Search

Context. The observations of the proplyds in the Orion Nebula Cluster, exhibiting clear evidence of ongoing photoevaporation, have provided clear proof of the role of externally induced photoevaporation in the evolution of circumstellar disks. NGC 6611 is an open cluster suitable for study of disk photoevaporation, due to its significant population of massive members and stars with disk. In a previous paper, we obtained evidence of the influence of the strong UV field generated by the massive cluster members on the evolution of disks around low-mass Pre-Main Sequence members. Our study was based on a multiband BVIJHKand X-ray catalog compiled for the purpose of selecting cluster members with and without disk.

Aims. We identify and characterize low-mass stars in the ~3 Myr old Trumpler 16 region by means of a deep ChandraX-ray observation, and study their optical and near-IR properties. We compare the X-ray activity of Trumpler 16 stars with the known characteristics of Orion and Cygnus OB2 stars.

Aims.The aim of this work is to identify the so far unknown low mass stellar population of the ~2 Myr old Cygnus OB2  star forming region, and to investigate the X-ray and near-IR stellar properties of its members.

Context.Mechanisms regulating the evolution of pre-main sequence stars can be understood by studying stellar properties such as rotation, disk accretion, internal mixing and binarity. To investigate such properties, we studied a sample of 332 candidate members of the massive and populous star forming region NGC 6530.

Context.The observation of young stars with circumstellar disks suggests that the disks are dissipated, starting from the inner region, by the radiation of the central star and eventually by the formation of rocky planetesimals, over a time scale of several million years. It was also shown that strong UV radiation emitted by nearby massive stars can heat a circumstellar disk up to some thousand degrees, inducing the photoevaporation of the gas. This process strongly reduces the dissipation time scale.

Context.We study the stellar population of the very young cluster NGC 2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only 5 Myr old, has already cleared most of its inter- and circumstellar dust, and with its small and uniform reddening offers a unique opportunity of studying its pre-main-sequence stellar population with minimal disturbance from a dense interstellar medium.

We present a combined X-ray, optical, and IR (2MASS) study of NGC 6530, complementing our previous studies of this cluster. We consider different indicators of IR excesses, which can be taken as an indicator of circumstellar disks and therefore of pre-main-sequence status. We used reddening-free indices to ensure that our results are unaffected by highly irregular, differential reddening. More than the study of the $\it JHK$bands alone (33 IR-excess stars found), we found it useful to compare various optical and IR colors, finding overall 333 stars with optical-IR excess. In the field of the previously studied ChandraACIS-I observation, we found 196 stars with optical-IR excess, of which 120 have not been previously found in X-rays. The total number of estimated cluster members thus becomes ≥1100. The estimated disk frequency in the ACIS field is ~20%. By considering different optical-IR excess indices, we find only partially overlapping subsamples, corresponding to different characteristics of their spectral energy distributions (longer- or shorter-wavelength IR excesses, or blue-violet excess). In a region displaced towards northwest with respect to the known cluster center, we find an unexpected concentration of stars with optical-IR excesses, most of which are not detected in X-rays. The spectral energy distribution of some of these objects shows more extreme excesses with respect to most ordinary T Tauri stars, which suggests circumstellar disks with very small inner holes and high accretion rates; some objects are best interpreted as being surrounded by reflection nebulosities, as found in some optically detected Class I objects in nearer star-forming regions. These reflection-nebulae candidates have the lowest X-ray detection rate among all subsamples considered here. Optical-IR excess stars in the north of NGC 6530 are nearly co-spatial with a sub-population of cluster stars older than the central cluster stars. This leads to the conclusion that in these northern regions of NGC 6530, far from massive cluster stars, star formation (and disk evolution) has proceeded rather undisturbed over longer time periods than near the cluster center, where most massive stars are found, and most stars lack substantial disks and strong accretion.

