IL7R⍺ is required for hematopoietic stem cell reconstitution of tissue-resident lymphoid cells

Traditional, adult-derived lymphocytes that circulate provide adaptive immunity to infection and pathogens. However, subsets of lymphoid cells are also found in non-lymphoid tissues and are called tissue-resident lymphoid cells (TLCs). TLCs encompass a wide array of cell types that span the spectrum of innate-to-adaptive immune function. Unlike traditional lymphocytes that are continuously generated from hematopoietic stem cells (HSCs), many TLCs are of fetal origin and poorly generated from adult HSCs. Here, we sought to understand the development of murine TLCs across multiple tissues and therefore probed the roles of Flk2 and IL7R⍺, two cytokine receptors with known roles in traditional lymphopoiesis. Using Flk2- and Il7r-Cre lineage tracing models, we found that peritoneal B1a cells, splenic marginal zone B (MZB) cells, lung ILC2s and regulatory T cells (Tregs) were highly labeled in both models. Despite this high labeling, highly quantitative, in vivo functional approaches showed that the loss of Flk2 minimally affected the generation of these cells in situ. In contrast, the loss of IL7R⍺, or combined deletion of Flk2 and IL7R⍺, dramatically reduced the cell numbers of B1a cells, MZBs, ILC2s, and Tregs both in situ and upon transplantation, indicating an intrinsic and more essential role for IL7Rα. Surprisingly, reciprocal transplants of WT HSCs showed that an IL7Rα-/- environment selectively impaired reconstitution of TLCs when compared to TLC numbers in situ. Taken together, our data revealed functional roles of Flk2 and IL7Rα in the establishment of tissue-resident lymphoid cells.

Parallel evolution of phenological isolation across the speciation continuum in serpentine-adapted annual wildflowers

Understanding the relative importance of reproductive isolating mechanisms across the speciation continuum remains an outstanding challenge in evolutionary biology. Here, we examine a common isolating mechanism, reproductive phenology, between plant sister taxa at different stages of adaptive divergence to gain insight into its relative importance during speciation. We study 17 plant taxa that have independently adapted to inhospitable serpentine soils, and contrast each with a nonserpentine sister taxon to form pairs at either ecotypic or species-level divergence. We use greenhouse-based reciprocal transplants in field soils to quantify how often flowering time (FT) shifts accompany serpentine adaptation, when FT shifts evolve during speciation, and the genetic versus plastic basis of these shifts. We find that genetically based shifts in FT in serpentine-adapted taxa are pervasive regardless of the stage of divergence. Although plasticity increases FT shifts in five of the pairs, the degree of plasticity does not differ when comparing ecotypic versus species-level divergence. FT shifts generally led to significant, but incomplete, reproductive isolation that did not vary in strength by stage of divergence. Our work shows that adaptation to a novel habitat may predictably drive phenological isolation early in the speciation process.

Scope for growth of cultivated Pacific and Gulf of California populations of lion's paw scallop Nodipecten subnodosus, and their reciprocal transplants

In the present work, the relative effects of the season (early vs. late summer), site (Gulf vs. Pacific study sites), the population of origin (Gulf vs. Pacific) and associated abiotic variables were determined in two geographically-separated Nodipecten subnodosus populations and their reciprocal transplants, through the scope for growth (SFG) and net growth efficiency (K2) using a portable open-flow system. Results indicate that both energy acquisition and expenditure were significantly affected by season, site, and population factors. Scallop energy acquisition and growth efficiency were highest during the earlier, cooler part of the summer and higher at the Pacific site where food availability was highest. Significantly higher respiration rates were measured for the Pacific scallop population, leading to significantly lower net growth efficiency than scallops of Gulf origin, which indicates a physiological advantage for Gulf scallops during the suboptimal growth conditions present during summer months on both coasts of the Baja California peninsula. It may have important implications for aquaculture enterprises in this region, and may also confirm the genetic divergence between these two geographically separated populations. The advantages of the open-flow system for in situ ecophysiological studies in aquatic organisms are discussed.

