3.2 Sex chromosomes and autosomes
The male and female Illumina paired-end DNA reads were mapped separately
to the genomic scaffolds to estimate the MRPM. In chrX1, chrX2, chrX3,
the MRPM of female reads length is 1,439,092, 1,333,387 and 1,051,602
and the MRPM of female reads length is 781,901, 726,210 and 576,946
respectively. The MRPM of female reads was nearly twice as high as that
of male reads in chrX1, chrX2 and chrX3. The other 10 chromosomes
exhibited no significant differences between females and males, with
female/male ratios ranging from 0.90 to 1.00 (Table S5). It has been
shown that the X chromosome is conserved in aphids while chromosomal
rearrangements are common on autosomes (Li et al. 2021, Mathers et al.
2021). The syntenic block between the S. chinensis assembly andAc. pisum from Macrosiphini (Li, et al., 2020), R. maidisfrom Aphidini (Chen et al. 2019), and E. lanigerum (Figure 3C)
from Eriosomatinae were identified. All comparisons reveal high levels
of genome rearrangements between autosomes. Interestingly, threeS. chinensis chromosomes were mapped the conserved X chromosome
of Macrosiphini and Aphidini, and two X chromosomes of E.
lanigerum . The observed multiple X chromosomes are consistent with
previous reports (Biello et al., 2020). We speculate that it may be
fragmentation of the X chromosome in S. chinensis and E.
lanigerum or the result of an ancient fusion event of the large X
chromosome in Aphidinae (Macrosiphini + Aphidini). The above evidence
strongly suggests that chrX1, chrX2 and chrX3 is sex chromosomes and the
karyotype of S. chinensis is XX+X.