Hypothesis Of Migration Research Articles

Pericytic-like angiotropism of glioma and melanoma cells.

We have identified in malignant melanoma an angiotumoral complex in which tumor cells occupy a pericytic location along the endothelium of microvessels without evidence of intravasation. We have suggested that this pericytic-like angiotropism could be a marker of an extravascular migration of tumor cells along the abluminal surface of vessels. The extravascular migratory metastasis proposed for melanoma has close analogies with glioma migration. To compare our hypothesis of extravascular migration by melanoma with the migration of glioma cells, we have used the B16 murine melanoma cell line and the GL26 murine glioma cell line in an in vivo murine brain tumor model and in vitro using endothelial cells that have formed capillary-like structures and have been cocultivated with tumor cells. In the brain tumors, a clear progression of glioma and melanoma cells was observed along the abluminal surface of vessels, where they occupied a pericytic location along the periendothelial laminin. In vitro, time-lapse videomicroscopy recorded the migration of tumor cells toward endothelial tubules. After 24 hours, both the melanoma cells and the glioma cells were localized along the external surfaces of the vascular tubules, occupying a pericytic-like location. These similarities between glioma and melanoma support the hypothesis of an extravascular migration of melanoma cells, particularly along the abluminal surface of vessels.

SYMPATRIC GROWTH MORPHS AND SIZE BIMODALITY IN THE GREEN SEA URCHIN (STRONGYLOCENTROTUS DROEBACHIENSIS)

Green sea urchins (Strongylocentrotus droebachiensis) have been a dominant structuring force in boreal, hard-bottom communities and recently have become an important fishery. Despite these roles, relatively little is known about the age, growth, and demographics of field populations. To begin to address these issues, we systematically sampled urchins during 1997 and 1998 at two sites (Allen Island [AI] and Schoodic Peninsula [SP]) in the Gulf of Maine, USA. Basic demographic information (density and size structure) was obtained and a subset of urchins aged by counting annual bands on the interambulacral plates. Size-at-age analyses showed that the population at AI consisted of two sympatric growth morphs, fast growing (fg) and slow growing (sg), each described by its own von Bertalanffy growth parameters for mean size and size variance. The two morphs also differed in maximum observed ages (sg, 6–11 yr, vs. fg, 16–18 yr) and in maximum test diameter sizes (sg, 30–35 mm, vs. fg, 50–70 mm). At SP, a single (fg) morph was identified which had growth characteristics similar to the fg morph at AI. An obviously bimodal size-frequency distribution at AI reflects the accumulation of urchins of each morph at asymptotic size. We advanced several hypotheses; including intrinsic factors, e.g., sex ratios, morphology, and genetics; and extrinsic factors, e.g., density, spatial distribution, settlement history, growth history, and migration; to explain the origin and coexistence of the two growth forms. Using primary and post hoc data analyses, we rejected sex ratio, density, spatial relations, and morphology. Analysis of annual growth and age structure indicated that the two morphs had different settlement histories, suggesting the mixing of two genetically distinct populations. Our post hoc analyses could not reject the genetics or growth history and migration hypotheses. Genetically different larvae, or else a phased process of differential settlement, migration, and growth (threshold migration hypothesis) are the most likely cause(s) for the two morphs. Our data indicate that field growth rates in green urchins are intrinsically highly variable, but also vary interannually probably in relation to their environment. Intrinsic variability provides a novel explanation for bimodality of size-frequency distributions in urchin populations. Interestingly, the sublegal size (<50 mm) and short life span of the sg morph preempts its recruitment into the fishery. Moreover, if the sg morph is widespread and genetically based, intensive harvesting may enhance selection for the sg morph. These findings may have serious implications for understanding the population structure and managing the green urchin fishery, and for assessing the grazing impact of these urchins in nearshore communities.

Neuronal migration, cerebral cortical development, and cerebral cortical anomalies.

