Biology subjects

Johnson, A. B.

Publications and source records attributed to Johnson, A. B..

8 recordsLinked to original sources

bioRxiv2026

APLNR regulates directed migration to spatially pattern mesoderm in human gastruloids

During human gastrulation, mesoderm progenitors migrate while acquiring distinct identities. Whether this movement contributes to subsequent patterning remains unclear. Here, we identify the apelin receptor (APLNR) as a regulator of directed migration of mesoderm in human gastruloids. TBXT+ cells ingress basally and disperse from the PS-like region toward epiblast-like cells. Tracking reveals that this dispersal reflects a directional bias rather than increased speed. APLNR is enriched in nascent mesoderm, while its ligand APELA is expressed in adjacent epithelial cells. Control gastruloids exhibit fourfold greater dispersal than APLNR-depleted gastruloids despite modest speed differences, consistent with a model of chemotaxis by an epithelial ligand field. Paraxial and lateral mesoderm differentiation competence is preserved following APLNR depletion, while signaling programs and central FOXC2+ paraxial organization are altered. These findings support a role for APLNR-dependent guidance in positioning competent mesoderm progenitors for subsequent patterning.

developmental biology↗

bioRxiv2026

Asymmetric gene flow across a desert contact zone in a riparian songbird

Secondary contact is a key point in the speciation process, and fine-scale geography can shape its outcomes. This is especially true for species restricted to fragmented habitats, such as riparian corridors through arid regions. Here we examine the role of disjunct riparian habitat in shaping secondary contact in Bells Vireo (Vireo bellii), a North American songbird species that contains distinct eastern and western forms. We recovered a unique, discontinuous contact zone along the Rio Grande in New Mexico, where two populations with greater nuclear and mitochondrial genetic affinity for the eastern lineage are separated by a population with an affinity for the western lineage. This point of primarily western ancestry on the Rio Grande corresponded with a stretch where several intermittently flowing tributaries join from the west and may have acted as gene flow corridors. Using a combination of empirical analyses of divergence and diversity across the genome and population genetic simulations, we uncovered evidence of neutral, genome-wide admixture driving the genomic architecture of divergence, rather than evidence for local adaptation or selective sweeps. In sum, this genomic study showed us how fine-scale dispersal corridors can cause idiosyncratic patterns of admixture when habitat is limiting in zones of secondary contact.

evolutionary biology↗

bioRxiv2024

Posterior parietal cortex maps progress along routes sharing the same meta-structure

Neurons of posterior parietal cortex were recorded as rats performed a working memory task within a network of intersecting paths. The specific routes utilized in task performance provided opportunity to contrast responses of posterior parietal cortex sub-populations to linear and angular velocity with more complex responses that map route progress. We found evidence for the presence of posterior parietal cortex neurons that generalize in their firing patterns across routes having the same shape but opposite action series. The results indicate that posterior parietal cortex has the capacity to generalize the mapping of route progress independent of the specific actions taken to move through those routes. We suggest that such encoding can form the basis for learning the meta-structural organization of a non-random path network structure, such as that commonly found in cities.

neuroscience↗

bioRxiv2023

Extraordinary levels of per- and polyfluoroalkyl substances (PFAS) in vertebrate animals at a New Mexico desert oasis: multiple pathways for wildlife and human exposure

Per- and polyfluoroalkyl substances (PFAS) threaten human and wildlife health, but their movement through food webs remains poorly understood. Contamination of the physical environment is widespread, but particularly concentrated at military installations. Here we measured 17 PFAS in wild, free-living mammals and migratory birds at Holloman Air Force Base (AFB), New Mexico, USA, where wastewater catchment lakes form biodiverse oases. PFAS concentrations were among the highest ever reported in animal tissues, and high levels have persisted for at least three decades. The hazardous long carbon-chain form, perfluorooctanosulfonic acid (PFOS), was most abundant, with liver concentrations averaging tens of thousands of ng/g wet weight (ww), reaching as high 97,000 ng/g (ww) in a 1994 specimen of white-footed mouse (Peromyscus leucopus) and 38,000 ng/g ww in a duck, the American wigeon (Mareca americana). Perfluorohexanesulfonic acid (PFHxS) averaged thousands of ng/g ww in the livers of birds and house mice, but one order of magnitude lower in the livers of upland desert rodent species. PFAS levels were strikingly lower at control sites, even for highly mobile migratory species. Tissue concentrations were correlated within individuals, and consistently higher in liver than in muscle or blood. Twenty of 23 vertebrate species sampled at Holloman AFB were heavily contaminated, representing multiple trophic levels and microhabitats, and implicating a range of pathways for PFAS spread: ingestion of surface water, sediments, and dust; foraging on aquatic invertebrates and plants by secondary consumers; and preying upon small vertebrates by higher level consumers, including consumption of game species by hunters. Unlike in other aquatic systems, piscivory was not an important pathway of PFAS uptake. In sum, legacy PFAS at a desert wetland have permeated the local food web across a period of decades, severely contaminating resident and migrant animals, and likely exposing humans via game meat consumption and outdoor recreation. Five highlightsO_LIA biodiverse, wetland food web at a military base is heavily contaminated with PFAS. C_LIO_LILittoral-zone mice and aquatic birds had high liver PFOS, up to 97,000 ng/g ww. C_LIO_LISpecies and ecological variation were high among 16 PFAS detected in animal tissues. C_LIO_LIGame species had dangerously contaminated meat and can transport it long distances. C_LIO_LIBiorepositories provide key temporal and spatial sampling for contaminant studies. C_LI Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=130 SRC="FIGDIR/small/561778v1_ufig1.gif" ALT="Figure 1"> View larger version (75K): org.highwire.dtl.DTLVardef@96a640org.highwire.dtl.DTLVardef@7cd7f4org.highwire.dtl.DTLVardef@45fb67org.highwire.dtl.DTLVardef@7c33a4_HPS_FORMAT_FIGEXP M_FIG C_FIG

ecology↗

bioRxiv2022

A CRISPR-engineered Isogenic Model Reveals Altered Neuronal Phenotypes of the 22q11.2 A-B Syndromic Deletion

