BACKGROUND: The flea beetle Omophoita octoguttata (Coleoptera, Chrysomelidae) is a member of a group in which the males completely lack meiotic recombination (male-specific achiasmy) and that have extraordinarily large X and Y chromosomes. We combined genome sequencing, including microdissected Y and X chromosomes, and cytogenetic in situ hybridization studies, to evaluate the potential role of satellite DNAs (satDNAs) in the differentiation of those gigantic sex chromosomes. RESULTS: We report flow cytometry results showing that this species has a very large genome size (estimated to be 4.61 and 5.47 pg, or roughly 4.6 and 5.5 gigabases, for males and females, respectively), higher than the estimates from two other Alticinae species without giant sex chromosomes, suggesting that these sequences have greatly expanded on both the sex chromosomes, and that the Y has not greatly shrunk like the ones of other insects such as Drosophila with male achiasmy. About 68% of this large genome is made up of repetitive DNAs. Satellite DNAs (OocSatDNAs) form ~ 8-9% of their genomes, and we estimate how much of the sex chromosome expansions occurred due to differential amplification of different satellite classes. Analysis of divergence between sequences in the X and Y chromosomes suggests that, during the past roughly 20 mya, different OocSatDNAs amplified independently, leading to different representations. Some are specific to the Y or X chromosome, as expected when males are achiasmate, completely preventing genetic exchanges between the Y and X.