Ageing effects in guppy sperm

Sexual selection is expected to drive intense selection pressure on traits selected for both before and after copulation, and so reduce variation. However, these features remain surprisingly very diverse and one of the main challenges in evolutionary biology is to explain this phenomenon.

The project 'The role of ageing in the maintenance of variability in male reproductive success' (AGEING AND RS) is researching sperm senescence in the guppy Poecilia reticulata as a possible contributor to diversity.

During the first phase of the project, scientists found that ovarian fluid collected from the female genital tract offsets the deleterious effects of ageing on sperm quality. Female guppies can store the sperm for up to six months.

However, during male storage, older sperm showed less motility than their freshly produced counterparts. As sperm motility is a major determinant of competitive fertilisation success in guppies, males with older sperm will not be favoured. As female storage mediates the effects of ageing on sperm, however, it may be that the female uses polyandry or multiple mates to select against aged sperm.

The AGEING AND RS team also found that ageing sperm reduces the chances of fertilisation when there is sperm from other males present. Significantly perhaps, this effect is not seen in the absence of competitor's sperm. Results also indicate that sperm age affects the resulting offspring.

Project findings suggest that sperm ageing is an important factor that may shape a number of male and female traits, extending to effects on the offspring. Future results promise to shed more light on how features of male reproductive success can remain diverse despite selection pressures.