Adaptive significance of environmental sex determination in an amphipod

Environmental sex determination (ESD) permits adaptive sex choice under patchy environmental conditions, where the environment affects sex-specific fitness and where offspring can predict their likely adult status by monitoring an appropriate environmental cue. For Gammarus duebeni, an amphipod with ESD, it has been proposed that this flexible sex determination system is adaptive because males gain more from large size. Under ESD, young which are born earlier in the season become mostly males and, experiencing longer to grow, are therefore larger at breeding than females which are born later in the season. In order to test the hypothesis that ESD is adaptive for this species we investigated the relationship between size and fitness for both males and females, in a population of G. duebeni known to have ESD. We measured size related pairing success and fecundity, and used these two measures to calculate the relative fitness gains achieved through an increase in size for either sex. The fitness of both males and females increased with size, but males gained more from an increase in size than did females, throughout the breeding season. The data support the adaptive explanation for the evolution and maintenance of ESD in this species.     

Intersexuality in the amphipod Gammarus duebeni results from incomplete feminisation by the vertically transmitted parasitic sex ratio distorter Nosema granulosis

We investigated the effects of parasitism and temperature on the production of intersexes in the amphipod Gammarus duebeni from a field population at Cumbrae, Scotland. There was significant temporal variation in intersex frequency which ranged from 0.5 to 5.2% in monthly field collections. Prevalence of Nosema granulosis, a feminising microsporidian parasite, also varied temporally and there was a significant correlation between parasite prevalence and intersex frequency in the field. Intersexes (16.3 ± 0.4 mg) were larger than true females (14.8 ± 0.1 mg) but produced fewer eggs thus demonstrating a cost of intersexuality. Intersexes were less likely to be paired than true females. In a breeding experiment, only females infected by N. granulosis produced intersex offspring. Temperature had no effect on intersex frequency. Intersexes therefore appear to be the result of incomplete feminisation by N. granulosis in this population. These results contrast with previous studies of G. duebeni from different populations which found that intersexes were the result of abnormal development under environmental sex determination. We suggest that intersexuality may be induced by both environmental and parasitic factors in populations of G. duebeni. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genetics of diapause in the comma butterfly Polygonia c‐album

The processes of local adaptation and ecological speciation can be better understood by studying the genetic background of life‐history decisions. The sex chromosomes host genes for many population differences in the Lepidoptera and therefore the inheritance of diapause determination in the butterfly Polygonia c‐album may be hypothesized to be sex‐linked. In the present study, Polygonia c‐album (L.) from Spain and Sweden and hybrid offspring are raised under an LD 17 : 7 h photocycle that induces most pure Swedish individuals to develop into the diapausing dark morph and most pure Spanish individuals into the light and directly‐developing morph. If inheritance of the daylength threshold for diapause is X‐linked, as is known to be the case for host‐plant preferences, females should follow the developmental path of their male parents' populations. However, female hybrids instead have a diapause propensity intermediate to that of their parental stocks and, consequentially, diapause determination is not X‐linked. However, male hybrids eclose as the diapausing morph to a higher extent than females and, moreover, this pattern is more pronounced in the Spanish female × Swedish male cross than in the reciprocal cross. Hence, it is concluded that the genetic determination of the critical daylength for diapause is mainly autosomal but with some influence of sex‐linked genes and/or parental effects, possibly as an effect of the importance of protandry for males. Such sex effects could provide a starting point for the evolution of population differences inherited on the sex chromosomes.     

Interaction between genetic,external and parasitic factors in sex determination of the crustacean amphipodGammarus duebeni

