RESUMEN
Environmentally cued hatching is well documented in anurans, enabling embryos to escape diverse threats. However, knowledge of anuran hatching mechanisms is limited and based largely on aquatic-breeding species without known plasticity in hatching timing. Generally, hatching gland cells produce a hatching enzyme that degrades the vitelline membrane. We investigated hatching and its regulation in terrestrial embryos of hourglass treefrogs, Dendropsophus ebraccatus, which accelerate hatching to escape dehydration. We specifically tested if changes in hatching gland cell development or hatching enzyme gene expression are associated with accelerated hatching. We measured perivitelline chamber size of well-hydrated eggs over development as an indicator of breakdown of the vitelline membrane and found that the size of the perivitelline chamber increased steadily until hatching, suggesting gradual hatching enzyme release and vitelline membrane degradation. Hatching gland cells peaked in abundance and began regression substantially prior to hatching, but we found no developmental differences in the abundance or surface area of hatching gland cells between dry and well-hydrated embryos. Hatching enzyme gene expression also peaked early in development then declined, with no difference between hydration treatments. In D. ebraccatus breakdown of the vitelline membrane appears gradual, mediated by hatching enzyme release starting long before hatching. However, hatching acceleration is not associated with ontogenetic changes in hatching gland cell development or hatching enzyme gene expression. This hatching process contrasts with that of red-eyed treefrogs, Agalychnis callidryas, which appear to release enzyme acutely at hatching, yet both species are capable of hatching to escape acute threats.
Asunto(s)
Anuros/fisiología , Embrión no Mamífero/fisiología , Óvulo/fisiología , Adaptación Fisiológica , Secuencia de Aminoácidos , Animales , Anuros/clasificación , Anuros/embriología , Metaloendopeptidasas/química , Metaloendopeptidasas/genética , Metaloendopeptidasas/metabolismo , Microscopía Electrónica , Filogenia , Reacción en Cadena en Tiempo Real de la Polimerasa , Homología de Secuencia de Aminoácido , Membrana Vitelina/metabolismoRESUMEN
Environmentally cued hatching allows embryos to escape dangers and exploit new opportunities. Such adaptive responses require a flexibly regulated hatching mechanism sufficiently fast to meet relevant challenges. Anurans show widespread, diverse cued hatching responses, but their described hatching mechanisms are slow, and regulation of timing is unknown. Arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, escape from snake attacks and other threats by very rapid premature hatching. We used videography, manipulation of hatching embryos and electron microscopy to investigate their hatching mechanism. High-speed video revealed three stages of the hatching process: pre-rupture shaking and gaping, vitelline membrane rupture near the snout, and muscular thrashing to exit through the hole. Hatching took 6.5-49â s. We hypothesized membrane rupture to be enzymatic, with hatching enzyme released from the snout during shaking. To test this, we displaced hatching embryos to move their snout from its location during shaking. The membrane ruptured at the original snout position and embryos became trapped in collapsed capsules; they either moved repeatedly to relocate the hole or shook again and made a second hole to exit. Electron microscopy revealed that hatching glands are densely concentrated on the snout and absent elsewhere. They are full of vesicles in embryos and release most of their contents rapidly at hatching. Agalychnis callidryas' hatching mechanism contrasts with the slow process described in anurans to date and exemplifies one way in which embryos can achieve rapid, flexibly timed hatching to escape from acute threats. Other amphibians with cued hatching may also have novel hatching mechanisms.