Granulopoiesis in the Control of Hematopoietic Stem Cell Self-Renewal and Niches

The bone marrow (BM) vasculature is a critical component of the hematopoietic stem cell (HSC) niche. While the mechanisms through which the vasculature regulates HSC are intensively studied little is known about how this vascular niche is regulated and its function in supporting other aspects of hematopoiesis. We have recently shown that, after myeloablation, BM neutrophils are both sufficient and necessary for the regeneration of the vascular niche. This crosstalk is dependent of TNFα secretion by the neutrophil and TNFR1/TNFR2 expression in the stroma (Bowers et al., 2018). We now demonstrate that the crosstalk between neutrophils and the vasculature is bidirectional and that bone marrow endothelial cells provide a niche for fast neutrophil production in response to stress. Emergency granulopoiesis (EG) is the process through which the hematopoietic system generates large neutrophil numbers-at the expense of lymphocyte production-in response to inflammation or infection. A key EG driver is IL1. This cytokine can induce a myeloid bias on HSC and this is thought to be the main mechanism for IL1-driven EG. While characterizing the BM response to IL1 treatment we noted a fourfold reduction in erythropoiesis. Since erythroid- and myeloid- lineage cells differentiate from a common myeloid progenitor downstream of the HSC these results indicated that IL1 acted on other cells during EG. Differentiation analyses showed that IL1 blocks B cell differentiation at the common lymphoid progenitor and Pro-B to Pre-B transition stages whereas loss of erythropoiesis is detected at the pre-MegE stage and remains suppressed through terminal reticulocyte differentiation. IL1 signals, exclusively, through the IL1 receptor (IL1R1). Using reciprocal transplants and mixed chimera experiments we demonstrate that IL1R1 expression in hematopoietic cells is completely dispensable for IL1-driven neutrophil production. Instead, IL1 acts on stromal cells to increase neutrophil production and diminish lymphocyte and erythrocyte production. Our analyses revealed that, in the stroma, only LepR+ perivascular cells and endothelial cells express IL1R1. Conditional Il1r1 deletion in LepR+ cells did not abrogate any of the hematopoietic phenotypes. However, conditional Il1r1 deletion in endothelial cells completely abrogated neutrophil production but did not prevent loss of erythroid and lymphoid lineages. Differentiation analyses showed that, after exposure to IL1, endothelial cells force the differentiation of immature, unipotent, neutrophil progenitors into terminally differentiated neutrophils. These studies thus demonstrate that endothelial cells provide a niche that is indispensable for neutrophil production during EG. This endothelial niche also uncouples neutrophil production from suppression of other hematopoietic lineages opening the door to targeting this pathway to selectively regulate neutropoiesis. Finally these data also highlight the bidirectional communication between the vascular niche and bone marrow neutrophils during stress. Bowers E, Slaughter A, Frenette PS, Kuick R, Pello OM, Lucas D. Granulocyte-derived TNFα promotes vascular and hematopoietic regeneration in the bone marrow. Nat Med. 2018;24:95-102 Disclosures No relevant conflicts of interest to declare.

Environmental exposure does not explain putative maladaptation in road-adjacent populations

While the ecological consequences of roads are well described, little is known of their role as agents of natural selection, which can shape adaptive and maladaptive responses in populations influenced by roads. This is despite a growing appreciation for the influence of evolution in human-altered environments. There, insights indicate that natural selection typically results in local adaptation. Thus populations influenced by road-induced selection should evolve fitness advantages in their local environment. Contrary to this expectation, wood frog tadpoles from roadside populations show evidence of a fitness disadvantage, consistent with local maladaptation. Specifically, in reciprocal transplants, roadside populations survive at lower rates compared to populations away from roads. A key question remaining is whether roadside environmental conditions experienced by early-stage embryos induce this outcome. This represents an important missing piece in evaluating the evolutionary nature of this maladaptation pattern. Here, I address this gap using a reciprocal transplant experiment designed to test the hypothesis that embryonic exposure to roadside pond water induces a survival disadvantage. Contrary to this hypothesis, my results show that reduced survival persists when embryonic exposure is controlled. This indicates that the survival disadvantage is parentally mediated, either genetically and/or through inherited environmental effects. This result suggests that roadside populations are either truly maladapted or potentially locally adapted at later life stages. I discuss these interpretations, noting that regardless of mechanism, patterns consistent with maladaptation have important implications for conservation. In light of the pervasiveness of roads, further resolution explaining maladaptive responses remains a critical challenge in conservation.