Combining the Trojan Y chromosome and daughterless carp eradication strategies

The Trojan Y chromosome (TYC) strategy and the daughterless carp (DC) strategy represent two autocidal genetic biocontrol methods for eliminating invasive fish by changing the sex ratio of the population. Each strategy is designed to reduce the number of females in a target population, ultimately leading to local extinction of the population. In the DC approach, the proportion of males in the population is increased as a result of introducing an autocidal fish containing a transgenic aromatase gene insertion into multiple autosome sites. In the TYC approach, matings of an autocidal fish containing two Y sex chromosomes results in an increased proportion of males in the population. A mathematical model based upon coupled ordinary differential equations was constructed to observe the effect of an autocidal fish with the combined genetic features of both strategies (TYCDC) on a target population. The model incorporated a fitness parameter associated with fish bearing aromatase inhibitor genes and for fish bearing two Y chromosomes. Under conditions where the fitness penalty of the autocidal fish was negligible, modeling results showed that a combined strategy produced a modest reduction in the time required for female eradication, and that fewer autocidal fish were required to achieve extinction. However, increasing the fitness penalty associated with the autocidal fish neutralized the benefits of the TYCDC strategy, and suggested that the effort and expense of a combined strategy may not be warranted if the fitness cost of the TYCDC autocidal fish is significant.     

The practicality of Trojan sex chromosomes as a biological control: an agent based model of two highly invasive Gambusia species

Invasive fish species are a primary threat to aquatic ecosystems. Owing to the high fecundity of some fish, conventional control methods (e.g. specific removal) can be ineffective and the use of poisons is not desirable due to their non-specificity. Trojan sex chromosomes (TSC) are a theoretical method of invasive species control, where sex-reversed fish that are only able to produce male offspring are released into the target population. These Trojan individuals subsequently breed, causing a male skewed population sex ratio and eventually population collapse. Previous publications have explored TSC as an invasive species control, but assume that wild-type and Trojan fish have equal fitness, an assumption that may not be valid. What is more, models from closely related fields suggest that differential fitness between Trojans and wild-type fish maybe influential in the efficacy of TSC as a bio-control. Here we use agent based modeling to test how effectively TSC can be used to control two common invasive species of mosquitofish (Gambusia affinis and G. holbrooki) when Trojans have compromised fitness. We manipulated the fecundity, probability of mating and offspring survival of Trojan fish. Overall, our models found that fecundity holds the most influence over how effectively TSC theory can be used to control fish populations. However, a recent meta-analysis demonstrates that the fecundity of sex-reversed fish is compromised. It may be possible to compensate for reduced fecundity by increasing the rate of Trojan introductions. Surprisingly, our models also found that Trojans are a more effective bio-control when consistently introduced into the same place, rather than being randomly distributed at introduction.     

Analysis of the Trojan Y-Chromosome eradication strategy for an invasive species

The Trojan Y-Chromosome (TYC) strategy, an autocidal genetic biocontrol method, has been proposed to eliminate invasive alien species. In this work, we analyze the dynamical system model of the TYC strategy, with the aim of studying the viability of the TYC eradication and control strategy of an invasive species. In particular, because the constant introduction of sex-reversed trojan females for all time is not possible in practice, there arises the question: What happens if this injection is stopped after some time? Can the invasive species recover? To answer that question, we perform a rigorous bifurcation analysis and study the basin of attraction of the recovery state and the extinction state in both the full model and a certain reduced model. In particular, we find a theoretical condition for the eradication strategy to work. Additionally, the consideration of an Allee effect and the possibility of a Turing instability are also studied in this work. Our results show that: (1) with the inclusion of an Allee effect, the number of the invasive females is not required to be very low when the introduction of the sex-reversed trojan females is stopped, and the remaining Trojan Y-Chromosome population is sufficient to induce extinction of the invasive females; (2) incorporating diffusive spatial spread does not produce a Turing instability, which would have suggested that the TYC eradication strategy might be only partially effective, leaving a patchy distribution of the invasive species.     

