Abstract
A loss of sperm viability and functionality during sperm transfer and storage within the female reproductive tract can have important fitness implications by disrupting fertilization and impairing offspring development and survival. Consequently, mechanisms that mitigate the temporal decline in sperm function are likely to be important targets of selection. In many species, ovarian fluid is known to regulate and maintain sperm quality. In this paper, we use the guppy Poecilia reticulata, a highly polyandrous freshwater fish exhibiting internal fertilization and sperm storage, to determine whether ovarian fluid (OF) influences the decline in sperm viability (the proportion of live sperm in the ejaculate) over time and whether any observed effects depend on male sexual ornamentation. To address these questions we used a paired experimental design in which ejaculates from individual males were tested in vitro both in presence and absence of OF. Our results revealed that the temporal decline in sperm viability was significantly reduced in the presence of OF compared to a saline control. This finding raises the intriguing possibility that OF may play a role in mediating the decline in sperm quality due to the deleterious effects of sperm ageing, although other possible explanations for this observation are discussed. Interestingly, we also show that the age-related decline in sperm viability was contingent on male sexual ornamentation; males with relatively high levels of iridescence (indicating higher sexual attractiveness) exhibited a more pronounced decline in sperm viability over time than their less ornamented counterparts. This latter finding offers possible insights into the functional basis for the previously observed trade-off between these key components of pre- and postcopulatory sexual selection.
Highlights
A number of studies have reported that ovarian fluid (OF) can have positive effects on the viability and/or motility of sperm (e.g. [1,2,3,4,5])
We report the first evidence of a differential pattern of sperm viability decline according to the presence or absence of ovarian fluid during incubation, and the first evidence that female-derived fluids positively affect sperm survival in any vertebrate with sperm storage
The conclusions from the present study are limited to the effects of OF on the short-term decline in sperm viability in vitro, but they raise a number of intriguing questions surrounding the possible adaptive functions of OF
Summary
A number of studies have reported that ovarian fluid (OF) can have positive effects on the viability and/or motility of sperm (e.g. [1,2,3,4,5]). A number of studies have reported that ovarian fluid (OF) can have positive effects on the viability and/or motility of sperm Studies investigating the effect of ovarian fluid (or more generally female-derived substances) on sperm survival are largely limited to insects Sperm viability is an important determinant of competitive fertilization success in poeciliid fishes [12] including the focal population of guppies used in the present study We explored whether male colour ornamentation was associated with the decline in sperm viability. We consider both colour traits (orange and iridescence) to determine whether any decline in sperm viability over time is dependent on male sexual ornamentation
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