Fertilization is not a new beginning: the relationship between sperm longevity and offspring performance

Abstract

Sperm are the most diverse cell type known: varying not only among- and within- species, but also among- and within-ejaculates of a single male. Recently, the causes and consequences of variability in sperm phenotypes have received much attention, but the importance of within-ejaculate variability remains largely unknown. Correlative evidence suggests that reduced within-ejaculate variation in sperm phenotype increases a male's fertilization success in competitive conditions; but the transgenerational consequences of within-ejaculate variation in sperm phenotype remain relatively unexplored. Here we examine the relationship between sperm longevity and offspring performance in a marine invertebrate with external fertilization, Styela plicata. Offspring sired by longer-lived sperm had higher performance compared to offspring sired by freshly-extracted sperm of the same ejaculate, both in the laboratory and the field. This indicates that within-ejaculate differences in sperm longevity can influence offspring fitness - a source of variability in offspring phenotypes that has not previously been considered. Links between sperm phenotype and offspring performance may constrain responses to selection on either sperm or offspring traits, with broad ecological and evolutionary implications.

Figure 1. Effect of sperm treatment (fresh sperm, fresh sperm diluted to 1% of the original concentration, and sperm stored for one hour before fertilization) on resulting fertilization success (a); and larval size (measured as area) (b).

Points represent least squares mean (± SE).

Figure 2. Relationship between sperm longevity and offspring hatching success. Points represent least squares mean (± SE).

Figure 3. Relationship between sperm longevity and offspring survival for two weeks in the field.

Black points represent the back-transformed estimates (± SE) from the mixed-effects logistic regression. Grey points are the estimated mean probabilities for each male (random effect values for each male back-transformed to the probability scale). Grey lines link means from each male.