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PubMed2003

The evolutionary demography of duplicate genes.

Although gene duplication has generally been viewed as a necessary source of material for the origin of evolutionary novelties, the rates of origin, loss, and preservation of gene duplicates are not well understood. Applying steady-state demographic techniques to the age distributions of duplicate genes censused in seven completely sequenced genomes, we estimate the average rate of duplication of a eukaryotic gene to be on the order of 0.01/ gene/million years, which is of the same order of magnitude as the mutation rate per nucleotide site. However, the average half-life of duplicate genes is relatively small, on the order of 4.0 million years. Significant interspecific variation in these rates appears to be responsible for differences in species-specific genome sizes that arise as a consequence of a quasi-equilibrium birth-death process. Most duplicated genes experience a brief period of relaxed selection early in their history and a minority exhibit the signature of directional selection, but those that survive more than a few million years eventually experience strong purifying selection. Thus, although most theoretical work on the gene-duplication process has focused on issues related to adaptive evolution, the origin of a new function appears to be a very rare fate for a duplicate gene. A more significant role of the duplication process may be the generation of microchromosomal rearrangements through reciprocal silencing of alternative copies, which can lead to the passive origin of post-zygotic reproductive barriers in descendant lineages of incipient species.

Animals↗
PubMed2006

Cryptic interspecific introgression and genetic differentiation within Gossypium aridum (Malvaceae) and its relatives.

Interspecific gene flow is increasingly recognized as an important evolutionary phenomenon in plants. A surprising observation is that historical introgression is often inferred between species that presently have geographic and reproductive barriers that would appear to prohibit the inferred sexual exchange. A striking example concerns Gossypium aridum (subsection Erioxylum); previous analyses have shown that populations from Colima (southwestern Mexico) have a chloroplast genome (cpDNA) similar to that of a different taxonomic subsection (Integrifolia) that presently is confined to Baja California and the Galapagos Islands, whereas other G. aridum populations share a cpDNA lineage with each other and with other species in subsection Erioxylum. To evaluate further the possibility that this cpDNA evidence reflects introgression as opposed to some other evolutionary process, as well as to explore patterns of genetic diversity and similarity in both subsections, we conducted amplified fragment-length polymorphism (AFLP) analysis using 50 populations representing all seven species in the two subsections. Genetic diversity is high in G. aridum, and is strongly correlated with geography, as are similarities among the five species in subsection Erioxylum. This subsection is genetically distant from the two species in subsection Integrifolia, whose populations are highly similar inter se. Populations of G. aridum from Colima are genetically distinct from the remainder of the species, and exhibit a comparatively high frequency of AFLP fragments that otherwise are diagnostic of the Integrifolia lineage. These data implicate intersubsectional introgression between presently allopatric and genetically isolated clades, giving rise to a morphologically cryptic, introgressant entity. Biogeographic considerations suggest that this history was initiated following migration of one or more seeds from Baja California to the Colima coast, perhaps during the Pleistocene. We suggest that cryptic and seemingly improbable interspecific introgression and molecular differentiation may be more common than appreciated in angiosperm evolution.

Analysis of Variance↗
PubMed2006

[Molecular variability in the commom shrew Sorex araneus L. from European Russia and Siberia inferred from the length polymorphism of DNA regions flanked by short interspersed elements (Inter-SINE PCR) and the relationships between the Moscow and Seliger chromosome races].

Genetic exchange among chromosomal races of the common shrew Sorex araneus and the problem of reproductive barriers have been extensively studied by means of such molecular markers as mtDNA, microsatellites, and allozymes. In the present study, the interpopulation and interracial polymorphism in the common shrew was derived, using fingerprints generated by amplified DNA regions flanked by short interspersed repeats (SINEs)-interSINE PCR (IS-PCR). We used primers, complementary to consensus sequences of two short retroposons: mammalian element MIR and the SOR element from the genome of Sorex araneus. Genetic differentiation among eleven populations of the common shrew from eight chromosome races was estimated. The NP and MJ analyses, as well as multidimensional scaling showed that all samples examined grouped into two main clusters, corresponding to European Russia and Siberia. The bootstrap support of the European Russia cluster in the NJ and MP analyses was respectively 76 and 61%. The bootstrap index for the Siberian cluster was 100% in both analyses; the Tomsk race, included into this cluster, was separated with the bootstrap support of NJ/MP 92/95%.

