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Abstract:
Population divergence is an important process in the evolution of lineages and can
occur rapidly through the interaction of random genetic drift with natural and sexual
selection. While natural selection operates on differences in fitness with respect to
the local environment, sexual selection acts on the reproductive success of
individuals through pre- and postcopulatory mate choice.
Recently separated populations of the Western European house mouse
Mus musculus domesticus were investigated for mating preferences. The study
system consisted of two populations, one sampled in the Cologne/Bonn region,
referred to as the “German population” and one from the Massif Central, termed here
the “French population”. These populations have been separated for at most 3,000
years. Although this time span is short in evolutionary terms, they already show
genetic differentiation.
To test whether population divergence is reflected in mate choice, I carried out four
replicates of a long-term experiment, in which individually tagged mice of both
populations were held for 6 month in a semi-natural enclosure. As controls, I
conducted cage experiments, where females could choose between males of both
populations during a 6 day period.
Paternities in the enclosure populations were determined by microsatellite typing of
all individuals and they were used as measures for mate choice and reproductive
success. The frequent monitoring of the populations during which animals were
examined individually allowed the assessment of their physical condition.
Furthermore, I examined the influence of a selfish genetic element, the t haplotype,
on pre- or postcopulatory mate choice for the different population backgrounds.
Finally, I analyzed whether the population divergence is also reflected in relative
frequencies of female strategies such as polyandry and communal breeding.
Founder animals of the long-term experiment did not follow a consistent mate choice
pattern, while individuals born in the enclosures showed a significant preference for
partners who had a father from the same population as themselves. In the controlled
cage experiment, there was no consistent preference pattern regarding population
background. However, female littermates that grew up in the same cage chose males
coming from one population, indicating an environmental influence. These findings
are discussed in the context of behavioral and genomic imprinting.
German and French founder animals differed slightly in reproductive success. Among
the F1 individuals, the comparison of reproductive success between individuals with
a mixed population background (i.e. with parents from different populations) versus
animals with a pure background (i.e. with parents from the same population) revealed
no significant differences. Nevertheless, when looking at the combination of
measures for reproductive success, such as offspring number, number of mating
events, and offspring per mating, in 5 out of 6 parameters “pure” individuals
outperformed the “mixed” individuals, which might indicate a slight decrease in hybrid
fitness.
No different influences were detected between German, French and hybrid animals
regarding the t haplotype or different frequencies of female multiple mating and
communal breeding. Influences of the t haplotype were restricted to a slight decrease
in offspring number in successful mating events between t/wt animals for all
combinations of population backgrounds. Contrary to theoretical assumptions and
other experiments, no evidence for an increased multiple mating frequency or
avoidance of partners with t/wt was found. Polyandry and communal breeding
seemed to be general strategies in females of pure as well as mixed population
backgrounds, and both strategies increased in frequency with an increasing
population density. Females displaying these strategies had a slightly higher
reproductive success in semi-natural conditions: Mothers with litters sired by several
males had a higher reproductive success than mothers with only single paternity
litters. A higher reproductive success was also detected for females which grew up in
communally reared litters.
Summarizing the outcome of the study, the recently diverged populations do not vary
in partner choice: no differences in mate choice or reproductive strategies were
observed. However, females preferred mates that had fathers from the same
population as themselves, a pattern which I will call the “father related assortative
mating pattern”. This suggests the presence of cues which enable the differentiation
between “one’s own population” and “the other population”. In addition, the results
gave insights into the benefits of costly female reproductive strategies.
