Multiple origins of Tibetan Y chromosomes

The genetic origin of Tibetans was investigated using Y chromosome markers. A total of three populations were studied, two from central Tibet speaking central Tibetan and one from Yunnan speaking Kham. Two dominant paternal lineages (>80%) were identified in all three populations with one possibly from central Asia (YAP+) and the other from east Asia (M122C). We conclude that Tibetan Y chromosomes may have been derived from two different gene pools, given the virtual absence of M122C in central Asia and YAP+ in east Asia, with drift an unlikely mechanism accounting for these observations.     

Early origins of the X and Y chromosomes: lessons from tilapia

Differentiated sex chromosome pairs in diverse species display certain common characteristics, normally comprising one largely heterochromatic genetically inactive chromosome and one euchromatic genetically active chromosome (e.g. the mammalian Y and X respectively). It is widely accepted that dimorphic sex chromosomes evolved from homologous pairs of autosomes. Although the exact mechanisms through which the pair diverged are not fully understood, an initial suppression of recombination in the sex-determining region is required by all of the major theories. Here we address the question of the mechanism by which this initial suppression of recombination occurs. Our model postulates that the stochastic, de novo accumulation of heterochromatin in the sex determining region can delay pairing of the sex chromosomes in meiosis, resulting in a decrease in recombination. Data to support this model is presented from the cichlid fish, Oreochromis niloticus. Although such a decrease would in most circumstances be evolutionarily disadvantageous, if the region concerned included the major sex determining gene and other gene(s) with sex-specific functions, then this would be selectively advantageous and could trigger the process(es) which, ultimately, lead to the differentiation of the sex chromosomes.     

Mapping replication origins in yeast chromosomes.

The replicon hypothesis, first proposed in 1963 by Jacob and Brenner, states that DNA replication is controlled at sites called origins. Replication origins have been well studied in prokaryotes. However, the study of eukaryotic chromosomal origins has lagged behind, because until recently there has been no method for reliably determining the identity and location of origins from eukaryotic chromosomes. Here, we review a technique we developed with the yeast Saccharomyces cerevisiae that allows both the mapping of replication origins and an assessment of their activity. Two-dimensional agarose gel electrophoresis and Southern hybridization with total genomic DNA are used to determine whether a particular restriction fragment acquires the branched structure diagnostic of replication initiation. The technique has been used to localize origins in yeast chromosomes and assess their initiation efficiency. In some cases, origin activation is dependent upon the surrounding context. The technique is also being applied to a variety of eukaryotic organisms.     

Phylogenetic distinctiveness of Middle Eastern and Southeast Asian village dog Y chromosomes illuminates dog origins

Modern genetic samples are commonly used to trace dog origins, which entails untested assumptions that village dogs reflect indigenous ancestry or that breed origins can be reliably traced to particular regions. We used high-resolution Y chromosome markers (SNP and STR) and mitochondrial DNA to analyze 495 village dogs/dingoes from the Middle East and Southeast Asia, along with 138 dogs from >35 modern breeds to 1) assess genetic divergence between Middle Eastern and Southeast Asian village dogs and their phylogenetic affinities to Australian dingoes and gray wolves (Canis lupus) and 2) compare the genetic affinities of modern breeds to regional indigenous village dog populations. The Y chromosome markers indicated that village dogs in the two regions corresponded to reciprocally monophyletic clades, reflecting several to many thousand years divergence, predating the Neolithic ages, and indicating long-indigenous roots to those regions. As expected, breeds of the Middle East and East Asia clustered within the respective regional village dog clade. Australian dingoes also clustered in the Southeast Asian clade. However, the European and American breeds clustered almost entirely within the Southeast Asian clade, even sharing many haplotypes, suggesting a substantial and recent influence of East Asian dogs in the creation of European breeds. Comparison to 818 published breed dog Y STR haplotypes confirmed this conclusion and indicated that some African breeds reflect another distinct patrilineal origin. The lower-resolution mtDNA marker consistently supported Y-chromosome results. Both marker types confirmed previous findings of higher genetic diversity in dogs from Southeast Asia than the Middle East. Our findings demonstrate the importance of village dogs as windows into the past and provide a reference against which ancient DNA can be used to further elucidate origins and spread of the domestic dog.     

