Our understanding of the mechanics of sexual selection has come a long way since 1871 and Darwin's second-most-famous book The Descent of Man, and Selection in Relation to Sex. At that time, the focus remained within the bounds of relatively polite pre-mating activities such as courtship rituals or duels between amorous males. However, since the 1970s researchers' eyes have been firmly drawn towards the intimate post-copulatory processes that occur inside the female body during and after mating. While our understanding of how male gametes compete in the race to fertilise eggs has advanced steadily, our understanding of the female's role in sperm selection, termed cryptic female choice, is less clear. One major barrier is that these cryptic choices happen inside the female body and are difficult to discern with any clarity. Now, in a paper published in Science, Scott Pitnick and his colleagues describe an extraordinary way of viewing internal processes, as they are played out within the female tract.
Sperm competition studies have long been impeded by the inability to determine which sperm belong to which male. The authors have now solved this problem by generating transgenic lines of Drosophila melanogaster that produce fully functioning sperm with heads that produce either green or red fluorescent protein tags so that they can be distinguished from other competitor male's sperm. The authors then filmed and tracked the progress of the fluorescent sperm of two competing males in the dissected organs of a female after mating.
As a first insight, the sperm's striking speed and manoeuvrability clearly exceeded the team's expectations. Pitnick and his colleagues also zoomed in on the fine detail of the interactions between the sperm. They saw that the ejaculate from one male displaces the sperm of predecessors from female storage organs (two spermathecae and a seminal receptacle); as a male's success is proportional to the number of sperm held in store by a female and males can effectively shift the odds of success in their favour by flushing out the competition. The team was also able to confirm the female's role in biasing fertilisation by showing that they can eject sperm from the uterus. However, there was no indication that the ejaculate of any male can damage or kill rival sperm.
By filming the progress of identifiable sperm in the female's reproductive tract, Pitnick and his colleagues have convincingly verified (and disproved) many predictions, based on theory and indirect observations, about the hitherto unseen and complex events that occur in the female reproductive tract prior to fertilisation. Building on their success in Drosophila, the team are applying their approach to other insect model systems to lift the lid on the fate that befalls sperm once they enter the female. Such direct investigation could herald a golden age in the study of sexual selection, as we can at last witness sperm cells skirmishing from front row seats in real time. Moreover, the technique could help decipher the all-too-cryptic choices that females make, with the hope of resolving controversies regarding the existence or importance of sperm selection by females.
