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.