A comparison of the Trojan Y Chromosome and daughterless carp eradication strategies

Two autocidal genetic biocontrol methods have been proposed as a means to eliminate invasive fish by changing the sex ratio of the population: the Trojan Y Chromosome (TYC) strategy and the Daughterless Carp (DC) strategy. Both strategies were modeled using ordinary differential equations that allow the kinetics of female decline to be assessed under identical modeling conditions. When compared directly in an ordinary differential equation (ODE) model, the TYC strategy was found to result in female extinction more rapidly than a DC strategy (in each of three models tested in which the Daughterless autocidal fish contained an aromatase inhibitor gene in either two or eight copies). The TYC strategy additionally required the introduction of fewer autocidal fish to the target population to achieve local extinction of females as compared to the DC approach. The results suggest that the relatively lower efficiency of female reduction associated with the DC approach is a consequence of a greater capacity to produce females and also a reduced capacity to produce males as compared to the TYC system.     

Control of introduced species using Trojan sex chromosomes

To control introduced exotic species that have predominantly genetic, but environmentally reversible, sex determination (e.g. many species of fish), Gutierrez and Teem recently modeled the use of carriers of Trojan Y chromosomes--individuals who are phenotypically sex reversed from their genotype. Repeated introduction of YY females into wild populations should produce extreme male-biased sex ratios and eventual elimination of XX females, thus leading to population extinction. Analogous dynamics are expected in systems in which sex determination is influenced by one or a few major genes on autosomes.     

Stochastic models for the Trojan Y-Chromosome eradication strategy of an invasive species

The Trojan Y-Chromosome (TYC) strategy, an autocidal genetic biocontrol method, has been proposed to eliminate invasive alien species. In this work, we develop a Markov jump process model for this strategy, and we verify that there is a positive probability for wild-type females going extinct within a finite time. Moreover, when sex-reversed Trojan females are introduced at a constant population size, we formulate a stochastic differential equation (SDE) model as an approximation to the proposed Markov jump process model. Using the SDE model, we investigate the probability distribution and expectation of the extinction time of wild-type females by solving Kolmogorov equations associated with these statistics. The results indicate how the probability distribution and expectation of the extinction time are shaped by the initial conditions and the model parameters.     

A model describing the effect of sex-reversed YY fish in an established wild population: The use of a Trojan Y chromosome to cause extinction of an introduced exotic species

A novel means of inducing extinction of an exotic fish population is proposed using a genetic approach to shift the ratio of male to females within a population. In the proposed strategy, sex-reversed fish containing two Y chromosomes are introduced into a normal fish population. These YY fish result in the production of a disproportionate number of male fish in subsequent generations. Mathematical modeling of the system following introduction of YY fish at a constant rate reveals that female fish decline in numbers over time, leading to eventual extinction of the population.     

Chromosome banding in Amphibia. XXVIII. Homomorphic XY sex chromosomes and a derived Y-autosome translocation in Eleutherodactylus riveroi (Anura, Leptodactylidae)

Extensive cytogenetic analyses on a population of the leptodactylid frog Eleutherodactylus riveroi in northern Venezuela revealed the existence of multiple XXAA male/XYAA female/XAA(Y) female sex chromosomes. The XAA(Y) karyotype originated by a centric (Robertsonian) fusion between the original, free Y chromosome and an autosome. 46.2% of the male individuals in this population are carriers of this Y-autosome fusion. In male meiosis the XAA(Y) sex chromosomes pair in the expected trivalent configuration. In the same population 53.8% of the male animals still possess the original, free XY sex chromosomes. E. riveroi is only the second vertebrate species discovered in which a derived Y-autosome fusion coexists with the ancestral free XY sex chromosomes. The free XY sex chromosomes, as well as the multiple XA(Y) sex chromosomes are still in a very primitive (homomorphic) stage of differentiation. With no banding technique applied it is possible to distinguish the Y from the X. Various banding techniques and in situ hybridizations have been carried out to characterize the karyotypes. DNA flow cytometric measurements show that the genome size of E. riveroi resembles that of other Eleutherodactylus species. The cytogenetic data obtained in E. riveroi are compared with those of the sole other vertebrate known to possess the extremely rare, multiple XXAA male/XYAA female/XAA(Y) female sex chromosomes. Surprisingly enough, this vertebrate again is a frog belonging to the genus Eleutherodactylus [E. ((maussi) biporcatus] which lives exactly in the same habitat in northern Venezuela as does E. riveroi.     