We present a ChandraACIS-I X-ray observation of the region near the Herbig early-Be star MWC 297, where we detect a tight group of point X-ray sources. These are probably physically associated to MWC 297, because of their obvious clustering with respect to the more scattered field-source population. These data are compared to earlier ASCA data with much poorer spatial resolution, from which the detection of strong quiescent and flaring emission from MWC 297 itself was claimed. We argue that this star, contributing only 5% to the total X-ray emission of the group, was probably not the dominant contributor to the observed ASCA emission, while the X-ray brightest star in the group is a much better candidate. This is also supported by the spectral analysis of the Chandradata, with reference to the ASCA spectra. We conclude that none of the X-ray data available for MWC 297 justify the earlier claim of strong magnetic activity in this star. The X-ray emission of MWC 297 during the Chandraobservation is even weaker than that found in other Herbig stars with the same spectral type, even accounting for its large line-of-sight absorption.

We present astrometry and BVIphotometry, down to $V\simeq22$, of the very young open cluster NGC 6530, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the Vvs. $B-V$and the Vvs. $V-I$color-magnitude diagrams (CMD) show that the upper main sequence is dominated by very bright cluster stars, while, because of the high obscuration of the giant molecular cloud surrounding the cluster, the blue envelopes of the diagrams at $V\gtrsim 14$are limited to the main sequence stars at the distance of NGC 6530. This particular structure of the NGC 6530 CMD allows us to conclude that its distance is about $d \simeq 1250$pc, significantly lower than the previous determination of $d=1800$pc. We have positionally matched our optical catalog with the list of X-ray sources found in a Chandra-ACIS observation, finding a total of 828 common stars, 90% of which are pre-main sequence stars in NGC 6530. Using evolutionary tracks of [CITE], mass and age values are inferred for these stars. The median age of the cluster is about 2.3 Myr; in the mass range (0.6-4.0)$\,M_\odot$, the Initial Mass Function (IMF) shows a power law index $x=1.22\pm0.17$, consistent with both the Salpeter index (1.35), and with the index derived for other young clusters; towards smaller masses the IMF shows a peak and then it starts to decrease.

We present an X-ray study of the high metallicity young open cluster Blanco 1 based on XMM-Newtondata. X-ray spectroscopy of cluster members is presented for the first time as well as new X-ray distribution functions of late-type stars. We detected all known dF and dG stars in the EPIC field and 80% and 90% of dK and dM stars, respectively. The X-ray spectral analysis of the X-ray brightest cluster stars and X-ray color analysis of a larger sample show that a model with two temperatures (at about 0.3 and 1 keV) explains the quiescent activity phase spectra.
We discuss also the nature of unidentified X-ray sources in the observed region and their X-ray spectral properties.

Study objectives:: To investigate the potential association between snoring and other symptoms indicative of sleep-disordered breathing and metabolic syndrome (MetS) in Hispanic adolescents and younger adults using a large population-based survey. Methods:: Sleep-related information, anthropometric measurements and fasting blood samples markers of MetS were obtained from subjects aged 15–40 years collected through the 2nd Chilean Health Survey. Regression models were constructed to evaluate the associations of snoring with MetS, hypertension and serum cholesterol levels. The modulating effect of sleep duration was accounted for in the models. Results:: A total of 2147 subjects (42% males, mean age 27.9±7.6 years) were included. Snoring and short sleep duration were present in 43.5 and 25% of the entire population, respectively. MetS was detected in 19.5% of the subjects. In the adjusted regression model, the odds of MetS among snoring subjects were 2.13 times higher (95% confidence interval (CI): 1.52–2.99; P<0.01), and 1.53-fold higher odds of elevated cholesterol also emerged (95% CI: 1.12–2.10; P<0.01). However, the odds of hypertension were not increased by the presence of snoring after adjusting for confounders. In addition, snoring was associated with an increase of 7.26 and 6.56?mg?dl-1for total and low-density lipoprotein cholesterol, respectively, even after adjusting for age, sex and body mass index. Short sleep duration was associated with a small albeit significant risk increase for high systolic blood pressure. Conclusions:: In this large population-based sample of young Hispanic adults and adolescents, snoring, but not sleep duration, emerged as an independent risk factor for dyslipidemia and MetS, but not for hypertension.