Sex determination inGammarus duebeni duebeni was analyzed by breeding experiments conducted with individuals obtained from various populations. Sex ratio of progeny depends on genetic factors and photoperiod. Generally, a preponderance of males was noted when offspring were raised under long-day photoperiods, whereas females prevailed under short-day photoperiods (Bulnheim, 1967, 1969). Based on these previous studies the critical daylength, the transition point at which the photoperiodic response switches, was estimated in specimens from a population of the Elbe estuary as ranging between 13 and 14 hours of light per day at 15° C. The susceptibility to photoperiod is assumed to depend on the balance between a system of male and female determining genetic factors that act on a polygenic basis. From various coastal brackish-water populations thelygenic (all-female) strains could be selected. This maternally inherited sex-ratio condition is caused by the transovarially transferred microsporidiansOctosporea effeminans andThelobania herediteria which, independently of each other, exert a feminizing influence on the host's offspring. Either perfectly or imperfectly thelygenic females may occur. The latter produce eggs that are not all infected; hence their progeny are bisexual. As a consequence of the sex-determining influence of the microsporidians males are generally not parasitized. However, some males associated withO. effeminans were found in a strain derived from a population at Bornholm (Baltic Sea). Also, males infested with one or both of the two parasite species occur in some populations ofG. d. celticus, indicating that in these sex determination is not governed by the microsporidians concerned. The feminizing influence of the parasites may be affected by environmental factors. An increase of the ambient salinity level to 25–30 results in a disappearance ofO. effeminans in the eggs released by infectedG. d. duebeni females. Thus, the normally acting switch mechanism of sex determination is re-established. In addition, long exposures to low temperatures (4° C) may have an adverse effect on the vegetative stages of both microsporidians. Owing to this, infected females may produce eggs which are not all parasitized. Consequently, mixed progeny may arise. Studies on the incidence rates of the two microsporidians. comprising 18 populations indicate that the parasites are widespread in their geographical range. Most populations studied in the Baltic Sea area and brackish-water habitats of the German North Sea coast are associated with either one or both of the two microsporidian parasites. Observations on the sex ratio of a population from the Elbe estuary, performed over several years, revealed considerable seasonal fluctuations. The percentages of infected females, however, remained at a fairly constant level during the study period. The results obtained are discussed with reference to the mechanisms of sex determination and differentiation as revealed in other amphipod species, in particular to the interaction of genetic and non-genetic factors and the occurrence of monogeny phenomena.     

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.     

Parental control of sex ratio in Gammarus duebeni,an organism with environmental sex determination

Parental sex ratio control was investigated in Gammarus duebeni, an amphipod with an environmentally mediated sex determining system. The effect on the F2 generation sex ratio of the photoperiodic conditions experienced by a) the P generation during and after copulation, b) the F1 generation before and after sex determination, and c) the F2 generation themselves during the period of sex determination, was tested. The photoperiodic conditions the F2 generation experienced during the period of sex determination had a significant effect on their sex ratio (more males were produced under long-day than under short-day conditions), but the photoperiodic conditions experienced by the F1 generation males and females or the P generation on the F1 male's side had no effect on the F2 sex ratio. However, the photoperiodic conditions the P generation on the F1 female's side experienced significantly affected the F2 sex ratio. When these animals experienced long-day conditions the F2 generation became female biased and when they experienced short-day conditions, male biased. It is proposed that the mechanism of control operates through the F1 generation mothers whilst in an embryonic stage of development in the P generation mother's brood pouch. The photoperiodically mediated effects of the embryonic F1 generation mother and the F2 generation on sex determination are additive. A mechanism by which both F1 generation maternal and F2 generation sex ratio control could operate in the field is proposed.     

Sex differences in incubation behaviour but not mortality risk in a threatened shorebird

Sex‐related variation in survival is common in birds and, as it influences effective population size and population growth, is important for conservation and species management. Here we assessed incubation behaviour and sex‐related survival in a threatened sexually monomorphic shorebird, the St Helena Plover Charadrius sanctaehelenae. Males incubated at night, the period of highest activity of cats, which are likely to be predators of breeding birds. In spite of behavioural differences between the sexes, adult survival was not significantly different between males and females, suggesting that sex‐biased behaviour need not lead to sex‐related survival, and thus behavioural differences may not impact upon the adult sex ratio.     

Zur Ökophysiologie,Sexualität und Populationsgenetik litoraler Gammaridea — ein Überblick