The Y chromosome that lost the male-determining function behaves as an X chromosome in the medaka fish, Oryzias latipes

The medaka, Oryzias latipes, has an XX/XY sex-determination system, and a Y-linked DM-domain gene, DMY, is the sex-determining gene in this species. Since DMY appears to have arisen from a duplicated copy of the autosomal DMRT1 gene approximately 10 million years ago, the medaka Y chromosome is considered to be one of the youngest male-determining chromosomes in vertebrates. In the screening process of sex-reversal mutants from wild populations, we found a population that contained a number of XY females. PCR, direct sequencing, and RT-PCR analyses revealed two different null DMY mutations in this population. One mutation caused loss of expression during the sex-determining period, while the other comprised a large deletion in putative functional domains. YY females with the mutant-type DMY genes on their Y chromosomes were fully fertile, indicating that the X and Y chromosomes were functionally the same except for the male-determining function. In addition, we investigated the frequencies of the sex chromosome types in this population over four successive generations. The Y chromosomes bearing the mutant-type DMY genes were detected every year with no significant differences in their frequencies. These results demonstrate that aberrant Y chromosomes behaving as X chromosomes have been maintained in this population.     

A review of the research on introduced freshwater fishes: new perspectives,the need for research,and management implications

Although freshwater fishes have a long history of human-induced introduction, recent globalization has accelerated worldwide introduction events even more, and those introduced fish species are now perceived to be a major threat to ecosystems. Over the last two decades, numerous studies have been published on introduced fish species; however, it has been challenging for researchers to understand the magnitude of the impact and the underlying mechanism of invasions. Recently, new perspectives in understanding invasive freshwater fish biology have been presented in a number of studies, which can be largely attributed to advances in analytical techniques and also to a growing need for proactive analysis in management strategies. The aim of this paper is to summarize new ecological perspectives, the need for research, and/or management implications with emphasis on technological advances in, for example, statistics, molecular analysis, modeling techniques, and landscape analysis addressed under the following five categories: introduction pathways, predicting spatial patterns, biotic homogenization, hybridization, and control and eradication. The conservation of native fish fauna and the management of introduced fish species will benefit from combining these new perspectives with fundamental studies such as those on life history and population biology.     

Genetic control of invasive fish: technological options and its role in integrated pest management

Genetic options for the control of invasive fishes were recently reviewed and synthesized at a 2010 international symposium, held in Minneapolis/St. Paul, MN, USA. The only option currently available “off-the-shelf” is triploidy, which can be used to produce sterile males for a release program analogous to those widely and successfully used for biological control of insect pests. However, the Trojan Y and several recombinant options that heritably distort pest population sex ratios are technologically feasible, are at or are close to proof-of-concept stage and are potentially much more effective than sterile male release programs. All genetic options at this stage require prolonged stocking programs to be effective, though gene drive systems are a potential for recombinant approaches. They are also likely to differ in their current degree of social acceptability, with chromosomal approaches (triploidy and Trojan Y) likely to be the most readily acceptable to the public and least likely to require changes in legislative or policy settings to be implemented. Modelling also suggests that the efficacy of any of these genetic techniques is enhanced by, and in turn non-additively enhance, conventional methods of pest fish control.     

Needles in a haystack. Genomic tools have become powerful tools for conservation biologists to monitor the spread of invasive species