Polymorphism and differentiation of rainbow trout Y chromosomes.

Most fish species show little morphological differentiation in the sex chromosomes. We have coupled molecular and cytogenetic analyses to characterize the male-determining region of the rainbow trout (Oncorhynchus mykiss) Y chromosome. Four genetically diverse male clonal lines of this species were used for genetic and physical mapping of regions in the vicinity of the sex locus. Five markers were genetically mapped to the Y chromosome in these male lines, indicating that the sex locus was located on the same linkage group in each of the lines. We also confirmed the presence of a Y chromosome morphological polymorphism among these lines, with the Y chromosomes from two of the lines having the more common heteromorphic Y chromosome and two of the lines having Y chromosomes morphologically similar to the X chromosome. The fluorescence in situ hybridization (FISH) pattern of two probes linked to sex suggested that the sex locus is physically located on the long arm of the Y chromosome. Fishes appear to be an excellent group of organisms for studying sex chromosome evolution and differentiation in vertebrates because they show considerable variability in the mechanisms and (or) patterns involved in sex determination.     

Homoeologic aberrations in human and chimpanzee Y chromosomes: inverted and satellited Y chromosomes

An inverted and a satellited Y chromosome detected in peripheral blood lymphocytes of a chimpanzee and a pygmy chimpanzee, respectively, were characterized cytogenetically by various banding techniques. A detailed comparison with an inverted and a satellited Y chromosome in man suggested that the corresponding aberrations were homoeologic.     

Extra Y chromosomes and alcoholism

Summary The authors examined the karyotype of 89 out of the 102 men 180 cm (511) tall or more among the 459 male inmates of four Swedish institutions for alcoholics. All 89 had a normal chromosome constitution.     

Nonfluorescent Y chromosomes. Cytologic evidence of origin

Summary Twelve presumptive structurally altered Y chromosomes were studied with Q-, G-, G-11, C-, Cd, and lateral asymmetric banding techniques and were compared with normal X and Y chromosmes and with an abnormal [i(Yq)] Y chromosome that exhibited intact fluorescence. Significant to this work is the fact that the Y chromosome has a small block of Giemsa-11 heterochromatin adjacent to the centromere on the long arm, while the X chromosome does not, which allows a distinction between the X-and Y-derived chromosomes. Two of the twelve altered chromosomes of either X or Y origin are small nonfluorescent rings. Each ring has a G-11-positive band of heterochromatin at the centromere, confirming Y origin. Each of the normal-length nonfluorescent presumed Ys and a Y with a fluorescent band in the center have one G-11 band at the centromere and another at an equal distance from the end of the long arm, the bands also being Cd positive, indicating that these chromosomes are pseudodicentric. The likely mechanism of origin is a break at the distal bright heterochromatin/ euchromatin junction (or within the bright segment in the chromosome with the bright center band), fusion of the sister chromatids at the breakpoints, and loss of the distal segment.     

Extra Y chromosomes and epilepsy

Summary The authors examined the karyotypes of the men 183 cm (6 ft.) tall or more among the 320 male residents of two Swedish institutions for the long-term care of epilepsy. All 14 men meeting this criterion proved to have the normal chromosome constitution-46, XY.     

The degeneration of Y chromosomes

Y chromosomes are genetically degenerate, having lost most of the active genes that were present in their ancestors. The causes of this degeneration have attracted much attention from evolutionary theorists. Four major theories are reviewed here: Muller's ratchet, background selection, the Hill Robertson effect with weak selection, and the 'hitchhiking' of deleterious alleles by favourable mutations. All of these involve a reduction in effective population size as a result of selective events occurring in a non-recombining genome, and the consequent weakening of the efficacy of selection. We review the consequences of these processes for patterns of molecular evolution and variation at loci on Y chromosomes, and discuss the results of empirical studies of these patterns for some evolving Y-chromosome and neo-Y-chromosome systems. These results suggest that the effective population sizes of evolving Y or neo-Y chromosomes are severely reduced, as expected if some or all of the hypothesized processes leading to degeneration are operative. It is, however, currently unclear which of the various processes is most important; some directions for future work to help to resolve this question are discussed.     

