Current Biology最新文献

Identifying new organisms that allow linking brain-wide circuit dynamics to complex adaptive behaviors is an ongoing challenge. A new study has demonstrated that Danionella cerebrum - a miniature teleost fish - is capable of multimodal sensory discrimination and displays oxytocin-dependent social interaction, thus opening the way for a wide range of detailed circuit investigations.

Recognition protects biological systems at all scales, from cells to societies. Social insects recognize their nestmates by colony-specific olfactory labels that individuals store as neural templates in their memory. Throughout an ant's life, learning continuously shapes the nestmate recognition template to keep up with the constant changes in colony labels.^1,2,3,4 Most explanations for template update rely on non-associative learning.⁵ Indeed, we know that ants become habituated to their colony's label: their reaction to the omnipresent chemical cues typical of their own nest fades.^3,6,7,8 However, non-associative habituation cannot explain the enormous variation in nestmate recognition behavior. For example, some ant species are more aggressive toward neighboring colonies than toward unfamiliar colonies (nasty neighbor effect^9,10,11,12). Social insects can learn associatively, for example, by associating an odor cue with a food reward.¹³ A recent model proposes that associative learning of non-nestmate odors leads to variation in the recognition templates among individuals, which then improves recognition at the group level.¹⁴ Here, we test whether associative learning of non-nestmate colony odors is possible.¹¹ Our results show that associative learning plays a crucial role in the formation of both nestmate and non-nestmate recognition templates and that the aggression received by an ant acts as an unconditioned stimulus that the ant likely associates with the odor label of its enemy. This type of template learning can help explain different patterns of variation in nestmate recognition, from nasty neighbor effects to task- and age-specific variation in aggression.^15,16.

Visual motion is a crucial cue for the brain to track objects and take appropriate actions, enabling effective interactions with the environment. Here, we study how the superior colliculus (SC) integrates motion information using asymmetric plaids composed of drifting gratings of different directions and speeds. With both in vivo electrophysiology and two-photon calcium imaging, we find that mouse SC neurons integrate motion direction by performing vector summation of the component gratings. The computation is constrained probabilistically by the possible physical motions consistent with each grating. Excitatory and inhibitory SC neurons respond similarly to the plaid stimuli. Finally, the probabilistically constrained vector summation also guides optokinetic eye movements. Such a computation is fundamentally different from that in the visual cortex, where motion integration follows the intersection of the constraints. Our studies thus demonstrate a novel neural computation in motion processing and raise intriguing questions regarding its neuronal implementation and functional significance.

Adaptive behavior in a dynamic environmental context often requires rapid revaluation of stimuli that deviates from well-learned associations. The divergence between stable value-encoding and appropriate behavioral output remains a critical component of theories of dopamine's function in learning, motivation, and motor control. Yet, how dopamine neurons are involved in the revaluation of cues when the world changes, to alter our behavior, remains unclear. Here, we make use of a complementary set of in vivo approaches to clarify the contributions of the mesolimbic dopamine system to the dynamic reorganization of reward- seeking behavior. Male and female rats were trained to discriminate when a conditioned stimulus would be followed by a sucrose reward by exploiting the prior, non-overlapping presentation of a another discrete cue-an occasion setter. Only when the occasion setter's presentation preceded the conditioned stimulus did the conditioned stimulus predict sucrose delivery, dissociating the average value of the conditioned stimulus from its immediate value, on a trial-to-trial basis. Activity of ventral tegmental area dopamine neurons was essential for rats to successfully update behavioral response to the occasion setter. Moreover, dopamine release in the nucleus accumbens following the conditioned stimulus only occurred when the occasion setter indicated it would predict reward and did not reflect its average expected value. Downstream of dopamine release, we found that neurons in the nucleus accumbens dynamically tracked the value of the conditioned stimulus. Together, these results help refine notions of dopamine function, revealing a prominent contribution of the mesolimbic dopamine system to the rapid revaluation of motivation.

Negative scaling relationships between both speciation and extinction rates, on the one hand, and the age or duration of organismal groups on the other, are pervasive and recovered in both molecular phylogenetic and fossil time series.^1,2,3,4 The agreement between molecular and fossil data hints at a universal cause and potentially at incongruence between micro- and macroevolution. However, the existence of negative rate scaling in fossil time series has not undergone the same level of scrutiny as in molecular data. Here, we analyze the marine animal fossil record across the last ∼538.8 Ma of the Phanerozoic to investigate the presence and strength of negative rate scaling. We find that negative rate scaling arises under commonly applied age range-based per capita rates, which do not control for sampling bias, but are severely reduced or absent when metrics are used that do correct for sampling. We further show by simulation that even moderately incomplete sampling of species occurrences through time may induce rate scaling. We thus conclude that there are no significant scaling relationships present in these fossil clades and that any apparent trend is caused by sampling artefacts and taxonomic practices. If rate scaling in molecular phylogenies is genuine, the absence of such a relationship in the fossil record will provide a valuable benchmark and constraint on what processes can cause it.

