4.2 Genetic basis of scabies mites to adapt an obligate and
permanent parasitic life
As a permanent ectoparasitic mite, scabies mites have evolved from
various levels. When we analyzed the mechanism that influenced
morphology of the species in Acari, we found the Hox gene family
divergence may cause the differences of body plan in superorder
Acariformes and Parasiformes, especially the absence of Zen andAbd-A . For Chelicerate species like spiders and mites, Zenand Abd-A were reported to function in body plan of the prosoma
and the opisthosoma (Hughes & Kaufman,
2002). Previous study showed that, the function of Zen has
transferred from a Hox-like role to a role in the extraembryonic tissues
due to the functional overlap with other Hox proteins
(Hughes & Kaufman, 2002;
Pick, 2016), thus pretended to be lost
for species in the Acariformes that generally have small genomes
(Table S25) . The expression of Abd-A and Abd-Breflects the differences in the insect abdomen, and Abd -Bacts to suppress Abd-A in posterior segments, and suppresses
posterior segmentation (Celniker et
al. , 1989; Jordi et al. , 1986;
Karch et al. , 1990). Previous data
supportted a model that inversion of the Abd-B locus results in
the loss of Abd-A , and correlated with reduced trunk segmentation
(Pace et al. , 2016), and in this
study, the Hox clusters and transcription direction in S.
scabiei , P. ovis and T. urticae implicated the same
mechanisms for the trunk segmentation of species in Acariforms. These
results indicated that the morphological evolution correlated with the
loss of specific Hox genes.
Genes in families specific to scabies mainly enriched in nutrition
absorption and digestion of proteins and lipids. And genes in families
of cytochrome P450 (CYP), carboxyl/cholinesterases (CCE), and multidrug
resistance proteins of the ATP-binding cassette transporter C group are
strikingly contracted, and members in these families were reported to
function in a wide range of detoxification events
(Dermauw et al. , 2013;
Enayati et al. , 2005;
X. Li et al. , 2007). Scabies mite
lives in the epidermis of the mammal skin, which is a relatively
enclosed stable environment. Compared with P. humanus andT. mercedesae that live on the surface of the hosts, scabies
mites have limited chances of encountering toxicants than other
ectoparasites, thus the strikingly contracted gene families that are of
important in metabolizing toxic xenobiotics in insects and the
acquisition of insecticide resistance indicated a less effective
detoxification system and an adaption to the enclosed environment.
Genetic basis of variant divergence and cross-infestivity
To date, there is no well-accepted standard to define subspecies or
variants of mites, especially by using limited gene markers. In this
study, we considered mites from different hosts as variants based on
results of PCA and phylogenetic analysis. Although there is no direct
evidence of host shift, comparative genomics analysis may provide clues
for the cross-infectivity of these mites. It is known that S.
scabiei reside at the interface of stratum lucidum and stratum
granulosum (Estes et al. , 1983;
Neste & Lachapelle, 1981;
Van Neste, 1984), and mites have to
burrow deep through stratum corneum to get to their destination,
therefore, the thickness of stratum corneum may partly reflected the
difficulties of mites to break through the barriers of the skin.
The selection signal between human mites/pig mites, human mite/dog mites
quantitatively reflected the adaption to the new host in shaping the
genome. Genes enriched in “cysteine-type peptidase activity” and
“apoptosis” mainly encode Sar s1 allergen scabies mite inactive
cysteine proteases (SMIPP-Cs). The function of these proteins is
promoting blood coagulation and changing the structure and density of
fibrin clots, making them resistant to fibrinolysis, thus protecting
scabies mites from the host immune system
(Fernando et al. , 2021). These
functions were thought to contribute to parasitic lifestyle. Neuroactive
ligand-receptor interaction (18 genes) is a function of environmental
information processing (Table S28 ). Most of the group 3
allergen SMIPP-S genes occur in tandem, studies have shown that these
proteins might function by binding and protecting target substrates from
cleavage by host immune proteases, thus preventing the host from
mounting an effective immune challenge
(Fischer et al. , 2009). Besides,
the presence of receptors, such as the olfactory receptor,
rhodopsin-like G protein-coupled receptor, and the 5‑hydroxytryptamine
receptor family, suggested that scabies mites in omnivorous pigs have
evolved to have a better ability to process environmental information,
and thus adapted to a more complicated pig skin environment.
A previous study reported that mites from scabies-infected dogs can
establish permanent infections on domestic rabbits and these mites can
then re-infect dogs (Nicoli et al. ,
2008). Interestingly, it is reported that the human and the pig stratum
corneum share very similar lipid types and percentage of lipids
(Hammond et al. , 2000), which
suggested the possibility of the transmission of scabies mites from
humans to dogs. Compared with the other three hosts, rabbits have the
shortest generation time (Table S13 ) and are more susceptible
to scabies mites. According to the statement of local farm workers, we
also learned that humans who come in contact with rabbit mites
experience intensive itching symptoms; however, because humans generally
apply drugs before the mites settle down on the skin, we do not know if
rabbit mites can establish a permanent infection on humans or vice
versa. The condition of pigs living with rabbits is very rare, thus few
reports have been published about the cross-infestivity between pig
mites and rabbit mite. The results of the pairwise selective sweeping
analysis provided clues to investigate cross-infestivity. Except in
humans, it is feasible to investigate the genetic basis of
cross-infestivity by performing artificial infection experiments to test
cross-infestivity and differentially expressed genes during host shift.