Timing of organogenesis underscores the evolution of neonatal life histories and powered flight in bats.

Bats have undergone one of the most drastic limb innovations in vertebrate history, associated with the evolution of powered flight. Knowledge of the genetic basis of limb organogenesis in bats has increased but little has been documented regarding the differences between limb organogenesis in bats and that of other vertebrates. We conducted embryological comparisons of the timelines of limb organogenesis in 24 bat species and 72 non-bat amniotes. In bats, the time invested for forelimb organogenesis has been considerably extended and the appearance timing of the forelimb ridge has been significantly accelerated, whereas the timing of the finger and first appearance of the claw development has been delayed, facilitating the enlargement of the manus. Furthermore, we discovered that bats initiate the development of their hindlimbs earlier than their forelimbs compared with other placentals. Bat neonates are known to be able to cling continuously with their well-developed foot to the maternal bodies or habitat substrates soon after birth. We suggest that this unique life history of neonates, which possibly coevolved with powered flight, has driven the accelerated development of the hindlimb and precocious foot.

On the sequence heterochrony of cranial ossification of bats in light of Haeckel's recapitulation theory.

Haeckel's recapitulation theory has been a controversial topic in evolutionary biology. However, we have seen some recent cases applying Haeckel's view to interpret the interspecific variation of prenatal ontogeny. To revisit the validity of Haeckel's recapitulation theory, we take bats that have undergone drastic morphological changes and possess a characteristic ecology as a case study. All members of Rhinolophoidea and Yangochiroptera can generate an ultrasonic pulse from the larynx to interpret surrounding objects (laryngeal echolocation) whereas Pteropodidae lacks such ability. It is known that the petrosal bone is particularly derived in shape and expanded in laryngeal echolocators. If Haeckel's recapitulation theory holds, the formation of this derived trait should occur later than those of other bones. Therefore, we compared the prenatal ossification timing of the petrosal in 15 bat species and five outgroup species. We found that the ossification of the petrosal is accelerated in laryngeal echolocators while it is the last bone to ossify in non-laryngeal echolocating bats and non-volant mammals, which runs counter to the prediction generated by Haeckel's recapitulation theory. We point out the evolutionarily labile nature of trait developmental timing and emphasize that Haeckel's recapitulation theory does not hold in many cases. We caution that generating predictions on ancestral conditions and evolutionary history leading from Haeckel's recapitulation theory is not well supported.

Embryonic staging of bats with special reference to Vespertilio sinensis and its cochlear development.

BACKGROUND: How bats deviate heterochronically from other mammals remains largely unresolved, reflecting the lack of a quantitative staging framework allowing comparison among species. The standard event system (SES) is an embryonic staging system allowing quantitative detection of interspecific developmental variations. Here, the first SES-based staging system for bats, using Asian parti-colored bat (Vespertilio sinensis) is introduced. General aspects of normal embryonic development and the three-dimensional development of the bat cochlea were described for the first time. Recoding the embryonic staging tables of 18 previously reported bat species and Mus musculus into the SES system, quantitative developmental comparisons were performed. RESULTS: It was found that limb bud development of V. sinensis is relatively late among 19 bat species and late limb development is a shared trait of vespertilionid bats. The inner ear cochlear canal forms before the semicircular canal in V. sinensis while the cochlear canal forms after the semicircular canal in non-volant mammals. CONCLUSIONS: The present approach using the SES system provides a powerful framework to detect the peculiarities of bat development. Incorporating the timing of gene expression patterns into the SES framework will further contribute to the understanding of the evolution of specialized features in bats.

Development of the hyolaryngeal architecture in horseshoe bats: insights into the evolution of the pulse generation for laryngeal echolocation.