Comparative investigations on the physiological capacities in the euryhaline amphipodsGammarus locusta, G. oceanicus, G. salinus, G. zaddachi andG. duebeni were reviewed. In order to assess the adaptations of these species to the abiotic conditions of their environment, the following criteria were examined: oxygen consumption in relation to ambient salinity and temperature levels, respiratory responses following osmotic stress, resistance capacities to oxygen deficiency, resistance to aerial exposure and the simultaneous presence of hydrogen sulphide. Covering the range from marine to typically brackish-water inhabitants, the 5 species show adaptive responses in the above-mentioned order. Respiration is less intensely modified by external factors, and oxygen consumption decreases. Accompanied by faster rates of acclimation to new steady states of performance, resistance capacities increase. The significance of the findings obtained is discussed in relation to the environmental requirements of the amphipods considered. Based on breeding experiments, the sex-determining systems reported thus far inGammarus species are outlined. As demonstrated inG. duebeni, a more or less pronounced influence of external factors such as photoperiod may become effective. A preponderance of males was noted when offspring were raised under long-day photoperiods, whereas females prevailed under short-day conditions. In terms of the critical daylength, the light per day was estimated as being between 13 and 14 h (Elbe estuary population). Feminizing microporidians (Octosporea effeminans, Thelohania herediteria), which are transovarially transmitted, can interfere with the system of sex determination and sex differentiation of the host. As reflected in variousG. duebeni populations, they cause a maternally transferred sex-ratio condition by the production of all-female broods, thereby mimicking extrachromosomal inheritance. An increase of the salinity level to 25–30‰ results in a disappearance ofO. effeminans. In both microsporidians, long exposure to low temperatures (≤4°C) produces eggs which are not all parasitized. Furthermore, intersexuality can be induced by changing environmental factors. Microsporidian species have no influence on sex differentiation inG. duebeni celticus, G. salinus, G. locusta andG. pulex. Patterns of relative electrophoretic mobilities of proteins and the distribution of allele frequencies at polymorphic gene loci can be utilized for species diagnosis and for the evaluation of the relationships between different taxa, particularly at and below the species level. As exemplified by studies on several gammarids from marine, brackish and freshwater environments, inter- and infraspecific gene-enzyme variation is described. Electrophoretic investigations on natural populations of the euryhaline amphipodsG. zaddachi, G. salinus, G. tigrinus and others from different geographic areas provided evidence of considerable biochemical genetic variation. InTalitrus saltator- andTalorchestia deshayesii-populations the extent of variability based on micro-and macrogeographic aspects is illustrated. The large-scale genetic divergence is demonstrated by comparison of samples obtained from the Baltic, North, Atlantic and northern Mediterranean Seas.      

Sex-specific effects of hybridization on reproductive fitness in Mytilus

Blue mussels of the genus Mytilus form extensive hybrid zones in the North Atlantic and elsewhere where the distributions of different species overlap. Mytilus species transmit both maternal and paternal mtDNA through egg and sperm, respectively, a process known as doubly uniparental inheritance (DUI), and some females produce offspring with extremely biased sex ratios. These two traits have been shown to be linked and maternally controlled, with sex determination involving nuclear–cytoplasmic interactions. Hybridization has been shown to disrupt DUI mitochondrial inheritance and sex ratio bias; however, the effect of hybridization on reproductive fitness has not previously been examined. We investigated this effect in M. edulis × M. trossulus crosses through histological examination of mature F1 progeny, and spawning of F1 hybrids to monitor survival of their progeny through to the D stage of larval development. For progeny produced from mothers with a strong bias toward female offspring (often 100%) in pure-bred crosses, there was a clear breakdown in female dominance of progeny and significantly more hermaphrodites in the hybrid crosses produced from sperm with the M-tr1 mitotype. We also found significant sex-specific differences among hybrid progeny, with females producing normal eggs while males and hermaphrodites evidenced impaired gonadal development with significantly greater numbers of Sertoli cells, phagocytic hemocytes, and degenerating germ cells, all associated with gonad resorption. Males from crosses where DUI was disrupted and where male progeny were homoplasmic for the female mtDNA were the most severely compromised. Allelic incongruity between maternal and paternal mitotypes in hybrid crosses was associated with significant disruption of male gonadal development.     

