Some like it hot, some like it cold: the heat shock response is found in New Zealand but not Antarctic notothenioid fishes

Previous research on Antarctic notothenioids has demonstrated that cells of cold-adapted Antarctic notothenioids lack a common cellular defense mechanism called the heat shock response (HSR), the induction of a family of heat shock proteins (Hsps) in response to elevated temperatures. The goal of this study was to address how widespread the loss of the HSR is within the Notothenioidei suborder and, specifically, to ask whether cold temperate non-Antarctic notothenioids possess the HSR. In general, Antarctic fish have provided an important opportunity for physiologists to examine responses to selection in the environment and to ask whether traits of the notothenioids represent cold adaptation, or whether the traits are related to history and are characteristics of the notothenioid lineage. Using in vivo metabolic labeling, results indicate that one of the two New Zealand notothenioids possess an HSR. The thornfish, Bovichtus variegatus Richardson, 1846, expressed heat shock proteins (Hsp) in response to heat stress, whereas the black cod, Notothenia angustata Hutton, 1875, did not display robust stress-inducible Hsp synthesis at the protein-level. However, further analysis using Northern blotting clearly demonstrated that mRNA for a common Hsp gene, hsp70, was present in cells of both New Zealand species following exposure to elevated temperatures. Overall, combined evidence on the HSR in notothenioid fishes from temperate New Zealand waters indicate that the loss of the HSR in Antarctic notothenioid fishes occurred after the separation of Bovichtidae from the other Antarctic notothenioid families, and that the HSR was most likely lost during evolution at cold and constant environmental temperatures.     

Evolution: like any other science it is predictable

Evolutionary biology rejoices in the diversity of life, but this comes at a cost: other than working in the common framework of neo-Darwinian evolution, specialists in, for example, diatoms and mammals have little to say to each other. Accordingly, their research tends to track the particularities and peculiarities of a given group and seldom enquires whether there are any wider or deeper sets of explanations. Here, I present evidence in support of the heterodox idea that evolution might look to a general theory that does more than serve as a tautology (‘evolution explains evolution’). Specifically, I argue that far from its myriad of products being fortuitous and accidental, evolution is remarkably predictable. Thus, I urge a move away from the continuing obsession with Darwinian mechanisms, which are entirely uncontroversial. Rather, I emphasize why we should seek explanations for ubiquitous evolutionary convergence, as well as the emergence of complex integrated systems. At present, evolutionary theory seems to be akin to nineteenth-century physics, blissfully unaware of the imminent arrival of quantum mechanics and general relativity. Physics had its Newton, biology its Darwin: evolutionary biology now awaits its Einstein.     

The long and the short of it: RNA-directed chromatin asymmetry in mammalian X-chromosome inactivation

Mammalian X-chromosome inactivation is controlled by a multilayered silencing pathway involving both short and long non-coding RNAs, which differentially recruit the epigenetic machinery to establish chromatin asymmetries. In response to developmentally regulated small RNAs, dicer, a key effector of RNA interference, locally silences Xist on the active X-chromosome and establishes the heterochromatin conformation along the silent X-chromosome. The 1.6 kb RepA RNA initiates silencing by targeting the PRC2 polycomb complex to the inactive X-chromosome. In addition, the nuclear microenvironment is implicated in the initiation and maintenance of X-chromosome asymmetries. Here we review new findings involving these various RNA species in terms of understanding Xist gene regulation and the establishment of X-chromosome inactivation.     

