Chromosomal Sex Determination Research Articles

Karyotypic analysis and FISH mapping of microsatellite motifs reveal highly differentiated XX/XY sex chromosomes in the pink-tailed worm-lizard (Aprasia parapulchella, Pygopodidae, Squamata)

BackgroundThe infraorder Gekkota is intriguing because it contains multiple chromosomal and environmental sex determination systems that vary even among closely related taxa. Here, we compare male and females karyotypes of the pink-tailed worm-lizard (Aprasia parapulchella), a small legless lizard belonging to the endemic Australian family Pygopodidae.ResultsWe applied comparative genomic hybridization to reveal an XX/XY sex chromosome system in which the Y chromosome is highly differentiated from the X in both gross morphology and DNA sequence. In addition, FISH mapping has revealed that two microsatellite repeat motifs, (AGAT)n and (AC)n, have been amplified multiple times on the Y chromosome.ConclusionXY karyotypes are found in other pygopodids (Delma inornata and Lialis burtonis), suggesting that the common ancestor of Pygopodidae also had XY sex chromosomes. However, the morphology and size of the Y chromosomes are different among the three species, suggesting that the processes underlying the evolution of sex chromosomes in the Pygopodidae involved chromosome rearrangements and accumulation and amplification of repeats.

Sex-determining chromosomes and sexual dimorphism: insights from genetic mapping of sex expression in a natural hybrid Fragaria × ananassa subsp. cuneifolia

We studied the natural hybrid (Fragaria × ananassa subsp. cuneifolia) between two sexually dimorphic octoploid strawberry species (Fragaria virginiana and Fragaria chiloensis) to gain insight into the dynamics of sex chromosomes and the genesis of sexual dimorphism. Male sterility is dominant in both the parental species and thus will be inherited maternally, but the chromosome that houses the sex-determining region differs. Thus, we asked whether (1) the cytotypic composition of hybrid populations represents one or both maternal species, (2) the sex-determining chromosome of the hybrid reflects the location of male sterility within the maternal donor species and (3) crosses from the hybrid species show less sexual dimorphism than the parental species. We found that F. × ananassa subsp. cuneifolia populations consisted of both parental cytotypes but one predominated within each population. Genetic linkage mapping of two crosses showed dominance of male sterility similar to the parental species, however, the map location of male sterility reflected the maternal donor in one cross, but not the other. Moreover, female function mapped to a single region in the first cross, but to two regions in the second cross. Aside from components of female function (fruit set and seed set), other traits that have been found to be significantly sexually dimorphic in the pure species were either not dimorphic or were dimorphic in the opposite direction to the parental species. These results suggest that hybrids experience some disruption of dimorphism in secondary sexual traits, as well as novel location and number of quantitative trait locus (QTL) affecting sex function.

Genes promoting and disturbing testis development.

Mammals have an XX/XY sex chromosomal sex determination system in which males represent the heterogametic sex. The Y-linked gene, SRY, determines sex by inducing the undifferentiated, bipotential gonads to differentiate as testes, which produce androgens and promote in this way the development of a male phenotype. Thus, in mammals, sex determination can be equated to testis determination, which involves several important cell processes, including Sertoli cell differentiation, mesonephric cell migration, testis cord formation, testis-specific vascularization, and myoid and Leydig cell differentiation. Many genes are currently known to be involved in testis development. Some of them, including SF1, WT1, GATA4 and FOG2, are necessary for the formation of the bipotential, undifferentiated gonad but also have important roles in testis differentiation. Others can be considered testis-promoting, differentaition and/or maintenance genes: these include SRY, SOX9, FGF9, PTGDS, SOX8, SOX3, NR0B1, PDGFRa, DMRT1, AMH, NGF, NTF3 and NGFR as the most important examples. Finally, there is a smaller group of genes which are involved in ovarian development and which can cause aberrant testis development if mutated, including RSPO1, WNT4, CTNNB1, FST, BMP2 and FOXL2. In this paper, we review our current knowledge on the function, spatio-temporal expression pattern and mutant sexual phenotypes associated with these genes, and discuss the various roles they play in gonad development.

