SEX RATIO SELECTION AND THE EVOLUTION OF ENVIRONMENTAL SEX DETERMINATION IN LABORATORY POPULATIONS OF MENIDIA MENIDIA

What happens when a population with environmental sex determination (ESD) experiences a change to an extreme environment that causes a highly unbalanced sex ratio? Theory predicts that frequency-dependent selection would increase the proportion of the minority sex and decrease the level of ESD in subsequent generations. We empirically modeled this process by maintaining five laboratory populations of a fish with temperature-dependent sex determination (the Atlantic silverside, Menidia menidia) in extreme constant temperature environments that caused highly skewed sex ratios to occur initially. Increases in the minority sex consistently occurred from one generation to the next across all five populations, first establishing and then maintaining a balanced sex ratio until termination of the experiment at 8 to 10 generations. The extent to which the level of ESD changed as balanced sex ratios evolved, however, was not consistent. Two populations that experienced high temperatures each generation displayed a loss of ESD, and in one of these ESD was virtually eliminated. This suggests that temperature-insensitive, sex-determining genes were being selected. In populations maintained in low temperature environments, however, the level of ESD did not decline. Instead, the response of sex ratio to temperature was adjusted upward or downward, perhaps by selection of sex-determining genes sensitive to higher (or lower) temperatures. The two different outcomes at low versus high temperatures occurred independent of the geographic origin of the founding population. Our results demonstrate that ESD is capable of evolving in response to selection.     

SEX RATIO VARIATION IN THE LESSONIA NIGRESCENS COMPLEX (LAMINARIALES,PHAEOPHYCEAE): EFFECT OF LATITUDE,TEMPERATURE, AND MARGINALITY1

Little is known about variation of sex ratio, the proportion of males to females, in natural populations of seaweed, though it is a major determinant of the mating system. The observation of sexual chromosomes in kelps suggested that sex is partly genetically determined. However, it is probably not purely genetic since the sex ratio can be modified by environmental factors such as salinity or temperature. In this paper, sex ratio variation was studied in the kelp Lessonia nigrescens Bory complex, recently identified as two cryptic species occurring along the Chilean coast: one located north and the other south of the biogeographic boundary at latitude 29°–30° S. The life cycle of L. nigrescens is characterized by an alternation of microscopic haploid gametophytic individuals and large macroscopic fronds of diploid sporophytes. The sex ratio was recorded in progenies from 241 sporophytic individuals collected from 13 populations distributed along the Chilean coast in order (i) to examine the effect of an environmental gradient coupled with latitude, and (ii) to compare marginal populations to central populations of the two species. In addition, we tested the hypothesis that the sex ratios of the two cryptic species would be affected differently by temperature. First, our results demonstrate that sex ratio seems to be mainly genetically determined and temperature can significantly modify it. Populations of the northern species showed a lower frequency of males at 14°C than at 10°C, whereas populations of the southern species showed the opposite pattern. Second, both species displayed an increased variation in sex ratio at the range limits. This greater variation at the margins could be due either to differential mortality between sexes or to geographic parthenogenesis (asexual reproduction).     

Countergradient variation in growth rate: compensation for length of the growing season among Atlantic silversides from different latitudes

Summary How do organisms adapt to the differences in temperature and length of the growing season that occur with latitude? Among Atlantic silversides (Menidia menidia) along the east coast of North America, the length of the first growing season declines by a factor of about 2.5 with increasing latitude. Yet body size at the end of the first growing season does not decline. High-latitude fish must, therefore, grow faster within the growing season than do low-latitude fish. This geographical pattern has a genetic basis. Laboratory experiments on fish from six different locations revealed a latitudinal gradient in the capacity for growth (i.e., maximum growth potential). In two subsequent experiments using fish from Nova Scotia (NS), New York (NY) and South Carolina (SC) that had been separately reared in a common environment for several generations, differences in growth rate among populations were highly significant. The rank order was NS>NY>SC, but the difference among populations depended on temperature. High-latitude fish outperformed those from low latitudes primarily at the high temperatures that low-latitude fish would be expected to experience most often in nature. These results suggest that instead of being adapted for growth at low temperatures, fish from high latitudes are adapted for rapid elevation of growth rate during the brief interval of the year when high temperatures occur. Selection on growth rate results from sizedependent winter mortality: the importance to winter survival of being large increases with latitude but the length of the growing season simultaneously decreases. The end result is countergradient variation in growth rate, a phenomenon that may be much more widespread than currently recognized.     

