Genome size and intron size in Drosophila.

Petrov, Lozovskaya, and Hartl (1996) demonstrated that unconstrained regions of the non-long terminal repeat retrotransposable element Helena lose DNA at an unusually high rate in species of the Drosophila virilis species group. More recent data also indicate a high rate of DNA loss of Helena in species of the Drosophila melanogaster species subgroup (Petrov and Hartl 1998). Based on these observations, the authors suggested that the paucity of pseudogenes in Drosophila is the product of rampant deletion of DNA in regions not subjected to selective constraints, and they further extrapolated that different deletion rates may contribute to the divergence in genome size among taxa. Their assumption is that such a high rate of deletion is not confined to Helena elements alone. The sizes of any unconstrained regions, such as introns and other noncoding regions, would also be decreased to the extent that selection allows, and consequently the genome size would be reduced. Supporting evidence was obtained from vertebrate genes (Hughes and Hughes 1995; Ogata, Fujibuchi, and Kanehisa 1996). In accordance with the difference in genome size, human genes have significantly longer introns than do avian or rodent homologs. The genome size of D. virilis is 0.34–0.38 pg per haploid genome, while those of D. melanogaster and Drosophila pseudoobscura are 0.18–0.21 pg (Powell 1997). Even taking into account the different proportions of the genome devoted to pericentromeric heterochromatin, the genome of D. virilis is considerably larger than that of D. melanogaster and D. pseudoobscura (Hartl and Lozovskaya 1995). The difference in the size of the euchromatic genome between D. virilis and D. melanogaster is about 36% (150 Mb vs. 110 Mb). If differences in genome size of such magnitude are due to different rates of accumulation of small deletions and insertions throughout the euchromatic genome, then we can predict that D. virilis genes should have longer introns than those of D. melanogaster and D. pseudoobscura. We compared the lengths of 115 complete introns collected from 42 homologous genes between D. melanogaster and D. virilis, and 60 introns from 22 homologous genes between D. melanogaster and D. pseudoobscura (11 genes are common in the three species).