Segmental pattern formation following amputation in the flagellum of the second antennae ofAsellus aquaticus(Crustacea, Isopoda)

A pan-metazoan concept for adult stem cells: the wobbling Penrose landscape

Adult stem cells (ASCs) in vertebrates and model invertebrates (e.g. Drosophila melanogaster) are typically long‐lived, lineage‐restricted, clonogenic and quiescent cells with somatic descendants and tissue/organ‐restricted activities. Such ASCs are mostly rare, morphologically undifferentiated, and undergo asymmetric cell division. Characterized by ‘stemness’ gene expression, they can regulate tissue/organ homeostasis, repair and regeneration. By contrast, analysis of other animal phyla shows that ASCs emerge at different life stages, present both differentiated and undifferentiated phenotypes, and may possess amoeboid movement. Usually pluri/totipotent, they may express germ‐cell markers, but often lack germ‐line sequestering, and typically do not reside in discrete niches. ASCs may constitute up to 40% of animal cells, and participate in a range of biological phenomena, from whole‐body regeneration, dormancy, and agametic asexual reproduction, to indeterminate growth. They are considered legitimate units of selection. Conceptualizing this divergence, we present an alternative stemness metaphor to the Waddington landscape: the ‘wobbling Penrose’ landscape. Here, totipotent ASCs adopt ascending/descending courses of an ‘Escherian stairwell’, in a lifelong totipotency pathway. ASCs may also travel along lower stemness echelons to reach fully differentiated states. However, from any starting state, cells can change their stemness status, underscoring their dynamic cellular potencies. Thus, vertebrate ASCs may reflect just one metazoan ASC archetype.

The genomics of phenotypically differentiated Asellus aquaticus cave, surface stream and lake ecotypes

Organisms well suited for the study of ecotype formation have wide distribution ranges, where they adapt to multiple drastically different habitats repeatedly over space and time. Here we study such ecotypes in a Crustacean model, Asellus aquaticus, a commonly occurring isopod found in freshwater habitats as diverse as streams, caves and lakes. Previous studies focusing on cave vs. surface ecotypes have attributed depigmentation, eye loss and prolonged antennae to several south European cave systems. Likewise, surveys across multiple Swedish lakes have identified the presence of dark-pigmented "reed" and light-pigmented "stonewort" ecotypes, which can be found within the same lake. In this study, we sequenced the first draft genome of A. aquaticus, and subsequently use this to map reads and call variants in surface stream, cave and two lake ecotypes. In addition, the draft genome was combined with a RADseq approach to perform a quantitative trait locus (QTL) mapping study using a laboratory bred F₂ and F₄ cave × surface intercross. We identified genomic regions associated with body pigmentation, antennae length and body size. Furthermore, we compared genome-wide differentiation between natural populations and found several genes potentially associated with these habitats. The assessment of the cave QTL regions in the light-dark comparison of lake populations suggests that the regions associated with cave adaptation are also involved with genomic differentiation in the lake ecotypes. These demonstrate how troglomorphic adaptations can be used as a model for related ecotype formation.

Built to break: the antenna of a primitive insect, Petrobius brevistylis (Archaeognatha)

Jumping bristletails (Archaeognatha) have flagellate antennae with frequent breakpoints, structures destined to break when undue mechanical stress is applied. In natural populations of Petrobius brevistylis at least 67% of animals had broken antennae. In twenty-four trials, all antennae broke at a breakpoint under the animal's weight. Breakpoints consist of two cylinders of laminated cuticle, one stuck inside the other. For half the length of the breakpoint the cylinders are separated by a non-laminated layer of cuticle; thus a tripartite cuticle is formed. During a breakage, the two cylinders slide apart along that non-laminated middle layer that seems to act like a lubricant. While the cylinders pull apart, a thin cuticle unfolds that closes off the new tip of the antenna formed by the outer breakpoint cylinder. Later on, the cylinder is not replaced by a new terminal antennomere. Instead, the antennomere directly proximal to a breakpoint, i.e. the one that after the break forms the last antennomere, already possess the same complement of sensilla as the original terminal antennomere. This includes sensilla basiconica Type A and B found exclusively on these antennomeres and the original terminal antennomere. Breakpoints are common to Archaeognatha and Thysanura, insects that moult throughout their lifetime.