Extreme tolerance and developmental buffering of UV‐C induced DNA damage in embryos of the annual killifish Austrofundulus limnaeus

Abstract

Free-living aquatic embryos are often at risk of exposure to ultraviolet radiation (UV-R). Successful completion of embryonic development depends on efficient removal of DNA lesions, and thus many aquatic embryos have mechanisms to reverse DNA lesions induced by UV-R. However, little is known of how embryos that are able to enter embryonic dormancy may respond to UV-R exposure and subsequent DNA damage. Embryos of the annual killifish Austrofundulus limnaeus are unique among vertebrates because their normal embryonic development includes (1) a complete dispersion of embryonic blastomeres prior to formation of the definitive embryonic axis, and (2) entry into a state of metabolic depression and developmental arrest termed diapause. Here, we show that developing and diapausing embryos of A. limnaeus have exceptional tolerance of UV-C radiation and can successfully complete embryonic development after receiving substantial doses of UV-C, especially if allowed to recover in full-spectrum light. Recovery in full-spectrum light permits efficient removal of the most common type of DNA lesion induced by UV-R: cyclobutane pyrimidine dimers. Interestingly, whole-mount embryo TUNEL assays suggest that apoptosis may not be a major contributor to cell death in embryos UV-C irradiated during dispersion/reaggregation or diapause. We also observed embryo mortality to be significantly delayed by several weeks in diapausing embryos irradiated and allowed to recover in the dark. These atypical responses to UV-R induced DNA damage may be due to the unique annual killifish life history and provide insight into DNA damage repair and recognition mechanisms during embryonic dormancy.

Mechanisms of animal diapause: recent developments from nematodes, crustaceans, insects, and fish

“…UV-C radiation induces the formation of cyclobutane pyrimidine dimers and 6-4 pyrimidinepyrimidone photoproducts in DNA that can lead to cell death or cell cycle arrest if not repaired (174,201). Not surprisingly, diapause II embryos have an extremely high tolerance of UV radiation-induced DNA damage that is 16-fold higher than other fish embryos (188). This tolerance is quite impressive compared with other vertebrates and invertebrates, including dauer larvae from a number of species of nematodes (67).…”

“…Survival of UV radiation in A. limnaeus embryos is not supported by accumulation of compounds that shield the embryos from damage, but rather from a high capacity for repair of DNA lesions. If allowed to recover in full-spectrum white light following UV-C irradiation, embryos can effectively and efficiently repair the DNA damage within 48 h (188). However, if kept in the dark, the DNA lesions are not repaired, suggesting an active role for photolyase-mediated DNA repair in these embryos.…”

“…stress is also of central importance to survival of dormant embryos (81,124,127,128,188). However, relatively little attention has been focused on maintenance of genome integrity during embryonic diapause, and how DNA damage may affect survival of dormant embryos.…”

Physiological strategies during animal diapause: lessons from brine shrimp and annual killifish

“…For example, diapause I is rare in Austrofundulus, and may only occur in embryos that are incubated at low temperatures (Wourms, 1972c). However, the dispersion and reaggregation phases of development that are associated with diapause I may act as a buffer against otherwise teratogenic levels of cellular damage (Wagner and Podrabsky, 2015). Entrance into diapause II is favored at temperatures of 25°C or below in the lab, and lower temperatures are associated with the dry season in their native habitat.…”

“…Entrance into diapause II is favored at temperatures of 25°C or below in the lab, and lower temperatures are associated with the dry season in their native habitat. Diapause II embryos have the highest tolerance to a number of environmental stresses, and in fact are able to withstand anoxia longer than any other vertebrate at 25°C (Podrabsky et al, 2001Machado and Podrabsky, 2007;Wagner and Podrabsky, 2015). An important parallel between the Artemia and Austrofundulus life histories is the presence of a postdiapause period of active development that retains the high tolerance to environmental stress observed in diapausing embryos.…”