Early detection and monitoring are key to controlling invasive species. Genomics and new sequencing technologies are now providing powerful new tools to track and combat relentless pestsIn the late 1960s, an Air France pilot and his family took a holiday in swampy Louisiana in the southeastern USA and were intrigued by the giant bullfrogs, Rana catesbeiana. The family introduced a dozen of the animals to a pond in the Bordeaux region of southwestern France, inadvertently starting a major invasion that affected thousands of lakes and creeks with devastating effects on the native fauna. “They eat everything and if you go to a pond where there are bullfrogs, there are no other amphibians [...] because bullfrogs prey on other animals or because they are spreading a disease—chytridiomycosis—that is absent from some European areas,” said conservation biologist Francesco Ficetola of the Department of Environmental Sciences at the University of Milano–Bicocca in Italy. “There was nothing like them in Europe,” he added.As part of his postdoctoral research, Ficetola studied the frog invasion in Europe at the Université de Savoie and the University of Grenoble in France. To monitor their distribution, he pioneered the use of environmental DNA (eDNA) to detect R. catesbeiana without observing the animals themselves. He and his colleagues took water samples, extracted eDNA and used primers monomorphic to nearly 400 bullfrog samples to demonstrate their presence in wetlands, even in low densities.“The significance was that we were able to detect the presence of the species without seeing or hearing the species,” Ficetola said. The eDNA technique has further potential: “As the environment can retain the molecular imprint of inhabiting species, our approach allows the reliable detection of secretive organisms in wetlands without direct observation. Combined with massive sequencing and the development of DNA barcodes that enable species identification, this approach opens new perspectives for the assessment of current biodiversity from environmental samples” (Ficetola et al, 2008)....molecular and genomic tools to identify, monitor and control invasive species have become increasingly sophisticated during the past five years...According to David Lodge, a biologist at the University of Notre Dame (South Bend, IN, USA), molecular and genomic tools to identify, monitor and control invasive species have become increasingly sophisticated during the past five years and are especially valuable in aquatic environments. “Hair traps and scat sampling have been used for mammals for a long time. You don''t have to catch the lynx, for example. You just put a piece of wire or tape on a trail and you get a little bit of hair and then you do genetic analysis to figure out which species it is. With enough sampling and genetic sequencing, you could even estimate the number of individuals in a population,” Lodge explained. Technological advances now make it possible to apply this technique to aquatic ecosystems to track everything from whales down to the smallest organisms.Most invasive species eventually find their ecological niche among the natives, but some can turn into pests, particularly if the new environment does not pose any threats for them such as predators or diseases. The American bullfrog in Europe, the cane toad, Bufo marinus, in Australia, or the zebra mussel, Dreissena polymorpha, in the Great Lakes have all wreaked havoc on the environment and have had an impact on commercial interests. In addition to standard control measures—such as killing animals or halting their further migration with barriers—scientists have also explored biological methods of tracking and controlling invasive species, but it is only in the past few years that these have become efficient enough to find the proverbial needle in a haystack.In the USA, these genomic tools are being applied to track Asian carp species in the Mississippi, its tributary Illinois River and the Great LakesIn the USA, these genomic tools are being applied to track Asian carp species in the Mississippi, its tributary Illinois river and the Great Lakes. The large, jumping carp have negative effects on the native fish and are even able to knock down boaters (http://www.youtube.com/watch?v=yS7zkTnQVaM). Christopher Dionigi, assistant director for National Policy and Programs of the US National Invasive Species Council (Washington, DC, USA), a cooperative of federal agencies created in 1999, said, “They''re large, they do jump out of the water, they can achieve very high densities in specific areas, and [have] impacted a lot of native fisheries, and also game fish.” The silver and bighead carp are harvesting phytoplankton and zooplankton, which are dietary mainstays for many other species. Lodge described the carp as “aquatic cows” that outgrow their predators and compete with other species for food....Lodge and his colleagues are using eDNA to tackle a raging political issue in the American Midwest: whether Asian carp are invading Lake Michigan around Chicago...Inspired by Ficetola''s eDNA work and his own research to develop detection tools for species in the ballast water of ships, Lodge and his colleagues are using eDNA to tackle a raging political issue in the American Midwest: whether Asian carp are invading Lake Michigan around Chicago—and potentially the rest of the Great Lakes—from the Mississippi River.Neighbouring states, led by Michigan, have taken the US Army Corps of Engineers and the Chicago Metropolitan Water Reclamation District to court to get Chicago to close locks on the Chicago Sanitary and Ship Canal, which was built to divert Chicago''s sewage away from Lake Michigan into the Mississippi instead. The main dispute is whether the canal allows the Asian carp to invade Lake Michigan.Since 2009, Lodge''s group has been conducting a risk assessment for the Army Corps to determine whether carp and other invasive species are able to move through the canal into the Great Lakes. The team collected more than 1,000 surface water samples, extracted DNA and used markers specific for Hypophthalmichthys nobilis and H. molitrix (Jerde et al, 2011). Subsequently, commercial fishermen caught an adult bighead carp within 13 km of Lake Michigan, only 4 km upstream from the nearest positive eDNA detection.“There has been a lot of hand-wringing about where the carp are. We wanted to see if we could detect the presence of Asian carp without even seeing them,” Lodge explained. “The most important thing we found (using eDNA) was DNA of both species in many places on the lake side of the electric barriers that the Army Corps of Engineers maintains in the canal [to keep the carp out of the lake]”....eDNA can help in early detection of invasive species, which is key to effective management on the basis of ''early detection, rapid response''He added that eDNA can help in early detection of invasive species, which is key to effective management on the basis of ''early detection, rapid response''. “Because eDNA evidence indicates that at least silver carp have entered Lake Michigan, surveillance is warranted within Great Lakes rivers that may be colonized and could support successful spawning,” Lodge said. “The eDNA method appears well-suited to rapid surveys across the large spatial scale that will be required in the Great Lakes.” He added that it might be possible to poison the carp when they aggregate to spawn.Nathan Bott, a molecular-diagnostic researcher at the South Australian Research and Development Institute (SARDI; Henley Beach, SA, Australia), is adapting a technique to extract DNA from soil to detect invasive species in the ballasts of ships. This approach involves identifying DNA from pests, some of which have the potential to destroy farmed or fished mussel, scallop and abalone. Bott has been working on assays for the Asian bag mussel, Musculista