Sex Ratios In Wild Populations Research Articles

The genome of New Zealand trevally (Carangidae: Pseudocaranx georgianus) uncovers a XY sex determination locus

BackgroundThe genetic control of sex determination in teleost species is poorly understood. This is partly because of the diversity of mechanisms that determine sex in this large group of vertebrates, including constitutive genes linked to sex chromosomes, polygenic constitutive mechanisms, environmental factors, hermaphroditism, and unisexuality. Here we use a de novo genome assembly of New Zealand silver trevally (Pseudocaranx georgianus) together with sex-specific whole genome sequencing data to detect sexually divergent genomic regions, identify candidate genes and develop molecular makers.ResultsThe de novo assembly of an unsexed trevally (Trevally_v1) resulted in a final assembly of 579.4 Mb in length, with a N50 of 25.2 Mb. Of the assembled scaffolds, 24 were of chromosome scale, ranging from 11 to 31 Mb in length. A total of 28,416 genes were annotated after 12.8 % of the assembly was masked with repetitive elements. Whole genome re-sequencing of 13 wild sexed trevally (seven males and six females) identified two sexually divergent regions located on two scaffolds, including a 6 kb region at the proximal end of chromosome 21. Blast analyses revealed similarity between one region and the aromatase genes cyp19 (a1a/b) (E-value < 1.00E-25, identity > 78.8 %). Males contained higher numbers of heterozygous variants in both regions, while females showed regions of very low read-depth, indicative of male-specificity of this genomic region. Molecular markers were developed and subsequently tested on 96 histologically-sexed fish (42 males and 54 females). Three markers amplified in absolute correspondence with sex (positive in males, negative in females).ConclusionsThe higher number of heterozygous variants in males combined with the absence of these regions in females support a XY sex-determination model, indicating that the trevally_v1 genome assembly was developed from a male specimen. This sex system contrasts with the ZW sex-determination model documented in closely related carangid species. Our results indicate a sex-determining function of a cyp19a1a-like gene, suggesting the molecular pathway of sex determination is somewhat conserved in this family. The genomic resources developed here will facilitate future comparative work, and enable improved insights into the varied sex determination pathways in teleosts. The sex marker developed in this study will be a valuable resource for aquaculture selective breeding programmes, and for determining sex ratios in wild populations.

Phenotypic/genotypic sex mismatches and temperature-dependent sex determination in a wild population of an Old World atherinid, the cobaltcap silverside Hypoatherina tsurugae.

Several New World atheriniforms have been recognized as temperature-dependent sex determined (TSD) and yet possess a genotypic sex determinant (amhy) which is primarily functional at mid-range temperatures. In contrast, little is known about the sex determination in Old World atheriniforms, even though such knowledge is crucial to understand the evolution of sex determination mechanisms in fishes and to model the effects of global warming and climate change on their populations. This study examined the effects of water temperature on sex determination of an Old World atheriniform, the cobaltcap silverside Hypoatherina tsurugae, in which we recently described an amhy homologue. We first assessed the occurrence of phenotypic/genotypic sex mismatches in wild specimens from Tokyo Bay for three years (2014-2016) and used otolith analysis to estimate their birth dates and approximate thermal history during the presumptive period of sex determination. Phenotypic sex ratios became progressively biased towards males (47.3%-78.2%) during the period and were associated with year-to-year increases in the frequency of XX-males (7.3%-52.0%) and decreases in XY/YY-females (14.5%-0%). The breeding season had similar length but was delayed by about 1month per year between 2014 and 2016, causing larvae to experience higher temperatures during the period of sex determination from year to year. Larval rearing experiments confirmed increased likelihood of feminization and masculinization at low and high temperatures, respectively. The results suggest that cobaltcap silverside has TSD, or more specifically the coexistence of genotypic and environmental sex determinants, and that it affects sex ratios in wild populations.

Relationships between external sexually dimorphic characteristics and internal gonadal morphology in a sex‐changing fish

AbstractThe mangrove rivulus fish (Kryptolebias marmoratus) is an emerging model for research in the biological sciences. The species is androdioecious, with populations consisting predominantly of self‐fertilizing hermaphrodites and a low abundance of males, but no females. Males arise either through environmental sex determination at the embryonic stage or environmentally driven sex change from hermaphrodite at the adult stage. Marked sexual dimorphisms in external morphological characters accompany gonadal transitions, including the loss of a caudal ocellus (‘eyespot’) and the development of orange color in males. How well external morphological features predict internal gonadal morphology, however, has not been systematically examined across the geographical range of this species. Our goal was to identify reliable external morphological predictors of animals possessing an ovotestis (hermaphrodite) or a testis (male) and to see if the strength of prediction is consistent throughout rivulus’ geographical range. We demonstrate that the state of the ocellus and, to a greater extent, orange coloration are key predictors of internal gonadal state. However, the degree to which these characteristics are predictive of internal gonadal state was region‐specific, with the ocellus and orange status of animals from Bahamas and Belize, respectively, having a lower probability of forecasting internal gonadal state than those from other regions. These results will help to streamline laboratory studies by allowing unambiguous assignment of sex, and will assist field studies by providing greater opportunity to determine sex ratios in wild populations.

