Cohen Kristina L, Piacentino Michael L, Warkentin Karen M
Department of Biology, Boston University, 5 Cummington Mall, Boston, MA, 02215, USA.
Department of Biology, Boston University, 5 Cummington Mall, Boston, MA, 02215, USA; Program in Molecular Biology, Cell Biology and Biochemistry, Boston University, 5 Cummington Mall, Boston, MA, 02215, USA.
Comp Biochem Physiol A Mol Integr Physiol. 2018 Mar;217:63-74. doi: 10.1016/j.cbpa.2017.10.020. Epub 2017 Oct 19.
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.
环境线索诱导孵化在无尾两栖类动物中已有充分记载，这使胚胎能够逃避各种威胁。然而，关于无尾两栖类动物孵化机制的知识有限，且主要基于孵化时间无已知可塑性的水生繁殖物种。一般来说，孵化腺细胞会产生一种孵化酶，该酶可降解卵黄膜。我们研究了沙漏树蛙（Dendropsophus ebraccatus）陆栖胚胎的孵化及其调控，这种树蛙会加速孵化以避免脱水。我们特别测试了孵化腺细胞发育或孵化酶基因表达的变化是否与加速孵化有关。我们测量了发育过程中水分充足的卵的卵黄周腔大小，以此作为卵黄膜破裂的指标，发现卵黄周腔大小在孵化前稳步增加，这表明孵化酶逐渐释放，卵黄膜逐渐降解。孵化腺细胞数量在孵化前达到峰值并开始大量减少，但我们发现干燥胚胎和水分充足的胚胎在孵化腺细胞数量或表面积上没有发育差异。孵化酶基因表达在发育早期也达到峰值，然后下降，不同水分处理之间没有差异。在沙漏树蛙中，卵黄膜的破裂似乎是渐进的，由孵化前很久就开始释放的孵化酶介导。然而，孵化加速与孵化腺细胞发育或孵化酶基因表达的个体发育变化无关。这种孵化过程与红眼树蛙（Agalychnis callidryas）不同，红眼树蛙似乎在孵化时急性释放酶，但这两个物种都能够孵化以逃避急性威胁。