Characterization and Genome Organization of a Repetitive Element Associated with the Nucleolus Organizer Region in Leporinuselongatus (Anostomidae: Characiformes)

Chromosome mapping and studies of the genomic organization of repetitive DNA sequences provide valuable insights that enhance our evolutionary and structural understanding of these sequences, as well as identifying chromosomal rearrangements and sex determination. This study investigated the occurrence and organization of repetitive DNA sequences in Leporinus elongatus using restriction enzyme digestion and the mapping of sequences by chromosomal fluorescence in situ hybridization (FISH). A 378-bp fragment with a 54.2% GC content was isolated after digestion with the SmaI restriction enzyme. BLASTN search found no similarity with previously described sequences, so this repetitive sequence was named LeSmaI. FISH experiments were conducted using L. elongatus and other Anostomidae species, i.e. L. macrocephalus,L. obtusidens, L. striatus, L. lacustris, L. friderici, Schizodon borellii, S. isognathus, and Abramites hypselonotus which detected signals that were unique to male and female L. elongatus individuals. Double-FISH using LeSmaI and 18S rDNA showed that LeSmaI was located in a nucleolus organizer region (NOR) in the male and female metaphases of L. elongatus. This report also discusses the role of repetitive DNA associated with NORs in the diversification of Anostomidae species karyotypes.

Disorders of sexual development and associated changes in the pituitary-gonadal axis in dogs

Normal sexual differentiation depends on completion of chromosomal sex determination, gonadal differentiation, and development of the phenotypic sex. An irregularity in any of these three steps can lead to a disorder in sexual development (DSD). We examined nine dogs with DSD by abdominal ultrasonography, laparotomy, histologic examination of the gonads, and reproductive tract, cytogenetic analysis, and mRNA expression of the SRY gene. We also determined the plasma concentrations of luteinizing hormone (LH), estradiol-17β, and testosterone before and after administration of gonadotropin-releasing hormone (GnRH) and compared these results with those obtained in anestrous bitches and male control dogs. The gonads of three dogs with DSD contained both testicular and ovarian tissue, while in the other six only testicular tissue was found. Each of the dogs had a uterus. Based on gynecologic examination, cytogenetic analysis, and the histology of the gonads, seven of the nine dogs appeared to be XX sex reversals. Three of these were XX true hermaphrodites and four were XX males; the other two dogs had incomplete XY gonadal dysgenesis. All seven XX sex-reversed dogs were found to be negative for the SRY gene by polymerase chain reaction. The basal plasma luteinizing hormone (LH) concentration was significantly higher in dogs with DSD than in anestrous bitches but not significantly different from that in male dogs. The basal plasma LH concentration increased significantly after GnRH administration in all dogs with DSD. The basal plasma estradiol concentration was significantly higher in dogs with DSD than in anestrous bitches but not significantly different from that in male dogs. The basal plasma testosterone concentration was lower in dogs with DSD than in male dogs. In all dogs with DSD both the basal and GnRH-induced plasma testosterone concentrations were above the upper limit of their respective ranges in the anestrous bitches. In conclusion, the secretion of LH and estradiol in these dogs with DSD, all of which had testicular tissue in their gonads, was similar to that in male control dogs. These results indicate that the basal and/or GnRH-stimulated plasma testosterone concentration might be used to detect the presence of testicular tissue in dogs with DSD.

Should Y stay or should Y go: The evolution of non‐recombining sex chromosomes

Gradual degradation seems inevitable for non-recombining sex chromosomes. This has been supported by the observation of degenerated non-recombining sex chromosomes in a variety of species. The human Y chromosome has also degenerated significantly during its evolution, and theories have been advanced that the Y chromosome could disappear within the next ~5 million years, if the degeneration rate it has experienced continues. However, recent studies suggest that this is unlikely. Conservative evolutionary forces such as strong purifying selection and intrachromosomal repair through gene conversion balance the degeneration tendency of the Y chromosome and maintain its integrity after an initial period of faster degeneration. We discuss the evidence both for and against the extinction of the Y chromosome. We also discuss potential insights gained on the evolution of sex-determining chromosomes by studying simpler sex-determining chromosomal regions of unicellular and multicellular microorganisms.

