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Senescence has always been a precious study field for biologists. It encloses the mystery of the research in combination with the practical application in human beings. This term is used in so many aspects, but in ecology it refers directly to the decrease in fitness with time in an organism life, reducing survival or fecundity (Abrams 1993). Typically used as synonym of 'ageing' (sometimes incorrectly since this does not imply the loss of efficiency in the living being), senescence is the response of cells for growing old. An obvious question that comes directly out is why this decrease in fitness has not been removed by natural selection. The answer to this has been object of research during last decades and still has no completely valid one. In this essay, I will discuss the different evolutionary theories that explain the maintenance of senescence in almost every living eukaryote organism and I will apply these to the senescence patterns shown by guppies (Poecilia reticulata Peters 1859), a species of fish which is present in South America and has been object of study by evolutionary ecologists for years.
Theories that explain the maintenance of senescence in organism are not completely satisfactory. We can find two classical ones in the literature. These are: the one focusing in the accumulation of mutations postulated by Medawar in 1952, or the one stating an antagonistic pleiotropy in some genes postulated by Williams in 1957 (Williams 1957, Reznick et al. 2002). A third theory arises when these genes have only effect on reproduction, this is the so-called 'disposable soma theory' and was first published by Kirkwood and Holliday in 1979 (Reznick et al. 2001). All these theories assume extrinsic mortality, (such as predation, hunting, disease, etc.) as the cause of shaping the evolution of senescence, as I will discuss later.
First hypothesis being published is based on the accumulation of mutations within DNA with age, as an irremediable effect of time passing by. As generally known, genetic material is susceptible of storing mutations due to several causes, and occasionally the cell mechanisms are unable to correct them, so that these mutations remain in the genome and accumulate with time. This storage would cause malfunction of several processes and would lead to cellular senescence, which in the end would exhibit the senescence of the whole organism. Medawar, to solve the problem of undetectable natural selection, stated that this would not select against since late mutation in life would not be transmissible to the offspring. In other words, early mutations are generally selected against due to their huge effect on fitness, but late ones would be obviated by natural selection since the organism does not contribute to future generations (this is a general statement, some exceptions have been reported, as the 'grandmother effect' present in human beings (Lahdenpera et al. 2004)). Basically, Medawar gives no importance to selection in the modelling of senescence since it has no effect removing these deleterious mutations, which in the end would be the final cause of senescence (Reznick et al. 2002).
In order to give solutions to the main problems this theory has been left unsolved, Williams proposed the 'Antagonistic pleiotropy' theory. Even though the cause of senescence would be also negative gene effects in cells (but not specifically spontaneous mutations, as Medawar implied), Williams pointed out the positive effect of natural selection in the shape of the evolution of senescence. This is explained by the dual effect that some genes would have, in particular the positive effect in youth against the negative effect in post reproductive stages; these tradeoffs are the bases of his postulation (Williams 1957). If this happens natural selection will favour the positive effect in early ages (pre and reproductive ones), due to the huge impact on fitness the improvement will have. However, as happens in Medawar's theory, the removal of the negative effects in older stages will be almost negligible. This means that the mutation in a gene which causes the creation of improved offspring will be strongly selected, in spite of its deleterious effects in older stages. This last trade-off creates the bases of the 'disposable soma theory' (Reznick et al. 2001). This is considered as a modification of the main theory and states the same as it but in a narrowly point of view. Figure 1 is attached to explain graphically these three hypotheses and their interaction with extrinsic mortality.
a, Extrinsic mortality in wild environments occurs to an extent that senescence-associated mortality is rare, undermining any idea that genes specifically for ageing have evolved. b, The 'selection shadow' at older ages may permit an accumulation of late-acting deleterious mutations (mutation-accumulation theory). c, Pleiotropic genes that benefit organisms early in life will be favored by selection even if they have bad effects at later ages (pleiotropy theory). d, Selection pressure to invest metabolic resources in somatic maintenance and repair is limited; all that is required is to keep the organism in sound condition for as long as it might survive in the wild (disposable-soma theory). Figure taken from Thomas B. L. (2000).
In order to illustrate the effects of different factors in the evolution of senescence, and how these theories can be applied for them, I have chosen the freshwater fish so-called 'guppy' (Poecilia reticulata) as model of study (see Figure 2 and table 1). They are ovoviviparous fishes, reproduction is continuous, females produce new offspring every three or four weeks and the longevity of reproducing females last 24-30 months (Reznick et al. 2001). Taxonomy is shown in table 1.
