Sex chromosomes and sex determination in weird mammals

Weird mammals are of two types. Highly divergent mammals, such as the marsupials and monotremes, have informed us of the evolutionary history of the Y chromosome and sex-determining gene, and the recently specialized rodents can help us predict its future. The Y chromosome has had a short but eventful history, and is already heading briskly for oblivion. It originated as a homologous partner of the X when it acquired a sex-determining gene (not necessarily SRY). Most of the genes on the Y, even those with a male-specific function, evolved from genes now on the X. At the mercy of a high rate of variability and the forces of drift and selection, the Y has lost genes at a rate of 3-6 genes/million years, sparing those that acquired critical male-specific functions. Even these genes have disappeared from one mammalian lineage or another as their functions were usurped by genes elsewhere in the genome. The mammalian testis-determining gene, SRY, is a typical Y-borne gene. It arose by truncation of a gene (SOX3) on the X that is expressed in brain development, and it may work by interacting with (inhibiting?) related genes, including SOX9. Variant sex-determining systems in rodents show that the action of SRY can change, as it evidently has in the mouse, and SRY can be inactivated, as in akodont rodents, or even completely superseded, as in mole voles.     

Sex chromosomes, sex determination, and sex-linked sequences in Microtidae

The Arvicolidae is a widely distributed rodent group with several interesting characteristics in their sex chromosomes. Here, we summarize the actual knowledge of some of these characteristics. This mammalian group has species with abnormal sex determination systems. In fact, some species present the same karyotype in both males and females, with total absence of a Y chromosome, and hence of SRY and ZFY genes. Other species present fertile, sex-reversed XY females, generally due to mutations affecting X chromosomes. Furthermore, in Microtus oregoni males and females are gonosomic mosaic (the females are XO in the soma and XX in the germ cells, while the males are XY in the soma and OY in the germ cells). Regarding sex chromosomes, some species present enlarged (giant) sex chromosomes because of the presence of large blocks of constitutive heterochromatin, which have been demonstrated to be highly heterogeneous. Furthermore, we also consider the alterations affecting composition and localization of sex-linked genes or repeated sequences. Finally, this rodent group includes species with synaptic and asynaptic sex chromosomes. In fact, several species with asynaptic sex chromosomes have been described. It is interesting to note that within the genus Microtus both types of sex chromosomes are present.     

Sex chromosomes and sex determination pathway dynamics in plant and animal models

In this review, we discuss and compare data obtained from animal and plant models, focusing our attention on the mechanisms that affect sex linkage and changes in sex‐determining pathways. Patterns in data across taxa suggest that sex bias and the dynamics that occurs within hybrid zones can play an important role in these processes that enable the spread of some otherwise handicapped genotypes. We discuss the data obtained from several main plant model species in the light of the patterns demonstrated in animal models. In several plant models, we discuss possible differences in the age of their sex‐determining pathways and the age of their current sex chromosomes. We also address an open question: how can an X/A ratio based sex‐determining system evolve from a sex‐determining system based on two genes on the Y chromosome that control two separate sex‐determining pathways (for the control of gynoecium suppression and anther promotion)? Taking inspiration from the well described mechanisms involved in sex determination dynamics in animals, we suggest a hypothetical stepwise scenario of change of the plant sex‐determining system based on two separate sex‐determining pathways (for the control of gynoecium suppression and anther promotion) into the other sex‐determining systems. We suppose that an intermediate step occurs before shift to X/A based sex determination. At that phase, sex determination in plants is still based on an active Y chromosome, although there exists already a connected control of both sex‐determining pathways. We suggest that this connection is enabled by the existence of the genes that control sexual dimorphism in the vegetative state of plant development, and that, in some circumstances, these genes can become sex‐determining genes. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 737–752.     