Some like it hot: body and weapon size affect thermoregulation in horned beetles

Body size and shape affect thermoregulatory properties of organisms, and in turn are believed to have shaped macroevolutionary patterns of morphological diversity across many taxa. However, it is less clear whether thermoregulation plays a role in shaping intraspecific morphological diversity such as sexual dimorphisms or the conditional expression of exaggerated secondary sexual traits. Here, we investigate individual thermoregulatory properties in two species of horned beetles that share similar ecologies and body size ranges, but differ substantially in degree of sexual and male dimorphism. We find that intraspecific variation in body size had an unexpectedly large effect on thermal preference behavior and the ability to passively regulate body temperature. Furthermore, we find that the presence or absence of exaggerated secondary sexual traits dramatically altered thermal preference behavior, consistent with a thermoregulatory cost of horn possession. Lastly, we show that the increase in surface area associated with the expression of enlarged horns is, by itself, insufficient to account for the radically altered thermoregulatory behavior observed in horn-bearing males, and discuss possible alternative, physiological explanations. These findings are among the first to link intra-and interspecific variation in body- and weapon size to thermal preferences within and between insect species.     

Folding into an autophagosome: ATG5 sheds light on how plants do it

Autophagosomes arise in yeast and animals from the sealing of a cup-shaped double-membrane precursor, the phagophore. The concerted action of about 30 evolutionarily conserved autophagy related (ATG) proteins lies at the core of this process. However, the mechanisms allowing phagophore generation and its differentiation into a sealed autophagosome are still not clear in detail, and very little is known in plants. This is due in part to the scarcity of structurally informative, real-time imaging data of ATG proteins at the phagophore site. Among these, the ATG5 complex directs anchoring of ATG8 to the phagophore, an event required for membrane expansion. Detailed real-time and 3D imaging of ATG5, ATG8, and an ER marker at the expanding phagophore allowed us to propose a model for autophagosome formation in plants. This model implies tight connections of the growing phagophore with the outer face of the cortical endoplasmic reticulum and prompts new questions on the mechanism of autophagosome biogenesis.     

Spatial heterogeneity in size and parasitism: How it arises in an estuarine snail population

Populations of Ilyanassa obsoleta, a snail abundant in estuaries along the Atlantic coast of North America, exhibit much unobvious spatial heterogeneity. Lifetime dispersal of snails stands to be great, as they live for decades and can make daily moves of tens of meters. They could distribute uniformly, but in many populations there are characteristic spatial distributions of differently sized and trematode-infected snails. This is the case where this work was done in the Savages Ditch habitat of Rehoboth Bay, Delaware, U.S.A.. The study was designed to explain how the spatial heterogeneity in that habitat arises as a consequence of snail movements. Individually marked snails were tested for trematode infections and released from five well-separated positions. Snails released from each position were in three groups: (1) natives, collected from the position area, and (2) large and (3) small “outlanders”, collected from a site in the habitat that was separated from all release positions, had enhanced tidal currents, and where both size classes were available. Locations of found, released snails were noted for up to four years. Depending on collection site, many native snails were parasitized. In any case, they always tended to disperse in the vicinity where initially collected. Large outlanders (∼ 21% infected) dispersed variably, moving back toward their source area from certain release positions, but not from others. Small outlanders (∼ 4% infected), almost regardless of where released, tended to disperse back toward their source area. Dispersals of these snail groups from the five release positions explains much about how the pattern of spatial heterogeneity seen in the habitat arises.     

A triallelic genetic male sterility locus in Brassica napus: an integrative strategy for its physical mapping and possible local chromosome evolution around it

Background and Aims

Spontaneous male sterility is an advantageous trait for both constructing efficient pollination control systems and for understanding the developmental process of the male reproductive unit in many crops. A triallelic genetic male-sterile locus (BnMs5) has been identified in Brassica napus; however, its complicated genome structure has greatly hampered the isolation of this locus. The aim of this study was to physically map BnMs5 through an integrated map-based cloning strategy and analyse the local chromosomal evolution around BnMs5.

Methods

A large F₂ population was used to integrate the existing genetic maps around BnMs5. A map-based cloning strategy in combination with comparative mapping among B. napus, Arabidopsis, Brassica rapa and Brassica oleracea was employed to facilitate the identification of a target bacterial artificial chromosome (BAC) clone covering the BnMs5 locus. The genomic sequences from the Brassica species were analysed to reveal the regional chromosomal evolution around BnMs5.

