The roles of genome size and cell size in body size variation among Hawaiian Drosophila. E.M. Craddock 1, T. Kacmarczyk 1, R.M. Dawley 2. 1) Div Natural Sci, Purchase Col, State Univ New York, Purchase, NY; 2) Dept Biology, Ursinus Col, Collegeville, PA.
The endemic Hawaiian drosophilid fauna includes the giants and dwarfs of the Drosophila world, with the largest flies being found in the picture-winged and antopocerus clades, and the smallest in the scaptomyzoid lineage. To investigate the cellular basis of these remarkable differences in adult body size, we used image analysis to make trichome counts in a defined region of the posterior compartment of the wing and thereby estimate cell areas. We found substantial interspecific variation in wing cell areas (> 3 fold), as well as cell size differences between conspecific males and females, that correlate broadly with patterns of body size variation. Along with sex differences in diploid genome size, flow cytometry of nuclei from homogenized heads revealed significant interspecific genomic size differences. Species with larger genomes tend to have larger cells and body sizes, but there are exceptions, i.e., some very large flies with large cells have small genomes. Karyotypic changes in Hawaiian Drosophila evolution, specifically addition of heterochromatic segments or even whole chromosome arms, have clearly contributed to the observed increases in genome, cell and body sizes. One of the giant picture wings, D. cyrtoloma, that has six arms added to the basic Hawaiian haploid karyotype of five rods and a dot chromosome, has a genome size double that of D. melanogaster and other picture wings in the same clade. The exceptionally large genome of this species is not unexpected, given its 70% satellite DNA content demonstrated by CsCl density ultracentrifugation. Additional factors such as developmental rate differences apparently complicate determination of species body sizes, implicating interspecific differences in adult DNA ploidy levels and possibly also in degree of underreplication of repetitive sequences in the heterochromatin.