In this book Professor Holmes discusses some of the evidence relating to one of the most baffling problems yet recognized by biologists-the factors involved in the regulation of growth and form. A wide range of possible influences, from enzymes to cellular competition, is considered. Numerous experiments and theories are described, with or without bibliographic citation. There is a list of references for each chapter and an index. The subject from a scientific standpoint is an exceedingly difficult one, for the reason that very little indeed is understood regarding such phenomena as differentiation. It follows that the problem offers fine opportunities for intellectual jousting by mechanists and vitalists, that hypotheses and theories must often be the weapons of choice, and philosophy the armor. Professor Holmes gives us a good seat from which to watch the combats, explains clearly what is going on, and occasionally slips away to enter the lists himself. This stereoscopic atlas of anatomy was designed as an aid in teaching neuro-anatomy for beginning medical students and as a review for physicians taking Board examinations in Psychiatry and Neurology. Each plate consists of a pair of stereoscopic photographs and a labelled diagram of the important parts seen in the photograph. Perhaps in this day of scarcity of materials, particularly of human brains hardened for dissection, photographs of this kind conceivably can be used as a substitute. Successive stages of dissection are presented in such a fashion that, used in conjunction with the dissecting manual, a student should be able to identify most of the important components of the nervous system without much outside help. The area covered is limited to the gross features of the brain and brain stem and perhaps necessarily does not deal with any of the microscopic structure. So much more can be learned from the dissection of the actual brain that it is doubtful if this atlas would be useful except where brains are not available. A good deal of effort has been spent on the preparation of this atlas, with moderately successful results.

The Integrative Action of the Nervous System.

Social isolation has been recognized as a major risk factor for morbidity and mortality in humans for more than a quarter century. The brain is the key organ of social connections and processes, however, and the same objective social relationship can be experienced as caring and protective or as exploitive and isolating. We review evidence that the perception of social isolation (i.e., loneliness) impacts brain and behavior and is a risk factor for broad-based morbidity and mortality. However, the causal role of loneliness on neural mechanisms and mortality is difficult to test conclusively in humans. Mechanistic animal studies provide a lens through which to evaluate the neurological effects of a member of a social species living chronically on the social perimeter. Experimental studies show that social isolation produces significant changes in brain structures and processes in adult social animals. These effects are not uniform across the brain or across species but instead are most evident in brain regions that reflect differences in the functional demands of solitary versus social living for a particular species. The human and animal literatures have developed independently, however, and significant gaps also exist. The current review underscores the importance of integrating human and animal research to delineate the mechanisms through which social relationships impact the brain, health, and well-being.

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Toward a neurology of loneliness.

Long-read sequencing technologies have substantially improved the assemblies of many isolate bacterial genomes as compared to fragmented short-read assemblies. However, assembling complex metagenomic datasets remains difficult even for state-of-the-art long-read assemblers. Here we present metaFlye, which addresses important long-read metagenomic assembly challenges, such as uneven bacterial composition and intra-species heterogeneity. First, we benchmarked metaFlye using simulated and mock bacterial communities and show that it consistently produces assemblies with better completeness and contiguity than state-of-the-art long-read assemblers. Second, we performed long-read sequencing of the sheep microbiome and applied metaFlye to reconstruct 63 complete or nearly complete bacterial genomes within single contigs. Finally, we show that long-read assembly of human microbiomes enables the discovery of full-length biosynthetic gene clusters that encode biomedically important natural products.

metaFlye: scalable long-read metagenome assembly using repeat graphs

Book reviewChronobiology: biological timekeeping

Conditions fish encounter during embryogenesis and early life history can leave lasting effects not only on morphology, but also on growth rate, life-history and behavioural traits. The ecology of offspring can be affected by conditions experienced by their parents and mother in particular. This review summarizes such early impacts and their ecological influences for a variety of teleost species, but with special reference to salmonids. Growth and adult body size, sex ratio, egg size, lifespan and tendency to migrate can all be affected by early influences. Mechanisms behind such phenotypically plastic impacts are not well known, but epigenetic change appears to be one central mechanism. The thermal regime during development and incubation is particularly important, but also early food consumption and intraspecific density can all be responsible for later life-history variation. For behavioural traits, early experiences with effects on brain, sensory development and cognition appear essential. This may also influence boldness and other social behaviours such as mate choice. At the end of the review, several issues and questions for future studies are given.

Early environment influences later performance in fishes.

Dominant-subordinate status emerges from agonistic encounters. The weakly electric fish, Gymnotus omarorum, displays a clear-cut example of non-breeding territorial aggression. The asymmetry in the behavior of dominants and subordinates is outstanding. Dominants are highly aggressive and subordinates signal submission in a precise sequence of locomotor and electric traits: retreating, decreasing their electric organ discharge rate, and emitting transient electric signals. The hypothalamic neuropeptide arginine-vasotocin (AVT) and its mammalian homolog arginine-vasopressin, are key modulators of social behavior, known to adapt their actions to different contexts. By analyzing the effects of pharmacological manipulations of the AVT system in both dominants and subordinates, we show evidence of distinct status-dependent actions of AVT. We demonstrate an endogenous effect of AVT on dominants' aggression levels: blocking the V1a AVT receptor induced a significant decrease in dominants' attack rate. AVT administered to subordinates enhanced the expression of the electric signals of submission, without affecting subordinates' locomotor displays. This study contributes a clear example of status-dependent AVT modulation of agonistic behavior in teleosts, and reveals distinctive activation patterns of the AVT system between dominants and subordinates.