Morphometrics, sex ratio, sexual size dimorphism, biomass, and population size of the Nile crocodile (Crocodylus niloticus) at its southern range limit in KwaZulu-Natal, South Africa

Animal body size and sex are requisite data for understanding population structure and demography. Little information exists regarding Nile crocodile, Crocodylus niloticus, morphometrics, sex ratios of wild populations, sexual size dimorphism and standing crop biomass. We captured 322 C. niloticus at Lake St Lucia and Ndumo Game Reserve, South Africa, the two largest crocodile populations at the southern extent of the species’ range in Africa, and measured a suite of physical characteristics to create predictive models of body length from other morphological attributes and body mass. Our sample included 118 hatchlings, 91 subadults and 113 adults. Strong positive allometric relationships were found between body length metrics (total length and snout-vent length) and other morphometrics. All morphometric regressions were linear, with the exception of the relationship of body length to body mass, which was logarithmic. Among relationships of cranial morphology and body length, we found considerable individual variation among all size classes. The mean head width-to-head length ratio was 1.9 ± 1.6, and mean head length-to-total length ratio was 0.14 ± 0.005. The sex ratios for non-hatchling individuals at both populations were essentially 1:1, but adult sex ratios were male biased. We calculated a total standing crop biomass of 96,867.18 kg (161.45 kg/km) and 52,640.40 kg (1504.01 kg/km) for C. niloticus at Lake St Lucia and Ndumo Game Reserve, respectively, and an estimated 3650 non-hatchling individuals for the province of KwaZulu-Natal. The data presented here will help inform crocodile management and population surveys in South Africa, where C. niloticus is an important apex predator that partitions aquatic resources and occasionally comes into conflict with human beings.

Female‐biased offspring sex ratios in birds at a mercury‐contaminated river

Mercury is a ubiquitous environmental pollutant that can negatively impact physiology and behavior of vertebrates, causing sub‐lethal changes in condition and reducing fitness. Here we examine its effect on offspring sex ratio. Previous studies demonstrate the ability of environmental contaminants to skew sex ratios in wild populations toward the production of females, and research in humans has demonstrated a decrease in male births following mercury exposure. We therefore hypothesized that female birds inhabiting the floodplain of a mercury‐contaminated river would produce broods more biased towards the production of females relative to birds from uncontaminated areas. We examined complete broods of three species: the aquatic‐feeding belted kingfisher Megaceryle alcyon, the terrestrial‐feeding eastern bluebird Sialia sialis, and the tree swallow Tachycineta bicolor, which feeds from both aquatic and terrestrial sources. Nestling sex ratios were shifted toward the production of females in all three species on mercury‐contaminated sites when compared to uncontaminated reference sites. These results may be explained by endocrine disruption or the Trivers–Willard theory of sex allocation. Our study is the first to examine the impact of mercury on offspring sex ratios in birds, and therefore contributes to our understanding of the potential for this persistent biomagnifying contaminant to alter fitness and effective population size in wildlife.

Some Like it Hot: Temperature Selection during the Critical Period of Thermolabile Sex Determination in Pejerrey ( Odontesthes bonariensis )

Introduction: Many fish species have thermolabile sex determination (TSD) and the sex ratios of wild populations may be influenced by the thermal conditions during the reproductive season. The pejerrey (Odontesthes bonariensis) is an Atherinopsid fish that shows a remarkable level of TSD at environmentally relevant temperatures [1]. Thus, gonadal sex is determined between 1 and 5 weeks after hatching and the frequency of males varies from 0% at temperatures below 19°C to 100% at 29°C [2]. As a first step to model the effects of temperature on the sex ratios of wild populations and to clarify the adaptive significance of TSD, we examined the thermal preference of pejerrey larvae during the time of gonadal sex determination and how sex ratios vary under a situation where larvae can freely select the temperature. Methods: Newly hatched pejerrey larvae were stocked in a horizontal thermal gradient consisting of five interconnected aquaria kept at constant temperatures of 17, 20, 23, 26, and 29°C. Larvae were able to move freely through the entire gradient. Similar amounts of live food (Artemia nauplii) were delivered to each aquarium three times a day regardless of the presence of larvae. Larval behavior was monitored 24-hour/day for 5 weeks using overhead cameras and the number of larvae and time spent at each temperature was calculated. Sex ratios were analyzed at the end of the experiments by light histology. Results and Conclusion: The behavioral analysis revealed a preference for high, masculinizing temperatures (26-29°C) and the resulting sex ratios were highly male-biased (80-100%). We also observed a diurnal rhythm of temperature selection, with fish preferring higher temperatures at night compared to daytime. These results provide important clues to understand the ecology of TSD for this species and predict the sex ratios of wild populations as a function of temperature. They could be also useful for modeling the effects of global warming on pejerrey resources.