Animals↗
PubMed2006

Restricted gene flow at specific parts of the shrew genome in chromosomal hybrid zones.

The species and races of the shrews of the Sorex araneus group exhibit a broad range of chromosomal polymorphisms. European taxa of this group are parapatric and form contact or hybrid zones that span an extraordinary variety of situations, ranging from absolute genetic isolation to almost free gene flow. This variety seems to depend for a large part on the chromosome composition of populations, which are primarily differentiated by various Robertsonian fusions of a subset of acrocentric chromosomes. Previous studies suggested that chromosomal rearrangements play a causative role in the speciation process. In such models, gene flow should be more restricted for markers on chromosomes involved in rearrangements than on chromosomes common in both parent species. In the present study, we address the possibility of such differential gene flow in the context of two genetically very similar but karyotypically different hybrid zones between species of the S. araneus group using microsatellite loci mapped to the chromosome arm level. Interspecific genetic structure across rearranged chromosomes was in general larger than across common chromosomes. However, the difference between the two classes of chromosomes was only significant in the hybrid zone where the complexity of hybrids is expected to be larger. These differences did not distinguish populations within species. Therefore, the rearranged chromosomes appear to affect the reproductive barrier between karyotypic species, although the strength of this effect depends on the complexity of the hybrids produced.

Animals↗
PubMed1989

Microhabitats of monogenean gill parasites on European eel (Anguilla anguilla).

The distribution of adult Pseudodactylogyrus anquillae and P. bini and postlarvae was recorded on glass-eels and pigmented eels of different size and infection intensity. The two species occupied different microhabitats on the host's gill apparatus but the preferred sites depended partly on host size. Postlarval migration was indicated. Mortality of young parasites, reinforcement of reproductive barriers and competition seems to be less likely explanations of the spatial distribution.

Anguilla↗
PubMed1994

[Studies of the miotic chromosomes, meiosis and spermatogenesis of a macaque hybrid].

The mitotic and meiotic chromosomes, as well as the spermatogenesis of a male hybrid between Macaca fascicularis and M. assamensis have been studied using chromosome banding, microspreading, and tissue sectioning techniques. The band patterns of the parental chromosome complements are found highly homologous. The observation on meiosis and testis histology indicates that the hybrid should be fertile. It is suggested that the reproductive barrier between M. fascicularis and M. assamensis is not complete and may resulted from ecological isolation. The taxonomic status of M. fascicularis and M. assamensis is discussed also.

Animals↗
PubMedDate not supplied

Double heterozygosity of chromosomal inversion in mice: a key to speciation.

Inversion can be very efficient as a reproductive barrier when the same chromosomes are involved in two different rearrangements leading to a double loop during the pachytene stage. The consequence can be a strongly reduced fertility of the double heterozygotes, leading to an irreversible dichotomy.

Animals↗
PubMed1995

Ascidian interspecific fertilization. III. Ultrastructural investigations of sperm-egg interaction.

Transmission electron microscope investigations on ascidian interspecific fertilization have been carried out on some crosses: Ascidia malaca female x Ascidiella aspersa male, Ascidia malaca female x Phallusia mammillata male, Ascidiella aspersa female x Ascidia malaca male. The morphological aspects of heterospecific sperm-egg interaction were compared with those of homospecific fertilization to complete the previous light and scanning electron microscope observations in this field. Sperm behaviour in homo- and heterospecific crosses mainly differs in the power of sperm passageway through the egg envelopes. Foreign spermatozoa, firstly interacting with the follicle cells (FCs), remain on the FC surface, leaving the interfollicular clefts empty. In homospecific fertilized eggs the initial events, also observed in hybrid crosses, are: attraction, entrapment and sperm mitochondrion translocation. But in homospecific crosses the FCs, by means of projections on their surface, facilitate the sperm path between the clefts for reaching the vitelline coat; the clefts are full of spermatozoa. These observations suggest a role of initial reproductive barrier for the FCs.

Animals↗
PubMed2005

Perspective: Reproductive isolation caused by natural selection against immigrants from divergent habitats.