Skewed adult sex ratios in Columbina ground doves from Venezuela

Although sex ratios at conception are close to 1:1 in most species of birds, skewed adult sex ratios (ASRs) are not uncommon in populations of birds, and occur frequently at local and temporal scales. ASRs are a key variable influencing population demography, breeding systems, and many aspects of the behavior of birds. However, factors contributing to variation in ASRs, particularly for tropical species of birds, remain poorly understood. By compiling information from field sites and records from bird collections, we found that the ASRs of four species of Columbina ground doves from Venezuela deviated significantly from parity. Males of all species outnumbered females at all field sites and in all museum samples. ASR, expressed as the proportion of males, ranged from 0.59 for Common Ground Doves (Columbina passerina) to 0.65 for Plain‐breasted Ground Doves (Columbina minuta) in the overall samples. Males outnumbered females by ~ 44% in Common Ground Doves and by 85% in Plain‐breasted Ground Doves. Our samples included birds collected as specimens over many decades and across broad geographic areas, suggesting that skewed ASRs are characteristic of Venezuelan ground doves. Because these ground doves do not exhibit pronounced sexual size dimorphism and have socially monogamous breeding systems, selection would be expected to favor equal investment in males and females at hatching. As such, we argue that greater post‐hatching mortality of females, rather than deviations in the sex ratio of embryos, is the main cause of the biased ASRs of Venezuelan ground doves.     

Geographic variation in sex‐chromosome differentiation in the common frog (Rana temporaria)

In sharp contrast with birds and mammals, sex‐determination systems in ectothermic vertebrates are often highly dynamic and sometimes multifactorial. Both environmental and genetic effects have been documented in common frogs (Rana temporaria). One genetic linkage group, mapping to the largest pair of chromosomes and harbouring the candidate sex‐determining gene Dmrt1, associates with sex in several populations throughout Europe, but association varies both within and among populations. Here, we show that sex association at this linkage group differs among populations along a 1500‐km transect across Sweden. Genetic differentiation between sexes is strongest (F_ST = 0.152) in a northern‐boreal population, where male‐specific alleles and heterozygote excesses (F_IS = ?0.418 in males, +0.025 in females) testify to a male‐heterogametic system and lack of X‐Y recombination. In the southernmost population (nemoral climate), in contrast, sexes share the same alleles at the same frequencies (F_ST = 0.007 between sexes), suggesting unrestricted recombination. Other populations show intermediate levels of sex differentiation, with males falling in two categories: some cluster with females, while others display male‐specific Y haplotypes. This polymorphism may result from differences between populations in the patterns of X‐Y recombination, co‐option of an alternative sex‐chromosome pair, or a mixed sex‐determination system where maleness is controlled either by genes or by environment depending on populations or families. We propose approaches to test among these alternative models, to disentangle the effects of climate and phylogeography on the latitudinal trend, and to sort out how this polymorphism relates to the ‘sexual races’ described in common frogs in the 1930s.     

Mating system of Bracon hebetor (Hymenoptera: Braconidae)

Abstract.

• 1 We report on the mating system of a field population of the parasitic wasp, Bracon hebetor, on a corn pile infested by the Indian meal moth, Plodia interpunctella. We demonstrate that the mating system is based upon male scramble competition polygyny with male aggregations on high places on the corn.
• 2 The sex ratio among adults was greater than 80% males on the surface of the corn, whereas below the surface the sex ratio was less than 45%. Males actively courted females on the surface, but there were no aggressive interactions among males during courtship or mating.
• 3 Approximately 20% of the females found on the surface of the corn had no sperm in their spermathecae, regardless of age, but the numbers of unmated females decreased later during the day.
• 4 In laboratory studies we showed that females from this population oviposit a female biassed sex ratio, and that only 14% of females were mated before dispersing from their place of emergence.
• 5 Thus sib-mating is unlikely in this gregarious parasitoid. This outcrossing mating system probably arose because of severe inbreeding depression that B.hebetor suffers via a sex locus: diploids that are heterozygous at the sex locus develop into females, but homozygous diploids are male and are generally inviable. The female biassed sex ratio may have evolved in B. hebetor in response to males being the more expensive sex, females dispersing more frequently from the population than males, or a fraction of females remaining unmated in the population.

     

Sex differences in growth rates of early life stage Japanese eels Anguilla japonica under experimental conditions

To assess the relationship between growth rate of body mass and sex in the Japanese eel Anguilla japonica in the early life stage; the growth rates of males and females were compared under experimental conditions. The mean growth rate of females was significantly slower than that of males. To assess the relative priority of growth rate and sex, growth was delayed by restricted feeding, resulting in a significantly higher proportion of females in the delayed than in the normal growth group. These findings indicate that the mean growth rate of A. japonica is slower in females than in males in the early life stage around sex determination and differentiation under experimental rearing conditions. Moreover, growth rate probably has priority over sex determination, with slow growth rate increasing the probability of being female.     