senhousia, northern Pacific sea star, Asterias amurensis, and European fan worm, Sabella spallanzanii.He said SARDI is in the process of purchasing a 454 sequencer to verify positive samples. “The idea is if we are getting positives in the ports, and particularly if the species in question is considered a significant threat, the authorities would want to conduct further surveys.” Bott explained that quantitative PCR can screen thousands of samples per day, and the 454 sequencer would be used to confirm the positives to deliver a rapid result. “If we can carry out a quick and relatively inexpensive experiment on the eDNA result with quantitative PCR using our 454, then we can save everyone a lot of time and money, and rather than concentrating on confirming the presence of the pest, a control strategy can be instigated.”Once an invasive species is identified, what can de done to remove it? The French took to shooting and trapping American bullfrogs, removing eggs and tadpoles and draining wetlands. The Americans built electrical fences to try to stop the Asian carp from entering the Great Lakes. The state of Illinois has even supported a fishing operation to catch Asian carp, shipping the fillets to China—a scheme that could potentially lead to perverse incentives to introduce more fish....biologists are refining an old concept for species control that disrupts the ratio of sexes in order to control or even kill off invasive speciesMeanwhile, biologists are refining an old concept for species control that disrupts the ratio of sexes in order to control or even kill off invasive species. The basic idea—creating sterile males, mainly by using irradiation, and releasing them by the millions—is not new. Sterile males compete with wild males for the females but do not produce offspring. Repeated introductions of these populations can control or even wipe out whole populations. Since its first application in the 1950s, there have been several success stories, including eradication of the screw-worm fly (Cochliomyia hominivorax) in North America and control of several species of fruit flies.In recent years, the concept has been improved by using molecular and genetic approaches. Ron Thresher, a marine ecologist who researches invasive species for the Australian Commonwealth Scientific and Industrial Research Organization in Hobart, Tasmania, Australia, said that common carp—bottom-feeders that foul water and dig up plants—were causing problems Down Under. However, he said that stakeholders, such as public and commercial fishing interests, resist biological controls—introducing predators, parasites or genetically modified viruses to make carp sterile—because these could have unintended consequences.As a result, his group has begun exploring another approach, while gently steering away from the transgenic controversies. “We''ve been careful to make it clear to people that we''re using genetic-modification technology but it''s not transgenic,” he said. “What we''re doing is taking a carp''s native genes and basically rearranging them a little bit. So it''s using wholly native fish genes at this point. This makes a large difference in terms of public acceptability.”Thresher said his group has been developing techniques that control gene activity with the goal of causing sterility in females or causing females to change sex. The approach has been proven in lab studies in zebra fish and is now being applied to carp. He estimates that it would take 50 years to eliminate the carp with this approach. “It''s generation and time dependent,” he said.Like Thresher, John Teem, a molecular biologist at the Florida Department of Agriculture and Consumer Services (Tallahassee, FL, USA), is tinkering with fish genetics and hormones to develop a Trojan fish with two Y chromosomes that would cause a population to collapse over time.Normally, a female has two X chromosomes and males have an X and a Y chromosome; it is the Y chromosome that determines ''maleness''. Teem said his strategy would create female fish with two Y chromosomes. This fish, when introduced into a target invasive population, will begin to mate with normal males (XY) producing only male progeny. Half of those males will be YY males that will themselves mate and produce only male progeny.“We''re trying to change the sex ratio of the population so that there are more males and fewer females at each generation. By flooding the system with Y chromosomes, more and more male fish are produced,” Teem explained. “Ultimately, the population collapses when there are no more females.” For a fish with a one-year mating cycle, he estimated that it would take 70 years to eradicate a population.Production of YY females involves selective breeding of fish that have been sex-reversed by hormone treatment as juveniles. The process uses techniques that are commonly employed in the aquaculture industry and does not rely on recombinant DNA technology. Teem said the approach could work on carp or other fish with an XY-chromosome sex determination, but he has been focusing on tilapia, which was introduced to control invasive aquatic plants in Florida but has now become a pest. “They''re very aggressive when they''re defending their nest,” he said. “They will attack other fish. If the native fish need that same territory to reproduce, tilapia can exclude them from that resource and negatively impact their reproduction.”...the idea of a tricorder has been around for decades and would be a useful tool to identify and monitor invasive species as well as any other plants and animalsUltimately, however, early detection and thorough monitoring is the best defence against invasions. Daniel Janzen, an evolutionary biologist from the University of Pennsylvania (Philadelphia, PA, USA), is working on a gadget that could enable the development of a dense surveillance system for invasive pests, using an army of volunteers. His tool resembles a species-level ''tricorder'', similar to that from the sci-fi franchise Star Trek, in which a character would use the portable device to detect life-forms and resources on an alien planet. Janzen said the idea of a tricorder has been around for decades and would be a useful tool to identify and monitor invasive species as well as any other plants and animals.Since 1978, Janzen has been studying moth and butterfly caterpillars in the northwestern corner of Costa Rica. By 2003, he had built a database of 3,000 species out of an estimated 12,500 in this region. That year, he connected with geneticist Paul Hebert, a researcher from the University of Guelph in Canada and inventor of the ''DNA-barcoding'' technique that uses short snippets of mitochondrial DNA to identify species. Their goal is to develop a hand-held personal reader the size and cost of a pocket comb. Janzen and Herbert envision that their ''barcorders'' would be used by farmers, game wardens, school children or anyone else.“I can''t think of any better way for anybody, everybody out there to be telling you whether invasive species are here, there and otherwise, than to have this device in their back pocket,” Janzen said. “How many farmers and other outdoors people are there in North America? If every one of them had a barcorder and every time he saw a weird weed growing in his wheat fields, he just pulled a little chip off a leaf and stuck it in his barcorder that would tell the USDA or some other central agency, ''Ah! This plant grows there.''”Bott, in South Australia, agreed that such devices would be a great help to monitor the spread of invasive species. “We can''t be everywhere at once sampling all these different areas. But having the ability to actually go down and test them on site would, in the future, really increase the throughput of our testing and make it a lot easier and possibly less expensive to test a wide range of species. It''s not a fantasy. It''s achievable.”     

