Insights Into the Formation and Diversification of a Novel Chiropteran Wing Membrane From Embryonic Development

BACKGROUND: Through the evolution of novel wing structures, bats (Order Chiroptera) became the only mammalian group to achieve powered flight. This achievement preceded the massive adaptive radiation of bats into diverse ecological niches. We investigate some of the developmental processes that underlie the origin and subsequent diversification of one of the novel membranes of the bat wing: the plagiopatagium, which connects the fore- and hind limb in all bat species. RESULTS: Our results suggest that the plagiopatagium initially arises through novel outgrowths from the body flank that subsequently merge with the limbs to generate the wing airfoil. Our findings further suggest that this merging process, which is highly conserved across bats, occurs through modulation of the programs controlling the development of the periderm of the epidermal epithelium. Finally, our results suggest that the shape of the plagiopatagium begins to diversify in bats only after this merging has occurred. CONCLUSIONS: This study demonstrates how focusing on the evolution of cellular processes can inform an understanding of the developmental factors shaping the evolution of novel, highly adaptive structures

Species-specific modifications of mandible shape reveal independent mechanisms for growth and initiation of the coronoid

BACKGROUND: The variation in mandibular morphology of mammals reflects specialisations for different diets. Omnivorous and carnivorous mammals posses large mandibular coronoid processes, while herbivorous mammals have proportionally smaller or absent coronoids. This is correlated with the relative size of the temporalis muscle that forms an attachment to the coronoid process. The role of this muscle attachment in the development of the variation of the coronoid is unclear. RESULTS: By comparative developmental biology and mouse knockout studies, we demonstrate here that the initiation and growth of the coronoid are two independent processes, with initiation being intrinsic to the ossifying bone and growth dependent upon the extrinsic effect of muscle attachment. A necessary component of the intrinsic patterning is identified as the paired domain transcription factor Pax9. We also demonstrate that Sox9 plays a role independent of chondrogenesis in the growth of the coronoid process in response to muscle interaction. CONCLUSIONS: The mandibular coronoid process is initiated by intrinsic factors, but later growth is dependent on extrinsic signals from the muscle. These extrinsic influences are hypothesised to be the basis of the variation in coronoid length seen across the mammalian lineage

Q&A:Morphological insights into evolution

Abstract In this question and answer article we discuss how evolution shapes morphology (the shape and pattern of our bodies) but also how learning about morphology, and specifically how that morphology arises during development, can shed light on mechanisms that might allow change during evolution. For this we concentrate on recent findings from our lab on how the middle ear has formed in mammals

Non-model systems in mammalian forelimb evo-devo

Mammal forelimbs are highly diverse, ranging from the elongated wing of a bat to the stout limb of the mole. The mammal forelimb has been a long-standing system for the study of early developmental patterning, proportional variation, shape change, and the reduction of elements. However, most of this work has been performed in mice, which neglects the wide variation present across mammal forelimbs. This review emphasizes the critical role of non-model systems in limb evo-devo and highlights new emerging models and their potential. We discuss the role of gene networks in limb evolution, and touch on functional analyses that lay the groundwork for further developmental studies. Mammal limb evo-devo is a rich field, and here we aim to synthesize the findings of key recent works and the questions to which they lead

Making a bat: The developmental basis of bat evolution.

Bats are incredibly diverse, both morphologically and taxonomically. Bats are the only mammalian group to have achieved powered flight, an adaptation that is hypothesized to have allowed them to colonize various and diverse ecological niches. However, the lack of fossils capturing the transition from terrestrial mammal to volant chiropteran has obscured much of our understanding of bat evolution. Over the last 20 years, the emergence of evo-devo in non-model species has started to fill this gap by uncovering some developmental mechanisms at the origin of bat diversification. In this review, we highlight key aspects of studies that have used bats as a model for morphological adaptations, diversification during adaptive radiations, and morphological novelty. To do so, we review current and ongoing studies on bat evolution. We first investigate morphological specialization by reviewing current knowledge about wing and face evolution. Then, we explore the mechanisms behind adaptive diversification in various ecological contexts using vision and dentition. Finally, we highlight the emerging work into morphological novelties using bat wing membranes

Transient role of the middle ear as a lower jaw support across mammals

Mammals articulate their jaws using a novel joint between the dentary and squamosal bones. In eutherian mammals, this joint forms in the embryo, supporting feeding and vocalisation from birth. In contrast, marsupials and monotremes exhibit extreme altriciality and are born before the bones of the novel mammalian jaw joint form. These mammals need to rely on other mechanisms to allow them to feed. Here we show that this vital function is carried out by the earlier developing, cartilaginous incus of the middle ear, abutting the cranial base to form a cranio-mandibular articulation. The nature of this articulation varies between monotremes and marsupials, with juvenile monotremes retaining a double articulation, similar to that of the fossil mammaliaform , while marsupials use a versican-rich matrix to stabilise the jaw against the cranial base. These findings provide novel insight into the evolution of mammals and the changing relationship between the jaw and ear