Cerebral cortical malformations are relatively common anomalies identified by neuroimaging and pathologically in patients with epilepsy and mental retardation. A disruption in neuronal migration during central nervous system development has been postulated as the pathogenesis for many of these disorders. Recently, the cell migration hypothesis has been proven accurate for lissencephaly, subcortical band heterotopia, and periventricular nodular heterotopia. Furthermore, advances in cellular and molecular biology have begun elucidating the fundamental mechanisms underlying these migration disorders. These data have resulted in redefining and recategorizing specific malformations based on their molecular genetic abnormality. In this review we shall discuss the current understanding of neuronal migration in the developing cerebral cortex, the evaluation of these patients, and attempt to describe the pathogenesis for several well-characterized human disorders of cell migration.

Distribution of the Cicadas (Homoptera: Cicadidae) of the Bahamas

While identifying material in the Florida State Collection of Arthropods (FCSA), I found specimens of cicadas that represent new distribution records for the cicadas that inhabit the Bahamas. The Bahamas are an archipelago of approximately 700 islands and islets and 2400 cays north of the Greater Antilles (Cohen 1998). These islands were almost completely inundated around seventy thousand years ago during the last interglacial thaw after the Sangamon Ice Age (Cranton & Saunders 1992). Thus, the terrestrial fauna and flora are of relatively recent origin. The first cicada to be described from the Bahamas was Diceroprocta bonhotei (Distant 1901). It has been attributed to New Providence and Andros Islands (Davis 1928). Specimens in the FCSA were collected in Nassau, New Providence, Bahamas which substantiates the distribution previously reported (Davis 1928). In addition, one specimen from the FCSA was collected on Eleuthera which extends the distribution of D. bonhotei approximately 80 km east-northeast of New Providence Island. I have also identified a single specimen of D. bonhotei in the K. C. Emerson Entomology Museum at Oklahoma State University from Norman's Cay which is one of the northern Exuma Cays approximately 75 km southeast of Nassau. These new records extend the known distribution of D. bonhotei to the eastern and central cays of the northern island group. The distribution of D. bonhotei is restricted to the Bahamas. Diceroprocta bonhotei is morphologically related to D. cleavesi Davis (1930) and D. caymanensis Davis (1939a) from the Cayman Islands, D. biconica (Walker) (Davis 1932; Davis 1935) and its variety from the Florida Keys, Cuba, the Isle of Pines, and Mexico (Metcalf 1963), andD. bicosta (Walker) (Davis 1928) reported from Cuba and Central America (Metcalf 1963). The morphological similarities suggest a common ancestry for the Diceroprocta species of the West Indies. A second cicada species found in the Bahamas, Ollanta caicosensis Davis, was originally described from South Caicos Island (type location), Acklin's Island, East Caicos Island, West Caicos Island, and Great Inagua Island (Davis 1939b). All but Acklin's Island are part of the Turks and Caicos Island group which forms the southeastern end of the Bahama Island chain. I have identified specimens in the FSCA that extend the known range of 0. caicosensis in the Commonwealth of the Bahamas. The newly identified specimens were collected on Mayaguana and Long Island. The Mayaguana specimens fill the gap in the distribution for the Turks and Caicos group to Acklin's Island. The Long Island specimen extends the known range of 0. caicosensis some 170 km northwest of Acklin's Island across the Crooked Island Passage. This places 0. caicosensis on islands of the Bahamas Platform proper. The genus Ollanta has a broad, discontinuous distribution. Ollanta species are found in the Bahamas (Davis 1939b), Central America (Metcalf 1963), and Hispaniola (Ramos 1983). The ancestor of 0. caicosensis may have migrated from the west onto the large island that would become the Bahamas during the last glacial period. Bahamian organisms that do not migrate easily show more affinities to the fauna of Cuba and Hispaniola than to that of the mainland United States (Cranton & Saunders 1992). This would support the hypothesis of an eastward migration of the ancestors of 0. caicosensis across Cuba or a northward migration from Hispaniola. The two Bahamian cicada species differ in size. Diceroprocta bonhotei is a relatively large species (body length 29-35 mm, wing span 89-110 mm) that inhabits the northwestern islands Ollanta caicosensis is a much smaller species (body length 18-22 mm, wing span 52-65 mm) that inhabits the southeastern half of the Bahamas and the Turks and Caicos. I wish to thank Julieta Brambila of the Florida State Collection of Arthropods and Donald Arnold of the K. C. Emerson Entomology Museum for their assistance during my visits.

