True polyandry and pseudopolyandry: why does a monandrous fly remate?

Abstract

BackgroundThe rate of female remating can have important impacts on a species, from affecting conflict and cooperation within families, to population viability and gene flow. However, determining the level of polyandry in a species can be difficult, with information on the mating system of many species being based on a single experiment, or completely absent. Here we investigate the mating system of the fruit fly Drosophila subobscura. Reports from England, Spain and Canada suggest D. subobscura is entirely monandrous, with no females remating. However, work in Greece suggests that 23% of females remate. We examine the willingness of female D. subobscura to remate in the laboratory in a range of conditions, using flies from both Greece and England. We make a distinction between pseudopolyandry, where a female remates after an ineffective first mating that is incapable of fertilising her eggs, and true polyandry, where a female remates even though she has received suitable sperm from a previous mating.ResultsWe find a low rate of true polyandry by females (4%), with no difference between populations. The rate of true polyandry is affected by temperature, but not starvation. Pseudopolyandry is three times as common as true polyandry, and most females showing pseudopolyandry mated at their first opportunity after their first failed mating. However, despite the lack of differences in polyandry between the populations, we do find differences in the way males respond to exposure to other males prior to mating. In line with previous work, English flies responded to one or more rivals by increasing their copulation duration, a response previously thought to be driven by sperm competition. Greek males only show increased copulation duration when exposed to four or more rival males. This suggests that the response to rivals in D. subobscura is not related to sperm competition, because sperm competition is rare, and there is no correlation of response to rivals and mating system across the populations.ConclusionsThese results illustrate the difficulties in determining the mating system of a species, even one that is well known and an excellent laboratory species, with results being highly dependent on the conditions used to assay the behaviour, and the population used.

Extreme cost of rivalry in a monandrous species: male–male interactions result in failure to acquire mates and reduced longevity

“…(i) Experiment 1: male response to social environment Previous work showed that the presence of a single rival affected copulation duration in both species [14,16,21]. Here, we examined the specificity of the male response to rivals by exposing males to various social environments.…”

“…The females used to alter social environment in the 'mated female' treatment were mated a day before exposure; they were conspecific and the same age as the males tested. As D. subobscura females are monandrous [16], they were thus inaccessible to the tested male. However, D. pseudoobscura females typically remate after 5 days, and so it is likely that they will have mated with the experimental male.…”

“…Drosophila pseudoobscura females mature at 3 days post eclosion, and males at 1 -2 days [26]. Both species have a highly male-biased operational sex ratio, owing to monandry in D. subobscura [16], and a 3 -5 day female remating latency in D. pseudoobscura [22]. Both species can live for three months in ideal laboratory conditions [27], but are much shorter lived as adults in nature [28,29].…”

“…For each species, males were placed either individually (treatment 'single'), or with one rival male (treatment 'one rival'), four rival males (treatment 'four rivals'), one heterospecific male (treatment 'heterospecific') or one conspecific mated female (treatment 'mated female'), from within 8 h of eclosion to 8 16). Numbers of trials were higher for D. subobscura to reduce the risk that the lower mating rate in this species would reduce the number of matings below which reasonable analysis of copulation duration and latency was difficult.…”

“…To this end, we examined the impact of social environment on a male's ability to acquire matings and on longevity. We compared social environment effects across two Drosophila species: the monandrous D. subobscura, and the closely related polyandrous species Drosophila pseudoobscura [16,19,20]. As males of a monandrous species are only subjected to pre-copulatory sexual selection while males of polyandrous species are subjected to both pre-and post-copulatory sexual selection, we expected that the presence of rivals before copulation would have a more profound impact on the fitness (reproductive success and survival) of males of the monandrous species than the polyandrous one.…”