CULTURAL INHERITANCE AS A MECHANISM FOR POPULATION SEX-RATIO BIAS IN REPTILES

Abstract.— Although natural populations of most species exhibit a 1:1 sex ratio, biased sex ratios are known to be associated with non‐Mendelian inheritance, as in sex‐linked meiotic drive and cytoplasmic inheritance (Charnov 1982; Hurst 1993). We show how cultural inheritance, another type of non‐Mendelian inheritance, can favor skewed primary sex ratios and propose that it may explain the female‐biased sex ratios commonly observed in reptiles with environmental sex determination (ESD). Like cytoplasmic elements, cultural traits can be inherited through one sex. This, in turn, favors skewing the primary sex allocation in favor of the transmitting sex. Female nest‐site philopatry is a sex‐specific, culturally inherited trait in many reptiles with ESD and highly female‐biased sex ratios. We propose that the association of nest‐site selection with ESD facilitates the maternal manipulation of offspring sex ratios toward females.     

GENETIC AND ENVIRONMENTAL BASIS OF VARIABLE SEX RATIOS IN LABORATORY STRAINS OF POECILIOPSIS LUCIDA

Skewed sex ratios are common among several species of Poeciliopsis, a viviparous fish from northwestern Mexico. Since previous, unrelated studies from this laboratory (Angus and Schultz, 1983) suggested that deviation from a 1:1 sex ratio might be influenced by temperature, two inbred strains of P. lucida were tested for temperature-dependent sex determination by comparing sex ratios of offspring from pregnant females held at different water temperatures. Different sex ratios were produced by the two strains at the same temperature: one strain produced almost all-male offspring at 30°C and female-biased sex ratios at 24°C, while the other strain produced a 1:1 sex ratio at both temperatures. At intermediate temperatures, the labile strain produced sequentially fewer males with decreasing temperatures. The other strain produced a consistent sex ratio regardless of temperature. Poeciliopsis lucida apparently has a genetic polymorphism for temperature-influenced sex determination. An hypothesis is offered for the evolutionary origin of environmental sex determination.     

SEX RATIOS AND CONDITIONS REQUIRED FOR ENVIRONMENTAL SEX DETERMINATION IN ANIMALS

Environmental sex determination (ESD) is a system of sexual determination that is influenced by a variable environment. Once sex is determined it is then fixed for life. The model of Charnov & Bull (1977) proposes that ESD is favoured by natural selection when an individual's fitness as a male or female is strongly influenced by environmental conditions and when the individual has little control over which environment it will experience. Adaptive sex ratio variation is considerably easier for organisms with ESD, and this feature is the ultimate cause for the evolution and maintenance of ESD. ESD is taxonomically widely expressed, and more cases are likely to be discovered. Both environmental and genotypic sex determination mechanisms are found in closely related species. Evidence of geographical variation in the degree and in the critical environmental values of ESD within the same species has also been discovered, e.g. in the fish Menidia menidia and in the crustacean Gammarus duebeni. The factors causing sex determination in invertebrates include temperature, daylength, nutrition, density, humidity, ionic composition of the environment, pH, carbon dioxide, UV light, metabolic products, parasites, exposure to the opposite sex of the same species, and in parasitoids also host size, age and type. In vertebrates temperature is the dominant factor causing sex determination, though in fish also pH, salinity, light, water quality and nutrition, and in turtles water potential of the substrate have some effect on the sex expression. Most of these factors influence growth through resource availability or developmental speed. In most cases of ESD in invertebrates and fish, the environmental factor has a gradual effect on the sex expression, in contrast to the typical steep threshold mode found in reptiles. These differences might be due to the fact that invertebrates exhibiting ESD are commonly parasitic or confined to aquatic environments, where less spatial microhabitat differentiation exists. Sex ratio data available from nature for animals with ESD are quite limited, except for reptiles. In the laboratory sex ratios can be varied more widely than what is observed in nature. There are a number of characteristic features some of which are found in each species exhibiting ESD: (1) Patchy environments, (2) variable sex ratios, (3) parthenogenesis in addition to bisexuality, (4) parasitism, (5) aquatic habitats, (6) sexual dimorphism, (7) females larger than males, and (8) local mate competition.     

