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We studied the convergence of two afferent pairs of single vestibular neurons by selective stimulation of the horizontal semicircular canal (HC) and saccular (SAC) nerves, and the HC and utricular (UT) nerves in decerebrate cats. All recorded neurons were classified as vestibulospinal (VS), vestibulo-oculospinal (VOS) or vestibulo-ocular (VO), by antidromic stimulation from the oculomotor/trochlear nuclei and the spinal cord: neurons that could not be activated from any test sites were classified as vestibular (V) neurons. Of a total of 125 neurons activated by stimulation of the HC/SAC nerves, 21(17%) received convergent inputs. Twelve of 21 neurons received monosynaptic excitatory inputs from both nerves. About half (9/21, 43%) of the convergent neurons were classified as VS neurons, the majority of which descended through the ipsilateral lateral vestibulospinal tract (i-LVST). The HC/SAC convergent neurons were located in the rostral part of the descending, the medial and the caudal-ventral part of the lateral vestibular nucleus. In 80 neurons studied by stimulation of the HC/UT nerves, both inputs converged in 12 (15%) neurons, more than half of which were VS neurons. Eight of 12 convergent neurons received excitatory inputs followed by inhibition from both the HC and UT nerves. A few convergent neurons (3/12) projected to the oculomotor/trochlear nucleus. Half of the convergent and non-convergent VS neurons descended to the spinal cord through the i-LVST, and the only one VOS convergent neuron via the medial vestibulospinal tract. Most of the convergent neurons were located in the lateral, the rostral part of the descending and medial vestibular nucleus. The percentages of HC/SAC and HC/UT convergence were half those of the posterior semicircular canal (PC), PC/SAC (33%) and PC/UT (33%) convergence, respectively. The convergent neurons receiving the HC and otolith inputs may contribute at least partly to the vestibulocollic reflex.
Convergent extension, the simultaneous narrowing and lengthening of a tissue, plays a major role in shaping and patterning the neural ectoderm in vertebrate embryos. In this paper, we characterize the cellular mechanism underlying convergent extension of the neural ectoderm in the Xenopus laevis late gastrula and neurula embryo. Neural ectoderm in X. laevis consists of two components, a superficial layer of epithelial cells overlying deep mesenchymal cells. To investigate the force contribution of the deep cells to convergent extension, we explanted single layers of neural deep cells from late gastrula stage embryos. These "neural deep cell explants" undergo active convergent extension autonomously, implying that these cells contribute force for neural convergent extension in vivo. Using time-lapse videorecording of these explants, we observed the neural deep cell behaviors (previously hidden behind an opaque epithelium) underlying convergent extension. We show that neural deep cells mediolaterally intercalate to form a longer, narrower tissue and that cell shape change and cell division contribute little to their convergent extension. Moreover, we characterize the neural deep cell motility driving mediolateral intercalation, also using time-lapse videorecordings. Analyses of these videos revealed that, on average, neural deep cells exhibit mediolaterally biased protrusive activity which is expressed in an episodic fashion. We propose that neural deep cells accomplish mediolateral intercalation by applying their protrusions upon one another, exerting traction, and pulling themselves between one another. This mechanism is similar to that previously described for convergent extension of the mesodermal cells. However, because the neural deep cells do not mediolaterally elongate during their convergent extension as the mesodermal cells do, we predict that a given intercalation will result in more extension for neural deep cells than for the mesodermal cells. Intercalation of neural cells also likely occurs in a more episodic manner than that of the mesodermal cells because the neural cells' mediolateral protrusive activity is episodic, whereas the protrusive activity of mesodermal cells is more continuous. These differences in protrusive activity and cell shape changes between the neural and mesodermal regions may reflect specializations of the same basic mechanism of mediolateral intercalation, tailored to accommodate other aspects of patterning and development of each tissue. These descriptions of the active cell motility underlying neural convergent extension in X. laevis are the first high-resolution video documentation of protrusive activity during neural convergent extension in any system. Our findings provide an important step in the investigation of neural convergent extension in X. laevis and further our understanding of convergent extension in general.
Uncovering the key genetic basis of complex phenotypic convergence in distantly related species has been a long-standing focus in evolutionary biology and genetics, and the convergent evolution of gliding in mammals offers a valuable opportunity to address this question. Here, we investigated the genomic basis of convergent evolution of gliding in mammals by analyzing both protein-coding genes and conserved non-coding elements (CNEs). We first de novo assembled and annotated two chromosome-level genomes of gliding mammals, the red and white giant flying squirrel (Petaurista alborufus) and sugar gliders (Petaurus breviceps), and conducted comprehensive comparative genomic analysis combined with another gliding mammal, the Sunda flying lemur (Galeopterus variegatus) and 14 background species. We found that the convergent evolution of protein-coding genes provided relatively limited but functionally relevant evidence linked to gliding phenotypes. By contrast, we found that gliding-accelerated CNEs (GACNEs) cluster near functionally equivalent genes and frequently aggregate into highly diverged yet functionally convergent hotspot regions. Across the three gliding lineages, both GACNEs and hotspot GACNEs show strong convergence in their functional enrichment profiles, suggesting a broad genetic basis underlying the convergent gliding phenotype. Furthermore, we identified 72 core transcription factors underpinning the genetic basis of gliding convergence, including EMX2 and ZFHX3, potentially involved in multiple aspects of gliding adaptation. Our study highlights the role of functional convergence in regulatory regions as a key mechanism in mammalian gliding convergence, offering valuable insights and strategies for uncovering the genetic basis of complex convergent traits, thereby advancing understanding of the molecular basis of convergent traits.
Resemblance between animal taxa may be due to convergence rather than to recent common ancestry. Constraints on biological materials and adaptation to particular habits or habitats will produce widespread convergence. How may we distinguish the two causes of resemblance? The relationship between convergence and taxonomy is discussed, demonstrating that the choice of taxonomic method will itself determine the extent to which convergence is perceived. In particular, cladistic analysis based on parsimony will tend to minimise and thus conceal convergence: neither the resulting cladogram nor a consistency index derived from it can be used to assess the prevalence of convergence. With any taxonomic system, there can be no substitute for evaluation of the morphological characters used. Complementary use of molecular characters shows promise: we wait further understanding of constraints in genetic evolution and of the possibilities of convergence at this level also. These general principles are illustrated with a range of examples from within and between invertebrate phyla: the phylogeny of Cnidaria and Platyhelminthes cannot be traced with certainty, but where the fossil record allows clear rooting, as for the echinoderms and in particular the echinoids, combination of morphological and molecular methods has made much progress. Sub-groups within a phylum, for example opisthobranch molluscs and the dipteran Phoridae, may show an uncontested phylogeny, and here studies have precisely identified convergence and shown that it may be the commoner cause of resemblance. Adaptation to exacting environments shown by terrestial and freshwater nemertines may also result in a predominance of convergent resemblance. Traditional grouping of phyla breaks down on re-examination of supposedly key characters, such as segmentation, body cavities, germ layers and symmetry, each of which must have had multiple origins: nor are developmental stages (especially not larvae) a reliable guide to relationships. Demarcation of phyla may be difficult, as with arthropods, and location of phyla is even more difficult, due to their early and rapid radiation. Over-simplified definition of characters has bedevilled invertebrate classification and the use of molecular data has not yet resolved the major controversies. The question "How common is convergence?' remains unanswered and may be unanswerable. Our examples indicate that even the minimum detectable levels of convergence are often high, and we conclude that at all levels convergence has been greatly underestimated.