爬行动物的温度依赖型性别决定

脊椎动物性别决定模式一直是进化生物学领域的热点问题,它对个体发育和自然种群性比组成都具有深刻的影响。性别决定模式根据主要成因可分为基因依赖型性别决定(GSD)和环境依赖型性别决定(ESD)2大类,其中温度依赖型性别决定(TSD)又是ESD中的主要性别决定模式。多数羊膜类脊椎动物具有稳定的GSD模式,而爬行动物的性别决定模式则丰富多样,即使是亲缘关系很近的物种也具有不同的模式。研究者们以爬行动物为模型动物开展了许多关于脊椎动物性别决定方面的工作。本文综述了近年来爬行动物TSD的最新研究进展,回顾了温度和性激素对TSD爬行类动物的影响及其进化适应意义,以及气候变化与TSD爬行类的关系,并提出了今后爬行动物TSD研究的重点。     

Plant sex determination and sex chromosomes

Sex determination systems in plants have evolved many times from hermaphroditic ancestors (including monoecious plants with separate male and female flowers on the same individual), and sex chromosome systems have arisen several times in flowering plant evolution. Consistent with theoretical models for the evolutionary transition from hermaphroditism to monoecy, multiple sex determining genes are involved, including male-sterility and female-sterility factors. The requirement that recombination should be rare between these different loci is probably the chief reason for the genetic degeneration of Y chromosomes. Theories for Y chromosome degeneration are reviewed in the light of recent results from genes on plant sex chromosomes.     

Environmental regulation of sex determination in reptiles

The various patterns of environmental sex determination in squamates, chelonians and crocodilians are described. High temperatures produce males in lizards and crocodiles but females in chelonians. Original experiments on the effects of incubation at 30 degrees C (100% females) or 33 degrees C (100% males) on development in Alligator mississippiensis are described. These include an investigation of the effect of exposing embryos briefly to a different incubation temperature on the sex ratio at hatching, and a study of the effects of 30 degrees C and 33 degrees C on growth and development of alligator embryos and gonads. A 7-day pulse of one temperature on the background of another was insufficient to alter the sex ratio dramatically. Incubation at 33 degrees C increased the rate of growth and development of alligator embryos. In particular, differentiation of the gonad at 33 degrees C was enhanced compared with 30 degrees C. A hypothesis is developed to explain the mechanism of temperature-dependent sex determination (TSD) in crocodilians. The processes of primary sex differentiation are considered to involve exposure to a dose of some male-determining factor during a specific quantum of developmental time during early incubation. The gene that encodes for the male-determining factor is considered to have an optimum temperature (33 degrees C). Any change in the temperature affects the expression of this gene and affects the dose or quantum embryos are exposed to. In these cases there is production of females by default. The phylogenetic implications of TSD for crocodilians, and reptiles in particular, are related to the life history of the animal from conception to sexual maturity. Those animals that develop under optimal conditions grow fastest and largest and become male. A general association between the size of an animal and its sex is proposed for several types of vertebrate.     

Sex,sex chromosomes and gene expression

The X chromosome has fewer testis-specific genes than autosomes in many species. This bias is commonly attributed to X inactivation in spermatogenesis but a recent paper in BMC Biology provides evidence against X inactivation in Drosophila and proposes that somatic tissue- and testis- but not ovary-specific genes tend not to be located on the X chromosome. Here, we discuss possible mechanisms underlying this bias, including sexual antagonism and dosage compensation.     

Sex-specific DNA in reptiles with temperature sex determination

Banded krait minor ("Bkm") satellite DNA, originating in the W-chromosome of the snake Bungarus fasciatus, has been found in the genome of diverse eukaryotic species including fruit fly, quail, and horse. Concentrations of Bkm have been found in the presumptive W-chromosome of snakes with isomorphic sex chromosomes and in the male-determining region of the Y-chromosome in mouse and man. We therefore asked whether Bkm-related DNA might be present in quantitative excess in DNA from males or females in two related species of sea turtle, Chelonia mydas, in which sex is determined by the temperature of the incubating egg, and Lepidochelys kempi, in which the critical sex-determining temperature has recently been described. Filter hybridization with the Bkm 2(8) probe revealed male-specific fragments in both species; female-specific fragments were also revealed in C. mydas. Sex-specific DNA sequences in temperature-sex-determined species such as Kemp's ridley and the green turtle were unexpected, but could be explained if there were an underlying genetic mode of sex determination in these animals, or alternatively, if temperature-influenced sex determination involved structural modifications in DNA adjacent to, or directly concerned with, the sex-determining genes. If these results are confirmed across a broader sample of sea turtles, the techniques described in this paper might be used routinely to identify gener in the young of these endangered animals, in which male and female are grossly indistinguishable.     