From the Old World to the New World: A Molecular Chronicle of the Phylogeny and Biogeography of Hystricognath Rodents

Hystricognath rodents include Old World Phiomorpha and New World Caviomorpha. These two groups have an enigmatic biogeographical history. Using a nuclear marker, the exon 28 of the von Willebrand Factor gene (vWF), we reconstructed the phylogenetic relationships among 23 Hystricognathi species. These taxa encompass the complete familial diversity of the Hystricognathi. Our results indicate a basal trifurcation of hystricognaths leading to Hystricidae, Phiomorpha s.s. (Bathyergidae, Thryonomyidae, and Petromuridae), and Caviomorpha. The monophyly of caviomorphs is robustly supported, confirming a single colonization event of South America by hystricognaths. Caviomorpha are divided into four lineages: Cavioidea, Erethizontoidea, Chinchilloidea, and Octodontoidea. Furthermore, we suggest that (1) Chinchillidae and Dinomyidae are sister clades, (2) Abrocomidae is a true Octodontoidea, and (3) Capromyidae, Echimyidae, and Myocastoridae cluster together. Surprisingly, Erethizontidae does not appear to be the most diverged caviomorph lineage. The molecular results are discussed in the light of previous paleontological and morphological observations. Local molecular clocks are used to estimate divergence dates among hystricognath lineages. An Asian origin is suggested for Caviomorpha, and a colonization route through Australia and Antarctica is indicated as an alternative to the hypothesis of a transatlantic migration of Caviomorpha from Africa to South America.

Distinguishing Migration From Isolation: A Markov Chain Monte Carlo Approach

A Markov chain Monte Carlo method for estimating the relative effects of migration and isolation on genetic diversity in a pair of populations from DNA sequence data is developed and tested using simulations. The two populations are assumed to be descended from a panmictic ancestral population at some time in the past and may (or may not) after that be connected by migration. The use of a Markov chain Monte Carlo method allows the joint estimation of multiple demographic parameters in either a Bayesian or a likelihood framework. The parameters estimated include the migration rate for each population, the time since the two populations diverged from a common ancestral population, and the relative size of each of the two current populations and of the common ancestral population. The results show that even a single nonrecombining genetic locus can provide substantial power to test the hypothesis of no ongoing migration and/or to test models of symmetric migration between the two populations. The use of the method is illustrated in an application to mitochondrial DNA sequence data from a fish species: the threespine stickleback (Gasterosteus aculeatus).

Littoral zone structures as Daphnia refugia against fish predators

The assumption that macrophytes can provide zooplankton a daytime refuge against fish predation is central to the diel horizontal migration hypothesis. However, previous observations and experiments have shown that largeߚbodied zooplankton avoid macrophytes. To directly test these contrasting roles of macrophytes, we measured the reaction of Daphnia pulex to macrophytes (Elodea canadensis or plastic) in the presence and absence of chemical cues from two commonly occurring European fishes, roach (Rutilus rutilus) and perch (Perca fluvialitis). In the same series of laboratory experiments, we then tested the ability of different densities of E. canadensis and plastic macrophytes to reduce predation by roach or perch. In the absence of fish, only 27% of daphnids occurred in the macrophytes. However, 70–80% of daphnids occurred in the macrophytes when either roach or perch occurred. In addition, significantly more daphnids occurred in plastic versus real macrophytes, indicating that some chemically mediated avoidance of real macrophytes by Daphnia occurred. In the presence of fish chemical cues from either roach or perch, the differences between real and plastic macrophytes quickly disappeared as daphnids moved into macrophytes. However, this behavior decreased mortality only from roach predation, because perch spent significantly more time than roach foraging among macrophytes. Thus, although daphnids sought macrophyte refuge in the presence of both fishes, the effectiveness of the refuge depended on macrophyte density and predator identity. These results support the idea that macrophytes can increase survival of Daphnia populations in shallow lakes, depending on the fish species present.

