Mutation accumulation and the catastrophic senescence of the Pacific salmon

Abstract

The bit-string model of biological aging is used to simulate the catastrophic senescence of Paci c Salmon. We have shown that reproduction occuring only once and at a xed age is the only ingredient needed to explain the catastrophic senescence according the mutation accumulation theory. Several results are presented, some of them with up to 10 8 shes, showing how the survival rates in catastrophic senescence are a ected by changes in the parameters of the model. PACS number(s): 05.50.+q; 89.60.+x; 07.05.Tp Senescence, or aging, is a process occuring in all higher organisms and it is related to a decrease in functional abilities. Several factors seems to be important to aging: the environment, metabolism, genetic factors and so forth 1,2]. Senescence can be characterized as the decrease in the survival probabilities with the age. At least two theories for the senescence are based on evolution 3]: the antagonistic pleiotropy (an optimality theory based on life strategy of increasing tness by increasing early performance at expense of late) and mutation accumulation (based on a greater mutation load on the later than the earlier ages). These theories, besides providing explanation for several aspects of senescence, also allow the use of methods of statistical physics (see ref. 4] for a review on earlier models). A dramatic manifestation of aging is the so-called catastrophic senescence of Paci c salmon, whereby they pass from sexual maturity to death in a few weeks. Semelparous individuals, which breed only once, usually present this feature 1{3], while for iteroparous individuals, which breed repeatedly, the senescence is more gradual. Jan 5] has shown how to introduce the catastrophic senescence in the PartridgeBarton model, but without taking into account explicitly the number of breeding attempts. In this work we show that, according to a model based on the mutation accumulation theory, this ingredient (semelparity) is the only responsible one for the catastrophic senescence.The bit-string model of life history, recently introduced 6], makes use of a balance between hereditary mutations and evolutionary selection pressure to simulate aging in a population. In this model, each individual of an initial population N (t = 0) is characterized by a string of 32 bits (\genome"), which contains the information when the e ect of a mutation will be present during the life of the individuum. The time is a discrete variable running from 1 to 32 steps (\years"). If at time (t = i) of the individuum lifetime, the i-th bit in the genome is set to one, it will su er the e ects of a deleterious mutation in that and all following years. At each year one bit of the genome is read, and the total number of mutations (bits 1) is computed; if it reaches a value greater than a threshold T, the individuum dies. At every year beyond the minimum reproduction age R the individuum produces b o springs. The genome of each baby differs from that of the parent by one randomly selected bit, toggled at birth. This mutation can be regarded as...

An unusual antagonistic pleiotropy in the Penna model for biological ageing

“…The most important characteristic of this dynamics is that the bits 1 accumulate, after many generations, at the end part of the genomes, that is, after the minimum reproduction age R. For this reason ageing appears: the survival probabilities per period decrease with age, in agreement with the mutation accumulation hypothesis and reality [16].…”

Stability in a population model without random deaths by the Verhulst factor

“…To limit growth, we allow individuals to breed only once in their lifetime (at age R). This behavior is observed in semelparous populations such as the mayflies and the Pacific salmon [19,20,21]. The simulation starts with N 0 = 20000 perfect individuals (no bad genes) and runs for 3000 iterations.…”

Evolution, ageing and speciation: Monte Carlo simulations of biological systems

“…The first big goal of the Penna model was the explanation of why some species like the salmon reproduce only once, always at the same age, and die a few days later [12]. The salmon, in particular, sometimes travels more than 1200 kilometers up river in order to reproduce, generally without eating after reaching sweet waters.…”