Key Results

BnMs5 was finally delimited to a 0·3-cM genetic fragment from an integrated local genetic map, and was anchored on the B. napus A8 chromosome. Screening of a B. napus BAC clone library and identification of the positive clones validated that JBnB034L06 was the target BAC clone. The closest flanking markers restrict BnMs5 to a 21-kb region on JBnB034L06 containing six predicted functional genes. Good collinearity relationship around BnMs5 between several Brassica species was observed, while violent chromosomal evolutionary events including insertions/deletions, duplications and single nucleotide mutations were also found to have extensively occurred during their divergence.

Conclusions

This work represents major progress towards the molecular cloning of BnMs5, as well as presenting a powerful, integrative method to mapping loci in plants with complex genomic architecture, such as the amphidiploid B. napus.     

Experimental infections of the monogenean Gyrodactylus turnbulli indicate that it is not a strict specialist

Parasites represent a threat to endangered fish species, particularly when the parasite can host switch and the new host is vulnerable. If the parasite is highly host specific then successful host switching should be a rare occurrence; however, the host range of many parasites which are assumed to be specialists has never been tested. This includes the monogenean Gyrodactylus turnbulli, a well-studied ectoparasite found caudally on its known host, the guppy, Poecilia reticulata. In this study, we monitored parasite establishment and reproduction on a range of poeciliids and more distantly related fish. Individually maintained fish were experimentally infected with a single parasite and monitored daily to establish whether G. turnbulli could survive and reproduce on other fish species. Gyrodactylus turnbulli can infect a wider range of hosts than previously considered, highlighting the fact that host specificity can never be assumed unless experimentally tested. Our findings also have significant implications for parasite transmission to novel hosts and provide further insight into the evolutionary origins of this ubiquitous group of fish pathogens. Previous molecular evidence indicates that host switching is the key mechanism for speciation within the genus Gyrodactylus. Until recently, most Gyrodactylus spp. were assumed to be narrowly host specific. However, our findings suggest that even so-called specialist species, such as G. turnbulli, may represent a threat to vulnerable fish stocks. In view of the potential importance of host switching under artificial conditions, we propose to describe this as 'artificial ecological transfer' as opposed to 'natural ecological transfer', host switching under natural conditions.     

I'm sexy and I glow it: female ornamentation in a nocturnal capital breeder

In many species, males rely on sexual ornaments to attract females. Females, by contrast, rarely produce ornaments. The glow-worm (Lampyris noctiluca) is an exception where wingless females glow to attract males that fly in search of females. However, little is known about the factors that promote the evolution of female ornaments in a sexual selection context. Here, we investigated if the female ornament of the glow-worm is a signal of fecundity used in male mate choice. In support of this, we found brightness to correlate with female fecundity, and males to prefer brighter dummy females. Thus, the glow emitted by females is a reliable sexual signal of female fecundity. It is likely that male preference for the fecundity-indicating ornament has evolved because of large variation among females in fecundity, and because nocturnal males cannot directly assess female size and fecundity. These results indicate that female ornamentation may evolve in capital breeders (i.e. those in which stored resources are invested in reproduction) when females vary significantly in fecundity and this variation cannot be assessed directly by males.     

Barentsz is essential for the posterior localization of oskar mRNA and colocalizes with it to the posterior pole

The localization of Oskar at the posterior pole of the Drosophila oocyte induces the assembly of the pole plasm and therefore defines where the abdomen and germ cells form in the embryo. This localization is achieved by the targeting of oskar mRNA to the posterior and the localized activation of its translation. oskar mRNA seems likely to be actively transported along microtubules, since its localization requires both an intact microtubule cytoskeleton and the plus end-directed motor kinesin I, but nothing is known about how the RNA is coupled to the motor. Here, we describe barentsz, a novel gene required for the localization of oskar mRNA. In contrast to all other mutations that disrupt this process, barentsz-null mutants completely block the posterior localization of oskar mRNA without affecting bicoid and gurken mRNA localization, the organization of the microtubules, or subsequent steps in pole plasm assembly. Surprisingly, most mutant embryos still form an abdomen, indicating that oskar mRNA localization is partially redundant with the translational control. Barentsz protein colocalizes to the posterior with oskar mRNA, and this localization is oskar mRNA dependent. Thus, Barentsz is essential for the posterior localization of oskar mRNA and behaves as a specific component of the oskar RNA transport complex.     

