Haploids--Hapless or Happening?

Science  29 Jun 2001:
Vol. 292, Issue 5526, pp. 2441-2443
DOI: 10.1126/science.1062890

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Why do animals have duplicate sets of chromosomes (diploid) instead of just one set (haploid)? The theory goes that diploidy is a superior state because if one copy of a gene gets mutated, the second copy can compensate. This dogma has been thrown into disarray with the first report of an animal, the false spider mite, that exists solely as a haploid ( Weeks et al .). As Otto and Jarne explain in their Perspective, the presence of a "feminizing" intracellular bacterium renders all of the mites female and the females then reproduce infected female offspring from unfertilized eggs (parthenogenesis). This negates the requirement for males and ensures that the genome remains haploid.