Genomic Divergence of Giant Sex Chromosomes in the Flea Beetle Alagoasa trifasciata (Coleoptera and Chrysomelidae)

Background/Objectives: Sex chromosomes may diverge genomically despite similar morphology. We investigated genomic differentiation and repetitive DNA organization in the giant sex chromosomes of Alagoasa trifasciata. Methods: Twenty specimens (12 males and eight females) were collected in São Carlos and the cytogenetic analyses included C-banding, fluorescence in situ hybridization (FISH), flow cytometry, and comparative genomic hybridization (CGH). Results: A. trifasciata exhibited 2n = 22, with ten autosomal pairs and giant, asynaptic X and Y chromosomes. Mean haploid genome sizes were 3.81 Gb in females and 3.69 Gb in males. The X chromosome lacked detectable C-positive heterochromatin, whereas the Y was almost entirely hetero-chromatic. Microsatellite mapping revealed chromosome-specific distributions, including GAG10 exclusively on the X and dispersed A30 signals along the Y. CGH identified male-enriched regions in the pericentromeric and distal portions of the Y chromo-some. Conclusions: Despite similar morphology, the X and Y chromosomes exhibit substantial genomic and chromatin differentiation. Differential repetitive DNA accumulation may contribute to their distinct evolutionary trajectories.