Université de Montréal

About: Université de Montréal is a education organization based out in Montreal, Quebec, Canada. It is known for research contribution in the topics: Population & Context (language use). The organization has 45641 authors who have published 100476 publications receiving 4004007 citations. The organization is also known as: University of Montreal & UdeM.

Reversal of grazing impact on plant species richness in nutrient-poor vs. nutrient-rich ecosystems

Abstract: To test the hypothesis that the impacts of grazers on plant species richness reverse under contrasting nutrient richness, we analyzed unpublished and published data from lake, stream, marine, grassland, and forest ecosystems. We analyzed data from 30 studies providing 44 comparisons of plant species richness under low vs. high grazing pressure in enriched or nutrient-rich and non-enriched or nutrient-poor ecosystems. All 19 comparisons from non-enriched or nutrient-poor ecosystems exhibited significantly lower species richness under high grazing than under low grazing. In contrast, 14 of 25 comparisons from enriched or nutrient-rich ecosystems showed significantly higher species richness under high grazing than under low grazing. However, nine of these 25 comparisons showed no significant impact of grazers on species richness, while two comparisons showed declines in species richness under high grazing. Based on all the comparisons, plant species richness decreases with high grazing in nutrient-poor ecosystems, while it increases with high grazing in nutrient-rich ecosystems. Although nutrient-rich ecosystems seemed to produce more variable responses to grazers than did nutrient-poor ecosystems, in rare cases high grazing produced a decline in species richness in nutrient-rich environments. We suggest that species richness declines with high grazing in nutrient-poor ecosystems because a limitation of available resources prevents regrowth of species after grazing, which may not be the case in nutrient-rich ecosystems. It is also possible that an increase in species richness under high grazing in nutrient-rich ecosystems may be due to an increase in the dominance of inedible species. Our observation of a grazer reversal of plant species richness under contrasting nutrient richness may have important implications for management of species diversity.

High Rates of Forward Transmission Events after Acute/Early HIV-1 Infection

Abstract: BACKGROUND A population-based phylogenetic approach was used to characterize human immunodeficiency virus (HIV)-transmission dynamics in Quebec. METHODS HIV-1 pol sequences included primary HIV infections (PHIs; <6 months after seroconversion) from the Quebec PHI cohort (1998-2005; n=215) and the provincial genotyping program (2001-2005; n=481). Phylogenetic analysis determined sequence interrelationships among unique PHIs (n=593) and infections from untreated (n=135) and treated (n=660) chronically infected (CI) potential transmitter populations (2001-2005). Clinical features, risk factors, and drug resistance for clustered and nonclustered transmission events were ascertained. RESULTS Viruses from 49.4% (293/593) of PHIs cosegregated into 75 transmission chains with 2-17 transmissions/cluster. Half of the clusters included 2.7+/-0.8 (mean+/-SD) transmissions, whereas the remainder had 8.8+/-3.5 transmissions. Maximum periods for onward transmission in clusters were 15.2+/-9.5 months. Coclustering of untreated and treated CIs with PHIs were infrequent (6.2% and 4.8%, respectively). The ages, viremia, and risk factors were similar for clustered and nonclustered transmission events. Low prevalence of drug resistance in PHI supported amplified transmissions at early stages. CONCLUSIONS Early infection accounts for approximately half of onward transmissions in this urban North American study. Therapy at early stages of disease may prevent onward HIV transmission.

The relation of risk factors to the development of atherosclerosis in saphenous-vein bypass grafts and the progression of disease in the native circulation. A study 10 years after aortocoronary bypass surgery.

Abstract: We examined 82 patients 10 years after saphenous-vein aortocoronary bypass surgery to determine their angiographic status and to relate those findings to the risk factors for coronary-artery disease. Of 132 grafts shown to be patent 1 year after surgery, only 50 were unaffected at 10 years. The remainder were narrowed (43) or occluded (39). Disease progression in coronary arteries without grafts was also frequent, both in vessels that were normal (15 of 32) and in those with minor stenosis (25 of 53). New lesions did not develop in 15 patients, whereas they did in 67--in the grafts, the native vessels, or both. There was no significant difference between the two groups in the incidence of hypertension, diabetes, or smoking, whereas plasma levels of very-low-density lipoproteins (VLDLs) and low-density lipoproteins (LDLs) were higher, and high-density lipoprotein (HDL) levels were lower in those with new disease than in those without. Univariate analysis showed that plasma cholesterol and triglyceride levels were significantly higher at the time of surgery and at the 10-year examination in those with new lesions. Multivariate analysis indicated that among the lipoprotein indexes, levels of HDL cholesterol and plasma LDL apoprotein B best distinguished the two groups. The findings indicate that atherosclerosis in these patients was a progressive disease, frequently affecting both the grafts and the native vessels, and that the course of such disease may be related to the plasma lipoprotein levels.

Comparison of the Mantel test and alternative approaches for detecting complex multivariate relationships in the spatial analysis of genetic data.

Abstract: The Mantel test is widely used to test the linear or monotonic independence of the elements in two distance matrices. It is one of the few appropriate tests when the hypothesis under study can only be formulated in terms of distances; this is often the case with genetic data. In particular, the Mantel test has been widely used to test for spatial relationship between genetic data and spatial layout of the sampling locations. We describe the domain of application of the Mantel test and derived forms. Formula development demonstrates that the sum-of-squares (SS) partitioned in Mantel tests and regression on distance matrices differs from the SS partitioned in linear correlation, regression and canonical analysis. Numerical simulations show that in tests of significance of the relationship between simple variables and multivariate data tables, the power of linear correlation, regression and canonical analysis is far greater than that of the Mantel test and derived forms, meaning that the former methods are much more likely than the latter to detect a relationship when one is present in the data. Examples of difference in power are given for the detection of spatial gradients. Furthermore, the Mantel test does not correctly estimate the proportion of the original data variation explained by spatial structures. The Mantel test should not be used as a general method for the investigation of linear relationships or spatial structures in univariate or multivariate data. Its use should be restricted to tests of hypotheses that can only be formulated in terms of distances.

Social Networks, Social Integration, and Social Engagement Determine Cognitive Decline in Community-Dwelling Spanish Older Adults

Abstract: Objectives. To examine the influence of social networks and social engagement on cognitive decline in a population-based cohort of elderly people, and to assess gender differences in the effect of social relations on cognition. Methods. A longitudinal study of community-dwelling people over 65 was carried out. Cognitive function (orientation and memory) in 1997 and cognitive decline (absent, mild, and severe) over 4 years (1993–1997) were assessed using an instrument previously validated for populations with a low level of education. The effect of social networks, social integration, and social engagement with friends, children, and relatives on cognitive function and cognitive decline was estimated by multiple linear and logistic regressions after adjusting for age, sex, education, depressive symptoms, systolic and diastolic blood pressure, and functional status. Results. Poor social connections, infrequent participation in social activities, and social disengagement predict the risk of cognitive decline in elderly individuals. The probability of cognitive decline was lower for both men and women with a high frequency of visual contact with relatives and community social integration. Engagement with friends seemed to be protective for cognitive decline in women but not in men. Discussion. This longitudinal study indicates that few social ties, poor integration, and social disengagement are risk factors for cognitive decline among community-dwelling elderly persons. The nature of the ties that influence cognition may vary in men and women.