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SUMMARYThe lineage-determining transcription factor ETV2 is necessary and sufficient for hematoendothelial fate commitment. We investigated how ETV2-driven gene regulatory networks promote hematoendothelial fate commitment. We resolved the sequential determination steps of hematoendothelial versus cardiac differentiation from mouse embryonic stem cells.Etv2was strongly induced and bound to the enhancers of hematoendothelial genes in a common cardiomyocyte-hematoendothelial mesoderm progenitor. However, onlyEtv2itself andTal1, not other ETV2-bound genes, were induced. Despite ETV2 genomic binding andEtv2andTal1expression, cardiomyogenic fate potential was maintained. A second wave of ETV2-bound target genes was up-regulated during the transition from the common cardiomyocyte-hematoendothelial progenitor to the committed hematoendothelial population. A third wave of ETV-bound genes were subsequently expressed in the committed hematoendothelial population for sub-lineage differentiation. The shift from ETV2 binding to productive transcription, not ETV2 binding to target gene enhancers, drove hematoendothelial fate commitment. This work identifies mechanistic phases of ETV2-dependent gene expression that distinguish hematoendothelial specification, commitment, and differentiation.HIGHLIGHTSThe hematoendothelial master TF ETV2 is expressed in a multipotent mesoderm progenitor.ETV2 binds to target genes in the mesoderm progenitor without target gene activation.ETV2 binding in the mesoderm progenitor does not diminish alternative cardiac fate potential.ETV2-bound target genes are activated upon hematoendothelial fate commitment.GRAPHICAL ABSTRACT

Segregation distorters violate Mendelian Inheritance by over-representing themselves in the progeny of carrier individuals and are commonly associated with chromosomal inversions. When distorting alleles are found on sex chromosomes, the progeny of carrier individuals will exhibit skewed sex ratios, as exemplified by the array ofSex-Ratio(SR) distorting chromosomes found in Drosophila. Because of the strong selective pressures such chromosomes are thought to inflict on genomes, segregation distorters and their associated inversions are expected to experience rapid turn-over. However, theSRX-chromosome ofDrosophila pseudoobscurais found at high frequencies in natural populations, forms stable latitudinal clines, appears to be unsuppressed, and shows evidence of being quite long-lived. Despite being a historically significant and well-studied segregation distortion system, the mechanisms allowing for the long-term persistence of theD.pseudoobscura SRchromosome remain unclear. Here, we perform a comparative genomic analysis betweenSRand uninverted standard X-chromosomes inD.pseudoobscurato study its evolutionary history and dynamics. We find a substantial level of differentiation between theSRand standard chromosome and estimate that the associated inversions have likely existed for the entire lifetime of the species (>2 million generations). Through direct recombination experiments and population genetic analyses, we infer that this high level of differentiation is maintained by a combination of suppressed recombination and epistatic selection. Finally, our data reveal a massive mutational target size for protein and expression level changes specific toSRgenerated by its three non-overlapping inversions. Together our results imply that the entireSRchromosome inD.pseudoobscurabehaves as a single co-adapted gene complex and has been maintained through a combination of suppressed recombination and epistatic selection. This finding highlights the dramatic consequences segregation distorters can have in shaping chromosome-wide patterns of recombination, nucleotide variation, and gene expression.

Observation towers in forests provide necessary data for satellite image product validation, such as net primary production and biomass. One of the key parameters in these calculations is tree height. Here we compared tree heights calculated using three different non-destructive measurement methods, using a Vertex III ultrasonic digital hypsometer in each case. Method 1 involved measuring tree height conventionally, from the ground. Methods 2 and 3 involved measuring tree height from the 4th and 5th floors, respectively, of an observation tower. Tree height measurements from the 4th floor were more stable than those from the ground. Tree height measured from the ground was greater than that measured from the 4th floor. The 4th and 5th floor measurements showed a good relationship (r2 = 0.88). From these results, we decided to measure tree height from the 5th floor of the tower because tree tips can be seen more clearly, thus reducing measurement time, and because future measurements require only remeasuring the angle on the 5th floor, further reducing the time required to gather tree-height data. Therefore, our measurement method is useful for application to field surveys and observations of tree-height change.