Element F expansion tracks parthenogenetic ability in Drosophila mercatorum

The dramatic transition to parthenogenetic reproduction is often accompanied by similarly dramatic changes in genome organisation. 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 these changes, we combined cytogenetic and comparative analyses to document variation in heterochromatin and large-scale rearrangements among 18 strains of Drosophila mercatorum capable of varying degrees of parthenogenetic reproduction. This revealed that element F is 4.4-fold larger (95% confidence interval 4.0-4.8) in strains capable of parthenogenesis but that multiple large chromosomal inversions (2.8-9 megabases) on element B likely arose earlier. We combined phylogenetic analysis with comparisons of these genomic changes and parthenogenetic ability in D. mercatorum strains collected in South America, North America, Hawaii, and Africa. This indicated that the heterochromatin expansion has arisen once and appears to gradually increase with the rate of parthenogenetic ability. However, since this increase was also present in obligate sexually reproducing strains, heterochromatin expansion was not a consequence of parthenogenesis. It is therefore possible that heterochromatin expansion and parthenogenetic ability are unrelated but are carried together; that a shared underlying mechanism promotes both; or that heterochromatin expansion itself drives the capacity for parthenogenetic reproduction.