The classification of reproductive isolating barriers laid out by Dobzhansky and Mayr has motivated and structured decades of research on speciation. We argue, however, that this classification is incomplete and that the unique contributions of a major source of reproductive isolation have often been overlooked. Here, we describe reproductive barriers that derive from the reduced survival of immigrants upon reaching foreign habitats that are ecologically divergent from their native habitat. This selection against immigrants reduces encounters and thus mating opportunities between individuals from divergently adapted populations. It also reduces the likelihood that successfully mated immigrant females will survive long enough to produce their hybrid offspring. Thus, natural selection against immigrants results in distinctive elements of premating and postmating reproductive isolation that we hereby dub "immigrant inviability." We quantify the contributions of immigrant inviability to total reproductive isolation by examining study systems where multiple components of reproductive isolation have been measured and demonstrate that these contributions are frequently greater than those of traditionally recognized reproductive barriers. The relevance of immigrant inviability is further illustrated by a consideration of population-genetic theory, a review of selection against immigrant alleles in hybrid zone studies, and an examination of its participation in feedback loops that influence the evolution of additional reproductive barriers. Because some degree of immigrant inviability will commonly exist between populations that exhibit adaptive ecological divergence, we emphasize that these barriers play critical roles in ecological modes of speciation. We hope that the formal recognition of immigrant inviability and our demonstration of its evolutionary importance will stimulate more explicit empirical studies of its contributions to speciation.

Animals↗
PubMed1998

The maternal and child health sites' practices regarding HIV education, counseling, and testing of women of reproductive age in Chicago: barriers to universal implementation.

OBJECTIVES: Women of reproductive age are increasingly at risk for human immunodeficiency virus (HIV) infection. Recent advances in reducing perinatal transmission have resulted in official guidelines on universal HIV education, counseling, and voluntary testing of women of reproductive age, especially pregnant women. This study assesses to what extent the maternal child health (MCH) sites are implementing these guidelines with their female patient population (including pregnant women) and examines the barriers that prevent them from implementing these guidelines. METHOD: The study uses survey data from 92 hospitals and community health centers offering MCH services in Chicago regarding their providers' practices on HIV education, counseling, and testing, implementation of zidovudine (ZDV) therapy to reduce perinatal transmission, and the barriers to implementing these services. In addition, 20 taped in-depth interviews were conducted with experts to examine the barriers to universal implementation. RESULTS: Almost half (45% of perinatal care and 50% of family planning providers) of the institutions are not consistently offering HIV testing. One-third of those institutions that offer testing are not offering pretest counseling. Thirty-nine percent of the perinatal care providers in these institutions are not providing posttest counseling to HIV-negative women. Over one-third (35%) of these institutions reported that they are not set up to implement ZDV therapy during labor and delivery. Almost half (49%) had no protocols for ZDV therapy in place. Barriers to implementation included lack of provider training, limited staff time, physician resistance, unavailability or avoidance to seek perinatal care by high-risk women, cost, absence of a statewide and hospital-specific plan, lack of reproductive choice focus in posttest counseling, lack of provider knowledge about the administration of ZDV or its availability during labor, and lack of consumer education on perinatal risk reduction. CONCLUSIONS: MCH sites and their providers need assistance to overcome many barriers they face to implement universal HIV education, counseling, and testing of women of reproductive age.

AIDS Serodiagnosis↗
PubMed2006

Human implantation: the last barrier in assisted reproduction technologies?

Implantation processes are highly complex involving the actions of numerous hormones, immunoglobulins, cytokines and other factors in the endometrium. They are also essential matters for the success of assisted reproduction. The nature of early embryonic development is of equal significance. It involves ovarian follicle growth, ovulation, fertilization and preimplantation growth. These processes are affected by imbalanced chromosomal constitutions or slow developmental periods. Post-implantation death is also a significant factor in cases of placental insufficiency or recurrent abortion. Clearly, many of these matters can significantly affect birth rates. This review is concerned primarily with the oocyte, the early embryo and its chromosomal anomalies, and the nature of factors involved in implantation. These are clearly among the most important features in determining successful embryonic and fetal growth. Successive sections cover the endocrine stimulation of follicle growth in mice and humans, growth of human embryos in vitro, their apposition and attachment to the uterus, factors involved in embryo attachment to uterine epithelium and later stages of implantation, and understanding the gene control of polarities and other aspects of preimplantation embryo differentiation. New aspects of knowledge include the use of human oocyte maturation in vitro as an approach to simpler forms of IVF, and new concepts in developmental genetics.