Sex determination and sex reversal

Sex determination in mammals is based on a genetic cascade that controls the fate of the gonads. Gonads will then direct the establishment of phenotypic sex through the production of hormones. Different types of sex reversal are expected to occur if mutations disrupt one of the three steps of gonadal differentiation: formation of the gonadal primordia, sex determination, and testis or ovary development.     

Multiple paternity is related to adult sex ratio and sex determination system in reptiles

The adult sex ratio (ASR, the proportion of males in the adult population) is an emerging predictor of reproductive behaviour, and recent studies in birds and humans suggest it is a major driver of social mating systems and parental care. ASR may also influence genetic mating systems. For instance male-skewed ASRs are expected to increase the frequency of multiple paternity (defined here as a clutch or litter sired by two or more males) due to higher rates of coercive copulations by males, and/or due to females exploiting the opportunity of copulation with multiple males to increase genetic diversity of their offspring. Here, we evaluate this hypothesis in reptiles that often exhibit high frequency of multiple paternity although its ecological and life-history predictors have remained controversial. Using a comprehensive dataset of 81 species representing all four non-avian reptile orders, we show that increased frequency of multiple paternity is predicted by more male-skewed ASR, and this relationship is robust to simultaneous effects of several life-history predictors. Additionally, we show that the frequency of multiple paternity varies with the sex determination system: species with female heterogamety (ZZ/ZW sex chromosomes) exhibit higher levels of multiple paternity than species with male heterogamety (XY/XX) or temperature-dependent sex determination. Thus, our across-species comparative study provides the first evidence that genetic mating system depends on ASR in reptiles. We call for further investigations to uncover the complex evolutionary associations between mating systems, sex determination systems and ASR.     

Sex determination: primitive Y chromosomes in fish

Recent analyses of the chromosomal regions that determine male development in sticklebacks and medaka have revealed several features associated with incipient Y chromosome evolution, including suppressed crossing over and the accumulation of repetitive DNA.     

蜜蜂性别决定与性比调控机理研究

叙述了 4个主要蜜蜂性别决定机理的假说 :即性位点假说、基因平衡假说、蜜蜂性别决定综合假说和性基因数量决定假说。然后就蜜蜂性比由蜂王操纵 ,或是由工蜂操纵进行了论述 ,并对蜜蜂性比调控机理研究提出了一些建议     

Evolution of the avian sex chromosomes and their role in sex determination

Is it the female-specific W chromosome of birds that causes the avian embryo to develop a female phenotype, analogous to the dominance mode of genic sex differentiation seen in mammals? Or is it the number of Z chromosomes that triggers male development, similar to the balance mode of differentiation seen in Drosophila and Caenorhabditis elegans? Although definite answers to these questions cannot be given yet, some recent data have provided support for the latter hypothesis. Moreover, despite the potentially common features of sex determination in mammals and birds, comparative mapping shows that the avian sex chromosomes have a different autosomal origin than the mammalian X and Y chromosomes.     

Exploring the evolution of environmental sex determination, especially in reptiles

Environmental sex determination has been documented in a variety of organisms for many decades and the adaptive significance of this unusual sex-determining mechanism has been clarified empirically in most cases. In contrast, temperature-dependent sex determination (TSD) in amniote vertebrates, first noted 40 years ago in a lizard, has defied a general satisfactory evolutionary explanation despite considerable research effort. After briefly reviewing relevant theory and prior empirical work, we draw attention to recent comparative analyses that illuminate the evolutionary history of TSD in amniote vertebrates and point to clear avenues for future research on this challenging topic. To that end, we then highlight the latest empirical findings in lizards and turtles, as well as promising experimental results from a model organism, that portend an exciting future of progress in finally elucidating the evolutionary cause(s) and significance of TSD.     

Analyzing the coordinated gene network underlying temperature-dependent sex determination in reptiles

Although gonadogenesis has been extensively studied in vertebrates with genetic sex determination, investigations at the molecular level in nontraditional model organisms with temperature-dependent sex determination are relatively new areas of research. Results show that while the key players of the molecular network underlying gonad development appear to be retained, their functions range from conserved to novel roles. In this review, we summarize experiments investigating candidate molecular players underlying temperature-dependent sex determination. We discuss some of the problems encountered unraveling this network, pose potential solutions, and suggest rewarding future directions of research.