Molecular parentage data: a study on competitive fertilizations in spiders

Sperm competition drives traits that enhance fertilization success. The
amount of sperm transferred relative to competitors is key for attaining
paternity. Female reproductive morphology and male mating order may also
influence fertilization, however the outcome for sperm precedence under
intense sperm competition remains poorly understood. In the polyandrous
spider Pisaura mirabilis, males offer nuptial gifts which prolong
copulation and increase sperm transfer, factors proposed to alter sperm
precedence patterns under strong sperm competition. First, we assessed the
degree of female polyandry by genotyping wild broods. A conservative
analysis identified up to 4 sires, with a mean of 2 sires per brood,
consistent with an optimal mating female rate. Then we asked whether
intense sperm competition shifts sperm precedence patterns from first male
priority, as expected from female morphology, to last male advantage. We
varied sexual selection intensity experimentally and determined
competitive fertilization outcome by genotyping broods. In double matings,
one male monopolised paternity regardless of mating order. A mating order
effect with first male priority was revealed when females were mated to 4
males, however this effect disappeared when females were mated to 6 males,
likely due to increased sperm mixing. The proportion of males that
successfully sired offspring drastically decreased with the number of
competitors. Longer copulations translated into higher paternity shares
independently of mating order, reinforcing the advantage of traits that
prolong copulation duration under intense competition, such as the nuptial
gift. Sperm competition intensity enhances the impact of competitive
sexual traits and imposes multiple effects on paternity.