Survival of water stress in annual fish embryos: dehydration avoidance andegg envelope amyloid fibers

Diapausing embryos of Austrofundulus limnaeus survive desiccating condition s by reducing evaporative water loss. Over 40% of diapause II embryos survi ve 113 days of exposure to 75.5% relative humidity. An early loss of water from the perivitelline space occurs during days 1-2, but thereafter, rates of water loss are reduced to near zero. No dehydration of the embryonic tis sue is indicated based on microscopic observations and the retention of bul k (freezable) water in embryos as judged by differential scanning calorimet ry. Such high resistance to desiccation is unprecedented among aquatic vert ebrates. Infrared spectroscopy indicates frequent intermolecular contacts v ia beta -sheet (14%) in hydrated egg envelopes (chorions). These beta -shee t contacts increase to 36% on dehydration of the egg envelope. Interestingl y, the egg envelope is composed of protein fibrils with characteristics of amyloid fibrils usually associated with human disease. These features inclu de a high proportion of intermolecular beta -sheet, positive staining and g reen birefringence with Congo red, and detection of long, unbranched fibril s with a diameter of 4-6 nm. The high resistance of diapause II embryos to water stress is not correlated with ontogenetic changes in the egg envelope .