Genomic gigantism: DNA loss is slow in mountain grasshoppers

Several studies have shown DNA loss to be inversely correlated with genome size in animals. These studies include a comparison between Drosophila and the cricket, Laupala, but there has been no assessment of DNA loss in insec ts with very large genomes. Podisma pedestris, the brown mountain grasshopp er, has a genome over 100 times as large as that of Drosophila and 10 times as large as that of Laupala. We used 58 paralogous nuclear pseudogenes of mitochondrial origin to study the characteristics of insertion, deletion, a nd point substitution in P. pedestris and Italopodisma. In animals, these p seudogenes are "dead on arrival"; they are abundant in many different eukar yotes, and their mitochondrial origin simplifies the identification of poin t substitutions accumulated in nuclear pseudogene lineages. There appears t o be a mononucleotide repeat within the 643-bp pseudogene sequence studied that acts as a strong hot spot for insertions or deletions (indels). Becaus e the data for other insect species did not contain such an unusual region, hot spots were excluded from species comparisons. The rate of DNA loss rel ative to point substitution appears to be considerably and significantly lo wer in the grasshoppers studied than in Drosophila or Laupala. This suggest s that the inverse correlation between genome size and the rate of DNA loss can be extended to comparisons between insects with large or gigantic geno mes (i.e., Laupala and Podisma). The low rate of DNA loss implies that in g rasshoppers, the accumulation of point mutations is a more potent force for obscuring ancient pseudogenes than their loss through indel accumulation, whereas the reverse is true for Drosophila. The main factor contributing to the difference in the rates of DNA loss estimated for grasshoppers, cricke ts, and Drosophila appears to be deletion size. Large deletions are relativ ely rare in Podisma and Italopodisma.