Integration of genotoxicity and population genetic analyses in kangaroo rats (Dipodomys merriami) exposed to radionuclide contamination at the Nevada Test Site, USA

We examined effects of radionuclide exposure at two atomic blast sites on kangaroo rats (Dipodomys merriami) at the Nevada Test Site, Nevada, USA, using genotoxicity and population genetic analyses. We assessed chromosome damage by micronucleus and flow cytometric assays and genetic variation by randomly amplified polymorphic DNA (RAPD) and mitochondrial DNA (mtDNA) analyses. The RAPD analysis showed no population structure, but mtDNA exhibited differentiation among and within populations. Genotoxicity effects were not observed when all individuals were analyzed. However, individuals with mtDNA haplotypes unique to the contaminated sites had greater chromosomal damage than contaminated-site individuals with haplotypes shared with reference sites. When interpopulation comparisons used individuals with unique haplotypes, one contaminated site had greater levels of chromosome damage than one or both of the reference sites. We hypothesize that shared-haplotype individuals are potential migrants and that unique-haplotye individuals are potential long-term residents. A parsimony approach was used to estimate the minimum number of migration events necessary to explain the haplotype distributions on a phylogenetic tree. The observed predominance of migration events into the contaminated sites supported our migration hypothesis. We conclude the atomic blast sites are ecological sinks and that immigration masks the genotoxic effects of radiation on the resident populations.

Methanesulfonate in the firn of King George Island, Antarctica

AbstractMethanesulfonate was investigated as a potential contributor to the sulfur budget, based on analysis of a firn core from Collins Ice Cap, King George Island, Antarctica (62°10′ S, 58°50′ W). The anion was found to be present at a mean concentration of 0.17 μeq L−1, with a maximum of 0.73 μeq L−1. Dating based on the δ18O profile suggests that the principal peaks of methanesulfonate are associated with snow deposited in summer and autumn. A careful examination of MSA, SO42− and nssSO42− profiles indicates that two of the three peaks in the MSA profile may result mainly from migration and relocation of MSA. The mechanism responsible for this might be similar to that for deep cores from other Antarctic glaciers, supporting the migration hypothesis proposed by prior researchers and extending it to near-temperate ice. Due to the post-depositional modification, the main part of the MSA profile of the firn is no longer indicative of the seasonal pattern of MSA in the atmosphere, and the basis for calculation of the MSA/nssSO42− ratio should be changed. The MSA/nssS042 ratio obtained by a new computation is 0.22, 10% higher than that ignoring the effect of MSA migration.

Maize, Matrilocality, Migration, and Northern Iroquoian Evolution

The co-occurrence of matrilocality and maize-based agriculture among historical northern Iroquoian groups of New York and southern Ontario has long been of interest to anthropologists and archaeologists. The traditional explanation of this association is that gradual evolution of maize-based agriculture through female labor enhanced female status in families, which resulted in matrilocality. Dean Snow (1995a) recently challenged this in situ hypothesis of matrilocality by arguing that the sudden appearance of maize-based agriculture and matrilocality can only be explained by the migration of ancestral Iroquoian agriculturists into areas already inhabited by other people. Matrilocality arose because it allowed a focus on external warfare by men against the hostile original inhabitants. In contrast, and based on a general model of maize agriculture evolution and the effects of postmarital residence patterns on that model, I argue that neither in situ development hypothesis nor Snow's migration hypothesis affect the coevolution of matrilocality and maize agriculture, and that their “sudden” appearance cannot be used as evidence in support of either hypothesis. I also show that current archaeological evidence for maize agriculture and matrilocality support a gradual coevolution of maize agriculture and matrilocality rather than the sudden appearance argued by Snow.