BACKGROUND: The hyolaryngeal apparatus generates biosonar pulses in the laryngeally echolocating bats. The cartilage and muscles comprising the hyolarynx of laryngeally echolocating bats are morphologically modified compared to those of non-bat mammals, as represented by the hypertrophied intrinsic laryngeal muscle. Despite its crucial contribution to laryngeal echolocation, how the development of the hyolarynx in bats differs from that of other mammals is poorly documented. The genus Rhinolophus is one of the most sophisticated laryngeal echolocators, with the highest pulse frequency in bats. The present study provides the first detailed description of the three-dimensional anatomy and development of the skeleton, cartilage, muscle, and innervation patterns of the hyolaryngeal apparatus in two species of rhinolophid bats using micro-computed tomography images and serial tissue sections and compares them with those of laboratory mice. Furthermore, we measured the peak frequency of the echolocation pulse in active juvenile and adult individuals to correspond to echolocation pulses with hyolaryngeal morphology at each postnatal stage. RESULTS: We found that the sagittal crests of the cricoid cartilage separated the dorsal cricoarytenoid muscle in horseshoe bats, indicating that this unique morphology may be required to reinforce the repeated closure movement of the glottis during biosonar pulse emission. We also found that the cricothyroid muscle is ventrally hypertrophied throughout ontogeny, and that the cranial laryngeal nerve has a novel branch supplying the hypertrophied region of this muscle. Our bioacoustic analyses revealed that the peak frequency shows negative allometry against skull growth, and that the volumetric growth of all laryngeal cartilages is correlated with the pulse peak frequency. CONCLUSIONS: The unique patterns of muscle and innervation revealed in this study appear to have been obtained concomitantly with the acquisition of tracheal chambers in rhinolophids and hipposiderids, improving sound intensity during laryngeal echolocation. In addition, significant protrusion of the sagittal crest of the cricoid cartilage and the separated dorsal cricoarytenoid muscle may contribute to the sophisticated biosonar in this laryngeally echolocating lineage. Furthermore, our bioacoustic data suggested that the mineralization of these cartilages underpins the ontogeny of echolocation pulse generation. The results of the present study provide crucial insights into how the anatomy and development of the hyolaryngeal apparatus shape the acoustic diversity in bats.

Anatomy and homology of the caudal auricular muscles in greater short-nosed fruit bat (Cynopterus sphinx).

Bats can be phylogenetically classified into three major groups: pteropodids, rhinolophoids, and yangochiropterans. While rhinolophoids and yangochiropterans are capable of laryngeal echolocation, pteropodids lack this ability. Delicate ear movements are essential for echolocation behavior in bats with laryngeal echolocation. Caudal auricular muscles, especially the cervicoauricularis group, play a critical role in such ear movements. Previously, caudal auricular muscles were studied in three species of bats with laryngeal echolocation, but to our knowledge, there have been no studies on non-laryngeal echolocators, the pteropodids. Here, we describe the gross anatomy of the cervicoauricularis muscles and their innervation in Cynopterus sphinx by using diffusible iodine-based contrast-enhanced computed tomography and 3D reconstructions of immunohistochemically stained serial sections. A previous study on bats with laryngeal echolocation reported that rhinolophoids have four cervicoauricularis muscles and yangochiropterans have three. We observed three cervicoauricularis muscles in the pteropodid C. sphinx. The number of cervicoauricularis muscles and their innervation pattern were comparable to those of non-bat boreoeutherian mammals and yangochiropterans, suggesting that pteropodids, and yangochiropterans maintain the general condition of boreoeutherian mammals and that rhinolophoids have a derived condition. The unique nomenclature had been previously applied to the cervicoauricularis muscles of bats with laryngeal echolocation, but given the commonality between non-bat laurasiatherians and bats, with the exception of rhinolophoids, maintaining the conventional nomenclature (i.e., M. cervicoauricularis superficialis, M. cervicoauricularis medius, and M. cervicoauricularis profundus) is proposed for bats.

Temporal and regulatory dynamics of the inner ear transcriptome during development in mice.

The inner ear controls hearing and balance, while the temporal molecular signatures and transcriptional regulatory dynamics underlying its development are still unclear. In this study, we investigated time-series transcriptome in the mouse inner ear from embryonic day 11.5 (E11.5) to postnatal day 7 (P7) using bulk RNA-Seq. A total of 10,822 differentially expressed genes were identified between pairwise stages. We identified nine significant temporal expression profiles using time-series expression analysis. The constantly down-regulated profiles throughout the development are related to DNA activity and neurosensory development, while the constantly upregulated profiles are related to collagen and extracellular matrix. Further co-expression network analysis revealed that several hub genes, such as Pnoc, Cd9, and Krt27, are related to the neurosensory development, cell adhesion, and keratinization. We uncovered three important transcription regulatory paths during mice inner ear development. Transcription factors related to Hippo/TGFß signaling induced decreased expressions of genes related to the neurosensory and inner ear development, while a series of INF genes activated the expressions of genes in immunoregulation. In addition to deepening our understanding of the temporal and regulatory mechanisms of inner ear development, our transcriptomic data could fuel future multi-species comparative studies and elucidate the evolutionary trajectory of auditory development.