Aneuploidy↗
PubMed2002

Barriers to sexual reproduction in Polygonum viviparum: a comparative developmental analysis of P. viviparum and P. bistortoides.

Polygonum viviparum is widely distributed in arctic and alpine regions of the northern hemisphere. Fruit set has never been observed in North American populations and has been reported only very rarely in Europe. Although this species is extremely well studied, the impediments to successful fruit production are unknown. We investigated the sexual reproductive process in P. viviparum growing in the southern Colorado Rocky Mountains. For comparison, we also examined this process in the sympatric congener P. bistortoides, in which reproduction is exclusively sexual. Lack of viable fruit production in P. viviparum has no single developmental explanation; defects occur in each of the processes and structures associated with sexual reproduction studied, yet, these processes and structures also appear to function normally in at least some flowers or individuals. Development is abnormal in many ovules of P. viviparum, however, comparison with P. bistortoides shows that these abnormalities do not contribute to differences in seed production between the two species. The virtual absence of sexual reproduction in P. viviparum appears to be due largely to a low rate of fertilization and to embryo/fruit abortion.

Arctic Regions↗
PubMed1999

MUC1/episialin: a critical barrier in the female reproductive tract.

The female reproductive tract must resist microbial infections as well as support embryonic development, implantation and placentation. Reproductive tract mucins, in general, and Muc1/episialin, in particular, play key roles in implantation related events and in protection from microbial infection. High levels of mucin expression in the lower reproductive tract presumably affords protection against infection while down-regulation of uterine mucins has been suggested to provide access to the uterine surface. The present studies demonstrate that mucins, particularly Muc1, are effective barriers to embryo attachment. Furthermore, a strain of female Muc1 null mice in normal housing displays chronic infection and inflammation of the lower reproductive tract and markedly reduced fertility rates. This phenotype is not observed when Muc1 nulls are housed in a pathogen-free environment indicating that this phenotype results from chronic microbial exposure. Only normal endogenous flora were isolated from the reproductive tracts of affected Muc1 null mice, suggesting that these bacterial species become opportunistic with loss of the mucin barrier. Staphylococcal adherence to lower reproductive tract epithelia was found to be mediated by cell surface mucin carbohydrates. Collectively, these studies demonstrate a critical barrier role for Muc1 in various aspects of female reproductive tract physiology.

Animals↗
PubMed2003

Blood-tissue barriers in the male reproductive tract of the dog: a morphological study using lanthanum nitrate as an electron-opaque tracer.

Blood-tissue barriers preventing an uncontrolled exchange of larger molecules between adjacent but metabolically separate compartments have been demonstrated in various organs. One prominent example is the blood-testis barrier which has been investigated in a number of species. A key function of this barrier is to shield developing germ cells from the immune system in order to avoid autoimmune reactions. This requirement also applies to the male excurrent duct system. Yet, very few investigations have addressed the morphology of the blood-epididymal barrier. The goal of the present study, therefore, was to revisit the blood-testis barrier in the dog and to identify the structures constituting the blood-epididymal barrier in this species. Lanthanum nitrate was used as a tracer for electron microscopy. In the testis, lanthanum had free access to the intercellular space of the seminiferous epithelium up to the Sertoli cell junctions. Similarly, epithelial tight junctions were found to represent the permeability barrier in the epididymis. The present study highlights species differences with respect to the blood-testis barrier and extends the knowledge of the blood-epididymal barrier by providing morphofunctional data in this domestic species.

Animals↗
PubMed2002

Expression of a blood-brain barrier-specific antigen in the reproductive tract of the male rat.