Sperm counts and sperm sex ratio in male infertility patients

In recent years, investigators have noted a trend toward a declining proportion of male births in many industrialized nations. While men bear the sex-determining chromosome, the role of the female partner as it pertains to fertilization or miscarriage may also alter the gender ratio. We attempted to determine a man's secondary sex ratio (F1 generation) by directly examining the sex chromosomes of his sperm. We examined our male infertility clinic database for all men who had undergone a semen fluorescence in situ hybridization (FISH). Patient demographic and semen parameters were recorded. Chi-squared analysis was used to compare gender ratios (Y chromosomes/total chromosomes). Multivariable logistic regression was used to predict the odds of possessing a Y-bearing sperm after accounting for demographic and semen parameters. A total of 185 men underwent sperm FISH. For the entire cohort, the proportion of Y chromosome-bearing sperm was 51.5%. Men with less than five million motile sperm had a significantly lower proportion of Y chromosome-bearing sperm (50.8%) compared to men with higher sperm counts (51.6%; P=0.02). After multivariable adjustment, a higher sperm concentration, total motile sperm count and semen volume significantly increased the odds of having a Y chromosome-bearing sperm (P<0.01). As a man's sperm production declines, so does the proportion of Y chromosome-bearing sperm. Thus, a man's reproductive potential may predict his ability to sire male offspring.

Large-Scale Introgression Shapes the Evolution of the Mating-Type Chromosomes of the Filamentous Ascomycete Neurospora tetrasperma

The significance of introgression as an evolutionary force shaping natural populations is well established, especially in animal and plant systems. However, the abundance and size of introgression tracts, and to what degree interspecific gene flow is the result of adaptive processes, are largely unknown. In this study, we present medium coverage genomic data from species of the filamentous ascomycete Neurospora, and we use comparative genomics to investigate the introgression landscape at the genomic level in this model genus. We revealed one large introgression tract in each of the three investigated phylogenetic lineages of Neurospora tetrasperma (sizes of 5.6 Mbp, 5.2 Mbp, and 4.1 Mbp, respectively). The tract is located on the chromosome containing the locus conferring sexual identity, the mating-type (mat) chromosome. The region of introgression is confined to the region of suppressed recombination and is found on one of the two mat chromosomes (mat a). We used Bayesian concordance analyses to exclude incomplete lineage sorting as the cause for the observed pattern, and multilocus genealogies from additional species of Neurospora show that the introgression likely originates from two closely related, freely recombining, heterothallic species (N. hispaniola and N. crassa/N. perkinsii). Finally, we investigated patterns of molecular evolution of the mat chromosome in Neurospora, and we show that introgression is correlated with reduced level of molecular degeneration, consistent with a shorter time of recombination suppression. The chromosome specific (mat) and allele specific (mat a) introgression reported herein comprise the largest introgression tracts reported to date from natural populations. Furthermore, our data contradicts theoretical predictions that introgression should be less likely on sex-determining chromosomes. Taken together, the data presented herein advance our general understanding of introgression as a force shaping eukaryotic genomes.

Triploid plover female provides support for a role of the W chromosome in avian sex determination

Two models, Z Dosage and Dominant W, have been proposed to explain sex determination in birds, in which males are characterized by the presence of two Z chromosomes, and females are hemizygous with a Z and a W chromosome. According to the Z Dosage model, high dosage of a Z-linked gene triggers male development, whereas the Dominant W model postulates that a still unknown W-linked gene triggers female development. Using 33 polymorphic microsatellite markers, we describe a female triploid Kentish plover Charadrius alexandrinus identified by characteristic triallelic genotypes at 14 autosomal markers that produced viable diploid offspring. Chromatogram analysis showed that the sex chromosome composition of this female was ZZW. Together with two previously described ZZW female birds, our results suggest a prominent role for a female determining gene on the W chromosome. These results imply that avian sex determination is more dynamic and complex than currently envisioned.

Polygenic sex determination system in zebrafish.