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Status-Dependent Vasotocin Modulation of Dominance and Subordination in the Weakly Electric Fish Gymnotus omarorum.

Environmental, seasonal, and social modulations of basal activity in a weakly electric fish.

This study was carried out in Uruguay (30–35°S), South America, with two complementary approaches. First, an extensive exploration of Uruguayan freshwaters allowed us to assess the distribution of the two major species of Gymnotiformes (out of 4) across sites. Gymnotus carapo was uniformly distributed in Uruguayan territory, whereas Brachyhypopomus pinnicaudatus was observed in the northern and eastern part of the country. There was a highly significant negative correlation between the relative abundance of Brachyhypopomus pinnicaudatus and pH and water conductivity. Moreover, these environmental factors are significant contributors to its spatial differences in relative abundance. Second, temperature, conductivity, photoperiod, and the structure of a Brachyhypopomus pinnicaudatus population were analyzed across seasons in a small lake over a two-year period. Water temperature and photoperiod exhibited important seasonal changes, whereas water conductivity remained low and relatively constant. The presence of sexually mature males, females, and the sudden increase of juveniles indicated the occurrence of the breeding season in November, December, and January, coinciding with high mean water temperatures and extreme photoperiod. These results agree with previous data that support the hypothesis of temperature as an important environmental factor for the onset of breeding in Gymnotiformes from the temperate zone.

Biogeography and Breeding in Gymnotiformes from Uruguay

Males and females commonly compete for limited resources. When interaction costs are similar for both sexes and there are no sexual differences in resource value estimation, a non-sex-biased dominance is expected. Moreover, only non-sex-biased assessment of contenders fighting ability (Resource Holding Potential, RHP) should influence contest decisions. To test these predictions, we evaluated non-breeding agonistic intra- and intersexual dyadic interactions in the weakly electric fish, Gymnotus omarorum. During the non-breeding season, resource value is not expected to depend on individuals’ reproductive status and should thus be equal for males and females. In addition, as G. omarorum presents no sexual differences in body size, interaction costs can be considered symmetric between sexes. We confirmed that body size differences, but not individuals’ gender, is the best predictor of dominance. We correlated RHP asymmetries with contest duration and evidenced that body size but not sex influences assessment in intrasexual and intersexual encounters. All dyads tested engaged in agonistic interactions (N = 33) in which a clear dominant emerged. The analysis of conflict phases evidenced the submissive role of electric displays. Electric organ discharge (EOD) interruptions appear early in the contest as an electric hiding attempt, whereas chirps are post-resolution signals of subordinate status. Interestingly, the decision of interrupting the EOD was also influenced by RHP asymmetries, whereas chirping activity was influenced by the intensity of the attacks received. Our results confirm that body size is the best RHP proxy in non-breeding intra- and intersexual contests of this monomorphic species and demonstrated a sequential pattern of submissive signalling by means of two different electric displays.

Non-sex-biased Dominance in a Sexually Monomorphic Electric Fish: Fight Structure and Submissive Electric Signalling

In contrast to most of the previous studies in gymnotiform reproduction, which have been conducted in the tropical region, this study examines a gymnotid from the temperate region in both the natural habitat and the laboratory. The gonadal histology of Brachyhypopomus pinnicaudatus is described for the first time. The male had a paired, lobular testis of the unrestricted spermatogonial type, and females a paired saccular cystovary. Analysis of gonads and their annual cycle enabled us to confirm the breeding season and to conclude that this species is a multiple spawner. Water temperature and photoperiod showed the expected annual cycles for the region. High temperature and a 14 h:10 h L:D photoperiod in the natural habitat coincided with (1) mature gonadal stages, (2) electrophysiological sexual dimorphism: males present a lengthened negative phase in their electric organ discharge (EOD) and (3) decreased temperature sensitivity of the EOD: the waveform does not change when temperature increases above 20°C. Acclimation to sustained high temperature (30 days, 28°C, 12 h:12 h L:D, low conductivity) induced gonad maturation along with EOD dimorphism. Our data show that high environmental temperature is enough to trigger sexual maturity in Brachyhypopomus pinnicaudatus from a temperate climate.

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Temperature induces gonadal maturation and affects electrophysiological sexual maturity indicators in Brachyhypopomus pinnicaudatus from a temperate climate.

Ana Silva

Papers

Status-Dependent Vasotocin Modulation of Dominance and Subordination in the Weakly Electric Fish Gymnotus omarorum.

Environmental, seasonal, and social modulations of basal activity in a weakly electric fish.

Biogeography and Breeding in Gymnotiformes from Uruguay

Non-sex-biased Dominance in a Sexually Monomorphic Electric Fish: Fight Structure and Submissive Electric Signalling

Temperature induces gonadal maturation and affects electrophysiological sexual maturity indicators in Brachyhypopomus pinnicaudatus from a temperate climate.