Dmrt1 polymorphism covaries with sex‐determination patterns in Rana temporaria

Patterns of sex‐chromosome differentiation and gonadal development have been shown to vary among populations of Rana temporaria along a latitudinal transect in Sweden. Frogs from the northern‐boreal population of Ammarnäs displayed well‐differentiated X and Y haplotypes, early gonadal differentiation, and a perfect match between phenotypic and genotypic sex. In contrast, no differentiated Y haplotypes could be detected in the southern population of Tvedöra, where juveniles furthermore showed delayed gonadal differentiation. Here, we show that Dmrt1, a gene that plays a key role in sex determination and sexual development across all metazoans, displays significant sex differentiation in Tvedöra, with a Y‐specific haplotype distinct from Ammarnäs. The differential segment is not only much shorter in Tvedöra than in Ammarnäs, it is also less differentiated and associates with both delayed gonadal differentiation and imperfect match between phenotypic and genotypic sex. Whereas Tvedöra juveniles with a local Y haplotype tend to ultimately develop as males, those without it may nevertheless become functional XX males, but with strongly female‐biased progeny. Our findings suggest that the variance in patterns of sex determination documented in common frogs might result from a genetic polymorphism within a small genomic region that contains Dmrt1. They also substantiate the view that recurrent convergences of sex determination toward a limited set of chromosome pairs may result from the co‐option of small genomic regions that harbor key genes from the sex‐determination pathway.     

Population biology and breeding cycle of the burrowing shrimp Callichirus seilacheri (Decapoda,Callianassidae) from the eastern tropical Pacific

Year-round continuous reproduction in tropical regions is an established paradigm in marine ecology. In this study, we tested this paradigm using the ghost shrimp Callichirus seilacheri from the tropical eastern Pacific as a model species. We also examined size-frequency distribution, sex ratio, and recruitment cycle to contribute to the biological knowledge of this species. To this end, a total of 456 individuals of C. seilacheri were collected during 12 months of sampling. Population structure was symmetrical for both sexes, and the overall sex ratio did not differ from evenness. Males outnumbered females in smaller size classes, though, revealing a potential sex-dependent mortality in small individuals. The breeding pattern followed the well-marked seasonal regime of the region, with ovigerous females registered during the rainy season. While natural variation in the seawater temperature had no influence on reproduction of this species, changes in water salinity possibly triggered the appearance of egg-bearing females in the population. Recruitment occurred throughout the year but was more intense during the dry season, following the appearance of ovigerous females. The adaptability of the life cycle of C. seilacheri to the seasonal climate provides further evidence that reproduction in tropical species is not always continuous.     

Change in offspring sex ratio over a very short season in Lincoln's Sparrows: the potential role of bill development

ABSTRACT The sex ratios of offspring are targets of natural selection that can affect parental energy expenditure and fitness, adult sex ratios, and population dynamics. Parents may manipulate offspring sex ratios based on sex differences in their offsprings' potential for reproductive success. In Lincoln's Sparrows (Melospiza lincolnii), male bill shape is associated with the quality of songs, and song quality predicts female preferences in a reproductive context. Males and females that hatch later relative to brood mates or later in the breeding season tend to develop bill shapes that are, for males, associated with low‐quality song. Because females do not sing and do not experience this selection pressure, we predicted that the sex of offspring produced late relative to their brood mates or relative to the season should be biased toward females. Using a molecular technique to sex nestlings, we found no effects of hatching order or any interaction between date of clutch initiation (season) and hatching order on offspring sex. However, we found a seasonal decline in the proportion of male offspring, from approximately 0.8 at the beginning to 0.4 at the end of a clutch initiation season only 19 d in duration. To our knowledge, this is the shortest period over which the offspring sex ratio has been shown to change in a bird population. Moreover, these findings are consistent with the hypothesis that sex differences in the potential attractiveness of offspring ultimately influence offspring sex ratios.     