Human dispersal into interior Alaska: antecedent conditions, mode of colonization, and adaptations

In spite of more than a half-century of exploration, no definitive evidence has yet come to light for human occupation in eastern Beringia preceding 12,000 yr BP. The oldest dates — between 11,500 and 12,000 yr BP — are from sites in interior and northern Alaska. Archaeological sites dating to this time period, such as the Broken Mammoth site in the central Tanana River Valley, have yielded evidence of pioneer colonization by groups with relatively little knowledge of lithic resources. Three possibly older cave sites — Bluefish Caves, Lime Hills Caves, and Trail Creek Caves — have stratigraphic and taphonomic problems that are not easily resolved. No sites in the glaciated coastal zone of southern Alaska are Pleistocene in date, and numerous objections can be raised to the viability of the coastal migration hypothesis, particularly in the western Gulf of Alaska region. For northern and interior Alaska, the earliest colonization appears to have been a “push–pull” phenomenon, linked to the dissolution of the Bering Land Bridge through a combination of rising sea levels and ameliorating climate. The climate of the “Birch-Poplar” rise in the terminal Pleistocene may have forced the extinction of obligate grazers such as mammoth and horse, but it seems to have favored other taxa such as bison and elk, at least until 9000 yr BP. Faunal data from the Broken Mammoth site in the central Tanana valley, with good organic preservation, demonstrate the utilization of a wide diversity of taxa, including small game, waterfowl, and fish. Faunal and sedimentological data give slight support to a Younger Dryas reversal, but this was dwarfed by the mid-Holocene period of dry, windy conditions during which interior Alaska may have been largely abandoned.

Genesis and evolution of hydrocarbons entrapped in the fluorite deposit of Koh-i-Maran, (North Kirthar Range, Pakistan)

The exceptional development of coeval hydrocarbon and aqueous fluid inclusions (FI) in fluorite from the MVT-type ore deposit of Koh-i-Maran, Baluchistan (North Kirthar range, Pakistan), provides samples which are representative of the ore-forming fluid and which support the hypothesis of petroleum migration in the province. Primary brines at 125°C (10 wt% equ. NaCl) and secondary CH 4-rich brines at 135°C (7 wt% equ. NaCl), are recognised to be associated with oil migration in the fluid inclusions. They support the model of a per ascensum MVT (Mississippi Valley Type) stratabound hydrothermal deposit. A pressure–temperature path of 120–125°C to 165–200 bars is calculated from microthermometric data and PVT modelling of hydrocarbon FI using the modified Peng–Robinson Equation of State (IFP software) from primary cogenetic inclusions (oil and brines). The composition of gas and oil fractions is obtained by a combination of Synchrotron FTIR microanalysis and gas chromatography performed on individual fluid inclusions. The oil entrapped as a coeval primary fluid phase is a light aliphatic normal oil in the range C 8–C 35 with a high CO 2 content. The brown solid phase found systematically in the oil is probably asphaltene resulting from precipitation after trapping of the heavy fraction, which commonly occurs by decreasing pressure and temperature and\\or by CO 2 injection. Later CH 4-rich brine influx probably modified part of the oil in the primary fluid inclusions because degraded oil is observed within such inclusions. Biomarkers obtained by GC-MS analysis indicate a terpane distribution quite similar to the nearest oil seepage in the Gokurt area. This result and the high CO 2 content of organic fluid inclusions indicate a restricted/confined sedimentary environment for the source rock, which could correspond to the Eocene Carbonate formation with type-II organic matter. A possible additional input of gas from the Sambar formation is suggested as feasible. The link between the fluid inclusion data and the geodynamic evolution lead us to propose a circulation of basinal fluids driven mainly by the fault system during dewatering in the foredeep. In Pakistan, they are coeval to major compressional NW–SE Oligocene episode in the thrust belt. The origin of the fluorine may be found in the basin sediments as well as near the basement. The brines originated in salt structures recognized in eocambrian at the decollement level, the source rock was already mature.