BackgroundDespite the popularity of zebrafish as a research model, its sex determination (SD) mechanism is still unknown. Most cytogenetic studies failed to find dimorphic sex chromosomes and no primary sex determining switch has been identified even though the assembly of zebrafish genome sequence is near to completion and a high resolution genetic map is available. Recent publications suggest that environmental factors within the natural range have minimal impact on sex ratios of zebrafish populations. The primary aim of this study is to find out more about how sex is determined in zebrafish.Methodology/Principal FindingsUsing classical breeding experiments, we found that sex ratios across families were wide ranging (4.8% to 97.3% males). On the other hand, repeated single pair crossings produced broods of very similar sex ratios, indicating that parental genotypes have a role in the sex ratio of the offspring. Variation among family sex ratios was reduced after selection for breeding pairs with predominantly male or female offspring, another indication that zebrafish sex is regulated genetically. Further examinations by a PCR-based “blind assay" and array comparative genomic hybridization both failed to find universal sex-linked differences between the male and female genomes. Together with the ability to increase the sex bias of lines by selective breeding, these data suggest that zebrafish is unlikely to utilize a chromosomal sex determination (CSD) system.Conclusions/SignificanceTaken together, our study suggests that zebrafish sex is genetically determined with limited, secondary influences from the environment. As we have not found any sign for CSD in the species, we propose that the zebrafish has a polygenic sex determination system.

Food availability and offspring sex in a monogamous seabird: insights from an experimental approach

Sex allocation theory predicts that parents should favor offspring of the sex that provides the greatest fitness return. Despite growing evidence suggesting that vertebrates are able to overcome the constraint of chromosomal sex determination, the general pattern remains equivocal, indicating a need for experimental investigations. We used an experimental feeding design to study sex allocation during 3 years in black-legged kittiwakes (Rissa tridactyla). Intense male–male competition for securing a breeding site is common in this species in which males are heavier and larger than females. Hence, we hypothesized that parents producing fledglings in better than average condition, as supplementarily fed pairs do, would increase their fitness return by producing sons. Conversely, producing daughters would be a better tactic for Unfed parents. Hence, we predicted that Fed parents produce more sons than Unfed parents. This prediction is particularly expected if sexual dimorphism arises as early as during chick rearing, suggesting strong selective pressures for optimal male development. Our results showed that 1) males were heavier and larger than females prior to fledging and that 2) Fed parents produced relatively more male hatchlings than Unfed parents. We interpret this result in terms of a Trivers–Willard-type process. Furthermore, our data revealed that Unfed parents significantly overproduced female hatchlings, whereas offspring sex ratio was balanced among Fed parents. Because the 3 reproductive seasons we considered were particularly poor food years, Unfed parents may have overproduced daughters to avoid the apparent higher reproductive costs of raising sons.

Systematics and evolution of the African pygmy mice, subgenus Nannomys: A review

African pygmy mice (subgenus Nannomys) are a group of small murine rodents that are widespread throughout Sub-Saharan Africa. Although this group has long been recognized for its extensive chromosomal diversity, the highly conserved morphology of its members has made taxonomic assignments problematic. Renewed interest resulting from a series of cytogenetic and molecular investigations has led to the identification of novel patterns of diversification in these rodents that are reviewed herein. These approaches have considerably improved species delimitation and provide tentative diagnostic criteria as well as preliminary phylogenetic relationships that will be refined as more taxa are investigated. Although sparse, ecological data suggest that pygmy mice may exhibit original reproductive traits that deserve further investigation. Chromosomal diversity undoubtedly remains one of the most interesting features of African pygmy mouse biology. They display several karyotypic traits that are rare in mammals: (i) their genomes tolerate the recurrent formation of tandem fusions and sex-autosome translocations, both of which are rare in other mammals due to their highly deleterious effects on fertility; (ii) they exhibit the first case of a whole arm exchange involving an X chromosome, and (iii) two species show novel means of sex chromosome determination – one exhibits XY females, whereas the other harbors populations in which males have no Y chromosome. The diversity of African pygmy mice offers a unique opportunity to study the processes involved in their radiation, and in a broader context, the evolution of sex chromosome determination in mammals.