We present a study of the X-ray sources present in the nearby L1551 star forming region, based on a deep XMM-Newtonobservation complemented with Chandradata for the brightest sources. Most known pre-main sequence stars in the region have been detected, most of them with sufficient statistics to allow a detailed study of the temporal and spectral characteristics of their X-ray emission. Significant temporal (and spectral) variability on both short and long time scales is visible for most of the stars. In particular XZ Tau shows large-amplitude variations on time scales of several hours with large changes in the intervening absorption, suggestive of the X-ray emission being eclipsed by the accretion stream (and thus of the X-ray emission being partly or totally accretion-induced). The coronal metal abundance of the WTTS sources is clustered around $Z \simeq 0.2$, while the CTTS sources span almost two orders of magnitudes in coronal Z, even though the photospheric abundance of all stars in the L1551 is likely to be very similar. Some individual elements (notably Ne) appear to be systematically enhanced with respect to Fe in the WTTS stars. The significant differences between the spectral and temporal characteristics of the CTTS and WTTS populations suggest that a different emission mechanism is (at least partly) responsible for the X-ray emission of the two types of stars.

We present the results of a ROSAT study of NGC 2422, a southern open cluster at a distance of about 470 pc, with an age close to the Pleiades. Source detection was performed on two observations, a 10-ks PSPC and a 40-ks HRI pointing, with a detection algorithm based on wavelet transforms, particularly suited to detecting faint sources in crowded fields. We have detected 78 sources, 13 of which were detected only with the HRI, and 37 detected only with the PSPC. For each source, we have computed the $0.2{-}2.0$keV X-ray flux. Using optical data from the literature and our own low-dispersion spectroscopic observations, we find candidate optical counterparts for 62 X-ray sources, with more than 80% of these counterparts being late type stars. We have assigned to the optical sources an astrometric and/or a photometric membership flag, depending on which data are available to us. The number of sources (38 of 62) with high membership probability counterparts is consistent with that expected for Galactic plane observations at our sensitivity. We have computed maximum likelihood X-ray luminosity functions (XLF) for F and early-G type stars with high membership probability. Heavy data censoring due to our limited sensitivity permits determination of only the high-luminosity tails of the XLFs; the distributions are indistinguishable from those of the nearly coeval Pleiades cluster.

We present the first results of an XMM-Newton EPICsurvey of NGC 2516, a southern low-metallicity open cluster with an age close to the Pleiades. The attained limiting sensitivity is of ~2.4 10-15erg s-1cm-2in the 0.1-4.0 keV bandpass. This has been achieved by summing the data of the MOS and PN cameras of two distinct observations for a total exposure time of ~33 ks and by analyzing the summed data set with the wavelet detection algorithm developed at Osservatorio Astronomico di Palermo (OAPA) that has yielded over 200 X-ray detections. Using data just from a single exposure or from a single camera would have reduced by a factor 2-4 our limiting sensitivity and would have resulted in 25-40% less X-ray detections. To date, 129 detections have as counterparts one or more of the 540 photometrically selected cluster members in the surveyed region, for a total of 147 likely detected members, with unique identification in 112 cases. We derive the X-ray luminosity functions (XLF) of NGC 2516 members of different spectral types and compare them with those of the more metal rich, approximately coeval Pleiades cluster, finding the NGC 2516 photometrically selected dG and dK stars less luminous than the Pleiades. The XLFs of the NGC 2516 and of the Pleiades dM stars are indistinguishable. We compare the XMM-Newtonresults with those recently obtained with Chandra.

Aims.We report a deep X-ray survey of the young (~140?Myr), rich open cluster NGC?2516 obtained with the EPIC camera on board the XMM-Newton satellite.Methods.By combining data from six observations, a high sensitivity, greater than a factor of?5 with respect to recent Chandra observations, has been achieved. Kaplan-Meier estimators of the cumulative X-ray luminosity distribution are built, statistically corrected for non members contaminants and compared to those of the nearly coeval Pleiades. The EPIC spectra of the X-ray brightest stars are fitted using optically thin model plasma with one or two thermal components.Results.We detected 431 X-ray sources and 234 of them have as optical counterparts cluster stars spanning the entire NGC?2516 Main Sequence. On the basis of X-ray emission and optical photometry, we indicate 20 new candidate members of the cluster; at the same time we find 49 X-ray sources without known optical or infrared counterpart. The X-ray luminosities of cluster stars span the range logLX(erg s-1)?=?28.4?30.8. The representative temperatures span the 0.3?0.6?keV (3.5?8?MK) range for the cool component and 1.0?2.0?keV (12?23?MK) for the hot one; similar values are found in other young open clusters like the Pleiades, IC?2391, and Blanco?1. While no significant differences are found in X-ray spectra, NGC?2516 solar type stars are definitely less luminous in X-rays than the nearly coeval Pleiades. The comparison with a previous ROSAT survey reveals the lack of variability amplitudes larger than a factor of?2 in solar type stars in a ? 11?yr time scale of the cluster and thus activity cycles like in the Sun are probably absent or have a different period and amplitude in young stars.

