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female choice; heterozygosity; mating strategies; sperm competition; sperm staggering; spermatheca
Abstract:
Although classically thought to be rare, female polyandry is widespread and may
entail significant fitness benefits. If females store sperm over extended periods of time,
the consequences of polyandry will depend on the pattern of sperm storage, and some
of the potential benefits of polyandry can only be realized if sperm from different
males is mixed. Our study aimed to determine patterns and consequences of polyandry
in an amphibian species, the fire salamander, under fully natural conditions. Fire salamanders
are ideal study objects, because mating, fertilization and larval deposition are
temporally decoupled, females store sperm for several months, and larvae are deposited
in the order of fertilization. Based on 18 microsatellite loci, we conducted paternity
analysis of 24 female-offspring arrays with, in total, over 600 larvae fertilized
under complete natural conditions. More than one-third of females were polyandrous
and up to four males were found as sires. Our data clearly show that sperm from multiple
males is mixed in the female’s spermatheca. Nevertheless, paternity is biased,
and the most successful male sires on average 70% of the larvae, suggesting a ‘topping
off’ mechanism with first-male precedence. Female reproductive success increased with
the number of sires, most probably because multiple mating ensured high fertilization
success. In contrast, offspring number was unaffected by female condition and genetic
characteristics, but surprisingly, it increased with the degree of genetic relatedness
between females and their sires. Sires of polyandrous females tended to be genetically
similar to each other, indicating a role for active female choice.