First record of supernumerary (B) chromosomes in electric fish (Gymnotiformes) and the karyotype structure of three species of the same order from the upper Paraná River basin

Cytogenetic studies were performed on the species Apteronotus prope albifrons Linnaeus, 1766, Rhamphichthys hahni Meinken, 1937 and Brachyhypopomus gauderio Giora & Malabarba, 2009, collected in the upper Paraná River floodplain, Porto Rico (PR), Brazil. Apteronotus prope albifrons showed a diploid number of 2n=24 chromosomes for both sexes and a karyotype formula of 14m+2sm+2st+6a (FN=42). Besides the standard karyotype, three specimens had one to three extra microchromosomes with inter- and intra-individual variations, which suggested the occurrence of B chromosomes in the species. The chromosomal data of Rhamphichthys hahni, described here for the first time, consists of 50 chromosomes and a formula comprised of 20m+24sm+6a (FN=94). Brachyhypopomus gauderio specimens demonstrated 2n=42 chromosomes infemales, all acrocentric, and 2n=41 chromosomes in males, with 40 acrocentric and 1 medium-sized metacentric chromosome. These differences concern with a multiple system of sexchromosome determination X1X1X2X2/X1X2Y (FN=42) in Brachyhypopomus gauderio. The analysis of nucleolar organizer regions by Ag-NOR and FISH 18S banding revealed a simple NOR system in Apteronotus prope albifrons and Rhamphichthys hahni and a multiple NOR system in Brachyhypopomus gauderio, that is unusual for Gymnotiformes fishes. Constitutive heterochromatin was mainly found in the pericentromere region in most of the chromosomes of the three species, although each species had its own peculiarities. The B chromosomes in Apteronotus prope albifrons demonstrated heterochromatin positioned in the centromeric and telomeric regions whereas Rhamphichthys hahni presented conspicuous blocks of heterochromatin on the long arms in three submetacentric pairs. Brachyhypopomus gauderio showed blocks of heterochromatin on the long arm in the interstitial and telomere positions. The finding of B chromosomes in Apteronotus prope albifrons represents the first description of these elements in the Gymnotiformes order. Although the karyotype of this species is similar with that described for populations in the Amazon basin, the presence of B chromosomes could represent a specific characteristic of this population. A comparative analysis of karyotypes of Rhamphichthys hahni with other species of the genus showed a relatively conservative structure suggesting 2n=50 as a common number in this group. The karyotype of Brachyhypopomus gauderio, a new species, provides an important reference for future chromosome studies of the Brachyhypopmus Mago-Lecia, 1994, and it might be also significant for cytotaxonomy in this group. The cytogenetic data also demonstrate the need of more comparative cytogenetic studies in the families of the highly diversified and taxonomically difficult complex Gymnotiformes.

Does the speciation clock tick more slowly in the absence of heteromorphic sex chromosomes?

Squamates may be an attractive group in which to study the influence of sex chromosomes on speciation rates because of the repeated evolution of heterogamety (both XY and ZW), as well as an apparently large number of taxa with environmental sex-determination.

Meiotic recombination, synapsis, meiotic inactivation and sperm aneuploidy in a chromosome 1 inversion carrier