Hydatid control in Australia: where it began, what we have achieved and where to from here

Echinococcus granulosus was imported into Australia with domestic livestock about 200 years ago. It spread rapidly through domestic animals and quickly became a public health problem in the new colony. Control was hampered by ignorance of the transmission pattern. The association between metacestodes and tapeworms was not elucidated until 63 years after the arrival of the First Fleet. Australian wildlife were highly susceptible to infection with E. granulosus and wildlife/domestic animal interaction facilated rapid infiltration of wildlife by E. granulosus. The wildlife reservoir has hampered hydatid control campaigns on mainland Australia but successful eradication has been achieved in the island state of Tasmania where there was no wildlife reservoir. The application of a new recombinant vaccine for sheep in control campaigns and the use of praziquantel baits for controlling infection in dingoes around bush campsites and picnic areas is discussed.     

Shy birds play it safe: personality in captivity predicts risk responsiveness during reproduction in the wild

Despite a growing body of evidence linking personality to life-history variation and fitness, the behavioural mechanisms underlying these relationships remain poorly understood. One mechanism thought to play a key role is how individuals respond to risk. Relatively reactive and proactive (or shy and bold) personality types are expected to differ in how they manage the inherent trade-off between productivity and survival, with bold individuals being more risk-prone with lower survival probability, and shy individuals adopting a more risk-averse strategy. In the great tit (Parus major), the shy–bold personality axis has been well characterized in captivity and linked to fitness. Here, we tested whether ‘exploration behaviour’, a captive assay of the shy–bold axis, can predict risk responsiveness during reproduction in wild great tits. Relatively slow-exploring (shy) females took longer than fast-exploring (bold) birds to resume incubation after a novel object, representing an unknown threat, was attached to their nest-box, with some shy individuals not returning within the 40 min trial period. Risk responsiveness was consistent within individuals over days. These findings provide rare, field-based experimental evidence that shy individuals prioritize survival over reproductive investment, supporting the hypothesis that personality reflects life-history variation through links with risk responsiveness.     

Structural and enzymatic characterization of physarolisin (formerly physaropepsin) proves that it is a unique serine-carboxyl proteinase

Previously, we purified and partially characterized physarolisin, a lysosomal acid proteinase from Physarum polycephalum, which had been suggested to be concerned with the morphological changes of the mold. In this study, a cDNA for the enzyme was cloned and sequenced, and the structural and enzymatic features were investigated. The enzyme shows a sequence similarity to the serine-carboxyl proteinase family (MEROPS S53). Indeed, diisopropylfluorophosphate (DFP) was shown to strongly inhibit the activity of the enzyme. However, the enzyme possesses several unique features distinct from the other members of the family, such as the two-chain structure and inhibition by diazoacetyl-D,L-norleucine methyl ester (DAN). The sites and mode of processing of the precursor to the mature enzyme were deduced, and the major DAN-reactive residue in the enzyme was identified to be Asp529. These features were suggested to be due to the unique local tertiary structure of the enzyme by molecular modeling. We now propose the name physarolisin for the enzyme.     

p66(Shc) gene has a pro-apoptotic role in human cell lines and it is activated by a p53-independent pathway

p66(Shc) protein has been proposed to be an indispensable factor for p53-dependent, mitochondria-mediated apoptosis in mice. Here, we show that p66(Shc) plays a pro-apoptotic role also in cell lines of human origin such as SaOs-2 and HeLa, where p53 is either absent or inactivated, thus, suggesting that p66(Shc) pro-apoptotic role is independent from the presence of a functional form of p53. The active form of p66(Shc) is phosphorylated in Serine 36. We confirm the importance of Serine 36 phosphorylation for p66(Shc) pro-apoptotic role, and our results suggest that the kinase involved in this process is activated independently from p53.     

