4 DISCUSSION
Compared with the most common pattern of oviparous reproduction in
insects, the reproduction of ovoviviparity is relatively unique, which
is defined by the ability to undergo pseudo-placental viviparous
reproduction (Meier et al. 1999), namely, nourishing intrauterine
offspring from a modified accessory gland and giving birth to larvae
(Majumder et al. 2014). This gland is highly specialized,
extending from where it connects to the uterus throughout the fat body
(Attardo et al. 2006; Ma et al. 1975; Meier et al.1999). The glandular secretions are mainly composed of fat transferred
from fat bodies during early larval development (Attardo et al.2006; Langley& Bursell 1980). The reproduction requires adaptive
evolution of the uterus to acclimatize developing larvae, as well as
adaptation of female accessory glands as nutrient synthesis and delivery
system (Watanabe et al. 2014). In this study, genes involved in
lipid metabolism are generally conserved, with gene expansions
associated with fatty acid synthase, fatty acid alpha-hydroxylase
activity, phospholipid metabolic process and intracellular cholesterol
transport (Additional file 2: Tables S24 and S25). This pattern leads to
fewer offspring per female, but a higher level of survival for the
offspring (Attardo et al. 2006; Majumder et al. 2014;
Meier et al. 1999).
Furthermore, genes involved in embryonic development have expanded
significantly, mainly encoding chorionic membrane formation, fibroblast
growth factor receptor activity and Dorso-ventral axis formation.
Previous study implied that differentiation of ventral follicular cells
is not a direct result of germline signal transduction, but relies on
indirect signals from the dorsal follicle cells, providing a link
between early and late events for dorsal-ventral axis formation inDrosophila embryos (Jordan et al. 2000). Despite the
failure to clearly clarify the possible genetic mechanism of the
reproductive pattern of S. peregrina , our study provides an
important theoretical basis for further exploring the reproduction of
ovoviviparity.
Besides, insect feeding behavior involves a broad range of activities,
such as initial activation, orientation, identification and feeding
(Ashworth& Wall 2010). The visual, olfactory, gustatory and neural
perceptions regulate the complicated physiological processes. Olfaction
plays an essential role in detecting and analyzing the semiochemicals
from the environment (Field et al. 2000; Li& Liberles 2015). A
complex and sensitive olfactory system has been developed during the
long-term evolution. Necrophagous flies can colonize and breed on the
decomposed corpses compared with herbivorous insects. It has been
demonstrated that olfactory cues can provide a functional description of
physiological mechanisms behind host choice (Carrasco et al.2015; Leal 2013). In this study, enrichment analysis exhibited
significantly expanded genes encoding olfactory receptor activity,
sensory perception of smell, sensory perception of taste (Additional
file 2: Table S24).
Moreover, genes that encode neuroactive ligand-receptor interaction have
expanded significantly in assembled genome. Previous study indicated
that neuropeptide F (NPF) is an abundant signaling peptide in D.
melanogaster , which play roles in feeding, reproduction, and
coordinates larval behavioral changes during development (Nassel&
Wegener 2011; Wu et al. 2003).
Hence
the mechanisms of host location by S. peregrina are of intrinsic
interest, and our study sheds insight into the physiological mechanisms
behind host choice.