With Whom Should an Endangered Population Mate?
Lessons from Drosophila melanogaster

Aaron May

It has been suggested that captively bred endangered species with low levels of genetic variation and low fitness should have an infusion of genetic variation from an outside population. There are also situations when few individuals remain in an endangered lineage and the only way to ensure the survival of its genes is to hybridize it to another lineage. Both of these scenarios call for the choosing of a "group" to mate with an endangered lineage. The commonly practiced hypothesis is that the endangered lineage should be mated to the most closely related lineage. Other possible hypotheses are that they should be mated to whomever has the highest levels of genetic variation or to an evolutionarily distant lineage. These hypotheses were tested by constructing hybrid populations from three distinct populations of Drosophila melanogaster with a known phylogeny and known levels of genetic variation. The matings were evaluated based upon their prospects for short and long term survival which were assessed through measures of fitness and genetic variation respectively. The hybrid and non-hybridized populations were maintained at a size of 32 individuals for onwards of 17 generations. Fitness and levels of heterozygosity at the allozymes studied were highest for hybrids that were the most evolutionarily distant. This suggests that genetic distance between populations may not provide a good indication of the fate of hybridized captive breeding programs. It was also found that the fitness of the hybrids was intermediate to that of the populations from which it was created. This suggests that fitness, rather than genetic distance, may be the best indicator of with whom an endangered species should mate. It was also found that the non-hybridized populations decreased in fitness over time while the hybrid populations' fitness remained relatively constant. Hybrids may be able to counteract the effects of inbreeding depression by the introduction of novel genetic variation and masking of deleterious alleles. This result provides more support to the claim that endangered populations would benefit from the infusion of genetic variation from other populations.