Species and sex identification of Formosa landlocked salmon using loop‐mediated isothermal amplification

Species and sex identification are among the most important parameters for conservation management. However, it is extremely difficult to perform such identification in Formosa landlocked salmon (Oncorhynchus masou formosanus). Both sexual dimorphism in landlocked dwarf form Formosa landlocked salmon and morphological difference among cherry salmon complex are minimal. We developed a simple, rapid and noninvasive method for identifying sex and species of this critically endangered species using a loop-mediated isothermal amplification (LAMP) technique. The LAMP assay showed the advantage of simple detection (evaluated by visual inspection), rapid reaction time (< 1 h), isothermal condition (less equipment required) and high efficiency (only 0.5-5 pg of DNA was required in the reaction mixture). Therefore, the method is more economical and practical than PCR. The LAMP assay can be easily performed in the field and is a valuable tool for detecting sex ratios in wild populations and identifying species in commercial imports. This is the first application of LAMP in identifying species and sex of salmonids as far as we know and clearly shows the potential application of LAMP in molecular ecology and conservation efforts.

A PCR-based method for sex identification of critically endangered Formosa landlocked salmon

The extinction risk in a small population depends largely on stochastic factors that affect effective population size. Unbalanced sex ratio, a form of demographic stochasticity, not only reduces the effective population size but also raises extinction risk. Sex identification is fundamental for conservation biology; however, it is extremely difficult to do in Formosa landlocked salmon Oncorhynchus masou formosanus. Because sexual dimorphism in landlocked dwarf form Formosa landlocked salmon is minimal, we developed a simple and noninvasive PCR-based method to identify sex in this critically endangered species. The OtY2m primers (derived from OtY2) amplified the male-specific marker (OtY2) in Formosa landlocked salmon. In addition, we provide a valuable tool for detection of sex ratios in wild populations for conservation management. This study contributes useful information to estimate the effective population size and analyze population viability of Formosa landlocked salmon.

Sex determination. Viviparous lizard selects sex of embryos.

No one suspected that temperature-dependent sex determination (TSD), whereby the sex of embryos depends on the temperature at which they develop, might occur in viviparous (live-bearing) reptiles, because thermoregulation in the mother results in relatively stable, raised gestation temperatures. But here we show that developing embryos of the actively thermoregulating viviparous skink Eulamprus tympanum are subject to TSD, offering the mother the chance to select the sex of her offspring and a mechanism to help to balance sex ratios in wild populations.

Sex Biases in Trapped Samples of Mustelidae

Sex ratios significantly different from 1:1 usually are observed in trap captures of mustelids. Although these ratios could be caused by skewed sex ratios in wild populations, trapped samples consistently are skewed toward males. This apparent sampling bias generally has been attributed to sexual dimorphism of home-range sizes in mustelids, postulated to result in greater exposure of males to traps. The magnitude of differences in rates of capture between sexes appears to be body-size dependent because small mustelids exhibit more strongly skewed sex ratios in trap captures than do large ones. We investigated possible causes of sexual differences in rates of capture of mustelids. Mathematical equations were derived to describe the effect of trap spacing in relation to home-range diameter as a cause of these sex biases. Large trap-spacing values in relation to home-range diameter result in these biases, but this “exclusion effect” (the effect of some smaller home ranges having no traps at all) is only one consequence of sexual dimorphism of home-range size. At low trap-spacing intervals, variations in home-range size result in differential numbers of traps in male and female home ranges (the “trap number effect”), although the density of traps is the same. Computer simulations were used to examine the interaction of known and postulated body-size-dependent factors, including home-range size, rate of travel, sensory acuity, and a body-size-independent factor, dimensionality of trap arrangement, as a possible cause of sexual differences in rates of encounter with traps. Rates of encounter with traps that favor capture of males and that produce greater differences in rates of capture between sexes in small species than in large ones were observed if home ranges were sexually dimorphic, traps were arranged in grids, rate of travel was proportional to the 0.25 power of body mass, and males perceived traps at the same distances as did females. Sex-specific behaviors, either resulting from sexual differences in territory packing, or in the way that animals respond to traps that they perceive, are hypothesized to be contributing causes of differences in rates of capture between sexes.