Development of ear asymmetry in the American barn owl (Tyto furcata pratincola)

Emergence of a left-right symmetric body plan in vertebrate embryos

External bilateral symmetry is a prevalent feature in vertebrates, which emerges during early embryonic development. To begin with, vertebrate embryos are largely radially symmetric before transitioning to bilaterally symmetry, after which, morphogenesis of various bilateral tissues (e.g somites, otic vesicle, limb bud), and structures (e.g palate, jaw) ensue. While a significant amount of work has probed the mechanisms behind symmetry breaking in the left-right axis leading to asymmetric positioning of internal organs, little is known about how bilateral tissues emerge at the same time with the same shape and size and at the same position on the two sides of the embryo. By discussing emergence of symmetry in many bilateral tissues and structures across vertebrate model systems, we highlight that understanding symmetry establishment is largely an open field, which will provide deep insights into fundamental problems in developmental biology for decades to come.

Cilia and Nodal Flow in Asymmetry: An Engineering Perspective

Over the past several years, cilia in the primitive node have become recognized more and more for their contribution to development, and more specifically, for their role in axis determination. Although many of the mechanisms behind their influence remain undocumented, it is known that their presence and motion in the primitive node of developing embryos is the determinant of the left-right axis. Studies on cilial mechanics and nodal fluid dynamics have provided clues as to how this asymmetry mechanism works, and more importantly, have shown that direct manipulation of the flow field in the node can directly influence physiology. Although relatively uncommon, cilial disorders have been shown to have a variety of impacts on individuals from chronic respiratory infections to infertility, as well as situs inversus which is linked to congenital heart disease. After first providing background information pertinent to understanding nodal flow and information on why this discussion is important, this paper aims to give a review of the history of nodal cilia investigations, an overview of cilia mechanics and nodal flow dynamics, as well as a review of research studies current and past that sought to understand the mechanisms behind nodal cilia's involvement in symmetry-breaking pathways through a biomedical engineering perspective. This discussion has the additional intention to compile interdisciplinary knowledge on asymmetry and development such that it may encourage more collaborative efforts between the sciences on this topic, as well as provide insight on potential paths forward in the field.

Development of the horizontal optocollic reflex in juvenile barn owls (Tyto furcata pratincola)

Adult barn owls and primates possess an almost symmetric monocular rotational horizontal optocollic reflex. In primates, the reflex is initially asymmetric and becomes symmetric with time after birth. The condition in barn owls has not been studied so far. Here, we present data on the development of this reflex in this bird. We tested juvenile barn owls from the time before they open their eyes after hatching to the time they reach adult feather length. Wide-field visual patterns served as stimuli. They were presented at different rotational speeds in binocular and monocular settings. The binocular horizontal optocollic responses of juvenile barn owls were symmetric and adult-like on the first day that the birds responded to the stimulus. The monocular responses showed different rates of development in respect to stimulus velocity and stimulus direction. For velocities up to 20 deg/s, the monocular reflex was also adult-like on the first day that the birds responded to the stimulus. An initially higher asymmetry for 30 deg/s compared to adults disappeared within about two weeks. The development at even higher velocities remained unclear.

Binaural Synthetic Aperture Imaging of the Field of Audition as the Head Rotates and Localisation Perception of Monophonic Sound Listened to through Headphones

A human listening to monophonic sound through headphones perceives the sound to emanate from a point inside the head at the auditory centre at effectively zero range. The extent to which this is predicted by synthetic-aperture calculation performed in response to head rotation is explored. The instantaneous angle between the auditory axis and the acoustic source, lambda, for the zero inter-aural time delay imposed by headphones is 90°. The lambda hyperbolic cone simplifies to the auditory median plane, which intersects a spherical surface centred on the auditory centre, along a prime meridian lambda circle. In a two-dimensional (2-D) synthetic-aperture computation, points of intersection of all lambda circles as the head rotates constitute solutions to the directions to acoustic sources. Geometrically, lambda circles cannot intersect at a point representing the auditory centre; nevertheless, 2-D synthetic aperture images for a pure turn of the head and for a pure lateral tilt yield solutions as pairs of points on opposite sides of the head. These can reasonably be interpreted to be perceived at the sums of the position vectors of the pairs of points on the acoustic image, i.e., at the auditory centre. But, a turn of the head on which a fixed lateral tilt of the auditory axis is concomitant (as in species of owl) yields a 2-D synthetic-aperture image without solution. However, extending a 2-D synthetic aperture calculation to a three-dimensional (3-D) calculation will generate a 3-D acoustic image of the field of audition that robustly yields the expected solution.

The early origin of a birdlike inner ear and the evolution of dinosaurian movement and vocalization

Reptiles, including birds, exhibit a range of behaviorally relevant adaptations that are reflected in changes to the structure of the inner ear. These adaptations include the capacity for flight and sensitivity to high-frequency sound. We used three-dimensional morphometric analyses of a large sample of extant and extinct reptiles to investigate inner ear correlates of locomotor ability and hearing acuity. Statistical analyses revealed three vestibular morphotypes, best explained by three locomotor categories-quadrupeds, bipeds and simple fliers (including bipedal nonavialan dinosaurs), and high-maneuverability fliers. Troodontids fall with Archaeopteryx among the extant low-maneuverability fliers. Analyses of cochlear shape revealed a single instance of elongation, on the stem of Archosauria. We suggest that this transformation coincided with the origin of both high-pitched juvenile location, alarm, and hatching-synchronization calls and adult responses to them.

