Popular Science: Is an old killifish reproductively more proficient than a young one?
Specifically, the authors investigated the phenomenon of reproductive senescence, which is a decline in reproductive performance associated with age. In organisms that continue growing even after reaching their maturity, such as molluscs, fish and reptiles, senescence is believed to be absent or to start long after reaching the reproductive age. On the other hand, some of these organisms live only shortly, and so it was predicted that senescence in them could be more rapid.
Jakub Žák and his supervisor Martin Reichard wanted to know the real situation, when the continuous growth ‘meets’ with a fast lifecycle within one organism - a fish with a short life expectancy. Will senescence be absent because it is continuously growing after reaching maturity, or will it experience reproductive senescence earlier or later, because of its fast lifecycle?
The authors chose killifish (Nothobranchius furzeri) for their experiments, a small fish measuring up to 7 cm, living in seasonally drying pools in south-east African savanna. The choice was quite obvious, as this species is a commonly used model for aging studies in vertebrates. When the killifish reach their maturity, they are only at 55% of their maximum possible body size. However so far no reproductive senescence was observed in them, both in nature and captivity.
The authors of the study observed 132 fish originating from two populations, a ‘laboratory’ one being held in captivity since 1968 (which represents more than 100 generations already) and a ‘wild’ population that has been kept in captivity only since 2011 when it was collected in southern Mozambique. These two populations are quite genetically similar (they are from a similar phylogeographic lineage), but the ‘laboratory’ population has already reduced genetic variability as a result of a long period of breeding in captivity.
Throughout the life of the fish, the research team measured, on a weekly basis, fish fecundity (the number of eggs laid), fertility (the number of fertilized eggs laid), fertilization rate (the proportion of eggs that were successfully fertilized), sexual maturation (the age when the eggs were laid for the first time), the sum of all eggs laid during the entire life of a female, and the maximum clutch size.
The authors found that despite a similar growth of the females from both populations, the ‘wild’ ones were more proficient in all aspects – longer lifespan, earlier maturity, higher fecundity and higher fertilization rate. The discrepancy between the populations could be a consequence of adaptation to different conditions in each population, in that the ‘laboratory’ population was not exposed to environmental stress, i.e., a seasonal drying of a pool, for a longer time and so it adapted to the relaxed conditions of captivity. On the other hand, the ‘wild’ population was in the previous generations under a strong pressure of rapid pool drying and in order to survive successfully, it had to invest significantly into reproduction. Reproductive senescence appeared in both populations long after the growth had stopped (approximately 9 weeks). A surprising finding was a decrease in fertilization as a result of eggs of lower quality produced by females, while males kept a high ability of fertilization until a very old age.
To sum up, female fecundity increased steadily after maturity and reproductive senescence appeared quite late in their life. The decrease in the fertilization rate was more apparent in females than in males.
The authors thus found out that, contrary to some expectations, reproductive senescence also exists in continuously growing animals such as fish. However, more investigation is needed to determine the situation is regarding other fish.