maternal influence on offspring sex. Here we demonstrate the association of maternally derived yolk hormone levels with the offspring sex ratio and describe two new aspects of temperature-dependent sex determination (TSD), i.e. seasonal variation in both thermal response and yolk steroid levels. Eggs from painted turtles (Chrysemys picta) were incubated at 28 degrees C. The hatchling sex ratio at 28 degrees C (i.e. the phenotypic reaction norm for sex at 28 degrees C) shifted seasonally from ca. 72% male to ca. 76% female. Yolk oestradiol (E2) increased seasonally while testosterone (T) decreased. The proportion of males in a clutch decreased as E2 levels increased and the E2:T ratio increased. These new findings are discussed in relation to heritability and adaptive explanations for the persistence of ESD in reptiles. Maternally derived yolk hormones may provide a mechanism for the seasonal shift in the sex ratio which in turn may help explain the persistence of ESD in reptiles. They may also explain those clutches of other reptiles with TSD that fail to yield only males at maximally masculinizing conditions.     

"Missing" G x E Variation Controls Flowering Time in Arabidopsis thaliana

Understanding how genetic variation interacts with the environment is essential for understanding adaptation. In particular, the life cycle of plants is tightly coordinated with local environmental signals through complex interactions with the genetic variation (G x E). The mechanistic basis for G x E is almost completely unknown. We collected flowering time data for 173 natural inbred lines of Arabidopsis thaliana from Sweden under two growth temperatures (10°C and 16°C), and observed massive G x E variation. To identify the genetic polymorphisms underlying this variation, we conducted genome-wide scans using both SNPs and local variance components. The SNP-based scan identified several variants that had common effects in both environments, but found no trace of G x E effects, whereas the scan using local variance components found both. Furthermore, the G x E effects appears to be concentrated in a small fraction of the genome (0.5%). Our conclusion is that G x E effects in this study are mostly due to large numbers of allele or haplotypes at a small number of loci, many of which correspond to previously identified flowering time genes.     

Sex determination and optimal development in the Moorish gecko,Tarentola mauritanica

Under temperature sex determination (TSD), sex is determined by temperature during embryonic development. Depending on ecological and physiological traits and plasticity, TSD species may face demographic collapse due to climate change. In this context, asymmetry in bilateral organisms can be used as a proxy for developmental instability and, therefore, deviations from optimal incubation conditions. Using Tarentola mauritanica gecko as a model, this study aimed first to confirm TSD, its pattern and pivotal temperature, and second to assess the local adaptation of TSD and variation of asymmetry patterns across four populations under different thermal regimes. Eggs were incubated at different temperatures, and hatchlings were sexed and measured. The number of lamellae was counted in adults and hatchlings. Results were compatible with a TSD pattern with males generated at low and females at high incubation temperatures. Estimated pivotal temperature coincided with the temperature producing lower embryonic mortality, evidencing selection towards balanced sex ratios. The temperature of oviposition was conservatively selected by gravid females. Asymmetry patterns found were likely related to nest temperature fluctuations. Overall, the rigidity of TSD may compromise reproductive success, and demographic stability in this species in case thermal nest choice becomes constrained by climate change.     

Temperature-Dependent Sex Determination in Sea Turtles in the Context of Climate Change: Uncovering the Adaptive Significance

The adaptive significance of temperature-dependent sex determination (TSD) in reptiles remains unknown decades after TSD was first identified in this group. Concurrently, there is growing concern about the effect that rising temperatures may have on species with TSD, potentially producing extremely biased sex ratios or offspring of only one sex. The current state-of the-art in TSD research on sea turtles is reviewed here and, against current paradigm, it is proposed that TSD provides an advantage under warming climates. By means of coadaptation between early survival and sex ratios, sea turtles are able to maintain populations. When offspring survival declines at high temperatures, the sex that increases future fecundity (females) is produced, increasing resilience to climate warming. TSD could have helped reptiles to survive mass extinctions in the past via this model. Flaws in research on sex determination in sea turtles are also identified and it is suggested that the development of new techniques will revolutionize the field.     

