Placozoans are a phylum of nonbilaterian marine animals currently represented by a single described species, Trichoplax adhaerens, Schulze 1883. Placozoans arguably show the simplest animal morphology, which is identical among isolates collected worldwide, despite an apparently sizeable genetic diversity within the phylum. Here, we use a comparative genomics approach for a deeper appreciation of the structure and causes of the deeply diverging lineages in the Placozoa. We generated a high-quality draft genome of the genetic lineage H13 isolated from Hong Kong and compared it to the distantly related T. adhaerens. We uncovered substantial structural differences between the two genomes that point to a deep genomic separation and provide support that adaptation by gene duplication is likely a crucial mechanism in placozoan speciation. We further provide genetic evidence for reproductively isolated species and suggest a genus-level difference of H13 to T. adhaerens, justifying the designation of H13 as a new species, Hoilungia hongkongensis nov. gen., nov. spec., now the second described placozoan species and the first in a new genus. Our multilevel comparative genomics approach is, therefore, likely to prove valuable for species distinctions in other cryptic microscopic animal groups that lack diagnostic morphological characters, such as some nematodes, copepods, rotifers, or mites.

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Production of reactive oxygen species (ROS) by NADPH oxidases (NOXs) impacts many processes in animals and plants, and many plant receptor pathways involve rapid, NOX-dependent increases of ROS. Yet, their general reactivity has made it challenging to pinpoint the precise role and immediate molecular action of ROS. A wellunderstood ROS action in plants is to provide the co-substrate for lignin peroxidases in the cell wall. Lignin can be deposited with exquisite spatial control, but the underlying mechanisms have remained elusive. Here, we establish a kinase signaling relay that exerts direct, spatial control over ROS production and lignification within the cell wall. We show that polar localization of a single kinase component is crucial for pathway function. Our data indicate that an intersection of more broadly localized components allows for micrometer-scale precision of lignification and that this system is triggered through initiation of ROS production as a critical peroxidase co-substrate.

Key words. Electron tomography, FIB-SEM tomography, focused ion beam scanning electron microscope. SummarySince the end of the last millennium, the focused ion beam scanning electron microscopy (FIB-SEM) has progressively found use in biological research. This instrument is a scanning electron microscope (SEM) with an attached gallium ion column and the 2 beams, electrons and ions (FIB) are focused on one coincident point. The main application is the acquisition of three-dimensional data, FIB-SEM tomography. With the ion beam, some nanometres of the surface are removed and the remaining block-face is imaged with the electron beam in a repetitive manner. The instrument can also be used to cut open biological structures to get access to internal structures or to prepare thin lamella for imaging by (cryo-) transmission electron microscopy. Here, we will present an overview of the development of FIB-SEM and discuss a few points about sample preparation and imaging.

In a striking case of evolutionary convergence, polarized cell layers with ring-like diffusion barriers have evolved in both plant and animal lineages independently. In plants, ring-like Casparian strips become localized by the CASPARIAN STRIP MEMBRANE DOMAIN PROTEINS (CASPs). The mechanism of this striking localization, however, has remained enigmatic. Here we present a genetic screen aimed at isolating determinants of CASP localization. One of the mutants, lord of the rings 2 (lotr2)/exo70a1, displays dramatic de-localization of CASPs into randomly localized microdomains. EXO70A1 is a subunit of the exocyst complex, a central component of secretion in eukaryotes. Irradiation of EXO70 subunit genes in plants has suggested specialization of this conserved complex. Intriguingly, lotr2/exo70a1 does neither affect secretion of the CASPs, nor that of other membrane proteins in the endodermis, thus separating exocyst activity in localization from a general defect in secretion. Our results establish EXO70A1 as a central player in Casparian strip formation, generating a transient positional information that will be translated into a precisely localized cell wall modification.