Characterization of the Avian Trojan Gene Family Reveals Contrasting Evolutionary Constraints

“Trojan” is a leukocyte-specific, cell surface protein originally identified in the chicken. Its molecular function has been hypothesized to be related to anti-apoptosis and the proliferation of immune cells. The Trojan gene has been localized onto the Z sex chromosome. The adjacent two genes also show significant homology to Trojan, suggesting the existence of a novel gene/protein family. Here, we characterize this Trojan family, identify homologues in other species and predict evolutionary constraints on these genes. The two Trojan-related proteins in chicken were predicted as a receptor-type tyrosine phosphatase and a transmembrane protein, bearing a cytoplasmic immuno-receptor tyrosine-based activation motif. We identified the Trojan gene family in ten other bird species and found related genes in three reptiles and a fish species. The phylogenetic analysis of the homologues revealed a gradual diversification among the family members. Evolutionary analyzes of the avian genes predicted that the extracellular regions of the proteins have been subjected to positive selection. Such selection was possibly a response to evolving interacting partners or to pathogen challenges. We also observed an almost complete lack of intracellular positively selected sites, suggesting a conserved signaling mechanism of the molecules. Therefore, the contrasting patterns of selection likely correlate with the interaction and signaling potential of the molecules.     

Environmental sex reversal,Trojan sex genes,and sex ratio adjustment: conditions and population consequences

The great diversity of sex determination mechanisms in animals and plants ranges from genetic sex determination (GSD, e.g. mammals, birds, and most dioecious plants) to environmental sex determination (ESD, e.g. many reptiles) and includes a mixture of both, for example when an individual’s genetically determined sex is environmentally reversed during ontogeny (ESR, environmental sex reversal, e.g. many fish and amphibia). ESD and ESR can lead to widely varying and unstable population sex ratios. Populations exposed to conditions such as endocrine‐active substances or temperature shifts may decline over time due to skewed sex ratios, a scenario that may become increasingly relevant with greater anthropogenic interference on watercourses. Continuous exposure of populations to factors causing ESR could lead to the extinction of genetic sex factors and may render a population dependent on the environmental factors that induce the sex change. However, ESR also presents opportunities for population management, especially if the Y or W chromosome is not, or not severely, degenerated. This seems to be the case in many amphibians and fish. Population growth or decline in such species can potentially be controlled through the introduction of so‐called Trojan sex genes carriers, individuals that possess sex chromosomes or genes opposite from what their phenotype predicts. Here, we review the conditions for ESR, its prevalence in natural populations, the resulting physiological and reproductive consequences, and how these may become instrumental for population management.     