Un análisis biogeográfico de los poliquetos (Annelida: Polychaeta) del golfo de Tehuantepec, México

En este trabajo se analizan las afinidades biogeográficas de 84 especies de anélidos poliquetos recolectados en la plataforma continental del golfo de Tehuantepec. Se encontraron cuatro componentes faunísticos dominantes: Amplia distribución (24,99%), Anfiamericano (21,42%), Pacífico (42,84%) y Disyunto (10,91%) definidos a partir de once grupos principales (Cosmopolita, Circuntropical, Anfiamericano Extendido, Anfiamericano en aguas cálidas, Pacífico Nororiental, Pacífico Oriental Tropical, Indopacífico, Transpacífico, Anfiamericano-Atlántico Oriental, Anfiamericano-Pacífico Occidental y Pacífico Oriental-Atlántico Oriental). Se discuten los patrones de distribución y la hipótesis de migración que pudieran explicar la distribución de los anélidos poliquetos presentes en esa área, y se compara la distribución de los grupos principales con estudios de Áreas cercanas del Pacífico, como Panamá, Colombia y la costa oeste de México.

Languages, geography and HLA haplotypes in native American and Asian populations.

A number of studies based on linguistic, dental and genetic data have proposed that the colonization of the New World took place in three separate waves of migration from North-East Asia. Recently, other studies have suggested that only one major migration occurred. It is the aim of this study to assess these opposing migration hypotheses using molecular-typed HLA class II alleles to compare the relationships between linguistic and genetic data in contemporary Native American populations. Our results suggest that gene flow and genetic drift have been important factors in shaping the genetic landscape of Native American populations. We report significant correlations between genetic and geographical distances in Native American and East Asian populations. In contrast, a less clear-cut relationship seems to exist between genetic distances and linguistic affiliation. In particular, the close genetic relationship of the neighbouring Na-Dene Athabaskans and Amerindian Salishans suggests that geography is the more important factor. Overall, our results are most congruent with the single migration model.

Hukou and non-hukou migrations in China: comparisons and contrasts.

The household registration (hukou) system in China was studied using China's 1990 census 1% microdata and interprovincial migration studies. In doing this, the socioeconomic characteristics and geographical patterns of long-distance hukou and non-hukou migratory flows were compared before developing a framework of dual migration circuits. The framework uses a statistical model to evaluate migration rates in relation to both origin and destination variables. It was found that these two types of migrants shared some general demographic characteristics, but displayed substantial socioeconomic differences. The hukou migrants that tended to originate in urban areas had an extremely high share of college-educated people and were employed in higher skilled jobs, while non-hukou migrants were mostly from rural areas with much lower educational attainment. Hukou labor migrants tended to move through government and formal channels, while non-hukou migrants relied on their own, often informal, source of jobs. Furthermore, a difference as to the migration mechanisms between hukou and non-hukou migrants was noted. Non-hukou migration rate were tied positively to the migration stock, a process consistent with a networked migration hypothesis, while hukou migration rates were not. The rural labor migrants moved away from provinces of high population pressure to those with more favorable land/labor ratios, in line with neoclassical predictions. Hukou migration moved in the opposite direction, reflecting a different set of factors at work. The analysis indicates that the hukou system remained a relatively powerful institution in structuring migration in the 1980s.

