4 ∣ DISCUSSION
Here we report a high-quality genome sequence assembled from an
important scale insect species. Among the 14,020 protein-encoding genes
predicted from this genome sequence, 6,189 genes were unique and 5,599
of them were supported by transcripts, indicating that these genes
encode real products (Supplementary Figure 11). Previous molecular
phylogenetic studies only used a number of nuclear and mitochondrial
genes to construct the evolutionary tree, which might have strong bias
in data sampling and could not well reflect the evolutionary
relationship between different species in insects
(Misof et al., 2014). The molecular
evolution tree based on whole genome information can avoid some of the
problems (Figure 2c). The large number of unique genes discovered from
this study provides a useful resource for future WWS studies and should
also be useful for future comparative studies with other organisms.
Expansion and reduction of genes are usually associated with adaptation
of species to specific ecological requirements in their respective
habitats (Harris & Hofmann, 2004;
Lespinet et al., 2002). Parasitic insect
species typically display expanded gene families that allow for feeding
of the hosts (Oliva et al., 2015;
Rays et al., 2013;
Zhao et al., 2015). Plant pathogens have
expanded gene families that function in cell wall degradation to
facilitate invasion to their host plants
(Morales-Cruz et al., 2015). Compared to
other insect genomes, there are several categories of genes that
underwent significant expansion in the WWS genome. One of these
categories corresponds to fatty acid metabolism, including fatty acid
synthases and low-density lipoprotein. UDP-glucuronosyltransferases,
involved in removing xenobiotics, are also expanded significantly, which
may have allowed the insect to eliminate endogenous and exotic toxic
chemicals.
One of the striking features of the WWS genome is its unique DNA
methylation pattern with the highest methylation level (Supplementary
Table 18) in the class of hexapoda that has been reported so far
(Phalke et al., 2009;
Regev et al., 1998;
Xiang et al., 2010). The high methylation
of the WWS genome may have allowed the insect for high phenotypic
plasticity to cope with environmental challenges since this females
insect species is immobile at the adult stage, and as such, hard to
evade harsh environmental conditions.
Two families of hormones, the ecdysteroids and the juvenile hormones,
control molting and metamorphosis during postembryonic life
(Gilbert, 1994). Ecdysone induces the
production of a new cuticle, whereas JH regulates the character of
molting (Truman & Riddiford, 1999).
During molts, JH levels are high, but fall dramatically to practically
undetectable in the pre-adult stage
(Belles, 2011). JH represses
metamorphosis, and its downregulation is required for metamorphosis to
occur (Riddiford, 2008). In our study,
20E titers in male development shows five distinct peaks, while females
only display two, consistent with the idea the male development is more
complex. Remarkably, females exhibit a pronounced peak of JH in the
1ststage nymph, with a corresponding drop in ecdysone. As such, the
determinant of female development appears to occur early in development,
since the JH titers are dramatically higher in FF compared to FM samples
and the relationship between JH and ecdysone is reversed in males
(Figure 4a-c). In fact, the female
1ststage nymph is the only time in either of the sexes where JH eclipses
absolute ecdysone concentrations. Therefore, we propose that the key
developmental events that lead to female development are established
already in 1st stage nymphs. Later in development, the
additional ecdysone peaks observed in males are likely associated with
the development of morphological features specific to males.
Insect metamorphosis is a fascinating adaptation that allows the
transformation of nymphs into reproductive adults. Ancestral insect
species did not undergo metamorphosis and there are still some extant
species that lack metamorphosis or only undergo partial metamorphosis
(Truman & Riddiford,
1999).
Both hemimetamorphosis and complete metamorphosis exist in the male and
female of white wax insect, respectively (Supplementary Figs. 2). This
suggests that WWS represents a transitional position between
hemimetamorphosis and holometabolic insects. The main benefits of
sexually dimorphic metamorphosis in WWS are likely the ability of larvae
and adults to utilize different habitats and food resources, and this
may have serves as a force to accelerate the evolution of rapid life
cycles and enhance their chance for survival effectively.
From this respect, WWS represents a fascinating model not only to study
the adaptive mechanisms of plant-sap feeding parasites but also to
examine the evolutionary relationships between hemimetaboly and
holometaboly in different insect orders. The phylogenetic status,
evolution of WWS gene families presented here provide a key case and
framework for furtherly understanding the evolution of insect
metamorphosis and functional herbivory adaption.
Our results provided comprehensive genome and epigenome resource of
scale insects and shed new insights into the different evolution paths
of metamorphosis between males and females within the same species. It
represents a conceptual framework for future studies that examine the
evolution of hexapod metamorphosis.