The Binaural Illusion of Wallach (1940) Apparent in Synthetic Aperture Images of the Field of Audition Generated as the Head Turns

Wallach (J. Exp. Psychol. 1940, 27, 339–368) predicted that a human subject rotating about a vertical axis through the auditory centre, having an acoustic source rotating around the same axis at twice the rotation rate of the human subject, would perceive the acoustic source to be stationary. His prediction, which he confirmed by experiment, was made to test the hypothesis that humans integrate head movement information that is derived from the vestibular system and visual cues, with measurements of arrival time differences between the acoustic signals received at the ears, to determine directions to acoustic sources. The simulation experiments described here demonstrate that a synthetic aperture calculation performed as the head turns, to determine the direction to an acoustic source (Tamsett, Robotics 2017, 6, 10), is also subject to the Wallach illusion. This constitutes evidence that human audition deploys a synthetic aperture process in which a virtual image of the field of audition is populated as the head turns, and from which directions to acoustic sources are inferred. The process is akin to those in synthetic aperture sonar/radar technologies and to migration in seismic profiler image processing. It could be implemented in a binaural robot localizing acoustic sources from arrival time differences in emulation of an aspect of human audition.

Ear asymmetry in Tengmalm's owl (Aegolius funereus): Two phases of asymmetrical development of the squamoso-occipital wing

Ear asymmetry is an adaptive characteristic present in the order of owls (Strigiformes). It developed independently up to seven times in this taxon and is accompanied by morphological adaptations in bones or soft tissues around or at the ear openings. Within all strigiform species, the Boreal or Tengmalm's owl (Aegolius funereus) possesses a particularly complex bilateral ear asymmetry that results from modifications of the neurocranium and some cartilaginous elements. While the ear asymmetry in adult birds has been described in detail, data on its development is scarce. Here we describe the development of the asymmetric squamoso-occipital wing of A. funereus from its embryonic origin up to adulthood. The asymmetry of the squamoso-occipital wing develops in two phases. Firstly, it originates as a cartilaginous structure in the last ten days before hatching. Its frontal margin shows a bilateral asymmetry from the beginning of its development while the rostro-ventral process stays symmetrical up to post-hatching day 35. Secondly, when the fledglings have already left the nest, the squamoso-occipital wing ossifies. Moreover, the rostro-ventral process on the right side grows towards the eyeball, while there is no relative displacement on the left side. Thus, the developmental process in A. funereus differs from that in the barn owl that develops its soft tissue asymmetry in one phase and completes the asymmetry before hatching. The new data presented here extend our knowledge of the mechanisms underlying the asymmetric skull development in owls.

New genome assembly of the barn owl (Tyto alba alba)

New genomic tools open doors to study ecology, evolution, and population genomics of wild animals. For the Barn owl species complex, a cosmopolitan nocturnal raptor, a very fragmented draft genome was assembled for the American species (Tyto furcata pratincola) (Jarvis et al. 2014). To improve the genome, we assembled de novo Illumina and Pacific Biosciences (PacBio) long reads sequences of its European counterpart (Tyto alba alba). This genome assembly of 1.219 Gbp comprises 21,509 scaffolds and results in a N50 of 4,615,526 bp. BUSCO (Universal Single-Copy Orthologs) analysis revealed an assembly completeness of 94.8% with only 1.8% of the genes missing out of 4,915 avian orthologs searched, a proportion similar to that found in the genomes of the zebra finch (Taeniopygia guttata) or the collared flycatcher (Ficedula albicollis). By mapping the reads of the female American barn owl to the male European barn owl reads, we detected several structural variants and identified 70 Mbp of the Z chromosome. The barn owl scaffolds were further mapped to the chromosomes of the zebra finch. In addition, the completeness of the European barn owl genome is demonstrated with 94 of 128 proteins missing in the chicken genome retrieved in the European barn owl transcripts. This improved genome will help future barn owl population genomic investigations.

EvoDevo in owl ear asymmetry-The little owl (Athene noctua)

Owls are well adapted to nocturnal hunting. This includes vision tuned to low level light conditions, silent flight, and asymmetrical ears. Asymmetrical ears facilitate sound localization and evolved up to seven times independently in the evolutionary history of owls. However, there are also owl species with a crepuscular or diurnal lifestyle, like the little owl (Athene noctua), that have symmetrical ears as adults. Here we show that a small, but significant ear asymmetry occurs in the embryonic development of little owls, despite the presence of symmetrical ears in adults. In the asymmetric period, the left ear opening is bigger in area than its counterpart on the right. The asymmetry in the little owl occurs in the same stages at which the asymmetry in the barn owl (Tyto furcata) develops, but in the little owl the asymmetry vanishes shortly before hatching. Asymmetries in the size of the ear openings are also found in the adults of other owl species, most of them belonging to the genus Strix. We interpret our finding as an indication of a secondarily evolved diurnal activity in little owls. Further, ear asymmetry might be more deeply rooted in the evolution of owls than previously assumed.