The cytotoxicity of the most common alloys used in orthodontic appliances was determined by cell culture testing. Human gingival fibroblasts were cultured on 304 and 316 stainless steel, on brazing alloy composed of palladium Pd, copper Cu, and silver Ag, and on plastic substrate control. Studies were carried out with SEM and radiolabeled precursor incorporation. Cells were cultured in MEM without serum but with the addition of 3Hthymidine to evaluate cell proliferation and 3Hglucosamine to evaluate glycosaminoglycan GAG synthesis and secretion in the culture medium. Moreover, gingival fibroblasts were cultured in the presence of some metal ions generally released by orthodontic appliances to evaluate the cytotoxic effects of single ions. Morphologic observations with SEM and radiolabeled incorporation studies showed that 304 and 316 stainless steel were more biocompatible than the brazing alloy. Among the metal ions tested, Ag and Pd, constituents of the brazing alloy, showed the highest cytotoxicity. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res, 51, 561–568, 2000.

Abstract: Lotus corniculatus plants transformed with a maize cDNA (G1L) encoding a sulphur-rich γ-zein were obtained by using two fusion genes: one with the CaMV 35S promoter, the other with the ribulose bisphosphate carboxylase small subunit (rbcS) promoter. The highest expression of G1L mRNA was found in plants transformed with G1L under the rbcS promoter. The steady state level of G1L mRNA in the leaves was generally directly correlated with the G1L copy number. However, due to a transcriptional block, no G1L mRNA was detected in some of the 35S-G1L multi-copy transformants. Analyses with methylation-sensitive restriction enzymes revealed that the T-DNA of the silenced 35S-G1L transformants was methylated. T-DNA copy number, G1L silencing activity, and the state of methylation were strictly correlated in primary transformants. A cross between two 35S-G1L transformed plants, one (S) with the T-DNA methylated and the other (NS) without, showed that: (1) the methylated state of T-DNA passed through meiosis; and (2) when T-DNA copies from the two parents were combined in the progeny, the unmethylated T-DNA copies of parent NS became methylated at different levels and G1L mRNA production was dependent on the degree of methylation.

Culture conditions have been established for callus induction and growth from different explants in L. angustissimus L. Calli were obtained from hypocotyls, leaves, stems, cotyledons and roots cultured on media containing 2,4-dichlorophenoxyacetic acid or a-naphthaleneacetic acid with kinetin, N6 - ?2 or benzyladenine in different combinations and concentrations. Only those calli induced in presence of a-naphthaleneacetic acid with benzyladenine or kinetin produced shoots. Calli induced from hypocotyl explants were the most efficient in regeneration of shoots. Transformation with an Agrobacterium rhizogenes binary vector carrying the plasmid pBI 121.1 is reported. The percentage of cotransformation was estimated by testing GUS activity in hairy roots. The integration of Ri T-DNA and the NPTII gene in transformed plants was confirmed by molecular analyses and in vitro culture of transgenic tissues in the presence of kanamycin.

Plant regeneration from leaf- and cotyledon-derived calli and from protoplast-derived tissue has been obtained in Lotus pedunculatus. Callus induction was achieved with 2,4-D and plant regeneration required the following two media sequences: bud formation was stimulated by IAA and BA and shoot growth by kinetin. Root formation occurred in the presence of IAA. Cotyledon protoplasts showed a low plating efficiency and plant regeneration was achieved via an intervening callus phase.