EVOLUTION OF INTRINSIC GROWTH AND ENERGY ACQUISITION RATES. II. TRADE-OFFS WITH VULNERABILITY TO PREDATION IN MENIDIA MENIDIA

Abstract The Atlantic silverside ( Menidia menidia ) exhibits countergradient latitudinal variation in somatic growth rate along the East Coast of North America. Larvae and juveniles from high-latitude populations display higher intrinsic rates of energy consumption and growth than genotypes from low-latitude populations. The existence of submaximal growth in some environments suggests that trade-offs must counter the oft-cited theoretical benefits of energy and growth maximization (e.g., "bigger is better,""faster is better") in the immature life stages. We hypothesized that energy and growth maximization trades off against investment in defense from predators. We conducted laboratory selection experiments to compare vulnerability to predation of silversides from: (1) fast-growing northern (Nova Scotia, NS) versus slow-growing southern (South Carolina, SC) source populations; (2) phenotypically manipulated fast-growing versus moderately-growing NS fish; and (3) recently fed versus unfed NS and SC fish. Tests involved fish drawn from common-garden environments and were conducted by subjecting mixed-treatment schools of size-matched silversides to natural, common piscine predators. NS silversides suffered significantly higher predation mortality than SC silversides. Parallel results were found in phenotypic manipulation of growth: NS silversides reared on a fast-growth trajectory (∼1.0 mm/day) were significantly more vulnerable to predation than those growing at a moderate rate (∼0.5 mm/day). Food consumption also affected vulnerability to predators: Silversides with large meals in their stomachs suffered significantly higher predation mortality than unfed silversides. Differences in predation vulnerability were likely due to swimming performance, not attractiveness to predators. Our findings demonstrate that maximization of energy intake and growth rate engenders fitness costs in the form of increased vulnerability to predation.     

Adaptive variation in energy acquisition and allocation among latitudinal populations of the Atlantic silverside

Understanding the evolution of growth rate requires knowledge of the physiology of growth. This study explored the physiological basis of countergradient variation (CnGV) in somatic growth across latitudinal populations of the Atlantic silverside, Menidia menidia. Energetics of northern (Nova Scotia, Canada) and southern (South Carolina, USA) genotypes were compared across resource levels, temperatures, and fish sizes to identify trade-offs to rapid growth. Offered unlimited resources, genotypes differed in both energy acquisition and allocation. Food consumption, growth, and efficiency of northern genotypes were consistently higher than in southern genotypes, across temperatures and body sizes. Feeding metabolism (specific dynamic action; SDA) was proportional to meal size, differing between genotypes to the extent that food consumption differed. Given limited resources, northern and southern genotypes displayed similar growth, efficiency, routine activity, and SDA across temperatures and fish sizes. Routine metabolism was equal at 17°C and 22°C, yet was significantly higher in northern fish at 28°C. Growth rates in M. menidia do not appear to trade off across environments or body sizes, i.e., at no temperature, ration, or size do southern fish outgrow northern conspecifics. Nor does submaximal growth result from increased costs of maintenance, tissue synthesis, or routine activity. Based on our findings, we propose that CnGV consumption and growth in M. menidia likely result from trade-offs with other energetic components, namely sustained and burst swimming. Received: 26 January 1999 / Accepted: 14 September 1999     

Geographic variation in critical photoperiod for diapause induction and its temperature dependence in Hyphantria cunea Drury (Lepidoptera: Arctiidae)

The fall webworm, Hyphantria cunea Drury (Lepidoptera: Arctiidae), was introduced from North America to Japan half a century ago. The critical photoperiod for diapause induction and its temperature dependence, as defined by the difference in the critical photoperiod between 20 and 25°C, were investigated in order to understand the mechanisms behind a shift from bi- to trivoltine life cycles. The critical photoperiod for diapause induction was shorter in the southern trivoltine populations than in the northern bivoltine populations, and this was more marked at 25°C than at 20°C. Although the critical photoperiod showed a positive correlation with the original latitude, the correlation was relatively low at both temperatures. Conversely, temperature dependence of the critical photoperiod for diapause induction correlated negatively with the original latitude. The trivoltine populations showed greater temperature sensitivity than the bivoltine populations. These results suggest that an increase in temperature sensitivity of the diapause response to photoperiods was involved in the shift to a trivoltine life cycle. The crossing experiments suggested that the photoperiodic control of diapause induction and its temperature dependence are under polygenic control without sex-linkage. Received: 29 October 1996 / Accepted: 26 February 1997     

Variation in sexual size dimorphism among populations: testing the differential‐plasticity hypothesis

