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University of Fribourg / Department of Biology / Unit Ecology & Evolution
The fitness consequences of population size and genetic diversity and their
relationship with the genetic load in Daphnia
Jennifer Lohr
Reduced population size, population bottlenecks and general population subdivision are important evolutionary
processes which have serious consequences for the adaptive potential and fitness of individuals. All of these
factors decrease the effective population size (the size of an ideal population that experiences genetic drift at the
rate of the population in question) of a given population and in turn decrease the population’s genetic diversity.
Despite the general importance of reduced effective population size in determining the course of evolution, few
studies to date have rigorously tested the spectrum of responses for population size ranging from small to large.
The goal of this dissertation was to consolidate theoretical and empirical studies, by investigating the fitness
differences between multiple populations of a single species. We used European and Scandinavian populations
of the water flea, Daphnia magna, as our model system, as these populations were shown previously to exhibit
large variation in population size.
In chapter 1 we begin by establishing the relationship between population size and genetic diversity within these
populations, using neutral loci (32 microsattelite markers). We then go on to test the relationship of population
size and genetic diversity with fitness. The results fell well in line with theoretical predications: the small
populations being genetically less diverse and having overall lower fitness. Specifically, they died earlier,
produced fewer offspring of smaller size and matured later. In addition, we used this data set to evaluate the
repercussions of small population size for the evolution of ageing. Generally, ageing evolves as the force of
natural selection declines with age, and thus, if we take populations of smaller size, where the efficacy of natural
selection is further reduced, we predict that these populations should have faster ageing. This is precisely the
result found here across our Daphnia populations. We further support this result by developing a mathematical
model, which builds on previous theoretical work to make specific predictions from population size to lifespan
and ageing. Our results speak to the importance of mutation accumulation via random genetic drift in
determining organismal fitness, as well as the variation in lifespan and ageing documented within natural
populations. Such large variation in rates of aging found across one species, suggests that ageing and lifespan are
rather plastic traits, which can evolve over short evolutionary time spans.
In chapter 2 we take this research a step further to gain a better understanding of the genetic background of the
relationship between population size, genetic diversity and fitness. We accomplished this by inbreeding and
outbreeding the same Daphnia populations, and measuring the fitness of their F2 offspring. This data set was
used to calculate inbreeding depression, hybrid vigour and the genetic load for each population. Again our
results fell nicely in line with theoretical predictions: inbreeding depression was positively correlated with
population size, whereas hybrid vigour and the genetic load were negatively correlated with population size.
Thus, smaller populations have fixed a larger number of deleterious recessive alleles via genetic drift and a
greater number of these alleles have been exposed into their homozygous state. The strong hybrid vigour found
in the small populations highlights the importance of migration for population persistence and for conservation
Finally, in chapter 3 we look briefly at the relationship between breeding system, fitness and population
persistence. Here we use a related species, Daphnia pulex, of which sexual and asexual populations coexists
within the Finnish archipelago. Contrary to expectations it is the presence of a microsporidian parasite, Gurleya
spp., which favors and facilitates the persistence of the asexual populations of this species. However, as the
asexual and sexual populations have vastly diverged phylogentic histories, with the asexual arriving in
Scandinavia from North America only recently, this appears to be a case of enemy or parasite release, where the
native Gurleya spp. is not adapted to the asexual Daphnia pulex, thus providing them an advantage.
Prof. Dr. Christoph Haag, thesis director
Prof. Dr. Christian Lexer, local expert
Dr. Daniel Levitis, MPI Rostock, external expert
Prof. Claudio de Virgilio, President of the jury