We have characterized the genetic consequences of somatic hybridization within the ribosomal DNA (rDNA) of three interspecific hybrids, each involving M. sativa as one of the parents. Restriction-fragment-length-polymorphisms (RFLPs) of rDNA spacers and fluorescent-in-situ-hybridization (FISH) of an 18S-gene probe to mitotic chromosomes were used to compare parental and hybrid species. The M. sativa-coerulea hybrid retained all six parental nucleolar-organizing regions (NORs) and all parental RFLPs representing a complete integration of rDNA. The M. sativa-arborea hybrid retained five of six parental NORs while losing half of the arborea-specific RFLPs, indicating that simple chromosome loss of one arborea NOR accounted for the RFLP losses. Dramatic alterations occurred within the M. sativa-falcata hybrid where five of six parental NORs were retained and new rDNA RFLPs were created and amplified differentially among somaclonal-variant plants. The molecular basis of the new RFLPs involved increased numbers of a 340-bp subrepeating element within the rDNA intergenic spacer (IGS), suggesting that recurrent cycles of unequal recombination occurred at high frequency within the rDNA in somatic lineages.

Seventeen ecotypes of the wild species Medicago polymorpha adapted to a Sardinian (Italy) environment have been evaluated for their response to tissue culture. The accession Samughero-Albi was the more respondent for callus induction and, together with Usassai, showed the highest regeneration capacity on media containing 1 mg l-1 2iP and 0.1 mg l-1 IAA. The morphogenetic response was also affected by the explant source. The hypocotyl-derived-calli were the best regenerating tissues. Regenerated plantlets were difficult to root and it was possible to obtain plants with a well developed root system only after 5–7 weeks of culture on media containing 2iP and IAA both at 0.2 mg l-1. Mesophyll cells were the best protoplast yielding source but only those isolated from roots were able to divide and to regenerate plants. Results are discussed in relation to the genotype specificity for the morphogenetic response and the feasibility of using M. polymorpha in the somatic hybridization with M. sativa.

Plant regeneration from explant and protoplast derived callus has been achieved in Medicago littoralis cv. Harbinger 1886, an annual legume resistant to the fungus Pseudopeziza medicaginis. Callus was induced from different tissue explants and the fastest growth rate was observed for hypocotyls in B5 medium with 2 mg l-1 2,4-dichlorophenoxyacetic acid and 0.5 mg l-1 N6-benzyladenine. Protoplasts were isolated from cotyledons and leaves of sterile plants and from callus; the first two kinds of protoplasts showed a plating efficiency of 5.6% and 5%, respectively, when embedded in agarose. Plant regeneration occurred on media containing % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9qq-f0-yqaqVeLsFr0-vr% 0-vr0db8meaabaqaciGacaGaaeqabaWaaeaaeaaakeaacaqGobWaaW% baaSqabeaacaqG2aaaaOGaaeOVfiaabs5adaahaaWcbeqaaiaaikda% aaGccaqG+waaaa!3F97!\[{\text{N}}^{\text{6}} {\text{\Delta }}^2 {\text{}}\]isopentenyl-adenine combined with indole-3-acetic acid or 1,2-benzisoxazole-3-acetic acid, and on media with N6-benzyladenine plus a-naphtaleneacetic acid; a cytokinin/auxin ratio higher than 1 induced embryos while a ratio around 1 stimulated shoot formation. Embryo development and rooting of shoots were performed in RL medium without growth regulators.

In alfalfa (Medicago sativa) regeneration is genotype-specific. In order to study the genetic control of somatic embryogenesis and to constitute a synthetic cultivar characterized by its high regeneration ability, 2 embryogenic plants selected from the cv. Adriana were selfed, intercrossed and also crossed in both directions with 5 non-embryogenic genotypes of the same cultivar.

Seventy-two plants regenerated from leaf-derived calli of a single plant of Lotus corniculatus have been evaluated for several morphological and agronomical traits. The analysis of selfed and polycross progenies of the regenerants indicates that the variation among regenerants was, at least in part, of genetic origin. Most of the mutations induced by tissue culture were recessive and were detected only after sexual propagation. Although in vitro culture had a depressive effect for most of the traits, the selfed progenies of 2 regenerants displayed higher values for leaflet width and seed yield than the selfed progeny of the initial plant. However the somaclonal variation did not increase the variation for any trait with respect to the variation of the donor cultivar of the initial plant.