Sexual size dimorphism (SSD) is a common phenomenon in animals and varies widely among species and among populations within species. Much of this variation is likely due to variance in selection on females vs. males. However, environmental variables could have different effects on females vs. males, causing variation in dimorphism. In this study, we test the differential‐plasticity hypothesis, stating that sex‐differential plasticity to environmental variables generates among‐population variation in the degree of sexual dimorphism. We examined the effect of temperature (22, 25, 28, and 31 °C) on sexual dimorphism in four populations of the cockroach Eupolyphaga sinensis Walker (Blattaria: Polyphagidae), collected at various latitudes. We found that females were larger than males at all temperatures and the degree of this dimorphism was largest at the highest temperature (31 °C) and smallest at the lowest temperature (22 °C). There is variation in the degree of SSD among populations (sex*population interaction), but differences between the sexes in their plastic responses (sex*temperature interaction) were not observed for body size. Our results indicated that sex‐differential plasticity to temperature was not the cause of differences among populations in the degree of sexual dimorphism in body size.     

Latitudinal differences in temperature effects on the embryonic development and hatchling phenotypes of the Asian yellow pond turtle,Mauremys mutica

The effect of incubation temperature on embryonic development and offspring traits has been widely reported for many species. However, knowledge remains limited about how such effects vary across populations. Here, we investigated whether incubation temperature (26, 28, and 30 °C) differentially affects the embryonic development of Asian yellow pond turtle (Mauremys mutica) eggs originating from low‐latitude (Guangzhou, 23°06′N) and high‐latitude (Haining, 30°19′N) populations in China. At 26 °C, the duration of incubation was shorter in the high‐latitude population than in the low‐latitude population. However, this pattern was reversed at 30 °C. As the incubation temperature increased, hatching success increased in the low‐latitude population but slightly decreased in the high‐latitude population. Hatchlings incubated at 30 °C were larger and righted themselves more rapidly than those incubated at 26 °C in the low‐latitude population. In contrast, hatchling traits were not influenced by incubation temperature in the high‐latitude population. Overall, 30 °C was a suitable developmental temperature for embryos from the low‐latitude population, whereas 26 and 28 °C were suitable for those from the high‐latitude population. This interpopulation difference in suitable developmental temperatures is consistent with the difference in the thermal environment of the two localities. Therefore, similarly to posthatching individuals, reptile embryos from different populations might have evolved diverse physiological strategies to benefit from the thermal environment in which they develop. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114 , 35–43.     

PHOTOPERIOD IN XANTHIUM POPULATIONS FROM TEXAS AND MEXICO

Photoperiodic responses of Xanthium strumarium L. originating between 19° N in Mexico and 34° N in Texas varied among seedlings grown from seed under controlled conditions. The critical night lengths form a gradient from 9.5 hr in northern Texas to 10.75 hr in southern Texas and northeastern Mexico. Populations with critical night lengths of 9.5 and 9.75 hr showed a longer interval to flower bud formation under cooler temperatures (24 C day/15 C night) than under warmer temperatures (30 C day/24 C night). Three of four populations with a 10.75-hr critical night length showed a shorter interval under the cooler temperature regime. Although the Texas populations demonstrate a strong correlation of photoperiodic response with latitude, the Mexico populations show diverse photoperiodic timing from approximately the same latitude. The study emphasizes that a combination of critical night length and ripeness-to-flower (maturity) response forms the basis for reproductive adaptation in different climatic regimes in Texas and Mexico.     

Coevolution of foraging behavior with intrinsic growth rate: risk-taking in naturally and artificially selected growth genotypes of <Emphasis Type="Italic">Menidia menidia</Emphasis>

Although there is accumulating evidence of growth-rate optimization by natural selection, the coevolution of growth rate and risk-taking behavior has not been sufficiently documented. The Atlantic silverside fish, Menidia menidia, displays countergradient variation in growth across a latitudinal gradient: genotypes from Nova Scotia (NS), for example, grow in length twofold faster than those from South Carolina (SC). Past work has established that fast growth is adaptive in northern climates, but the trade-off is poorer swimming performance and higher susceptibility to predators. We compared escape behavior and willingness to forage under threat of predation among growth genotypes reared and tested under common-garden conditions. When chased with a predator model, NS fish occupied shelter more quickly than SC fish. When food was supplied after a chase, NS fish reemerged from the shelter much more quickly than SC fish and immediately commenced feeding, whereas many SC fish displayed timid behavior and did not feed. When food was absent following a chase, however, NS fish remained in the shelter longer than did SC fish and both displayed timid behavior. Hence, the fast-growing NS genotype was bolder than SC fish in the presence of food, but shyer in the absence of food. These behaviors are adaptive given the physiological constraints intrinsic to each genotype. Experiments on captive populations of silversides that had been artificially selected for fast or slow growth confirmed that foraging behavior is genetically correlated with intrinsic growth rate, although in these trials the fast-growth genotype was always more bold, regardless of food availability, as would be expected in the absence of predators. We conclude that risk-taking foraging behavior coevolves adaptively with intrinsic growth rate in M. menidia.     

