Abstract
The mating system strongly influences genetic variation, reproductive isolation, speciation, and invasiveness. In haplodiploid species, inbreeding is expected to produce high homozygosity and purge deleterious mutations, potentially reducing inbreeding depression but increasing outbreeding depression at shallow genetic distances. In bark, ambrosia, and seed beetles (Scolytinae), haplodiploidy and inbreeding have been linked to small population persistence and invasiveness. However, observed heterozygosity and outbreeding in natural populations challenge these assumptions. To examine the relationship between genetic distance and fitness, we established F4 inbred lines of the date stone beetle, Coccotrypes dactyliperda, from populations in Israel and the United States. Genetic distances were estimated using mitochondrial genes and nuclear markers, and 814 experimental crosses were conducted to assess fecundity and hatch rates. Contrary to previous reports, we observed both inbreeding and outbreeding depression, especially in sibling matings and in extremely distant crosses. A non-linear model revealed reduced fitness at both ends of the relatedness spectrum, with highest fecundity at intermediate distances. Mitochondrial lineages showed the strongest inbreeding depression, although lineage assignments varied between mitochondrial and nuclear markers. Our results emphasize the importance of quantifying genetic distance continuously rather than categorically. The discordance between mitochondrial and nuclear markers suggests that incompatibility may involve unmeasured loci. These findings highlight the complexity of genetic structure and the need for further studies on the fitness consequences of genetic distance in haplodiploid species.