Lose it to use it

In this issue, Wang et al. (2021. J. Cell Biol. https://doi.org/10.1083/jcb.201911114) describe a phenomenon in which neuromuscular junction synapse elimination triggers myelination of terminal motor axon branches. They propose a mechanism initiated by synaptic pruning that depends on synaptic activity, cytoskeletal maturation, and the associated anterograde transport of trophic factors including Neuregulin 1-III.

Neuromuscular junctions (NMJs) are a favorite model system to study the development, maintenance, and function of neuronal synapses because of their accessibility, size, and simplicity. Although many synaptic mechanisms discovered at the peripheral NMJ have provided important insights into synaptic mechanisms in the central nervous system (CNS), the phenomena of synapse elimination and refinement remain poorly understood in both. In the peripheral nervous system (PNS), synapse elimination is an essential developmental step that removes redundant presynaptic inputs to the muscle fiber. In addition, peripheral motor axon terminals must become myelinated to facilitate rapid and synchronized acetylcholine release to the muscle fiber. However, whether these two essential events during PNS development are coordinately regulated remains unknown.The immature rodent NMJ is first innervated by many axons which are then removed until the synapse reaches a dually innervated state (1). These two axons then further compete for synaptic territory, leaving one “winner” that eventually occupies the motor endplate by the end of the second postnatal week. To determine the relationship between synapse elimination and myelination, Wang et al. (2) used the formation of paranodal junctions between axons and Schwann cells as a surrogate for myelination and then determined whether axons that occupied NMJs in a singly or dually innervated state were more or less likely to be myelinated. They found that when the NMJ is dually innervated, myelination of the terminal axon branch is inhibited; neither synaptic occupancy of the competing axons nor axon diameter influenced myelination. However, once synapse elimination at the NMJ is complete, i.e., a single axon terminal innervates the motor endplate, the winner branch becomes myelinated. Thus, synapse elimination precedes myelination of the terminal axon branch, and competition between dually innervated NMJs restricts myelination.What mechanisms regulate the coordinated maturation of the motor neuron, Schwann cell, and muscle circuit? Since previous studies showed that synapse elimination at the NMJ depends on muscle activity (3), Wang et al. (2) inhibited synapse elimination by blocking acetylcholine receptors with α-bungarotoxin (α-Btx). This inhibition of motor endplate and muscle activity increased not only the number of dually innervated NMJs, but also significantly decreased myelination of terminal axon branches of singly innervated NMJs. Thus, neuromuscular activity must induce retrograde signaling mechanisms that promote not only synapse elimination but also myelination.During synapse elimination, the microtubule cytoskeleton of retracting axons is degraded and reduced (4). In contrast, axons that singly innervate NMJs have a higher microtubule content. α-Btx–dependent block of neuromuscular transmission reduced microtubule content in axons that singly innervate NMJs. Thus, α-Btx treatment simultaneously reduces both microtubule content and myelination.To determine if a mature microtubule-based cytoskeleton is causally related to myelination, Wang et al. used spastin knockout (spastin^KO) mice to artificially stabilize microtubules. Although spastin^KO mice had delayed axon branch removal, stabilization of the microtubule cytoskeleton increased myelination of axons that dually innervated NMJs. Thus, the brake that synaptic competition normally places on terminal branch myelination can be overcome by increasing the mass and maturity of the microtubule cytoskeleton.How does axonal microtubule stability influence terminal axon myelination? Microtubules participate in the anterograde and retrograde transport of diverse cargoes including mitochondria and growth factors. To determine if anterograde axonal transport promotes myelination of axons that singly innervate NMJs, Wang et al. used a dominant-negative mutant of kinesin-1 heavy chain which binds cargo, but lacks the protein’s motor domain, thereby impairing transport. After confirming transport inhibition by tracking impaired movement of the β1 subunit of voltage gated sodium channels, they found that myelination and node of Ranvier formation were significantly delayed in singly innervated NMJs expressing the dominant negative kinesin. Taken together, these results