Cordilleran‐type orogeny and episodic growth of continents: Insights from the circum‐Pacific continental margins

A new material migration hypothesis for the plate subduction zone orogeny, so‐called ‘the cordilleran‐type orogeny’, is proposed on the basis of geological constraints as well as mechanics of accretionary wedges. The major tectonic processes of the hypothesis comprise: (i) episodic, extensive magmatism along the margin of an overriding plate; (ii) supply of voluminous igneous and eroded materials through forearc to trench, with an increase in the net sediment influx into trench; (iii) accelerated accretion of sediments beneath an overriding older accretionary wedge; and (iv) upward material migration within the wedge and resultant exhumation of high‐P/T metamorphic rocks near the inland margin of the wedge. This hypothesis was validated by the test using available geo‐ and thermo‐chronological data from two classical types of subduction‐related orogens in Southwest Japan and California. The hypothesis, coupled with the thermochronologic point of view, requires the reconsideration of coevality of paired metamorphism. The temporal pairing is to be observed between the beginning of the regional high‐T/P metamorphism and that of the uplift and exhumation of high‐P/T metamorphism, with some time lag needed for material migration. Where the temporal pairing is examined therefore, the formation age of igneous rocks and related high‐T/P metamorphic rocks should be compared to the exhumation age of high‐P/T metamorphic rocks. The episodic, extensive magmatism that triggers the cordilleran‐type orogeny shows a temporal correlation in the mid‐Cretaceous for most circum‐Pacific continental margins. The resultant widespread formation of accretionary complexes is also observed in the western part of the circum‐Pacific margins. The deduced mid‐Cretaceous circum‐Pacific orogeny accompanied a gross increase in the continental crust production rate, and was approximately coeval with the Pangea breakup and the Central Pacific superplume episode, implying the orogeny as a part of the mid‐Cretaceous pulsation of the Earth.

Spatial distribution of Prosopis glandulosa var. torreyana in vegetation stripes of the southern Chihuahuan Desert

Abstract Mosaics consisting of vegetation stripes surrounded by bare areas have been described in several arid and semiarid ecosystems. The dynamics of the system depends on the redistribution of rainwater which is preferentially stored and evapotranspired in the vegetated stripes. A process of plant `colonization' in the upslope fringe of the stripes has been described in some cases and a consequent upslope migration of the stripes has been inferred, but not confirmed in all cases quoted in the literature. In this paper, we studied the spatial distribution of mesquite ( Prosopis glandulosa var. torreyana ) and the soil parameters in three vegetation stripes and their associated bare areas in the southern Chihuahuan Desert. The spatial distribution of mesquites of different sizes do not coincide with that expected under the hypothesis of an uniform upslope stripe migration, but soil data suggest that current bare areas had been vegetated some time ago. Dispersion and establishment abilities enhanced by overgrazing may explain the observed mesquite distribution, but the presence of trees with high basal diameters in any part of the stripes suggests stripe permanence at the same site and no upslope migration. These results point to the conflicting evidence on stripe migration that has been already found in other areas. The most probable scenario in our study area is that of a general long-term change of form of the stripes taking place at very variable speeds in different stripes, including the possibility that some of them remain stationary for prolonged periods, and showing different histories of colonization according to the life-history of the different species concerned. The speed and regularity of the process would show a very high temporal and spatial variability due to the interaction of climatic, geomorphologic and biotic interactions.

Genetic variation among Asian populations of rice planthoppers,Nilaparvata lugensandSogatella furcifera(Hemiptera: Delphacidae): mitochondrial DNA sequences

