Adaptive significance of queen mating frequency in ants : In search of benefits of genetic variability

University dissertation from Uppsala : Acta Universitatis Upsaliensis

Abstract: Fitness effects of polyandry, multiple mating by females, were studied on two ant speciesLasius niger and Formica truncorum. Particular emphasis was put on evaluating diploid male load, which happens when homozygosity in the sex determination locus leads to intendedfemales developing into sterile, diploid males. It has been hypothesised that under certainconditions diploid male load selects for polyandry.The power of spermatheca-PCR (genotyping stored sperm in the spermatheca of a femaleinsect) in identifying multiply mated females was scrutinised. Microsatellite amplificationsfrom artificial sperm mixes with known composition showed that alleles at and above 10%representation are reliably amplified and detected.Microsatellite markers were developed for L. niger and female survival and fecundityin relation to her mate number were studied in two populations: Uppsala, Sweden and Lausanne, Switzerland. Annual sexual production did not correlate with higher female mating frequency in either population. Colony survival appeared to be higher for multiple mated females in Lausanne but not in Uppsala. The possible higher survival of multiple mated queens is unlikely to be due to diploid male load as there was no sign of diploid males among broods of 479 queens which were kept in laboratory.Diploid male load in the colonies of the ant Formica truncorum was measured. Thecolony size was significantly negatively affected by diploid male production. However, theobserved frequencies of diploid males in the colonies did not comply with the theoreticalpredictions. Until the determinants of diploid male production in F.truncorum are clarified itis not possible to settle whether diploid male load selects for multiple mating in this species.A theoretical model was constructed to examine how the selection on mating frequencyfrom diploid male load changes when allelic diversity in the sex determination locus increases.Fitness of single and double mated females was compared. The fitness difference between themate number classes rapidly decreases when the number of alleles in the sex determinationlocus increases. Moreover, even when the selection pressure from diploid male production ismost extreme, the fitness differences between single and double mated females are small.Diploid male load probably is not strong enough a force to drive mating frequency evolutionin eusocial insects.

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