Plants of Lotus corniculatus regenerated from single genotype-derived calli showed for several agronomic and morphological traits a variation similar to that present in the original population. Comparisons of the results obtained under field conditions during the 2nd and 3rd year from plant regeneration show: a) the maintenance of significant variance within callus-derived population for several traits examined; b) the total disappearance of ‘minus variants’ indicating a time-related decrease of depressive in vitro effects.

From two lines of Medicago sativa characterized by a high regeneration capability, calli resistant to culture filtrate of Fusarium oxysporum f. sp. medicaginis have been selected. In these calli regeneration capability was greatly reduced and only one plant per callus was recovered. Regenerated plants have been evaluated for resistance to culture filtrate and for in vivo resistance to the pathogen. Three plants out of eight were resistant to the fungus and a high correlation between resistance to culture filtrate and in vivo resistance was observed.

The widely cultivated forage legume alfalfa (Medicago sativa L.) was transformed with the agropine type Agrobacterium rhizogenes NCPPB 1855. Sterile root and callus cultures were derived from tumorous hairy roots which were easily obtained independent of the plant variety or genotype. Plant regeneration, via somatic embryogenesis, was achieved only when a selected alfalfa line, characterized by high regenerative capability, was utilized. Genetic transformation was confirmed by the presence of agropine and T-DNA. Phenotypic alterations, mainly affecting the root system, were observed in transformed plants. The possibility that T-DNA-induced variations could be useful in the improvement of M. sativa is discussed.

Abstract: Several dicotyledonous species were infected with an Agrobacterium rhizogenes binary vector harbouring the plasmid 121.Sn which contains the maize gene Sn under the constitutive promoter CaMV35S. In maize, Sn transactivates the anthocyanin pathway in different tissues. The aim of this work was to test the efficiency of this gene to regulate the anthocyanin pathway in heterologous systems and verify its suitability as a reporter gene. The pigmentation of the hairy roots was compared with hairy roots stained for β-glucuronidase activity, which were used as a control. In two polymorphic genotypes of Lotus angustissimus, DNA integration and expression were assayed. The maize gene is competent to induce anthocyanin pigmentation in different species, but the complexity of the regulatory mechanisms of anthocyanin synthesis restricts the use of Sn as a reporter gene.

Abstract:  The aim of this study was to produce somatic hybrids between tetraploid (2n=4x=32) M. sativa and diploid (2n=2x=16) M.  falcata and analyse their genomic structure. Protoplasts from genotypes selected for regeneration ability from the cultivar Rangelander of M. sativa and Wisfal-1 of M. falcata were electrofused. Seven somatic hybrid calli were produced and one of them regenerated plants. The hybrid nature of these plants and their genetic composition were assessed with morphological, cytological, and molecular analyses. The resulting plants were hyper-aneuploid (2n=33) and contained one extra long chromosome, indicating that a translocation had taken place. The presence of both types of parental sequences in the RAPDs analysis confirmed the true hybrid nature of the plants. Rearrangements within the parental genomes and the presence of somaclonal variation among hybrid plants were observed through an RFLP analysis of the nucleolar organizing region (NOR). The possible causes for the gross genomic alterations, and the suitability of this method for transferring useful agronomic traits from wild species to cultivated alfalfa, are discussed.

Interspecific somatic hybrid plants were obtained by symmetrical electrofusion of mesophyll protoplasts of Medicago sativa with callus protoplasts of Medicago arborea. Somatic hybrid calli were picked manually from semi-solid culture medium after they were identified by their dual color in fluorescent light. Twelve putative hybrid calli were selected and one of them regenerated plants. The morphogenesis of the somatic hybrid calli was induced by the synthetic growth regulator 1,2 benzisoxazole-3-acetic acid. Somatic hybrid plants showed intensive genome rearrangements, as evidenced by isozyme and RFLP analysis. The morphology of somatic hybrid plants was in general intermediate between the parents. The production of hybrids by protoplast fusion between sexually incompatible Medicago species is related to the in vitro respon siveness of the parental protoplasts. The possibility of using somatic hybrid plants in alfalfa breeding is discussed.