Sex reversal of pejerrey (Odontesthes bonariensis), a species with temperature-dependent sex determination,in a seminatural environment

This study examined the changes in sex ratios and sex reversal rates in pejerrey Odontesthes bonariensis that occur with the progression of the spawning season in a seminatural setting. Four groups of hatchery-produced pejerrey larvae were stocked in floating cages in La Salada de Monasterio lake (Pampas region), a natural habitat of this species, and reared from hatching beyond gonadal sex determination with minimum human interference. Cage 1 was stocked at the beginning of the spring spawning season and the other cages were stocked with monthly delays until cage 4 in early summer. The genotypic (amhy+, XY/YY; amhy−, XX) and phenotypic (testis, male; ovary, female) sex ratios and proportions of genotype/phenotype mismatched individuals were estimated and their relation to water temperature and daylength during the experiment was analysed by generalized linear modelling. Water temperature varied between 11 and 30.5°C, and daylength duration between 11 h 22 min and 14 h 35 min. Sex genotyping revealed nearly balanced sex ratios of XY/YY (46%–49.1%) and XX (50.9%–54%) fish in cages 2–4 whereas the genotypic sex ratio in cage 1 was clearly biased towards XY/YY fish (60.6%). Phenotypic males ranged from 42% to 54.4% in cages 1–3. Cage 4, in turn, had significantly more phenotypic males (66%). The percentage of XX males (phenotypic male/genotypic female) was 23.1% in cage 1, decreased to a minimum of 5.4% in cage 2 and gradually increased in cages 3 and 4 to a maximum of 40.7% in the latter. The percentages of XY/YY females (phenotypic female/genotypic male) were highest in cage 1 (30%) and decreased progressively in the other cages to a significantly lower value (4.3%) in cage 4. These results generally support the findings of laboratory studies on the effect of temperature on the sex determination of this species and also provide novel evidence of a XX genotype-specific masculinizing effect of short daylength.     

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.     

Identifying homomorphic sex chromosomes from wild‐caught adults with limited genomic resources

We demonstrate a genotyping‐by‐sequencing approach to identify homomorphic sex chromosomes and their homolog in a distantly related reference genome, based on noninvasive sampling of wild‐caught individuals, in the moor frog Rana arvalis. Double‐digest RADseq libraries were generated using buccal swabs from 30 males and 21 females from the same population. Search for sex‐limited markers from the unfiltered data set (411 446 RAD tags) was more successful than searches from a filtered data set (33 073 RAD tags) for markers showing sex differences in heterozygosity or in allele frequencies. Altogether, we obtained 292 putatively sex‐linked RAD loci, 98% of which point to male heterogamety. We could map 15 of them to the Xenopus tropicalis genome, all but one on chromosome pair 1, which seems regularly co‐opted for sex determination among amphibians. The most efficient mapping strategy was a three‐step hierarchical approach, where R. arvalis reads were first mapped to a low‐coverage genome of Rana temporaria (17 My divergence), then the R. temporaria scaffolds to the Nanorana parkeri genome (90 My divergence), and finally the N. parkeri scaffolds to the X. tropicalis genome (210 My). We validated our conclusions with PCR primers amplifying part of Dmrt1, a candidate sex determination gene mapping to chromosome 1: a sex‐diagnostic allele was present in all 30 males but in none of the 21 females. Our approach is likely to be productive in many situations where biological samples and/or genomic resources are limited.     

Mating frequency, genetic structure, and sex ratio in the intermorphic female producing ant species Myrmecina nipponica

1. Myrmecina nipponica has two types of colonies: a queen colony type, in which the reproductive females are queens and new colonies are made by independent founding, and an intermorphic female colony type, in which reproductive females belong to a wingless intermediate morphology between queen and worker, and where colonies multiply through colonial budding. 2. The mating frequencies of reproductive females in both types indicate monoandry. The relatedness among nestmates in both types was almost 0.75, however relatedness between mother and daughter in intermorphic female colonies was slightly higher than that of queen colonies. 3. The sex ratio (corrected investment female ratio) was 0.70 at the population level, suggesting that the sex ratio is controlled by workers in this species, however the ratio differed greatly between the two types of colonies. Queen colonies (n = 37) had a female‐biased sex ratio of 0.77 while intermorphic female colonies (n = 33) had a ratio of 0.56. 4. Each reproductive intermorphic female was accompanied by an average of 2.9 workers (including virgin intermorphic females) in the colonial budding, and when the investment to those workers was added to the female investment, the sex ratio reached 0.81. 5. The frequency distribution of sex ratio was bimodal, with many colonies producing exclusively males or females, however mean estimated relatedness within colonies was almost 0.75. These data are inconsistent with the genetic variation hypothesis, which is one of the predominant hypotheses accounting for the between‐colony variation in sex ratio.