4 DISCUSSION
M. dirhodum is an important pest of wheat and other cereals
worldwide. Previous studies of this species have mainly focused on
occurrence regularity (Honek et al., 2018), control methods (Cambier et
al., 2001; Chopa et al., 2012) and reproductive strategies (Wratten,
1977), whereas more in-depth studies are limited due to the lack of
high-quality genome data. Therefore, we used PacBio long HiFi reads and
Hi-C technology to produce a chromosomal-level genome for M.
dirhodum . This is the first high-quality chromosome-level genome ofM. dirhodum , which will be very helpful for cloning, functional
verification and evolutionary analysis of genes in this important
species or even in other hemipteran insects.
The k -mer analysis showed that the M. dirhodum genome
harbored a moderate level of heterozygosity and a high level of
repetitive sequence content, which is similar to other Aphidinae insects
with low or moderate level heterozygosity (Jiang et al., 2019; Chen et
al., 2019). BUSCO assessment revealed that 96.9% of the complete BUSCOs
could be found in the current assembled M. dirhodum genome. This
percentage is higher than those in the genomes of some other species,
such as S. miscanthi (90.2%) (Jiang et al., 2019), R.
maidis (94.5%) (Chen et al., 2019), A. pisum (93.5%)
(International Aphid Genomics Consortium., 2010) and E. lanigerum(96.8%) (Mathers et al., 2021). Considering the moderate level of
heterozygosity and the high-level repetitiveness of the genome, the
current result represents a high-quality genome assembly of M.
dirhodum . Nine chromosomes were finally obtained after Hi-C-assisted
assembly, supporting a 2n = 18 karyotype for M. dirhodum, which
is identical to S. miscanthi (Jiang et al., 2019). Furthermore, a
total of 18,003 protein-coding genes were predicted in the genome ofM. dirhodum , which was comparable to that of several other
Aphidinae species, such as 16,006
protein-coding genes in S. miscanthi (Jiang et al., 2019) and
19,097 protein-coding genes in D. noxia (Nicholson et al., 2015),
but less than R. maidis (Chen et al., 2019), A. pisum(International Aphid Genomics Consortium., 2010), M. persicae(Jiang et al., 2013) and E. lanigerum (Mathers et al., 2021),
which have 26,286, 36,195, 23,910 and 28,186 protein-coding genes,
respectively.
Wing polymorphism is an evolutionarily successful feature in a wide
variety of insect species, including Hemiptera, Coleoptera, Hymenoptera,
Orthoptera, Diptera, Lepidoptera, Isoptera, Psocoptera and Dermaptera
(Xu et al., 2017; Zhang et al., 2019). Insect wing polymorphisms have
been widely studied, and some progress has been made, especially in the
rice plant hopper Nilaparvata lugens , in which two putative
insulin receptors (InRs), InR1 and InR2, were identified. Interestingly,
InR1 and InR2 play fully opposite roles in wing morph determination by
regulating the activity of Foxo. In detail, knockdown of InR2 at the
nymph stage led to a strong trend toward long-winged adults, whereas
dysfunction of InR1 resulted in the development of short-winged adults
(Xu et al., 2015). In the present study, two transcripts annotated as
insulin receptors (insulin-like receptors and insulin-like peptide
receptors) were identified in M. dirhodum , which is consistent
with the annotation of the A. pisum genome (International Aphid
Genomics Consortium., 2010). Comparative transcriptome analysis showed
that the insulin-like receptor was
downregulated in third-instarM. dirhodum wingless nymphs and that the insulin-like peptide
receptor was upregulated in fourth-instar wingless nymphs compared to
winged individuals. In addition, one transcript annotated as insulin
receptor substrate 1 was downregulated in both third- and fourth-instar
wingless nymphs, and one transcript annotated as insulin gene enhancer
protein isl-1 was upregulated in both third-instar and fourth-instar
wingless nymphs compared to the winged individuals. No DEGs associated
with the insulin signaling pathway were identified between the wingless
and winged M. dirhodum adults. These results suggest that insulin
signaling may play an important role in the early wing dimorphism ofM. dirhodum . Foxo was identified to be upregulated in winglessM. dirhodum in third- and fourth-instar nymphs and adults, which
is another important factor in the regulation of wing dimorphism.
Vellichirammal et al. (2017) revealed that ecdysone signaling plays a
critical role in transgenerational plasticity in wing dimorphism inA. pisum . Their results showed that more winged offspring would
be produced by injection of ecdysone or its analog, while fewer winged
offspring were produced by knockdown of EcR or treatment with an EcR
antagonist. Our results showed that EcR is downregulated in winglessM. dirhodum from third-instar nymphs to adults compared to winged
nymphs, suggesting that the decrease in ecdysone is not conducive to the
production of winged individuals. Moreover, DEGs annotated as juvenile
hormone epoxide hydrolase, brc, FTZ-F1, Wnt, Dpp, Ubx, Srf and Fl were
also detected between the wingless and winged M. dirhodum . These
findings provide a valuable reference for revealing the mechanism of
wing dimorphism.