AbstractMany species of insects associated with cultivated rice do not over-winter in Korea and Japan, but migrate into these areas each year. To understand better the origins of these immigrations as well as the geographic structure of rice pests in Asian rice growing regions, intraspecific variation in two species of delphacid planthoppers,Nilaparvata lugens(Stål) andSogatella furciferaHorvath, was examined. An 850 base pair region of mitochondrial DNAcytochrome oxidase-I (CO-I) was sequenced from a total of 71 individuals collected from 11 localities in seven countries: Korea, Philippines, China, Bangladesh, Malaysia, Vietnam and Thailand. InN. lugens, three haplotypes were found and all populations sampled shared a dominant haplotype. Localities in Korea contained two haplotypes and localities in China and the Philippines contained three. However, in samples from the Indochina peninsula no variation was detected either within or between populations, consistent with a hypothesis of regular migration and gene flow. These populations did not contain some haplotypes found in Korea, suggesting they were not the source of yearly immigration into Korea and, by extension, Japan. Populations from China did share haplotypes with Korea, which was consistent with the hypothesis that China was the source for yearly immigration into Korea. There was insufficient resolution to distinguish among populations in China. ForN. lugens, the data suggested that populations south of the Red River Valley in Vietnam experienced regular mixing and were distinct from populations to the north which contributed to yearly immigrations. InS. furcifera, there was less differentiation among populations. Two haplotypes were found in all populations except Malaysia. The results for both species were consistent with seasonal weather data and indicated that more detailed analysis of DNA sequence data will be fruitful.

The role of subduction in the evolution of the Apennines foreland basin

Seismic tomographic images of the Mediterranean region lead to the formulation of an hypothesis involving detachment (or alternatively break-off) of subducted lithosphere, propagating horizontally in the along strike direction: lateral migration of slab detachment [Wortel, M.J.R., Spakman, W., (1992). Structure and dynamics of subducted lithosphere in the Mediterranean region. Proc. K. Ned. Akad. Wet. 95, 325-347]. On the basis of this hypothesis variations in vertical loads are expected along strike of the convergent plate margin involved. We investigate whether such variations in vertical loads can be inferred from the Plio-Pleistocene foreland basin along the eastern side of the Apennines. Our aim is to test the hypothesis of a laterally migrating slab detachment beneath Italy. We employ a finite difference method to obtain the flexural downbending of the Adriatic plate resulting from known loads, such as topography and sediments. The unknown slab pull is inferred by constraining the model to fit the horizon that is assumed to reflect flexural downbending in two-dimensional sections perpendicular to the strike of the Apennines. Our method forms a consistent treatment of all loads involved in two-dimensional lithospheric flexure. If we assume that the downgoing Adriatic plate behaves purely elastically, we conclude that there is a trade-off relation between slab pull and elastic plate thickness for sections in north Italy. For these sections gravity anomaly data cannot constrain a best elastic model. If we make the more realistic assumption of a continental lithosphere with a depth-dependent rheology, we obtain a good match for sections in the north. In the south, however, we find that a match cannot be obtained. This may imply that the data reflect more than purely flexural processes — probably thrusting. The conclusion is that a trend in vertical loads along strike of the Apennines cannot be resolved from Plio-Pleistocene deflection data. Therefore, the hypothesis of lateral migration of slab detachment can neither be shown to be consistent nor inconsistent with the observations.

Dynamical Effects of Planetary Migration on Primordial Trojan-Type Asteroids

The hypothesis of planetary migration is tested by numerically integrating orbits of Trojan-type asteroids in the gravitational field of the Sun, the migrating parent planet, and at least one extra migrating planet. A linear model and an exponential model for migration are used. Migration speeds are defined by several total migration times for the linear model and by two different timescales for the exponential model. The migration process is found to develop great instability on Saturn Trojans. It is a much more stable process for Jupiter and the two outermost major planets' Trojans. The instability produced by migration is also found to be strongly dependent on the initial orbits of the planets. Migration can explain why Saturn Trojans have not been found up to date. It can also explain the absence of these bodies for Uranus and Neptune if initial positions for Neptune closer to the Sun are assumed. In order that migration does not create too much instability for Jupiter Trojans, initial positions for the pair Jupiter-Saturn should not be as close as usually supposed. For the exponential model and timescale 2.5 × 106 yr, there is a significant trend of Jupiter Trojan survivors to the L4 point. The results of this work and previous ones about planetary migration can impose some constraints on possible initial positions for the major planets.