The 22q11.2 deletion syndrome (22q11.2DS), associated with congenital and neuropsychiatric anomalies, is the most common copy number variant (CNV)-associated syndrome. Patient-derived, induced pluripotent stem cell (iPS) models have provided important insight into the mechanisms of phenotypic features of this condition. However, patient-derived iPS models may harbor underlying genetic heterogeneity that can confound analysis of pathogenic CNV effects. Furthermore, the [~]1.5 Mb "A-B" deletion at this locus is inherited at higher frequency than the more common [~]3 Mb "A-D" deletion, but remains under-studied due to lack of relevant models. To address these issues, here we leveraged a CRISPR-based strategy in Cas9-expressing iPS cells to engineer novel isogenic models of the 22q11.2 "A-B" deletion. After in vitro differentiation to excitatory neurons, integrated transcriptomic and cell surface proteomics identified deletion-associated alterations in surface adhesion and cell signaling. Furthermore, implantation of iPS-derived neuronal progenitor cells into the cortex of neonatal mice found accelerated neuronal maturation within a relevant microenvironment. Taken together, our results suggest pathogenic mechanisms of the 22q11.2 "A-B" deletion in driving neuronal and neurodevelopmental phenotypes, both in vitro and in vivo. We further propose that the isogenic models generated here will provide a unique resource to study this less-common variant of the 22q11.2 microdeletion syndrome.

genetics↗

bioRxiv2021

Blood variation implicates respiratory limits on elevational ranges of Andean birds

The extent to which species ranges reflect intrinsic physiological tolerances is a major, unsolved question in evolutionary ecology. To date, consensus has been hindered by the limited tractability of experimental approaches across most of the tree of life. Here, we apply a macrophysiological approach to understand how hematological traits related to oxygen transport shape elevational ranges in a tropical biodiversity hotspot. Along Andean elevational gradients, we measured traits that affect blood oxygen-carrying capacity--total and cellular hemoglobin concentration and hematocrit--for 2,355 individuals of 136 bird species. We used these data to evaluate the influence of hematological traits on elevational ranges. First, we asked whether hematological plasticity is predictive of elevational range breadth. Second, we asked whether variance in hematological traits changed as a function of distance from the midpoint of the elevational range. We found that the correlation between hematological plasticity and elevational range breadth was slightly positive, consistent with a facilitative role for plasticity in elevational range expansion. We further found reduced local variation in hematological traits near elevational range limits and at high elevations, patterns consistent with intensified natural selection, reduced effective population size, or compensatory changes in other cardiohematological traits with increasing distance from species-specific optima for oxygen availability. Our findings suggest that constraints on hematological plasticity and local genetic adaptation to oxygen availability promote the evolution of the narrow elevational ranges that underpin tropical montane biodiversity.

ecology↗

bioRxiv2021

Complementary Maps for Location and Environmental Structure in CA1 and Subiculum

The dorsal subiculum lies among a network of interconnected brain regions that collectively map multiple spatial and orientational relationships between an organism and the boundaries and pathways composing its environment. A unique role of the subiculum in spatial information processing has yet to be defined despite reports of small neuron subpopulations that encode relationships to specific boundaries, axes of travel, or locations. We examined the activity patterns among populations of subiculum neurons during performance of a spatial working memory task performed within a complex network of interconnected pathways. Compared to neurons in hippocampal sub-region CA1, a major source of its afferents, subiculum neurons were far more likely to exhibit multiple firing fields at locations that were analogous with respect to path structure and function. Subiculum neuron populations were also found to exhibit a greater dynamic range in scale of spatial representation and for persistent patterns of spiking activity to be aligned to transitions between maze segments. Together, the findings indicate that the subiculum plays a unique role in spatial mapping, one that complements the location-specific firing of CA1 neurons with the encoding of emergent and recurring structural features of a complex path network.

neuroscience↗

bioRxiv2020

Detecting turnover among complex communities using null models: A case study with sky-island haemosporidian parasites

Turnover in species composition between sites, or beta diversity, is a critical component of species diversity that is typically influenced by geography, environment, and biotic interactions. Quantifying turnover is particularly challenging, however, in multi-host, multi-parasite assemblages where undersampling is unavoidable, resulting in inflated estimates of turnover and uncertainty about its spatial scale. We developed and implemented a framework using null models to test for community turnover in avian haemosporidian communities of three sky islands in the southwestern United States. We screened 776 birds for haemosporidian parasites from three genera (Parahaemoproteus, Plasmodium, and Leucocytozoon) by amplifying and sequencing a mitochondrial DNA barcode. We detected infections in 280 birds (36.1%), sequenced 357 infections, and found a total of 99 parasite haplotypes. When compared to communities simulated from a regional pool, we observed more unique, single-mountain haplotypes and fewer haplotypes shared among three mountain ranges than expected, indicating that haemosporidian communities differ to some degree among adjacent mountain ranges. These results were robust even after pruning datasets to include only identical sets of host species, and they were consistent for two of the three haemosporidian genera. The two more distant mountain ranges were more similar to each other than the one located centrally, suggesting that the differences we detected were due to stochastic colonization-extirpation dynamics. These results demonstrate that avian haemosporidian communities of temperate-zone forests differ on relatively fine spatial scales associated with adjacent sky-islands. Null models are essential tools for detecting turnover in complex, undersampled, and poorly known systems.

ecology↗