Grab Your Sunglasses, it’s too Bright for Tadpoles

By: Emily Collins

In recent years, the concept of global warming due to human influence has shifted from a theory to a fact. As a consequence of human activity, the stratospheric ozone layer is depleting, resulting in increased levels of ultraviolet radiation (UVR) in the atmosphere. There are two types of UVR that reach the earth’s surface, ultraviolet A (UVA) and ultraviolet B (UVB). To understand how the increasing levels of UVR affect life on earth, scientists Niclas Lundsgaard, Rebecca Cramp, and Craig Franklin conducted a laboratory experiment to investigate how short-term exposure to different concentrations and irradiance (brightness) of ultraviolet B radiation (UVBR) would affect the metamorphic development of the Australian green tree frog Litoria caerulea. There are increased levels of concentration and irradiance of UVBR in regions of amphibian decline perhaps highlighting a correlation; as very little is known about how UVBR exposure during early life might affect post-metamorphosis through long-term carryover effects. Currently, research has been focused on early life exposure without considering the long-term effects. The aim of Lundsgaard, Cramp, and Franklin’s study was to determine which parameters of UVBR exposure affect amphibian health long-term; whether it be concentration or irradiance.

UVBR can cause detrimental cellular effects on DNA, specifically on telomers. Telomers are non-coding regions of DNA located on the ends of chromosomes that function to maintain genome stability. Telomers are shortened following each cellular division cycle, decreasing genome stability, and therefore shortening the life span of the organism. UVBR damages DNA in a way that disrupts the cellular division cycle, leading to increased shortening of telomeres and consequently, premature aging. As telomere length is directly influenced by environmental stress, it was measured and used to determine carryover effects. Carryover effects are persistent consequences that remain after exposure, similar to finding sand in your shoes after leaving the beach.

In order to test for carryover effects, tadpoles of the Litoria caerulea species were exposed to a variety of different UVBR concentrations and irradiance levels and grown into frogs in the absence of UVBR to establish a temporal gap between UVBR exposure and physiological carryover effects. The UVBR exposures were centred on midday and were in addition to 12-hour light:12-hour dark non-UVBR background lighting. The UVBR exposure consisted of 1-hour, 4-hour, and 8-hour low, medium, and high irradiance levels with the concentration remaining constant throughout. The temperature cycled during and after the treatments ranged between 21 to 31°C, modelled after summer conditions from the collection site.

Following metamorphosis, the juvenile frogs were analyzed to determine how UVBR exposure affected size, condition, performance, and relative telomere length. To determine performance, foraging and jumping efficiency were analyzed and to determine relative telomere length, the frogs were euthanized and analyzed. It was concluded that there was no significant interaction between UVBR concentration and irradiance on the progression to the average age, however, there was a significant effect of UVBR irradiance on mass and overall size at metamorphosis. On average, larvae that were exposed to high levels of irradiance had a 20% decrease in mass as compared to larvae that were exposed to the low and medium irradiance level treatments. Regardless of concentration, all subjects that received the high irradiance treatment suffered from detrimental carryover effects affecting overall fitness including a reduced immune system, reduction in foraging efficiency, and jumping performance.

Interestingly and contrary to the scientist’s hypothesis, the medium and high UVBR concentration treatments actually improved progression into metamorphosis when administered at low irradiance, these treatments resulted in twice as many larvae progressing into frogs. This is likely a result of UVBR increasing vitamin D3 levels in larvae which acts to aid in bone mineralization, and muscular and nerve function. In terms of telomere length, it was determined that there was no interaction between concentration or irradiance on telomere length, however, subjects that were treated with medium concentrations of UVBR had the shortest telomere length. While it is known that UVBR can cause telomere shortening, this is the first study to show that telomere shortening is not linked to carryover effects post-exposure.

The conclusion of the study was that higher concentrations of UVBR exposure in larvae resulted in improved rates of metamorphosis, and exposure at higher levels of irradiance resulted in frogs metamorphosing smaller in size and in poorer conditions. The tie between amphibian decline and UVBR still remains unknown although it was determined that the majority of the detriment from UVBR exposure was not immediate but rather manifested long-term, post-exposure. So next time you see tadpoles, consider giving them your sunglasses!

Lundsgaard, N.U., Cramp, R. L., Franklin, C. E. (2022) Early exposure to UV radiation causes telomere shortening and poorer condition later in life. Journal of Experimental Biology. 225 (17), jeb243924