Sperm allocation strategies in a sperm heteromorphic insect

Theories predict that in polyandrous species,the focal male should increase sperm allocation per mate in the presence of rivals to gain greater share of paternity,but in the presence of additional mates,he should reduce sperm allocation per mate to save sperm for insemination of more mates.However,empirical findings are often inconsistent and reasons behind are unclear.Furthermore,many studies use copulation duration as an estimate of the number of sperm transferred.Yet,empirical evidence for such assumption is largely lacking.Here,we used a sperm heteromorphic insect Ephestia kuehniella whose males produce two types of sperm,eupyrenes(fertile)and apyrenes(nonfertile),to test these postulations.We allowed focal males to detect chemical and acoustic but no tactile cues from rivals or additional mates both before and during mating and measured copulation duration and sperm allocation in successive copulations.We demonstrate that males transfer significantly more eupyrenes per mate in the presence of rivals and that the sperm allocation pattern persists in successive copulations under this condition.However,males do not adjust apyrene allocation in response to rivals probably because apyrenes play a relatively minor role in male reproductive success.Contrary to a previous study,focal males do not respond to additional mates most likely due to the lack of tactile cues in the present study.We reveal that sperm allocation is not a function of copulation duration in this insect for spermatophore formation and delivery occupy most of copulation duration and sperm transfer is complete near the end of copulation.

Juvenile socio-sexual experience determines lifetime sperm expenditure and adult survival in a polygamous moth, Ephestiakuehniella

Male animals often adjust their sperm investment in response to sperm competition environment.To date,only a few studies have investigated how juvenile sociosexual settings affect sperm production before adulthood and sperm allocation during the first mating.Yet,it is unclear whether juvenile sociosexual experience(1)determines lifetime sperm production and allocation in any animal species;(2)alters the eupyrene:apyrene sperm ratio in lifetime ejaculates of any lepidopteran insects,and(3)influences lifetime ejaculation patterns,number of matings and adult longevity.Here we used a polygamous moth,Ephestia kuehniella,to address these questions.Upon male adult emergence from juveniles reared at different density and sex ratio,we paired each male with a virgin female daily until his death.We dissected each mated female to count the sperm transferred and recorded male longevity and lifetime number of matings.We demonstrate for the first time that males ejaculated significantly more eupyrenes and apyrenes in their lifetime after their young were exposed to juvenile rivals.Adult moths continued to produce eupyrene sperm,contradicting the previous predictions for lepidopterans.The eupyrene:apyrene ratio in the lifetime ejaculates remained unchanged in all treatments,suggesting that the sperm ratio is critical for reproductive success.Male juvenile exposure to other juveniles regardless of sex ratio caused significantly shorter adult longevity and faster decline in sperm ejaculation over successive matings.However,males from all treatments achieved similar number of matings in their lifetime.This study provides insight into adaptive resource allocation by males in response to juvenile sociosexual environment.