suggest that synapse elimination promotes maturation of the microtubule cytoskeleton which allows more efficient delivery of promyelinating signals to the terminal branch.What could these promyelinating signals be? One obvious candidate is Neuregulin 1 type III (Nrg1-III), which has long been known to promote myelination of peripheral nervous system axons (5). Consistent with this idea, conditional deletion of Nrg1-III dramatically reduced the number of myelinated axon terminals that singly innervate NMJs but did not alter the number of dually innervated NMJs. In contrast, overexpression of Nrg1-III in a transgenic mouse removed the competition-dependent block on myelination resulting in more myelination of both dually and singly innervating axon terminals. In these same transgenic Nrg1-III mice, among those NMJs that were singly innervated, their corresponding axons had higher levels of Nrg1-III. Remarkably, even in these same transgenic overexpressers, inhibition of muscle activity reduced the amount of Nrg1-III found on singly innervated axons, consistent with the observed impairment of the microtubule-based cytoskeleton after α-Btx treatment. ERK1/2 and AKT are downstream effectors of Nrg1-III in Schwann cells and implicated in the myelination pathway. Immunostaining of Schwann cells ensheathing singly innervating axon terminals revealed higher levels of pERK and pAKT.Taken together, the experiments performed by Wang et al. (2) suggest that as multiple axons actively compete for synaptic dominance at the NMJ, the myelination of their terminal branches is delayed. Upon synapse elimination, neuromuscular activity promotes a retrograde signal that increases maturity of the microtubule cytoskeleton. Maturation of the microtubule-based cytoskeleton facilitates the transport of promyelinating signals like Nrg1-III which, when presented to Schwann cells, results in myelination of the “winner” terminal axon branch of a singly innervated NMJ (Fig. 1).Open in a separate windowFigure 1.Synapse elimination promotes myelination of terminal motor axon branches. During early development, NMJs are innervated by multiple axons that compete for endplate territory. During this time, the terminal branches of the axons are not myelinated, and the tubulin cytoskeletal network remains immature. Synaptic activity induces elimination of redundant connections, which leads the winner axon’s microtubule-based cytoskeleton to mature and increase, while the microtubule cytoskeleton is degraded in the retracting axon. The maturity of the cytoskeleton allows for kinesin dependent anterograde transport of Neuregulin 1-III, which then initiates a promyelination signaling cascade via AKT and ERK activation.To the best of our knowledge, this is the first demonstration of plasticity of myelination downstream of activity and synapse refinement in the peripheral motor nervous system. Many studies in the CNS demonstrate that de novo myelination occurs in response to neuronal activity and learning paradigms (6, 7), although the mechanisms responsible remain unknown. Thus, synapse refinement and elimination-dependent myelination may be a paradigm to uncover mechanisms of learning- and activity-dependent myelination in the CNS. Functionally, the addition of myelin to the terminal motor axon branch promotes efficient neurotransmitter release through faster action potential propagation, improved metabolic support of the axon, and more efficient depolarization of the presynaptic terminal by clustered Na⁺ channels at the terminal heminode (8). Whether any or all of these benefits also exist in the CNS remains unknown.This is also the first demonstration of postsynaptic activity driving myelination of a presynaptic axon. Although it is clear that a retrograde signal from the muscle promotes the further maturation and subsequent myelination of the terminal axon, the identity of this cue is unknown. One interesting candidate for a muscle-derived competition and axonal maturation cue is the neurotrophin brain-derived neurotrophic factor (BDNF), which is released during muscle activity (9). Consistent with this idea, BDNF promotes axon maturation by stimulating both actin polymerization and microtubule assembly (10). It will be interesting to test the role of trophic factors in activity-dependent synapse elimination and subsequent myelination in both the CNS and PNS.In conclusion, Wang et al. (2) is an excellent addition to a growing body of research that demonstrates how neuronal activity promotes and modulates myelination. Furthermore, it stands as another example of how using simple model systems, such as the NMJ, may provide insights and have important implications for much more complicated biological systems.