The endothelial barrier antigen (EBA) is a protein expressed specifically by the endothelial cells of the rat brain barrier vessels. This antigen has been described as a 'barrier protein' and is used as a marker for the competent blood-brain barrier. A blood-testis barrier has also been described. However, unlike the blood-brain barrier, which is formed by endothelial cells, the blood-testis barrier is formed mainly by the Sertoli cells, which provide an isolated environment for spermatogenic cells within the seminiferous tubules. Testicular blood vessels express the erythroid glucose transporter protein and other markers, which are strongly expressed in brain blood vessels, and may contribute to the blood-testis barrier. This study was carried out to determine whether Sertoli cells or testicular blood vessels express EBA. Tissues of other organs were used as controls for EBA expression. EBA was expressed by the endothelial cells in most microvessels of the testis, and in a few vessels of the epididymis, seminal vesicle, prostate gland, vas deferens and bladder-neck region. Furthermore, EBA was strongly and consistently detected in epithelial cells of the rete testis and dorsolateral prostate gland, and in a few epithelial cells of the ventral prostate gland, the seminal vesicle and the coagulating gland. However, Sertoli cells, which are the main site of the blood-testis barrier, were negative for EBA. In conclusion, EBA may have a wider role in rat tissues than has been previously appreciated.

Animals↗
PubMedDate not supplied

[Morphological barrier in mechanisms of reproductive isolation acting in areas of sympatry of closely related species Ixodes persulcatus-I.pavlovskyi and I. persulcatus-I. ricinus (Ixodidae)].

A morphometric analysis of different male and female organs used in the process of copulation, including an attachment of the spermatophore to the genital opening of female and placing the spermatophore into the vestibular part of vagina, was carried out for the pairs of ixodid tick species Ixodes persulcatus--I. pavlovskyi and I. persulcatus--I. ricinus in areas of their sympatry. In two disjunctive areas of sympatry of I. persulcatus--I. pavlovskyi (South-East Altai and Southern Primorye), the morphological barrier is displayed in the precopulation mechanism of reproductive isolation and does act only in the pair I. pavlovskyi female--I. persulcatus male. It is proved by the reliable prevalence of the hypostome diameter (in widest part) of I. persulcatus male over the genital opening length of I. pavlovkyi female. Because of this reason, the copulation between these partners cannot take place. On the other hand, there are no any morphological obstacles for a copulation within each species examined and in the combination I. persulcatus female--I. pavlovskyi male, and in any combination of the partners I. persulcatus--I. ricinus in sympatry areas (samples from two locations in the north-west of the Eastern-European Plane have been examined). Morphological peculiarities causing differences in host preference of tick species, that in turn realizes the isolation mechanisms between species, are discussed. Postcopulation non-genetic components of the reproductive isolation mechanism have been found in examined species.

Animals↗
PubMedDate not supplied

Barriers to sexual and reproductive health care: urban male adolescents speak out.

CONTEXT: Risky sexual behaviors among adolescent males put them at risk for sexually transmitted diseases, HIV/AIDS, and unplanned fatherhood, yet few facilities in the United States provide focused sexual and reproductive health services to these young men. A general acknowledgement exists that the development of such services is needed, yet there is little research to guide providers in making existing services more attractive to young males and in developing new sexual health services for this population. OBJECTIVES: This research aimed to explore attitudes and perceptions of urban black male adolescents regarding the availability of and access to reproductive healthcare. METHODS: Eighteen black male adolescents participated in three focus group discussions held in a central New Jersey city. Transcripts of the discussions were analyzed using the constant comparative method. Resulting categories were grouped into themes, which reflected the adolescents' perceptions and experiences. Member checks were used to verify findings. RESULTS: The adolescents felt that obtaining sexual health services was a stressful experience fraught with both internal and external barriers. Internal barriers included a fear of stigma and a loss of social status, shame, and embarrassment. External barriers included disrespectful providers, a lack of privacy/confidentiality, and challenges in accessing and negotiating the healthcare system. The young males described an idealized clinic environment as informal, welcoming, and respectful. DISCUSSION AND CONCLUSIONS: Providers should focus on improving the quality of care in existing clinics, particularly in the areas of access, privacy, and confidentiality, and on developing adolescent-friendly clinics focusing on male services. Adolescents should be encouraged to visit clinics prior to an acute need for services. There also is a need for providers who are comfortable with and able to communicate with male adolescents.

Adolescent↗