Meeting Abstract
Red-eyed treefrogs, Agalychnis callidryas, are an excellent example of environmentally cued hatching. Undisturbed, these arboreal embryos typically hatch at 6–7d, but they can hatch as early as 4d to escape pathogens, dehydration, and predators, responding to attacks in seconds. We showed that A. callidryas hatch by acutely releasing hatching enzyme from large hatching gland cells (HGCs) concentrated on their snout, which appear at 4d. More recently, using a strong hypoxia/flooding cue, we found that embryos can hatch at 3d, before these glands develop. Using scanning electron microscopy, we found another, earlier developing and smaller type of HGCs with a more dispersed spatial distribution. We used in situ hybridization to visualize hatching enzyme gene expression over development and found it matches the spatial distributions of both types of HGCs observed in SEM. Expression was also visible at stages before HGCs are externally visible, indicating that transcription begins before HGCs are fully developed. We then used histology to assess localization of the hatching enzyme mRNA in the two types of HGCs. The smaller, earlier-appearing HGCs more closely resemble HGCs observed in other hylids in their morphology, spatial distribution, and developmental timing. Thus, they may be the ancestral HGC type and the large HGCs a derived cell type that enables rapid escape from predators. A. callidryas is the first anuran observed to have two types of hatching gland cells, but other species with dramatic cued hatching responses have not been investigated. Our studies of another treefrog have shown that both gradual and acute release of hatching enzyme can occur in the same species in different contexts. Hatching mechanisms in anurans are clearly more diverse than previously known.
