3.4 Specialized metabolism to adapt an obligate parasitic life
The scabies mite burrows into the skin of its host, and then resides in
the lower epidermis close to the dermis, using host skin and
intracellular fluid (lymph) that seeps into the burrow as food
(Estes et al. , 1983;
Neste & Lachapelle, 1981;
Van Neste, 1984). Compared with the
other eight species genes in specific families to S. scabiei were
significantly enriched in protein metabolic-related GO categories,
especially in proteolysis, serine-type peptidase activity, serine-type
endopeptidase activity, cysteine-type peptidase activity, peptidase
activity, and endopeptidase activity (Figure 4 ). KEGG analysis
results showed categories associated with the digestive system (Protein
digestion and absorption, and Pancreatic secretion), lipid metabolism
(Fatty acid metabolism, and Biosynthesis of unsaturated fatty acids),
transport and catabolism (Lysosome) and cell growth and death
(Apoptosis) (Figure 4 ) were also highly enriched. These results
reflected that S. scabiei var. cuniculi may underwent
natural selection pressure by living in mammalian skin, evolved to have
better ability of digesting proteins to break through the skin barrier
and migrate in the epidermis, and most importantly, to meet the
requirement of nutrition.
Besides, genes in the families that were shared only by ectoparasite
scabies mite and Tropilaelaps mite are highly enriched in the
biological process of cellular nitrogen compound metabolic process and
nitrogen compound metabolic process (Figure S8-S9 ), probably
related to carnivorous diets of both scabies mite andTropilaelaps mite. These genes were also enriched in KEGG pathway
of human diseases, such as Alzheimer’s disease, Arrhythmogenic right
ventricular cardiomyopathy (ARVC), Dilated cardiomyopathy, and Cardiac
muscle contraction (Figure S8; Table S15 ). We speculated that
the enhancement of heart muscle-like function is an adaption of the
aerobic nature of scabies mites in the hypoxic skin environment
(Arlian et al. , 1988).
Scabies mites have been known to well adapted to live in the epidermis
of the skin, protecting them from the environment. According to
expansion and contraction analysis, three gene families were expanded,
and 34 gene families were contacted. The primary contributor to the
acquisition of new functions and physiology is gene duplication
(Nasvall et al. , 2012). The three
expanded gene families including the vitamin D-binding protein (46
genes), synaptotagmin (12 genes) and HAUS complex (7 genes). The vitamin
D-binding protein is the major binding/transport protein for all vitamin
D metabolites (Bouillon et al. ,
2020), genes in synaptotagmin family encode proteins that regulate
calcium-dependent membrane fusion events
(Wolfes & Dean, 2020), both were
associated with enhanced absorption ability of nutrients. Among the 34
contracted gene families in scabies mites, genes were over-represented
in the process of materials transport (Tables S16-17 ). We
discovered four gene families that implicated in digestion,
detoxification and transport of xenobiotics, including cytochrome P450
(CYP), carboxyl/cholinesterases (CCE), glutathione-S-transferase genes
and ATP-binding cassette transporter C group and found unique scabies
mite composition (Tables 2-3; Table S19-S20) . A total of only
25 cytochrome P450 (CYP) genes in Sarcoptes scabiei , followed byPediculus humanus (36 genes), T. mercedesae (42 genes),
and Apis mellifera (46 genes) (Table 2 ); of which the
number of genes in CYP4 clan being close to the number of
counterparts in A. mellifera (4 genes), but far less than the
number found in other mites (16~23 genes); members of
CYP2 clan (three genes) and Mito clan (three genes) have orthologs inDrosophila melanogaster (Figure S10 ), indicating that
scabies mites only retain the genes that support basic life. A notable
contraction is also found in the family of carboxyl/cholinesterases
(CCE) that include 11 genes compared with other three mites (31-69
genes), and 6 genes in subclass acetylcholinesterase (AChEs) compared
with 22–59 genes in other mites (Table 3 ). Although the total
number of ABC transporters remain the same, we discovered 14 multidrug
resistance proteins that belong to the ATP-binding cassette (ABC)
transporters (class C), while other mites have 20–39 genes
(Table S18 and Figure S11 ). Finally, scabies mites have 13
glutathione-S -transferase genes (Table S19 and Figure
S12 ) that can be assigned to four subfamilies: Delta/Epsilon (4 genes),
Mu (6 genes), Zeta (1 gene), and Kappa (2 genes), interestingly, both mu
and kappa appear to be tandem repeats in the genome. The relatively
small number of P450 genes in honey bee was thought to be a consequence
of the social organization of beehive, which shields the queen and
larvae from environmental exposure to toxins
(Claudianos et al. , 2006). It is
likely that the extremely contracted detoxification genes in scabies
mites might be the consequences of long time living in the epidermis of
the skin, protect mites from environmental exposure to toxins.