3.3 Specialized morphology to adapt an obligate parasitic life
The skin of mammalian species is composed of epidermis, dermis and
hypodermis (Figure 3a ). As a permanent parasitic mite species,
scabies mites burrow into the epidermis of the skin, and resident at the
interface of stratum lucidum and stratum granulosum (Figure3b ). Belonging to a member of chelicerate, the adult scabies
mite body plan comprises the anterior prosoma bearing the chelicerae,
pedipalps, and the four pairs of walking legs, and the posterior
opisthosoma (Figure 3c ). After long time of adaptation, the
four walking legs of scabies mites has become extremely stubby to
adapted to its parasitic environment in the epidermis, while for other
mites such as surface living and free-living mites, the length of
walking legs remains long (Figure 3c ). Interestingly, when we
observed the morphology of species in class Acari, we found that for
species in the superorder Acariformes, the anterior and posterior two
pairs of legs were distantly distributed and the proportion of the
abdominal segments were reduced, while for species in the superorder
Parasitiformes and the velvet spider, all the legs were very close to
each other and the abdominal segment is relatively longer (exceptMetaseiulus occidentalis , which seems a transactional species)
(Figure 3c; Figure 3d, left panel ). Therefore, we analyzed the
structurally and functionally conserved Hox genes of these related
species, including lab , pb , Hox3/Zen , Dfd ,Scr , ftz , Antp , Ubx , Abd-A , andAbd-B , hoping to find potential clues to explain this phenomenon.
The results showed that S. scabiei has eight of ten canonical Hox
genes that are present in the arthropod ancestor, except Zen andAbd-A , and no gene duplication events (Table S14 ). The
loss of Zen and Abd-A seems common and unique for species
in superorder Acariformes (Figure 3d, central and right panel ).