Phylloxera and Aphids Show Distinct Features of Genome Evolution Despite Similar Reproductive Modes.

Genomes of aphids (family Aphididae) show several unusual evolutionary patterns. In particular, within the XO sex determination system of aphids, the X chromosome exhibits a lower rate of interchromosomal rearrangements, fewer highly expressed genes, and faster evolution at nonsynonymous sites compared with the autosomes. In contrast, other hemipteran lineages have similar rates of interchromosomal rearrangement for autosomes and X chromosomes. One possible explanation for these differences is the aphid's life cycle of cyclical parthenogenesis, where multiple asexual generations alternate with 1 sexual generation. If true, we should see similar features in the genomes of Phylloxeridae, an outgroup of aphids which also undergoes cyclical parthenogenesis. To investigate this, we generated a chromosome-level assembly for the grape phylloxera, an agriculturally important species of Phylloxeridae, and identified its single X chromosome. We then performed synteny analysis using the phylloxerid genome and 30 high-quality genomes of aphids and other hemipteran species. Unexpectedly, we found that the phylloxera does not share aphids' patterns of chromosome evolution. By estimating interchromosomal rearrangement rates on an absolute time scale, we found that rates are elevated for aphid autosomes compared with their X chromosomes, but this pattern does not extend to the phylloxera branch. Potentially, the conservation of X chromosome gene content is due to selection on XO males that appear in the sexual generation. We also examined gene duplication patterns across Hemiptera and uncovered horizontal gene transfer events contributing to phylloxera evolution.