Expansion of chromosome F heterochromatin in parthenogenetic Drosophila mercatorum

The dramatic transition to parthenogenetic reproduction is often accompanied by similarly dramatic changes in genome organization. Genomic changes may precede or succeed the onset of parthenogenesis and may directly or indirectly contribute to the success of this reproductive transition and fitness of parthenogenetic animals. To gain greater understanding of genomic changes accompanying the transition to parthenogenetic reproduction, we have characterized genomic differences between a sexually reproducing and parthenogenetic strain of Drosophila mercatorum. This revealed three large (2.6-9 Mbp) inversions present on Muller element B and a 30-fold length expansion of heterochromatin on Muller element F. Comparisons of these genomic changes and parthenogenetic ability in 18 D. mercatorum strains collected in South America, North America, Hawaii, and Africa indicates no clear correlation of these genomic changes with geographical origin. However, these changes are minimal in strains collected in Brazil suggesting this is the origin for the ancestral strain and that both chromosome inversions and increased heterochromatin have likely preceded the transition to parthenogenetic reproduction. Parthenogenetic ability correlates most strongly with expansion of chromosomal element F. We speculate that this increased heterochromatic environment of genes on element F influences gene expression to either enhance parthenogenesis directly or redirect the activity of another factor contributing to it, in a manner analogous to the known influence of heterochromatin on gene activity in position effect variegation.