ON THE POSSIBLE USE OF ANNUAL KILLIFISHES AS MODELS FOR AGING RESEARCH: A COMMENT ON HERRERA AND JAGADEESWARAN

To the Editor: In their article, Herrera and Jagadeeswaran have proposed the use of the annual fishes Nothobranchius rachovii and Cynolebias (Austrolebias after the last taxonomic revision) nigripinnis as models for aging research (1). The authors are seemingly unaware of the seminal work of Cooper, Markofsky, and others (2–7) who showed aging-related degenerations in several organs of the annual killifish Nothobranchius guentheri and Nothobranchius korthause and introduced Nothobranchius as a natural model of rapid aging in vertebrates. Herrera and Jagadeeswaran have analyzed the survival curves of Nothobranchius rachovii from a commercial source, and reported a maximum life span of 9.5 months but could not show age-dependent increase in mortality rate or expression of any aging biomarker. If researchers can be enticed to a new model system based on this information, we think that readers of the Journals of Gerontology will be interested to learn that other species of annual fishes exist with much shorter life spans. In 2003, we reported that Nothobranchius furzeri has a maximum survival in laboratory conditions of 12 weeks (8). This life span is comparable to that of wild-type Drosophila, a popular model for aging studies. Moreover, Nothobranchius furzeri has an explosive growth (sexual maturity is attained at 3–4 weeks of age), and its age-dependent mortality rate increases exponentially with age (8). The huge difference in growth and life span between the two species is not surprising in light of Nothobranchius ecology. Nothobranchius fish are adapted to colonize the temporal ponds of Eastern Africa. When the ponds fill due to the floods, Nothobranchius embryos hatch, reach sexual maturity, and breed before their ephemeral habitat disappears. Extrinsic death probability for Nothobranchius is basically rectangular and is determined by the length of the rainy season. Nothobranchius rachovii originates from the city of Beira (9), with annual rainfall of approximately 1600 mm/year (data from www.worldweather.org). Nothobranchius furzeri, in contrast, originates from the semi-arid lowveld at the border between Mozambique, South Africa, and Zimbabwe (10). Average annual rainfall in that area is about 340 mm/year (data refer to Breitbridge, from www.worldweather.org) but is erratic and can be as low as 200 mm/year in lean years (10). Therefore, the temporal profile of extrinsic mortality rate is drastically different for the two species. Evolutionary theories of aging predict that an increase in extrinsic mortality rate results in faster growth and, as a consequence, accelerated aging (11). The predictions of the theory perfectly fit the differences in growth rate and life span that we observe by comparing Nothobranchius furzeri and Nothobranchius rachovii. We agree with Herrera and Jagadeeswaran that the short life span of Nothobranchius fishes can represent a decisive advantage for aging research. Even more so, if the correct species is selected. Decision Editor: James R. Smith, PhD