A novel cellular automata approach: seed input/output of the alien species Leucaena leucocephala in the soil and the effects of climate changes

The potential impact of the soil seed bank of a given species and subsequent population persistence will depend on the robustness of the seeds against depletion. Invasiveness is frequently linked to this behavior and successful control of invasive species depends on reducing the size of their seed banks. We propose a hybrid model that simulates the seed bank dynamics in association with their physical dormancy (PD) based on experimental data on seed bank depletion of a Leucaena leucocephala population during the dispersal phase. The hybrid model uses cellular automata (CA) and Monte Carlo techniques. The sensitivity analysis revealed that the model delivers reproducible and reliable results. Then, we discuss the ability of the hybrid model to predict the effect of seasonal climate change. A 2.0 °C increase in temperature was enough to cause a considerable impact in the seed depletion curve, and after 1 year, ~?75% of the seeds had already disappeared from the seed bank. The proposed hybrid CA model can be used to study soil-stored seed banks with physical dormancy, and could be applied to other Fabaceae species with similar seed traits. We believe that this computational model contributes to a better understanding of seed bank dynamics and will aid in the control of invasive species and for testing different scenarios to reduce or eliminate seed banks that have already been formed.

     

HIV-mediated phosphatidylinositol 3-kinase/serine-threonine kinase activation in APCs leads to programmed death-1 ligand upregulation and suppression of HIV-specific CD8 T cells

Recent evidence demonstrates that HIV-1 infection leads to the attenuation of cellular immune responses, which has been correlated with the increased expression of programmed death (PD)-1 on virus-specific CD8(+) T cells. PD-1 is induced upon T cell activation, and its prolonged expression facilitates CD8(+) T cell inhibitory signals when bound to its B7 family ligands, PD-ligand (L)1/2, which are expressed on APCs. Importantly, early reports demonstrated that blockade of the PD-1/PD-L interaction by Abs may help to counter the development of immune exhaustion driven by HIV viral persistence. To better understand the regulation of the PD-1 pathway during HIV infection, we examined the ability of the virus to induce PD-L expression on macrophages and dendritic cells. We found a direct relationship between the infection of APCs and the expression of PD-L1 in which virus-mediated upregulation induced a state of nonresponsiveness in uninfected HIV-specific T cells. Furthermore, this exhaustion phenotype was revitalized by the blockade of PD-L1, after which T cells regained their capacity for proliferation and the secretion of proinflammatory cytokines IFN-γ, IL-2, and IL-12 upon restimulation. In addition, we identify a critical role for the PI3K/serine-threonine kinase signaling pathway in PD-L1 upregulation of APCs by HIV, because inhibition of these intracellular signal transducer enzymes significantly reduced PD-L1 induction by infection. These data identify a novel mechanism by which HIV exploits the immunosuppressive PD-1 pathway and suggest a new role for virus-infected cells in the local corruption of immune responses required for viral suppression.     

Are seed germination and ecological breadth associated? Testing the regeneration niche hypothesis with bromeliads in a heterogeneous neotropical montane vegetation

The role of seed germination in contributing to species ecological breadth and geographic distribution is still a matter of debate. Here, we attempted to relate seed germination requirements with ecological breadth in 12 bromeliad species from heterogeneous montane vegetation in southeastern Brazil. Seeds were set to germinate under both light and dark conditions at a broad range of temperatures to determine the breadth of the germination niche. We ran a RLQ analysis based on the matrices of species occurrence, environmental parameters and germination traits and found a significant association between germination traits, and the characteristics of sites where adult plants occur. The variation of germination responses to environmental factors was not random with habitat-generalist plants having broader germination niches and habitat-specialist plants having narrower germination niches. The RLQ analysis showed that substrate moisture and light environment were the most important factors correlated with germination traits. Phylogenetic niche conservatism appears to play a role in the patterns found here, especially in the Tillandsioideae. There is an association between the regeneration niche of a species and its ecological range, and this also provides support for the idea that the regeneration niche may help assemble plant species into heterogeneous, species-rich communities.