Abstract
Although pregenital abdominal outgrowths occur only rarely in pterygote insects, they are interesting from the evolutionary viewpoint because of their potential homology to wings. Our previous studies of early development of an epizoic dermapteran, Arixenia esau revealed that abdominal segments of the advanced embryos and larvae, growing inside a mother’s uterus, are equipped with paired serial outgrowths. Here, we focus on the origin and functioning of these outgrowths. We demonstrate that they bud from the lateral parts of the abdominal nota, persist till the end of intrauterine development, and remain in contact with the uterus wall. We also show that the bundles of muscle fibers associated with the abdominal outgrowths may facilitate flow of the haemolymph from the outgrowths’ lumen to the larval body cavity. Following completion of the intrauterine development, abdominal outgrowths are shed together with the larval cuticle during the first molt after the larva birth. Using immunohistochemical and biochemical approaches, we demonstrate that the Arixenia abdominal outgrowths represent an evolutionary novelty, presumably related to intrauterine development, and suggest that they are not related to serial wing homologs.
Highlights
Two basic reproductive strategies, i.e. the oviparity and viviparity can be found in various vertebrate and invertebrate lineages[1,2,3,4]
Till the end of the second intraovarian stage of embryogenesis, i.e. before the “hatching”, the Arixenia embryos develop surrounded by a chorion, amnion and serosa[5] (Fig. S1)
To find out which part(s) of the abdominal segment participate(s) in the formation of the outgrowths we examined such dissected embryos fixed before and after the dorsal closure
Summary
I.e. the oviparity and viviparity can be found in various vertebrate and invertebrate lineages[1,2,3,4]. After hatching (that is liberation from the chorion) the outgrowths adhere to the uterus epithelium This leads to the formation of a series of small contact sites between the mother and embryo tissues that collectively constitute a “scattered” placenta-like organ. We show that the outgrowths bud from the lateral parts of abdominal nota (terga) of early embryos and remain attached to these exoskeletal elements till the end of the intrauterine development They degenerate and become shed together with the cuticle of the 1st instar larva during the first larval molt after the birth. These observations, together with immunohistochemical and biochemical data imply that Arixenia abdominal outgrowths represent de novo formed protrusions of the dorsal exoskeletal plates (sclerites) and are not evolutionarily related to (serial) wing homologs and/or their elements. We suggest that coordinated contractions of these fibers are responsible for the flow (pumping) of the haemocelic fluid from the outgrowths’ lumen to the larval haemocel
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