Passage IV

Scientists investigated the effect of ultraviolet-B radiation (UV-B) on the eggs of 3 species of amphibians: Hyla regilla, Rana cascadae, and Bufo boreas. H. regilla populations are stable, but R. cascadae and B. boreas populations are declining.

Experiment 1

First, 1,800 newly laid eggs of H. regilla were collected from a high mountain lake. Next, 150 of the eggs were placed in each of 12 artificial enclosures: 4 that were covered with a filter, FB, that blocked UV-B; 4 that were covered with a filter, FT, that transmitted UV-B; and 4 that were not covered with any filter. Then, a set of these 3 types of enclosures was placed in the lake at each of 4 sites. All of the above procedures were repeated for the other 2 species. The enclosures were monitored until all the eggs had either hatched or died. The percent of total eggs that hatched for each species and type of enclosure is shown in Figure 1.

Experiment 2

Additional B. boreas eggs were collected and divided into 8 groups (Groups 1-8), each containing 30 eggs. Group 1 was not exposed to UV-B. Groups 2-8 were each exposed to a single dose of UV-B between 820 joules per square meter (J/㎡) and 4,100 J/㎡ (see Table 1). The percent of eggs that were surviving in each group was recorded each day for 10 days (see Figure 2).

Table 1 and Figure 2 adapted from J. Herkovits, J. L. D'Eramo, and O. Fridman, "The Effect of UV-B Radiation on Bufo arenarum Embryos Survival and Superoxide Dismutase Activity." ©2006 by MDPI.

Experiment 3

UV-B damages cells by causing cyclobutane pyrimidine dimers (CBPDs) to be produced in their DNA. Photolyase is an enzyme found in some cells that repairs DNA by removing CBPDs. The photolyase activity (the rate at which CBPDs are removed from DNA) present in the eggs of each of the 3 species was recorded (see Figure 3).

Figures t and 3 adapted from Andrew R. Blaustein et al., "UV Repair and Resistance to Solar UV-B in Amphibian Eggs: A Link to Population Declines?" ©1994 by the National Academy of Sciences of the United States of America.