Figure 2: Picture of two guppies in a natural environment. Source: Wikipedia
Taxonomy of Poecilia reticulata
Guppies have been object of study in this specific field thanks to their known life history, which makes them one of the most appropriate model animals for this within the vertebrates (Reznick et al. 2001). Fieldwork executed by Reznick and colleagues has been the bases of my study. Their research tries to understand the evolution of senescence in different populations of P. reticulata since their privileged locations in South America set the bases for proper study in natural environments, high-predation and low-predation ones.
The patterns of senescence of guppies are, then, shaped by the extrinsic mortality that populations experience (see figure 3). High-predation localities is reported having higher mortality rate than low-predation ones (Reznick et al. 1996), and classical theories predict these rates would produce earlier senescence in high-predation populations, as well as earlier maturation and improved offspring (more amount of newborns per litter and higher reproduction frequency). In order to prove these postulates, data was compiled studying a population of guppies from high-predation environments that was introduced in low-predation ones in 1981, so that comparison between them was valid and mortality rate was a reflection of senescence. Their results showed that females from high-predation populations experienced increase of the mortality rate when they are 6 months old whereas females from low-predation localities did not show increase until they are 16 months old (Bryant et al. 2004). There were also studies which the introduction of predator set the bases for the comparison (Reznick et al. 1987, 1990), or the completely alienation of guppies from their natural environment made able the laboratory study, controlling all the factors under strict supervision (Reznick et al. 2006). All these studies showed the same results, as it was said before. Table 2 summarizes the results of Reznick et al. (1990). However, oppositely as it was believed, high-predation population of guppies, when alienated from their natural predatory environment, showed longer lifespan. Specifically, it was the reproductive lifespan which was enlarged since it is the one which can contribute to individual fitness (Reznick et al. 2006).
Figure 3: The predicted pattern of results if guppies from high-predation sites senesce more rapidly than those from low-predation sites. Senescence is interpreted as a decline in fitness related variables with age. Theory predicts that this decline should be more rapid in guppies from high-predation environments. Figure taken from Reznick et al. (2001).
Guppies were normally found below the waterfall with Crenicichla alta and other predators. Guppies were introduced over the barrier waterfall 11 years before this experiment. This introduction decreased mortality rates experienced by the guppies. The age at maturity of the Introduction site fish has increased, as predicted by theory. Other aspects of the life history have also evolved in the predicted fashion. These data are summarized from the results reported by Reznick, Bryga, and Endler (1990). Ns - Not significant; * p < 0.05; † p < 0.01. Table taken from Reznick (1997).
Returning to the main point, and in order to summarize the patterns of variation in senescence, it is clear from these experiences that guppies from high-predation environments show more rapid senescence, mature earlier and have higher investments on reproduction earlier in life. On the other hand, low-predation ones show the opposite of these features.
These results are consistent with classical senescence evolutionary theories. They predict that high mortality rate (extrinsic) will shape how senescence evolves, towards earlier ages. Nevertheless, the causes that lead to this conclusion are completely different. Medawar's theory focuses on the accumulation of mutation in DNA and predicts that, due to the lower life expectancy present in high-predation environments, guppies would be able to accumulate more deleterious mutations in old ages (Reznick et al. 2001). Selection would not act against since really few organisms would live until older stages and these would have practically zero impact on fitness.
On the other hand, Williams' theory postulates the positive effect of natural selection in the shape of senescence evolution. This theory states that selection would favour earlier senescence as consequence of earlier and stronger investment on life (antagonistic pleiotropy). If the trade-off that we are considering is between 'improved reproduction' and 'late investment on life' we are talking about Kirkwood's disposable soma theory, which is a part of Williams' antagonistic pleiotropy theory. Any improvement in reproduction would be strongly favoured by selection, but late deleterious effects would be obviated because they do not contribute on individual fitness. This trade-off explains the experimental results, since greater reproduction investment in guppies from high-predation environments and earlier ages for first mating are observed, as well as higher intrinsic mortality rate in females, reported as senescence (Reznick et al. 2001, Bryant et al. 2004).
Senescence patterns of guppies and the evolutionary theories to explain them have been exposed in this essay, however, discussion should lead to more important issues such as how we can measure real senescence and how the environmental model biases these results. Late studies prove that considering the mortality rate as a reflection of senescence can lead to really wrong conclusions; and other parameters, based on phenotype and genetics, should be observed as well. Also, the evolution of senescence should not be only considered shaped by extrinsic mortality, other factors such as density dependence and condition dependence can influence too (Bronikowski et al. 2005). In fact, the guppies statistical experiences are biased by the incapability of distinguishing senescence, since other factors (e.g. disease) can be the cause of mortality and not the object of this study (Bryant et al. 2004). This field of study is continuously remodelling and improvements are needed for future research. However, this data shows the perfect bases for further studies and an extraordinary example for explaining senescence patterns in Poecilia reticulata.