Multi-locus genotyping of stored sperm reveals female remating rates in wild populations of the Queensland fruit fly.

Female insects commonly have more than one mate during a breeding period ('polyandry'), storing and using sperm from multiple males. In addition to its evolutionary significance, insect polyandry has practical implications for pest management that relies on the sterile insect technique (SIT). The Queensland fruit fly, Bactrocera tryoni (Froggatt), is a major horticultural pest in Australia, and outbreaks are managed by SIT in some regions. The present study provides the first evidence for polyandry in female B. tryoni from field populations from New South Wales (NSW) and Queensland (QLD) through multi-locus genotyping (ten microsatellite markers in four fluorescent multiplexes) of the stored sperm in ovipositing females. Polyandry level was significantly higher in the NSW collection (80.0 %) than the QLD collection (26.1 %), suggesting substantial regional and/or temporal variation. These findings have important implications for the use of SIT to suppress B. tryoni populations and to eradicate outbreaks.

Patterns of sperm use by twice-mated female Queensland fruit flies.

Multiple mating by females, polyandry, is common in insects, including in tephritid fruit flies. Female insects that remate commonly store sperm of multiple males. How the sperm of different males contribute to paternity is an important element of sexual selection. Sexual behavior and reproduction of the Queensland fruit fly (Qfly), Bactrocera tryoni, has been extensively investigated both in relation to understanding this economically important species' reproductive biology and in relation to implications for Sterile Insect Technique (SIT), whereby sterile flies are released to constrain reproduction of pest populations. Despite numerous studies of pre- and postcopulatory sexual selection in Qfly, there have been no direct studies of paternity patterns in polyandrous female Qflies. We used two morphologically distinguishable lines to investigate patterns of sperm use in Qfly. The two lines showed comparable mating performance evidenced by similar mating and remating frequency, copula duration, and proportion of second mate paternity (P2) between reciprocal crosses. The mechanism of sperm usage, with P2 close to 0.5 immediately after the second mating followed by gradual decrease of P2 as females aged, is most consistent with stratification or repositioning of sperm. Patterns observed in the present study are compared with the available information from other tephritid fruit flies, and are discussed in relation to this species' reproductive biology, known patterns of sperm storage, and SIT.

Patterns of sperm storage in twice-mated Queensland fruit flies.

Polyandry, whereby females mate with more than one male in a reproductive cycle, can result in sperm competition or cryptic female choice, and have fitness implications for both sexes. Understanding patterns of sperm storage in twice-mated females can provide valuable insights to mechanisms that mediate sperm use and paternity. In the Queensland fruit fly, Bactrocera tryoni (Qfly), and other insects that are managed by the Sterile Insect Technique (SIT), polyandry can reduce the efficacy of this pest control method. Patterns of sperm storage in twice-mated Qflies were studied by developing three fly lines that are homozygous for different alleles of a microsatellite marker (Bt32) and using a combination of quantitative real time polymerase chain reaction (qPCR) and capillary electrophoresis-based techniques to quantify and genotype sperm in each spermatheca. Female Qflies consistently stored fewer sperm from their second mate than from their first mate. Further, asymmetry between the spermathecae in the distribution of sperm stored from the first mate appears to in part determine the distribution of sperm stored from the second mate, likely because of constraints in storage capacity in the two spermathecae. Implications of these findings for elucidating pattern of sperm competition in this species, and for SIT, are discussed.