Effects of genomic homozygosity on total fitness in an invertebrate: lethal equivalent estimates for Drosophila melanogaster

Estimates of susceptibility to inbreeding depression for total fitness are needed for predicting the cost of inbreeding and for use in population viability analyses, but no such valid estimates are available for any wild invertebrate population. I estimated the number of lethals equivalents for total fitness in recently wild-caught populations of Drosophila melanogaster using published data on the total fitness of homozygosity versus heterozygosity for each of the major chromosomes (the X, second, and third) under competitive conditions. As there are no data for the fitness effects of homozygosity for the small fourth chromosome which represents 1.0% of the euchromatic genome, this was accounted for by attributing the homozygosity for the three large chromosome to an inbreeding coefficient of 0.99 when computing lethal equivalents for total fitness. Total genomic homozygosity is predicted to be essentially lethal in D. melanogaster. The corresponding haploid lethal equivalents estimate for total fitness was 5.04. The lethal equivalent value lies within the range for vertebrates but tends to be higher than for most outbreeding plants which are often purged as they exhibit up to 20% selfing (by definition). As D. melanogaster has its genome sequenced and annotated and has lethal equivalent estimates for total fitness for individual chromosomes as well as its total genome, it provides an excellent opportunity for evaluating genomic estimates of mutation load.