Somatic hybrid plants produced by protoplast fusion between tetraploid Medicago sativa (2n= 4x=32) and the diploid species Medicago coerulea (2n= 2x=16) have been RFLP fingerprinted to establish their nuclear composition. Although all of the chromosomes were present, molecular analysis revealed an incomplete incorporation of the alleles of the diploid parent in the fusion products. In the polycross progeny the alleles of both parents segregated in a Mendelian mode. Cytological observations indicated that in the somatic hybrid population minor abnormalities are present; these are restricted mainly to the formation of univalents and lagging chromosomes. Meiosis appeared to be more stable than has been previously reported in the hexaploids of alfalfa. The somatic hybrids grown in the field had a rather vigorous aspect, particularly with respect to the vegetative organs. Forage yield was comparable to that of thmore productive parent. The results are discussed with a view to utilizing the somatic hybrids as starting material for breeding alfalfa at the hexaploid level.

Symmetric somatic hybrid plants have been produced by electrofusion of leaf protoplasts of Medicago sativa and callus protoplasts of Medicago coerulea. The selection of hybrid individuals has been performed at the cellular level by recording the positions of single heterocaryons immobilized in a semisolid culture medium. The hybrid nature of the heterokaryons was assessed in fluorescent light on the basis of their color. Hybrid minicalli were picked up manually and grown first on propagating, and then on regenerating, media. Six putative hybrid calli were selected and two of them regenerated several plants. The hybrid nature of the regenerants was confirmed by cytological and isozyme analysis. Among the several morphological traits taken into account for the characterization of somatic hybrid plants, some were intermediate, some lower, and some higher, with respect to the parents. The somatic hybrid plants were fertile and set seed. The production of somatic hybrid plants in the genus Medicago is discussed in relation to the regenerating capability of parental protoplasts.

Context. NGC 6611 and its parental cloud, the Eagle Nebula (M 16), are well-studied star-forming regions, thanks to their large content of both OB stars and stars with disks and the observed ongoing star formation. In our previous studies of the Eagle Nebula, we identified 834 disk-bearing stars associated with the cloud, after detecting their excesses in NIR bands from Jband to 8.0 $\rm \mu m$.

Context. X-rays from very young stars are powerful probes to investigate the mechanisms at work in the very first stages of the star formation and the origin of X-ray emission in very young stars.Aims. We present results from a 500 ks long observation of the Rho Ophiuchi cloud with a XMM-Newton large program named DROXO, aiming at studying the X-ray emission of deeply embedded young stellar objects (YSOs).Methods. The data acquired during the DROXO program were reduced with SAS software, and filtered in time and energy to improve the signal to noise of detected sources; light curves and spectra were obtained.Results. We detected 111 sources, 61 of them associated with ρOphiuchi YSOs as identified from infrared observations with ISOCAM. Specifically, we detected 9 out of 11 Class I objects, 31 out of 48 Class II and 15 out 16 Class III objects. Six objects out of 21 classified Class III candidates are also detected. At the same time we suggest that 15 Class III candidates that remain undetected at logLx[erg/s] <28.3 are not related to the cloud population. The global detection rate is ~64%. We have achieved a flux sensitivity of ~5 × 10-15erg s-1cm-2. The $L_\mathrm X$to $L_\mathrm{bol}$ratio shows saturation at a value of ~-3.5 for stars with $T_\mathrm{eff}$≤5000 K or 0.7 $M_\odot$as observed in the Orion Nebula. The plasma temperatures and the spectrum absorption show a decline with YSO class, with Class I YSOs being hotter and more absorbed than Class II and III YSOs. In one star (GY 266) with infrared counterpart in 2MASS and Spitzer catalogs we have detected a soft excess in the X-ray spectrum, which is best fitted by a cold thermal component less absorbed than the main thermal component of the plasma. This soft component hints at plasma heated by shocks due to jets outside the dense circumstellar material.

Context. X-rays from very young stars are powerful probes to investigate the mechanisms at work in the very first stages of the star formation and the origin of X-ray emission in very young stars.