From tadpole to adult frog locomotion

Metamorphosis of a unicellular green alga in the presence of acetate and a spatially structured three-dimensional environment

Photosynthetic protists, named microalgae, are key players in global primary production. The green microalga Chlamydomonas reinhardtii is a well-studied model organism. In nature, it dwells in acetate-rich paddy rice soil, which is not mimicked by standard liquid laboratory conditions. Here, we maintained the algae in a liquid environment with spatially structured 3-D components (S3-D) and acetate recreating natural conditions. We perform transcriptome sequencing, immunoblotting, fluorescence and electron microscopy, and Raman microspectroscopy to characterize the algae in S3-D vs homogeneous conditions. The algae undergo a metamorphosis-like process when transitioned from homogeneous aquatic to a lifestyle simulating acetate-rich rice soil. These conditions result in reduced cell size and cilia length, an enlarged eyespot and many cells with double-layered cell walls. RNA-Seq reveals alterations in c. 2400 transcripts. Four key photoreceptors including CRY-DASH1 and phototropin governing plastid metabolism along with its eyespot are altered in their protein expression. Consequently, photosynthetic pigments, lipids and starch levels vary as do starch distribution patterns. Fitness against antagonistic bacteria is enhanced concurrently with the downregulation of an involved Ca2+ channel transcript. This study highlights the profound impact of S3-D initiating processes inaccessible under homogeneous laboratory conditions. Thus, overexpression lines for certain photoreceptors and starch are naturally created.

Unravelling drivers on the morphological diversification of the terminal phalanx in hyloid frogs

The anuran locomotor system integrates traits that are influenced by phylogenetic, ecological, and development constraints. Given their significance to locomotion, we studied terminal phalange morphology in the Hyloidea group. We aim to deduce if morphological variability stems from phylogenetic, ecological, or life-cycle constraints. We explore the influence of size on variation and assess if evolutionary rates and shape disparities differ among the groups under consideration. Finally, we optimized phalangeal morphology within the phylogenetic framework to delineate evolutionary trends. We included 424 specimens of 128 species representing 17 families of Hyloidea and two of non-hyloid anurans. Configuration of the terminal phalanx was quantified using geometric morphometrics and characterized through qualitative traits. We established four categories based on microhabitats and locomotor abilities. Our life-cycle categorization distinguishes species by their consistent or changing microhabitat across larval and adult stages. The results show a complex scenario, where certain clades occupy distinct regions of morphospace, but there is also a relationship between phalangeal shape, microhabitats, and locomotor abilities. However, both the phylogenetic signal and the relationship with microhabitats and locomotor abilities are not particularly robust. Species inhabiting arboreal microhabitats develop convergent traits to thrive in this niche, such as rounded proximal epiphysis and the claw-shaped phalanx. Morphological disparity was higher in walkers, which also includes arboreal species, prompting further questions on the demands of locomotion in vertical substrates.

Spinal neuron diversity scales exponentially with swim-to-limb transformation during frog metamorphosis

Vertebrates exhibit a wide range of motor behaviors, ranging from swimming to complex limb-based movements. Here we take advantage of frog metamorphosis, which captures a swim-to-limb-based movement transformation during the development of a single organism, to explore changes in the underlying spinal circuits. We find that the tadpole spinal cord contains small and largely homogeneous populations of motor neurons (MNs) and V1 interneurons (V1s) at early escape swimming stages. These neuronal populations only modestly increase in number and subtype heterogeneity with the emergence of free swimming. In contrast, during frog metamorphosis and the emergence of limb movement, there is a dramatic expansion of MN and V1 interneuron number and transcriptional heterogeneity, culminating in cohorts of neurons that exhibit striking molecular similarity to mammalian motor circuits. CRISPR/Cas9-mediated gene disruption of the limb MN and V1 determinants FoxP1 and Engrailed-1, respectively, results in severe but selective deficits in tail and limb function. Our work thus demonstrates that neural diversity scales exponentially with increasing behavioral complexity and illustrates striking evolutionary conservation in the molecular organization and function of motor circuits across species.

Neuropeptide Modulation Enables Biphasic Internetwork Coordination via a Dual-Network Neuron

