Leaving X: How scale insects repeatedly evolved alternatives to chromosomal sex determination.

Two independent losses of the X sex chromosomal system were recovered in 17 newly sequenced scale insect species, occurring in Monophlebidae and at the base of PGE families.

• Why it matters: Sex determination turnover is puzzling because many taxa keep systems for hundreds of millions of years, while scale insects show repeated transitions among distinct mechanisms.
• What they did: The team sequenced 17 species, reconstructed transitions, mapped genomic rearrangements, and compared ortholog conservation, finding only one orthogroup lost in both transitions and several orthogroups gained.
• The result: The data suggest turnover can occur with limited gene-content change, while extreme sex-biased expression predates turnover and persists on some formerly X-linked sequences, refining hypotheses and enabling new genomic studies.

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