Nest site choice compensates for climate effects on sex ratios in a lizard with environmental sex determination

Theoretical models suggest that in changing environments natural selection on two traits, maternal nesting behaviour and pivotal temperatures (those that divide the sexes) is important for maintaining viable offspring sex ratios in species with environmental sex determination (ESD). Empirical evidence, however, is lacking. In this paper, we provide such evidence from a study of clinal variation in four sex-determining traits (maternal nesting behaviour, pivotal temperatures, nesting phenology, and nest depth) in Physignathus lesueurii, a wide-ranging ESD lizard inhabiting eastern Australia. Despite marked differences in air and soil temperatures across our five study sites spanning 19° latitude and 1200 m in elevation, nest temperatures did not differ significantly among sites. Lizards compensated for climatic differences chiefly by selecting more open nest sites with higher incident radiation at cooler sites. Clinal variation in the onset of nesting also compensated for climatic differences, but to a lesser extent. There was no evidence of compensation through pivotal temperatures or nest depth. More broadly, our results extend to the egg stage the life history prediction that behaviour is the chief compensatory mechanism for climatic differences experienced by species spanning environmental extremes. Furthermore, our study was unique in revealing that nest site choice influenced mainly the daily range in nest temperatures, rather than mean temperatures, in a shallow-nesting reptile. Finally, indirect evidence suggests that the cue used by nesting lizards was radiation or temperature (through basking or assessing substrate temperatures), not visual detection of canopy openness. We conclude that maternal nesting behaviour and nesting phenology are traits subject to sex ratio selection in P. lesueurii, and thus, must be considered among the repertoire of ESD species for responding to climate change.     

EVOLUTION OF INTRINSIC GROWTH AND ENERGY ACQUISITION RATES. I. TRADE-OFFS WITH SWIMMING PERFORMANCE IN MENIDIA MENIDIA

Abstract Latitudinal populations of the Atlantic silverside, Menidia menidia , show substantial genetic variation in rates of energy acquistion and allocation. Reared in common environments, silversides from northern latitudes consume more food, grow faster and more efficiently, store more energy, and produce greater quantities of eggs than their southern conspecifics. The persistence of seemingly inferior southern genotypes in the face of ostensibly superior northern genotypes suggest that there are hidden evolutionary trade-offs associated with these elevated acquisition and allocation rates. We tested the hypothesis that rapid growth and high levels of food consumption trade-off against locomotory performance in M. menidia . We compared both aerobic (prolonged and endurance) and anaerobic (burst) swimming capacities between intrinsically fast-growing fish from the north (Nova Scotia, NS) and intrinsically slow-growing fish from the south (South Carolina, SC) and between growth-manipulated phenotypes within each population. We also compared swimming speeds and endurance between fasted and recently fed fish within populations. Maximum prolonged and burst swimming speeds of NS fish were significantly lower than those of SC fish, and swimming speeds of fast-growing phenotypes were lower than those of slow-growing phenotypes within populations. Fed fish had lower burst speeds and less endurance than fasted fish from the same population. Thus, high rates of growth and the consumption of large meals clearly diminish swimming performance, which likely increases vulnerability to predation and decreases survival and relative fitness. The submaximal growth rate of southern M. menidia appears to be adaptive, resulting from balancing selection on rates of somatic growth.     

Latitudinal variation in suppression of flower bud formation at high temperature in four native dandelions (Taraxacum spp.) with different distributions and genetic structures in Japan