A reciprocal translocation radically reshapes sex‐linked inheritance in the common frog

X and Y chromosomes can diverge when rearrangements block recombination between them. Here we present the first genomic view of a reciprocal translocation that causes two physically unconnected pairs of chromosomes to be coinherited as sex chromosomes. In a population of the common frog (Rana temporaria), both pairs of X and Y chromosomes show extensive sequence differentiation, but not degeneration of the Y chromosomes. A new method based on gene trees shows both chromosomes are sex‐linked. Furthermore, the gene trees from the two Y chromosomes have identical topologies, showing they have been coinherited since the reciprocal translocation occurred. Reciprocal translocations can thus reshape sex linkage on a much greater scale compared with inversions, the type of rearrangement that is much better known in sex chromosome evolution, and they can greatly amplify the power of sexually antagonistic selection to drive genomic rearrangement. Two more populations show evidence of other rearrangements, suggesting that this species has unprecedented structural polymorphism in its sex chromosomes.     

Heterogeneous Histories of Recombination Suppression on Stickleback Sex Chromosomes

How consistent are the evolutionary trajectories of sex chromosomes shortly after they form? Insights into the evolution of recombination, differentiation, and degeneration can be provided by comparing closely related species with homologous sex chromosomes. The sex chromosomes of the threespine stickleback (Gasterosteus aculeatus) and its sister species, the Japan Sea stickleback (G. nipponicus), have been well characterized. Little is known, however, about the sex chromosomes of their congener, the blackspotted stickleback (G. wheatlandi). We used pedigrees to obtain experimentally phased whole genome sequences from blackspotted stickleback X and Y chromosomes. Using multispecies gene trees and analysis of shared duplications, we demonstrate that Chromosome 19 is the ancestral sex chromosome and that its oldest stratum evolved in the common ancestor of the genus. After the blackspotted lineage diverged, its sex chromosomes experienced independent and more extensive recombination suppression, greater X–Y differentiation, and a much higher rate of Y degeneration than the other two species. These patterns may result from a smaller effective population size in the blackspotted stickleback. A recent fusion between the ancestral blackspotted stickleback Y chromosome and Chromosome 12, which produced a neo-X and neo-Y, may have been favored by the very small size of the recombining region on the ancestral sex chromosome. We identify six strata on the ancestral and neo-sex chromosomes where recombination between the X and Y ceased at different times. These results confirm that sex chromosomes can evolve large differences within and between species over short evolutionary timescales.     

Polymorphism for Y-Linked Suppressors of Sex-Ratio in Two Natural Populations of Drosophila Mediopunctata

In several Drosophila species there is a trait known as ``sex-ratio': males carrying certain X chromosomes (called ``SR') produce female biased progenies due to X-Y meiotic drive. In Drosophila mediopunctata this trait has a variable expression due to Y-linked suppressors of sex-ratio expression, among other factors. There are two types of Y chromosomes (suppressor and nonsuppressor) and two types of SR chromosomes (suppressible and unsuppressible). Sex-ratio expression is suppressed in males with the SR(suppressible)/Y(suppressor) genotype, whereas the remaining three genotypes produce female biased progenies. Now we have found that ~10-20% of the Y chromosomes from two natural populations 1500 km apart are suppressors of sex-ratio expression. Preliminary estimates indicate that Y(suppressor) has a meiotic drive advantage of 6% over Y(nonsuppressor). This Y polymorphism for a nonneutral trait is unexpected under current population genetics theory. We propose that this polymorphism is stabilized by an equilibrium between meiotic drive and natural selection, resulting from interactions in the population dynamics of X and Y alleles. Numerical simulations showed that this mechanism may stabilize nonneutral Y polymorphisms such as we have found in D. mediopunctata.     