On the Embryonic Development of the Nasal Turbinals and Their Homology in Bats.

Multiple corrugated cartilaginous structures are formed within the mammalian nasal capsule, eventually developing into turbinals. Due to its complex and derived morphology, the homologies of the bat nasal turbinals have been highly disputed and uncertain. Tracing prenatal development has been proven to provide a means to resolve homological problems. To elucidate bat turbinate homology, we conducted the most comprehensive study to date on prenatal development of the nasal capsule. Using diffusible iodine-based contrast-enhanced computed tomography (diceCT), we studied in detail the 3D prenatal development of various bat species and non-bat laurasiatherians. We found that the structure previously identified as "maxilloturbinal" is not the true maxilloturbinal and is only part of the ethmoturbinal I pars anterior. Our results also allowed us to trace the evolutionary history of the nasal turbinals in bats. The turbinate structures are overall comparable between laurasiatherians and pteropodids, suggesting that pteropodids retain the ancestral laurasiatherian condition. The absence of the ethmoturbinal I pars posterior in yangochiropterans and rhinolophoids has possibly occurred independently by convergent evolution.

Embryonic evidence uncovers convergent origins of laryngeal echolocation in bats.

Bats are the second-most speciose group of mammals, comprising 20% of species diversity today. Their global explosion, representing one of the greatest adaptive radiations in mammalian history, is largely attributed to their ability of laryngeal echolocation and powered flight, which enabled them to conquer the night sky, a vast and hitherto unoccupied ecological niche. While there is consensus that powered flight evolved only once in the lineage, whether laryngeal echolocation has a single origin in bats or evolved multiple times independently remains disputed. Here, we present developmental evidence in support of laryngeal echolocation having multiple origins in bats. This is consistent with a non-echolocating bat ancestor and independent gain of echolocation in Yinpterochiroptera and Yangochiroptera, as well as the gain of primitive echolocation in the bat ancestor, followed by convergent evolution of laryngeal echolocation in Yinpterochiroptera and Yangochiroptera, with loss of primitive echolocation in pteropodids. Our comparative embryological investigations found that there is no developmental difference in the hearing apparatus between non-laryngeal echolocating bats (pteropodids) and terrestrial non-bat mammals. In contrast, the echolocation system is developed heterotopically and heterochronically in the two phylogenetically distant laryngeal echolocating bats (rhinolophoids and yangochiropterans), providing the first embryological evidence that the echolocation system evolved independently in these bats.

Prenatal cranial bone development of Thomas's horseshoe bat (Rhinolophus thomasi): with special reference to petrosal morphology.

Cochlear morphology has been regarded as one of the key traits to understand the origin and evolution of echolocation in bats, given its functionality and performance for receiving echolocation sonar. While numerous researchers have compared adult-stage morphology, few have studied the prenatal development of the cochlea. Here, we provide the first detailed three-dimensional description of the prenatal cranial development in bats, using Rhinolophus thomasi as a model, with particular interest to the petrosal which houses the cochlea. Results revealed that among all cranial bones the onset of the ossification of the petrosal is earlier in R. thomasi when compared to other reported mammals. Generally, the cochlea reaches adult size and shape before or around birth in placental mammals including bats, but we found that its shape and size growths continue until maturity in Rhinolophus species. The relationship of cochlear size and skull size is maintained constant throughout the postnatal ontogeny to adulthood in Rhinolophus, a pattern previously reported neither in any other bats nor other mammals. The peculiar developmental pattern in Rhinolophus possibly allows them to form their characteristically large cochlea and facilitate their distinctive echolocation behavior. A recent study reported that non-echolocating Pteropodidae shares a similar prenatal cochlear size to laryngeal echolocating bats. The apparent resemblance of fetal cochlear size was proposed to be a vestigial signal of large cochlear size in the last common ancestor of bats and thus as supporting evidence for the single origin of laryngeal echolocation. However, results from the present observations suggest that limited aspects of the cochlear development were captured in this previous investigation and that the resulting interpretations may be questionable. We point out that diversity and patterns of cochlear development among bats are still not resolved, and the controversy on the origins of laryngeal echolocation is still open to discussion.