Isodicentric Y chromosomes and secondary microchromosomes

We report here on two mosaic patients with both an idic(Yp) and a microchromosome. FISH with the DYZ3 alphoid repeat demonstrated that the isodicentrics effectively exhibited two alphoid clusters whereas the small markers had a Y-centromere. These data, along with 4 previous observations, indicate that such microchromosomes effectively result from functional dicentricity of isodicentric Y-chromosomes and represent the excision of one centromere plus various amounts of adjacent chromatin. Other than a real infrequency of such a concurrence or a very low proportion of the cell line(s) containing the microchromosome, the paucity of observations points to a high rate of underdiagnosis as revealed by two idic(Y) instances in which the microchromosome was detected only in samples assessed by FISH.     

African and Asian perspectives on the origins of modern humans.

The ways in which the cultural evidence - in its chronological context - can be used to imply behavioural patterning and to identify possible causes of change are discussed. Improved reliability in dating methods, suites of dates from different regional localities, and new, firmly dated fossil hominids from crucial regions such as northeast Africa, the Levant, India and China, are essential for clarification of the origin and spread of the modern genepool. Hominid ancestry in Africa is reviewed, as well as the claims for an independent origin in Asia. The cultural differences and changes within Africa, West and South Asia and the Far East in the later Middle and early Upper Pleistocene are examined and compared, and some behavioural implications are suggested, taking account of the evolutionary frameworks suggested by the 'multiregional evolution' and 'Noah's Ark' hypotheses of human evolution. A possible explanation is proposed for the cultural differences between Africa, West Asia and India on the one hand, and southeast Asia and the Far East on the other. The apparent hiatus between the appearance of the first anatomically modern humans, ca. 100 ka ago, and the appearance of the Upper Palaeolithic and other contemporaneous technological and behavioural changes around 40 ka ago, is discussed. It is suggested that the anatomical changes occurred first, and that neurological changes permitted the development of fully syntactic language some 50 ka later. The intellectual and behavioural revolution, best demonstrated by the 'Upper Palaeolithic' of Eurasia, seems to have been dependent on this linguistic development - within the modern genepool - and triggered the rapid migration of human populations throughout the Old World.     

Spatial distribution of chromosomes 1 and Y in human spermatozoa.

The positions of chromosomes 1 and Y inside human spermatozoa were determined by differential staining techniques. In 85/100 cells the two chromosomes were in close contact and in association with a vacuole. This observation is in contrast to previous findings for chromosome No. 9 and the Y-chromosome whose positions do not appear to be correlated.     

The position of the Nazlet Khater specimen among prehistoric and modern African and Levantine populations

The morphometric affinities of the 33,000 year old skeleton from Nazlet Khater, Upper Egypt are examined using multivariate statistical procedures. In the first part, principal components analysis is performed on a dataset of mandible dimensions of 220 fossils, sub-fossils and modern specimens, ranging in time from the Late Pleistocene to recent and restricted in space to the African continent and Southern Levant. In the second part, mean measurements for various prehistoric and modern African and Levantine populations are incorporated in the statistical analysis. Subsequently, differences between male and female means are examined for some of the modern and prehistoric populations. The results indicate a strong association between some of the sub-Saharan Middle Stone Age (MSA) specimens, and the Nazlet Khater mandible. Furthermore, the results suggest that variability between African populations during the Neolithic and Protohistoric periods was more pronounced than the range of variability observed among recent African and Levantine populations. Results also demonstrate a general reduction in the degree of sexual dimorphism during the Holocene. However, this pattern of reduction pattern varies by geographic location and is not uniform across the African continent.