Genotype x environment interaction in QTL analysis of an intervarietal almond cross by means of genetic markers

Besides QTL location and the estimation of gene effects, QTL analysis based on genetic markers could be used to comprehensively investigate quantitative trait-related phenomena such as pleiotropy, gene interactions, heterosis, and genotype-by-environment interaction (G x E). Given that the G x E interaction is of great relevance in tree improvement, the objective of the research presented here was to study the effect of years on QTL detection for 15 quantitative traits by means of isozymatic markers in a large progeny group of an intervarietal cross of almond. At least 17 putative QTLs were detected, 3 of which had alleles with opposite effects to those predicted from the parental genotypes. Only 3 QTLs behaved homogeneously over the years. Three possible causes are discussed in relation to this lack of stability: the power of the test statistic being used, the low contribution of the QTL to the genetic variation of the trait, and a differential gene expression dependent on the year (G x E). Most cases showing lack of stability involved traits whose heritability estimates change drastically from year to year and/or whose correlation coefficients between years are low, suggesting the presence of G x E as the most likely cause. A marker-assisted selection scheme to improve late flowering and short flowering duration is suggested for an early and wide screening of the progeny.     

SEX ALLOCATION BASED ON RELATIVE AND ABSOLUTE CONDITION

Traditional models predict that organisms should allocate to sex based on their condition relative to the condition of their competitors, tracking shifts in mean condition in fluctuating environments, and maintaining an equilibrium sex ratio. In contrast, when individuals are constrained to define their condition absolutely, environmental fluctuations induce fluctuating sex ratios and the evolutionary loss of condition‐dependent sex allocation in short‐lived organisms. Here, we present a simulation model of temperature‐dependent sex determination (TSD) in fluctuating environments that specifically examines the importance of relativity in defining individual condition. When relativity in condition is allowed to evolve, short‐lived organisms evolve switchlike TSD reaction norms and define their condition relative to the annual temperature distribution, thus preventing biased cohort sex ratios in extreme years. Long‐lived organisms also evolve switchlike reaction norms, but define condition less relatively and experience biased cohort sex ratios. The predictions are supported by data from painted turtles, where TSD reaction norms exhibit pivotal temperatures of sex determination that partially track mean annual temperature. Examining relativity in amniotic vertebrates provides a conceptual framework for multifactorial sex determination and suggests new ways of exploring adaptive hypotheses of sex allocation by focusing on the importance of frequency‐dependent selection on sex.     

Sex ratios in naturally incubating Macrochelys temminckii nests,a species with type II temperature-dependent sex determination

The alligator snapping turtle, Macrochelys temminckii, exhibits type II temperature-dependent sex determination (TSD), wherein females are produced at high and low incubation temperatures. This TSD pattern is well studied at constant temperatures, but little work has focused on sex ratios in natural nests that experience daily and seasonal temperature fluctuations. We monitored nesting activity of reintroduced Macrochelys temminckii at Tishomingo National Wildlife Refuge in 2010–2011. Nests located prior to predation were excavated to determine clutch size and the eggs were reburied with a temperature data logger to collect nest temperatures. Overall, 24% of nests were protected with wire mesh prior to predation, and the average clutch size in intact nests was 22.4 eggs. Nest predation rates in the study population will likely approach 100% if nest protection efforts do not continue. Temperature profiles were used to compare estimated sex ratios using two methods—mean nest temperature during middle third of incubation and the degree-day model—to actual sex ratios in naturally incubated Macrochelys temminckii nests. The sex ratio in all 2010 recruits was female-biased (91.8% female); 2011 nests did not produce any hatchlings, likely the result of severe drought. The predicted sex ratios based on mean nest temperature and the degree-day model matched actual sex ratios in the warmer nests (0% male), but the degree-day model estimate proved more accurate in the cooler nest. A strongly skewed population sex ratio could become a threat to this reintroduced population if the strongly female-biased sex ratio in 2010 reflects a long-term trend.     

Marker-based quantitative genetics in the wild?: the heritability and genetic correlation of chemical defenses in eucalyptus

Marker-based methods for estimating heritability and genetic correlation in the wild have attracted interest because traditional methods may be impractical or introduce bias via G x E effects, mating system variation, and sampling effects. However, they have not been widely used, especially in plants. A regression-based approach, which uses a continuous measure of genetic relatedness, promises to be particularly appropriate for use in plants with mixed-mating systems and overlapping generations. Using this method, we found significant narrow-sense heritability of foliar defense chemicals in a natural population of Eucalyptus melliodora. We also demonstrated a genetic basis for the phenotypic correlation underlying an ecological example of conditioned flavor aversion involving different biosynthetic pathways. Our results revealed that heritability estimates depend on the spatial scale of the analysis in a way that offers insight into the distribution of genetic and environmental variance. This study is the first to successfully use a marker-based method to measure quantitative genetic parameters in a tree. We suggest that this method will prove to be a useful tool in other studies and offer some recommendations for future applications of the method.