Disrupted meiotic behaviour of inversion carriers may be responsible for suboptimal sperm parameters in these carriers. This study investigated meiotic recombination, synapsis, transcriptional silencing and chromosome segregation effects in a pericentric inv(1) carrier. Recombination (MLH1), synapsis (SYCP1, SYCP3) and transcriptional inactivation (γH2AX, BRCA1) were examined by fluorescence immunostaining. Chromosome specific rates of recombination were determined by fluorescence in-situ hybridization. Furthermore, testicular sperm was examined for aneuploidy and segregation of the inv(1). Our findings showed that global recombination rates were similar to controls. Recombination on the inv(1) and the sex chromosomes were reduced. The inv(1) associated with the XY body in 43.4% of cells, in which XY recombination was disproportionately absent, and 94.3% of cells displayed asynapsed regions which displayed meiotic silencing regardless of their association with the XY body. Furthermore, a low frequency of chromosomal imbalance was observed in spermatozoa (3.4%). Our results suggest that certain inversion carriers may display unimpaired global recombination and impaired recombination on the involved and the sex chromosomes during meiosis. Asynapsis or inversion-loop formation in the inverted region may be responsible for impaired spermatogenesis and may prevent sperm-chromosome imbalance. In infertile men who are carriers of chromosome abnormalities, adequate sperm production can be impaired. Meiosis, a cell division process in which spermatozoa are produced, can be impaired by the interaction of the abnormal chromosome with its homologous pair or with other chromosomes. In meiosis, a process called ‘recombination’ occurs, where homologous chromosomes pair up and exchange genetic material. Recombination is important in ensuring equal distribution of chromosomes during meiosis. When these processes are disrupted, sperm production can be impaired. In this study, we investigated meiosis in an infertile man carrying an inversion on chromosome 1, a particular type of chromosome abnormality. We used fluorescence microscopy techniques to examine recombination, the fidelity of chromosome pairing (synapsis) and chromosome distribution in the meiotic cells obtained from testicular tissue. Spermatozoa was also studied for proper chromosome distribution. Meiotic cells and spermatozoa from five fertile men that did not have any chromosome abnormalities were used as a reference group. Our results showed that recombination involving the affected chromosome, and between the sex-determining chromosomes were reduced compared with the reference. The abnormal chromosome associated with the sex chromosomes in 43.4% of the meiotic cells, in which recombination was noticeably absent between the sex chromosomes. Despite these findings, the frequency of chromosomally abnormal spermatozoa was not greater than the reference. Our findings suggest that certain chromosome inversions may have impaired recombination on the involved and the sex chromosomes. However, this impairment may be a protective factor against the production of chromosomally abnormal spermatozoa.

Comparative analysis of the location of rDNA in the Palaearctic bushcricket genus Isophya (Orthoptera: Tettigoniidae: Phaneropterinae)

The present study focused on the evolution of the karyotype in 21 taxa of the genus Isophya, which was done by mapping the location on the chromosomes of ribosomal RNA (rRNA) coding genes using fluorescence in situ hybridization (FISH) with an 18S rDNA probe and using silver staining (AgNO3) to evaluate the activity of major rDNA clusters. Since the chromosome number and sex determination do not vary in this genus, the above markers were used in a detailed comparison of the cytogenetic features of species of Isophya. The species analyzed were placed into three groups based on the location of rDNA on their chromosomes: (1) rDNA-FISH signals only on the two long pairs of autosomes, (2) rDNA-FISH signals on one long and one short pair of autosomes, and (3) rDNA-FISH signals on three to five different sized pairs of autosomes. These groupings partly correspond to the morphological groupings proposed in earlier studies. One long pair of autosomes frequently carried rDNA in all the Isophya species and probably is a plesiomorphic character for these taxa. The cytogenetic mapping revealed great variability among Isophya species in the chromosomal location of major rDNA clusters. Our results suggest that the observed variation in the number of rDNA clusters can be an important species-group specific phylogenetic marker. Analysis of 18S rDNA hybridization signals showed that the evolutionary dynamics of rDNA in this genus is remarkably high and accompanied by changes in the structure of chromosomes bearing rDNA at an inter- and intra-specific level. The telomeric sequence (TTAGG)n hybridized with the termini of most of chromosomes, however, some chromosome ends lacked signals probably due to a low copy number of telomeric repeats.

The charms of sex chromosomes in snakes

determined and the 1:1 sex ratio is maintained. In many species of reptiles, environmental factors such as temperature play an important role in determining the sex ratio (Head et al. 1987). For example, in alligators, crocodiles and turtles, during the early stages of embryogenesis, temperature has a determining effect on the sex ratio. In amphibians, the sex ratio is usually determined genetically. Thus, the existence of specialized sex-determining chromosomes poses a range of fascinating and fundamental questions, although few of them have been satisfactorily answered. Why, for example, do some species show morphological differentiation of sex chromosomes but not others? If the sex ratio can be maintained by a set of genes, as in the case of amphibians, then what is the need for specializing one entire chromosome exclusively for this singular sex determination function and maintenance of sex ratio at such an enormous cost that most genes, barring a few specifically involved in sex determination, have been lost from the sex-determining chromosome? And to compensate for this, why did yet another mechanism of dosage compensation have to be invented? What is the mechanism involved? Can we answer these questions?