The control of the response of flowering to temperature plays a key role in successful range‐expansion of plants. A previous study showed that the suppression of flower‐bud formation at high temperature in Taraxacum officinale decreases genetically with latitude from north to south in Japan. The present study investigated whether similar trait variation occurs among populations of native Taraxacum species in Japan. Seedlings of T. albidum (a low‐ and mid‐latitude allopolyploid), T. japonicum (a mid‐latitude diploid) and T. venustum (a high‐latitude autopolyploid) were grown at three temperatures. Time to flower‐bud appearance increased with temperature in T. japonicum and T. venustum, but did not increase in T. albidum. Time to flower‐bud appearance did not differ significantly among the three species at 14°C, but it was shorter in T. albidum than in the other two species at 19°C and 24°C. The early appearance of buds of T. albidum was confirmed by another experiment in which plants of 18 populations from the three species and T. platycarpum (a mid‐latitude diploid) grown at 19°C were used. The results clearly indicate that high‐temperature suppression of flower‐bud formation was lower in low‐latitude species than in high‐latitude species. This interspecific variation is analogous to the intraspecific variation in T. officinale. Time to bud appearance of five populations in T. albidum was homogeneous within and between the populations. The results suggest that the five populations are monoclonal and lack the sensitivity of suppression of flower‐bud formation to high temperature.     

Climate change,sex reversal and lability of sex‐determining systems

Sex reversal at high temperatures during embryonic development (e.g., ZZ females) provides the opportunity for new genotypic crosses (e.g., ZZ male × ZZ female). This raises the alarming possibility that climatic warming could lead to the loss of an entire chromosome—one member of the sex chromosome pair (the Y or W)—and the transition of populations to environmental sex determination (ESD). Here we examine the evolutionary dynamics of sex‐determining systems exposed to climatic warming using theoretical models. We found that the loss of sex chromosomes is not an inevitable consequence of sex reversal. A large frequency of ZZ sex reversal (50% reversal from male to female) typically divides the outcome between loss of the ZW genotype and the stable persistence of ZZ males, ZW females and ZZ females. The amount of warming associated with sex chromosome loss depended on several features of wild populations—environmental fluctuation, immigration, heritable variation in temperature sensitivity and differential fecundity of sex‐reversed individuals. Chromosome loss was partially or completely buffered when sex‐reversed individuals suffered a reproductive fitness cost, when immigration occurred or when heritable variation for temperature sensitivity existed. Thus, under certain circumstances, sex chromosomes may persist cryptically in systems where the environment is the predominant influence on sex.     

Sex determination in flatfishes: Mechanisms and environmental influences

Flounder of the genus Paralichthys exhibit a unique mode of sex determination where both low and high temperatures induce male-skewed sex ratios, while intermediate temperatures produce a 1:1 sex ratio. Male differentiation is thus easily induced in genetic females creating a combination of genetic (GSD) and environmental sex determination (ESD). Since male flounder become reproductively fit at substantially smaller body sizes than females, temperature or other environmental variables that elicit lower growth rates may also influence sex differentiation toward male development. This review covers our current knowledge of sex determination and differentiation in flatfishes including possible adaptive significance of ESD and involvement of factors such as aromatase (cyp19).     

Evidence of thermolabile sex determination in pejerrey*

Temperature regimes of 17 ± 1°C and 21 ±1°C early in development of pejerrey Odontesthes bonariensis produced nearly all females, whereas at 25 ± 1°C variable, sometimes male-biased sex-ratios were obtained. The critical period of thermolabile sex determination seemed to occur between 25 and 50 days post-hatch (about 11 and 21 mm s.i.) at low temperatures (17–20°C) and between 0 and 25 days (about 7 and 15 mm) at high temperatures (22–25°C). The likelihood of expression of temperature-dependent sex determination in natural populations and the possible adaptive significance of environmental sex determination in pejerrey are discussed.     

Pattern and scale of geographic variation in environmental sex determination in the Atlantic silverside,Menidia menidia

The Atlantic silverside, Menidia menidia (Pisces: Atherinidae), exhibits an exceptionally high level of clinal variation in sex determination across its geographic range. Previous work suggested linear changes in the level of temperature‐dependent sex determination (TSD) with increasing latitude. Based on comparisons at 31 sites encompassing the entire species’ range, we find that the change in level of TSD with latitude is instead highly nonlinear. The level of TSD is uniformly high in the south (Florida to New Jersey), then declines rapidly into the northern Gulf of Maine where genotypic sex determination (GSD) predominates and then rebounds to moderate levels of TSD in the northern‐most populations of the Gulf of St. Lawrence. Major latitudinal breakpoints occur in central New Jersey (40^oN) and the northern Gulf of Maine (44^oN). No populations display pure TSD or GSD. Length of the growing season is the likely agent of selection driving variation in TSD with a threshold at 210 days. Because gene flow among populations is high, such distinct patterns of geographic variation in TSD/GSD are likely maintained by contemporary selection thereby demonstrating the adaptive fine tuning of sex determining mechanisms.