Background

Interferon gamma (IFNγ) is a pro-inflammatory cytokine, which may be up-regulated after trauma to the peripheral or central nervous system. Such changes include reactive gliosis and synaptic plasticity that are considered important responses to the proper regenerative response after injury. Also, IFNγ is involved in the upregulation of the major histocompatibility complex class I (MHC class I), which has recently been shown to play an important role in the synaptic plasticity process following axotomy. There is also evidence that IFNγ may interfere in the differentiation and survival of neuronal cells. However, little is known about the effects of IFNγ absence on spinal cord neurons after injury.

Methods

We performed a unilateral sciatic nerve transection injury in C57BL/6J (wild type) and IFNγ-KO (mutant) mice and studied motoneuron morphology using light and electron microscopy. One week after the lesion, mice from both strains were sacrificed and had their lumbar spinal cords processed for histochemistry (n = 5 each group) and transmission electron microscopy (TEM, n = 5 each group). Spinal cord sections from non-lesioned animals were also used to investigate neuronal survival and the presence of apoptosis with TUNEL and immunohistochemistry.

Results

We find that presumed motoneurons in the lower lumbar ventral horn exhibited a smaller soma size in the IFNγ-KO series, regardless of nerve lesion. In plastic embedded sections stained with toluidine blue, the IFNγ-KO mice demonstrated a greater proportion of degenerating neurons in the ventral horn when compared to the control series (p < 0.05). Apoptotic death is suggested based on TUNEL and caspase 3 immunostaining. A sciatic nerve axotomy did not further aggravate the neuronal loss. The cellular changes were supported by electron microscopy, which demonstrated ventral horn neurons exhibiting intracellular vacuoles as well as degenerating nuclei and cytoplasm in the IFNγ-KO mice. Adjacent glial cells showed features suggestive of phagocytosis. Additional ultrastructural studies showed a decreased number of pre-synaptic terminals apposing to motoneurons in mutant mice. Nevertheless, no statistical difference regarding the input covering could be detected among the studied strains.

Conclusion

Altogether, these results suggest that IFNγ may be neuroprotective and its absence results in neuronal death, which is not further increased by peripheral axotomy.

Disentangling the mechanisms that shape community assembly across diversity gradients is a central matter in ecology. While many studies have explored community assembly through species average trait values, there is a growing understanding that intraspecific trait variation (ITV) can also play a critical role in species coexistence. Classic biodiversity theory hypothesizes that higher diversity at species-rich sites can arise from narrower niches relative to species-poor sites, which would be reflected in reduced ITV as species richness increases. To explore how ITV in woody plant communities changes with species richness, we compiled leaf trait data (leaf size and specific leaf area) in a total of 521 woody plant species from 21 forest communities that differed dramatically in species richness, ranging from boreal to tropical rainforests. At each forest, we assessed ITV as an estimate of species niche breadth and we quantified the degree of trait overlap among co-occurring species as a measure of species functional similarity. We found ITV was relatively invariant across the species richness gradient. In addition, we found that species functional similarity increased with diversity. Contrary to the expectation from classic biodiversity theory, our results rather suggest that neutral processes or equalizing mechanisms can be acting as potential drivers shaping community assembly in hyperdiverse forests.

Numerous studies have shown reduced performance in plants that are surrounded by neighbours of the same species1,2, a phenomenon known as conspecific negative density dependence (CNDD)3. A long-held ecological hypothesis posits that CNDD is more pronounced in tropical than in temperate forests4,5, which increases community stabilization, species coexistence and the diversity of local tree species6,7. Previous analyses supporting such a latitudinal gradient in CNDD8,9 have suffered from methodological limitations related to the use of static data10-12. Here we present a comprehensive assessment of latitudinal CNDD patterns using dynamic mortality data to estimate species-site-specific CNDD across 23 sites. Averaged across species, we found that stabilizing CNDD was present at all except one site, but that average stabilizing CNDD was not stronger toward the tropics. However, in tropical tree communities, rare and intermediate abundant species experienced stronger stabilizing CNDD than did common species. This pattern was absent in temperate forests, which suggests that CNDD influences species abundances more strongly in tropical forests than it does in temperate ones13. We also found that interspecific variation in CNDD, which might attenuate its stabilizing effect on species diversity14,15, was high but not significantly different across latitudes. Although the consequences of these patterns for latitudinal diversity gradients are difficult to evaluate, we speculate that a more effective regulation of population abundances could translate into greater stabilization of tropical tree communities and thus contribute to the high local diversity of tropical forests.

Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations are critical for host-tree performance. However, how mycorrhizal associations correlate with the latitudinal tree beta-diversity remains untested. Using a global dataset of 45 forest plots representing 2,804,270 trees across 3840 species, we test how AM and EcM trees contribute to total beta-diversity and its components (turnover and nestedness) of all trees. We find AM rather than EcM trees predominantly contribute to decreasing total beta-diversity and turnover and increasing nestedness with increasing latitude, probably because wide distributions of EcM trees do not generate strong compositional differences among localities. Environmental variables, especially temperature and precipitation, are strongly correlated with beta-diversity patterns for both AM trees and all trees rather than EcM trees. Results support our hypotheses that latitudinal beta-diversity patterns and environmental effects on these patterns are highly dependent on mycorrhizal types. Our findings highlight the importance of AM-dominated forests for conserving global forest biodiversity.

Old-growth tropical forests harbor an immense diversity of tree species but are rapidly being cleared, while secondary forests that regrow on abandoned agricultural lands increase in extent. We assess how tree species richness and composition recover during secondary succession across gradients in environmental conditions and anthropogenic disturbance in an unprecedented multisite analysis for the Neotropics. Secondary forests recover remarkably fast in species richness but slowly in species composition. Secondary forests take a median time of five decades to recover the species richness of old-growth forest (80% recovery after 20 years) based on rarefaction analysis. Full recovery of species composition takes centuries (only 34% recovery after 20 years). A dual strategy that maintains both old-growth forests and species-rich secondary forests is therefore crucial for biodiversity conservation in human-modified tropical landscapes.

Tropical forests are known for their high diversity. Yet, forest patches do occur in the tropics where a single tree species is dominant. Such "monodominant" forests are known from all of the main tropical regions. For Amazonia, we sampled the occurrence of monodominance in a massive, basin-wide database of forest-inventory plots from the Amazon Tree Diversity Network (ATDN). Utilizing a simple defining metric of at least half of the trees ≥ 10 cm diameter belonging to one species, we found only a few occurrences of monodominance in Amazonia, and the phenomenon was not significantly linked to previously hypothesized life history traits such wood density, seed mass, ectomycorrhizal associations, or Rhizobium nodulation. In our analysis, coppicing (the formation of sprouts at the base of the tree or on roots) was the only trait significantly linked to monodominance. While at specific locales coppicing or ectomycorrhizal associations may confer a considerable advantage to a tree species and lead to its monodominance, very few species have these traits. Mining of the ATDN dataset suggests that monodominance is quite rare in Amazonia, and may be linked primarily to edaphic factors.