Indirect evidence from DNA sequence diversity for genetic degeneration of the Y-chromosome in dioecious species of the plant Silene: the SlY4/SlX4 and DD44-X/DD44-Y gene pairs

The action of natural selection is expected to reduce the effective population size of a nonrecombining chromosome, and this is thought to be the chief factor leading to genetic degeneration of Y-chromosomes, which cease recombining during their evolution from ordinary chromosomes. Low effective population size of Y chromosomes can be tested by studying DNA sequence diversity of Y-linked genes. In the dioecious plant, Silene latifolia, which has sex chromosomes, one comparison (SlX1 vs. SlY1) indeed finds lower Y diversity compared with the homologous X-linked gene, and one Y-linked gene with no X-linked homologue has lower species-wide diversity than a homologous autosomal copy (SlAp3Y vs. SlAp3A). To test whether this is a general pattern for Y-linked genes, we studied two further recently described X and Y homologous gene pairs in samples from several populations of S. latifolia and S. dioica. Diversity is reduced for both Y-linked genes, compared with their X-linked homologues. Our new data are analysed to show that the low Y effective size cannot be explained by different levels of gene flow for the X vs. the Y chromosomes, either between populations or between these closely related species. Thus, all four Y-linked genes that have now been studied in these plants (the two studied here, and two previously studied genes, have low diversity). This supports other evidence for an ongoing degeneration process in these species.     

Does localized control of invasive eastern gambusia (Poeciliidae: Gambusia holbrooki) increase population growth of generalist wetland fishes?

While invasive fish management is heavily focussed on containment measures when introductions occur, examples from invasive species management in terrestrial systems suggest that there may also be considerable conservation benefits in implementing localized control programmes. We conducted a field‐based experiment to assess the effectiveness of removing a globally significant invasive fish, eastern gambusia Gambusia holbrooki, from natural wetland habitats of south‐eastern Australia. With recent work suggesting the impacts of eastern gambusia may be minimal for species with generalist life‐history strategies, we hypothesized that the removal of eastern gambusia will reduce localized population growth of the invasive species, but will have little influence on the population growth of more generalist sympatric wetland fish species. We used a predictive modelling approach to investigate changes in eastern gambusia populations following removal activities, and how sympatric fish species responded to such changes. Although eastern gambusia rapidly populated habitats, we demonstrated that control actions substantially reduced the rate of population increase over the four‐month study period. This suggests that control may be an effective localized strategy to suppress eastern gambusia densities. There was however, no evidence of any response to the removal actions by any of the three sympatric fish species investigated – carp gudgeon (Hypseleotris spp.), Australian smelt (Retropinna semoni) and the invasive common carp (Cyprinus carpio). These results support previous work which suggests that the flexible life‐history strategies and behavioural traits of all three species allow co‐existence with eastern gambusia. The study highlights the importance of understanding the potential outcomes of control options which is particularly pertinent for established aquatic invasive species where information on control effectiveness, population dynamics and/or ecosystem response is currently lacking.     

Evidence for Emergence of Sex-Determining Gene(s) in a Centromeric Region in Vasconcellea parviflora

Sex chromosomes have been studied in many plant and animal species. However, few species are suitable as models to study the evolutionary histories of sex chromosomes. We previously demonstrated that papaya (Carica papaya) (2n = 2x = 18), a fruit tree in the family Caricaceae, contains recently emerged but cytologically heteromorphic X/Y chromosomes. We have been intrigued by the possible presence and evolution of sex chromosomes in other dioecious Caricaceae species. We selected a set of 22 bacterial artificial chromosome (BAC) clones that are distributed along the papaya X/Y chromosomes. These BACs were mapped to the meiotic pachytene chromosomes of Vasconcellea parviflora (2n = 2x = 18), a species that diverged from papaya ∼27 million years ago. We demonstrate that V. parviflora contains a pair of heteromorphic X/Y chromosomes that are homologous to the papaya X/Y chromosomes. The comparative mapping results revealed that the male-specific regions of the Y chromosomes (MSYs) probably initiated near the centromere of the Y chromosomes in both species. The two MSYs, however, shared only a small chromosomal domain near the centromere in otherwise rearranged chromosomes. The V. parviflora MSY expanded toward the short arm of the chromosome, whereas the papaya MSY expanded in the opposite direction. Most BACs mapped to papaya MSY were not located in V. parviflora MSY, revealing different DNA compositions in the two MSYs. These results suggest that mutation of gene(s) in the centromeric region may have triggered sex chromosome evolution in these plant species.