Sperm competition and genetic diversity in the agile antechinus (Dasyuridae : Antechinus agilis)

The mouse-like marsupial Antechinus agilis is common to south-eastern Austr alia with breeding and life-history traits being highly synchronous. Mating activity is confined to a 10- to 15-day period in August, at the end of wh ich all males die as a result of a stress-induced suppression of the immune system. Ovulation occurs at this time and females rely on stored sperm fro m specialized crypts in the lower isthmus of the oviduct for successful fer tilization. Here we report a high incidence of mixed paternity litters, whi ch can be attributed to sperm from multiple males being stored in the isthm ic crypts. Data from this study also suggest a possible second male siring advantage in controlled ex situ sperm competition mating trials, irrespecti ve of the delay between the two males being given mating access to the fema le or of the mating time relative to ovulation. In determining paternity th rough DNA profiling, population genetic data were obtained that showed sign ificant differences in the genetic heterozygosity between unrelated adults, half siblings and full siblings. We suggest that the isthmic crypts, in ad dition to storing viable sperm, are capable of releasing a mix of sperm tha t increases the likelihood of mixed paternity litters. This allows all mati ng males the opportunity to sire young, increases female reproductive fitne ss and overall maintains high levels of population genetic heterozygosity i n the face of total male mortality annually.