Disentangling the effect of genes, the environment and chance on sex ratio variation in a wild bird population

Sex ratio theory proposes that the equal sex ratio typically observed in birds and mammals is the result of natural selection. However, in species with chromosomal sex determination, the same 1 : 1 sex ratio is expected under random Mendelian segregation. Here, we present an analysis of 14 years of sex ratio data for a population of song sparrows (Melospiza melodia) on Mandarte Island, at the nestling stage and at independence from parental care. We test for the presence of variance in sex ratio over and above the binomial variance expected under Mendelian segregation, and thereby quantify the potential for selection to shape sex ratio. Furthermore, if sex ratio variation is to be shaped by selection, we expect some of this extra-binomial variation to have a genetic basis. Despite ample statistical power, we find no evidence for the existence of either genetic or environmentally induced variation in sex ratio, in the nest or at independence. Instead, the sex ratio variation observed matches that expected under random Mendelian segregation. Using one of the best datasets of its kind, we conclude that female song sparrows do not, and perhaps cannot, adjust the sex of their offspring. We discuss the implications of this finding and make suggestions for future research.

Comparative Genetic Mapping Points to Different Sex Chromosomes in Sibling Species of Wild Strawberry (Fragaria)

Separate sexes have evolved repeatedly from hermaphroditic ancestors in flowering plants, and thus select taxa can provide unparalleled insight into the evolutionary dynamics of sex chromosomes that are thought to be shared by plants and animals alike. Here we ask whether two octoploid sibling species of wild strawberry--one almost exclusively dioecious (males and females), Fragaria chiloensis, and one subdioecious (males, females, and hermaphrodites), F. virginiana--share the same sex-determining chromosome. We created a genetic map of the sex chromosome and its homeologs in F. chiloensis and assessed macrosynteny between it and published maps of the proto-sex chromosome of F. virginiana and the homeologous autosome of hermaphroditic diploid species. Segregation of male and female function in our F. chiloensis mapping population confirmed that linkage and dominance relations are similar to those in F. virginiana. However, identification of the molecular markers most tightly linked to the sex-determining locus in the two octoploid species shows that, in both, this region maps to homeologues of chromosome 6 in diploid congeners, but is located at opposite ends of their respective chromosomes.

Inheritable Model of Male Pseudohermaphroditism in Canine Somatic Cell Nuclear Transfer.

Normal mammalian sex differentiation depends upon three genetically controlled steps: chromosomal sex determination, gonadal differentiation, and phenotypical sex development. However, two case studies have reported anomalies in the sexual differentiation of mouse and wolf that have occurred during SCNT procedure for unknown reasons. This evokes questions about the early event of reprogramming in SCNT when the chromosomal sex is clearly determined in genetic donor cells, which can never be overcome by technical inflexibilities of sex.During our canine somatic cell nuclear transfer (SCNT) using skin fibroblasts from a male adult as a genetic donor, we report here the birth of one sex reversed cloned dog out of 6 healthy live offspring. Chromosomal, genetic, and histological analyses revealed that the sex-reversed clone has a 78 XY, SRY positive with female external genitalia, which includes an oviduct and a uterus. Interestingly enough, the bilateral testis was in the shape of a hypotrophic ovary and was positioned in the location of normal ovaries. A complete bicornuate uterus was also present. Histological analyses showed that the testis consists of a gonocyte free semiferous tubule-like structure that was stained positive with vimentin, and inhibin alpha, the sertoli cell markers.To study further whether it can be reversible or not, we recloned him, and the result was that the sex reversal condition remained. Thus we believe that we have established the cell line for canine model of inheritable male pseudohermaphroditism and its sex differentiation mechanism. However, we have not observed any opposite sex reversal cases in female genotypes. Further molecular analyses on SRY, SOX-9, MISIIR, AR showed no mutations in their full or partial sequences, which were associated with disorders of sex development in previous reports. In summary, this report describes the first SRY positive XY sex reversal with an almost complete female phenotype including clinical signs; ultrasonographic findings in the abdomen; plasma concentrations of testosterone, oestradiol and progesterone before after administration of GnRH; cytogenetic analysis; PCR analysis for presence of the SRY gene and histology of the genital tract. The results will be discussed with regard to possible causes of the sex reversal. (poster)