Linked rhythmic behaviors, such as respiration/locomotion or swallowing/chewing, often require coordination for proper function. Despite its prevalence, the cellular mechanisms controlling coordination of the underlying neural networks remain undetermined in most systems. We use the stomatogastric nervous system of the crab Cancer borealis to investigate mechanisms of internetwork coordination, due to its small, well-characterized feeding-related networks (gastric mill [chewing, ∼0.1 Hz]; pyloric [filtering food, ∼1 Hz]). Here, we investigate coordination between these networks during the Gly1-SIFamide neuropeptide modulatory state. Gly1-SIFamide activates a unique triphasic gastric mill rhythm in which the typically pyloric-only LPG neuron generates dual pyloric-plus gastric mill-timed oscillations. Additionally, the pyloric rhythm exhibits shorter cycles during gastric mill rhythm-timed LPG bursts, and longer cycles during IC, or IC plus LG gastric mill neuron bursts. Photoinactivation revealed that LPG is necessary to shorten pyloric cycle period, likely through its rectified electrical coupling to pyloric pacemaker neurons. Hyperpolarizing current injections demonstrated that although LG bursting enables IC bursts, only gastric mill rhythm bursts in IC are necessary to prolong the pyloric cycle period. Surprisingly, LPG photoinactivation also eliminated prolonged pyloric cycles, without changing IC firing frequency or gastric mill burst duration, suggesting that pyloric cycles are prolonged via IC synaptic inhibition of LPG, which indirectly slows the pyloric pacemakers via electrical coupling. Thus, the same dual-network neuron directly conveys excitation from its endogenous bursting and indirectly funnels synaptic inhibition to enable one network to alternately decrease and increase the cycle period of a related network.

Switching neuron contributions to second network activity

Network flexibility is important for adaptable behaviors. This includes neuronal switching, where neurons alter their network participation, including changing from single- to dual-network activity. Understanding the implications of neuronal switching requires determining how a switching neuron interacts with each of its networks. Here, we tested 1) whether "home" and second networks, operating via divergent rhythm generation mechanisms, regulate a switching neuron and 2) if a switching neuron, recruited via modulation of intrinsic properties, contributes to rhythm or pattern generation in a new network. Small, well-characterized feeding-related networks (pyloric, ∼1 Hz; gastric mill, ∼0.1 Hz) and identified modulatory inputs make the isolated crab (Cancer borealis) stomatogastric nervous system (STNS) a useful model to study neuronal switching. In particular, the neuropeptide Gly1-SIFamide switches the lateral posterior gastric (LPG) neuron (2 copies) from pyloric-only to dual-frequency pyloric/gastric mill (fast/slow) activity via modulation of LPG-intrinsic properties. Using current injections to manipulate neuronal activity, we found that gastric mill, but not pyloric, network neurons regulated the intrinsically generated LPG slow bursting. Conversely, selective elimination of LPG from both networks using photoinactivation revealed that LPG regulated gastric mill neuron-firing frequencies but was not necessary for gastric mill rhythm generation or coordination. However, LPG alone was sufficient to produce a distinct pattern of network coordination. Thus, modulated intrinsic properties underlying dual-network participation may constrain which networks can regulate switching neuron activity. Furthermore, recruitment via intrinsic properties may occur in modulatory states where it is important for the switching neuron to actively contribute to network output.NEW & NOTEWORTHY We used small, well-characterized networks to investigate interactions between rhythmic networks and neurons that switch their network participation. For a neuron switching into dual-network activity, only the second network regulated its activity in that network. In addition, the switching neuron was sufficient but not necessary to coordinate second network neurons and regulated their activity levels. Thus, regulation of switching neurons may be selective, and a switching neuron is not necessarily simply a follower in additional networks.

Changes in thermal sensitivity of Rhinella arenarum tadpoles (Anura: Bufonidae) exposed to sublethal concentrations of different pesticide fractions (Lorsban® 75WG)

The intensive use of agrochemicals and the rapid increase of global temperatures have modified the thermal conditions of aquatic environments, thus increasing amphibians' vulnerability to global warming and positioning them at great risk. Commercial formulations of chlorpyrifos (COM) are the pesticides most widely used in agricultural activities, with a high toxic potential on amphibians. However, little is known about the separate effects of the active ingredient (CPF) and adjuvants (AD). We studied the thermal sensitivity at different concentrations and pesticide fractions in Rhinella arenarum tadpoles, on thermal tolerance limits (CTmax = Critical thermal maximum and CTmin = Critical thermal minimum), swimming speed (Ss), Optimum temperature (Top), and Thermal breadth 50 (B50). Our results demonstrate that the pesticide active ingredient, the adjuvants, and the commercial formulation of chlorpyrifos differentially impair the thermal sensitivity of R. arenarum tadpoles. The pesticide fractions affected the heat and the cold tolerance (CTmax and CTmin), depending on the concentrations they were exposed to. The locomotor performance (Ss, Top, and B50) of tadpoles also varied among fractions, treatments, and environmental temperatures. In the context of climate change, the outcomes presented are particularly relevant, as mean temperatures are increasing at unprecedented rates, which suggests that tadpoles inhabiting warming and polluted ponds are currently experiencing deleterious conditions. Considering that larval stages of amphibians are the most susceptible to changing environmental conditions and the alarming predictions about environmental temperatures in the future, it is likely that the synergism between high temperatures and pesticide exposure raise the threat of population deletions in the coming years.