Kelp forests are declining in many parts of the northeast Pacific.^1,2,3,4 In small populations, genetic drift can reduce adaptive variation and increase fixation of recessive deleterious alleles,^5,6,7 but natural selection may purge harmful variants.^8,9,10 To understand evolutionary dynamics and inform restoration strategies, we investigated genetic structure and the outcomes of genetic drift and purging by sequencing the genomes of 429 bull kelp (Nereocystis luetkeana) and 211 giant kelp (Macrocystis sp.) from the coastlines of British Columbia and Washington. We identified 6 to 7 geographically and genetically distinct clusters in each species. Low effective population size was associated with low genetic diversity and high inbreeding coefficients (including increased selfing rates), with extreme variation in these genetic health indices among bull kelp populations but more moderate variation in giant kelp. We found no evidence that natural selection is purging putative recessive deleterious alleles in either species. Instead, genetic drift has fixed many such alleles in small populations of bull kelp, leading us to predict (1) reduced within-population inbreeding depression in small populations, which may be associated with an observed shift toward increased selfing rate, and (2) hybrid vigor in crosses between small populations. Our genomic findings imply several strategies for optimal sourcing and crossing of populations for restoration and aquaculture, but these require experimental validation. Overall, our work reveals strong genetic structure and suggests that conservation strategies should consider the multiple health risks faced by small populations whose evolutionary dynamics are dominated by genetic drift.

As one of the most influential environmental factors, light fundamentally shapes plant physiology and growth traits.^{1,2,3,4,5,6,7,8,9,10} The hypocotyl is critical for the morphological establishment of the seedling, and its length displays remarkable plasticity upon perception of changes in the light conditions.^{4,5,8,9,10,11,12,13,14,15} Although remodeling of the primary cell walls is well-documented to play an important role in hypocotyl growth, how the hypocotyl elongation rate is swiftly repressed at the dark-to-light transition remains elusive.^{16,17,18,19,20,21,22,23,24,25} Here, we show that expression of an Arabidopsis microRNA, miR775, is quickly inhibited at the dark-to-light transition by ELONGATED HYPOCOTYL 5 (HY5), an essential negative regulator of hypocotyl elongation that is degraded in the dark and accumulates in the light.²⁶ We found that this repression allows the miR775-targeted GALACTOSYLTRANSFERASE 9 (GALT9) to accumulate in the transverse walls of hypocotyl cells within 10 min of light exposure. Genetic analysis coupled with time-lapse photography demonstrates that GALT9 is both necessary and sufficient for controlling the differential hypocotyl growth rates at the dark-to-light transition. Immunohistochemical analysis and coherent Raman microscopy reveal that the polarized accumulation of GALT9 confers a rapid increase in the pectin content of the transverse walls. Atomic force microscopy (AFM) confirms that polarized pectin accumulation mediated by the HY5-miR775-GALT9 repression cascade correlates with rapid asymmetric increases in cell wall rigidity and hence decreases in cell elongation in the light. Together, these findings add new insights into the cellular mechanism governing differential hypocotyl growth at the dark-to-light transition and should also benefit the general understanding of polarized cell expansion in plants.

Beetles that feed on the nutritionally depauperate and recalcitrant tissues provided by the leaves, stems, and roots of living plants comprise one-quarter of herbivorous insect species. Among the key adaptations for herbivory are plant cell wall-degrading enzymes (PCWDEs) that break down the fastidious polymers in the cell wall and grant access to the nutritious cell content. While largely absent from the non-herbivorous ancestors of beetles, such PCWDEs were occasionally acquired via horizontal gene transfer (HGT) or by the uptake of digestive symbionts. However, the macroevolutionary dynamics of PCWDEs and their impact on evolutionary transitions in herbivorous insects remained poorly understood. Through genomic and transcriptomic analyses of 74 leaf beetle species and 50 symbionts, we show that multiple independent events of microbe-to-beetle HGT and specialized symbioses drove convergent evolutionary innovations in approximately 21,000 and 13,500 leaf beetle species, respectively. Enzymatic assays indicate that these events significantly expanded the beetles' digestive repertoires and thereby contributed to their adaptation and diversification. Our results exemplify how recurring HGT and symbiont acquisition catalyzed digestive and nutritional adaptations to herbivory and thereby contributed to the evolutionary success of a megadiverse insect taxon.

Animals construct diverse behavioral repertoires by moving a limited number of body parts with varied kinematics and patterns of coordination. There is evidence that distinct movements can be generated by changes in activity dynamics within a common pool of motoneurons or by selectively engaging specific subsets of motoneurons in a task-dependent manner. However, in most cases, we have an incomplete understanding of the patterns of motoneuron activity that generate distinct actions and of how upstream premotor circuits select and assemble such motor programs. In this study, we used two closely related but kinematically distinct types of saccadic eye movement in larval zebrafish as a model to examine circuit control of movement diversity. In contrast to the prevailing view of a final common pathway, we found that in the oculomotor nucleus, distinct subsets of motoneurons were engaged for each saccade type. This type-specific recruitment was topographically organized and aligned with ultrastructural differences in motoneuron morphology and afferent synaptic innervation. Medially located motoneurons were active for both saccade types, and circuit tracing revealed a type-agnostic premotor pathway that appears to control their recruitment. By contrast, a laterally located subset of motoneurons was specifically active for hunting-associated saccades and received premotor input from pretectal hunting command neurons. Our data support a model in which generalist and action-specific premotor pathways engage distinct subsets of motoneurons to elicit varied movements of the same body part that subserve distinct behavioral functions.