Chemosensory traits are key components of speciation in many insects, and changes in these traits can influence insect behavior and subsequently mediate survivability. Although evaluating changes in chemosensory gene is a necessary beginning for understanding how chemosensory traits contribute to adaptability, surprisingly few studies have compared how sister taxa differ in this field. Moreover, chemosensory gene identification within Coleoptera groups has rarely been reported. These genes have been characterized in whole genome level only the flour beetle T. castaneum [37]. Several species have been identified partly by RNA-seq, such as two bark beetles (I. typographus and D. ponderosae) [20], two leaf beetles (P. aenescens and P. maculicollis) [27], scarab beetle Hylamorpha elegans [50], and a leaf beetle C. lapponica [49]. In the present study, we identified the chemosensory gene families at whole genome level in a pair of blister beetles (Meloidae: Hycleus) in the Meloidae and provided the evolution analysis of chemosensory genes within Coleoptera groups.
Non-receptor gene families
The OBP and CSP families mediate the first critical step of chemosensation in insects. Thus, these genes have received considerable attention [27,47,50–55]. When chemical signals enter the sensillum lymph, OBPs/CSPs bind it and involve in peripheral olfactory processing [27]. Our analysis of these gene families obtained 29 and 30 OBPs and 10 and 10 CSPs in H. phaleratus and H. cichorii, respectively. These totals are comparable to the totals observed in previous reports, such as in the D. ponderosae transcriptome (31 OBPs and 11 CSPs) [20], the Pyrrhalta genus transcriptome (31 and 36 OBPs and 9 and 10 CSPs) [27] and the leaf beetle C. lapponica (32 OBPs and 12 CSPs)[49]. An expansion of the minus-C OBPs was found in this Hycleus genus (27 of minus-C OBPs) comparison to D. melanogaster, the similar pattern reported in Tribolium and bark beetles [20], two beetles in the Pyrrhalta genus [27] and the leaf beetle C. lapponica [49]. The expanded Minus-C OBPs may one specific evolution event in Coleoptera chemosensory, and it suggests that these genes might play an important role in chemosensation.
Receptor gene families
The OR family, which was studied mostly in these three receptors. Based on previous studies [20,37], seven groups of beetle ORs were recovered in this study. Notably, some new patterns have been found in this study (Fig. 1). Such as, the group 7 was a specific expansion only in bark beetles (DponOR and ItypOR) was reported before [20]; this clade was also expansion in blister beetles was be found in our result. The group 3 and 5 almost the flour beetle specific ORs was reported before [20, 25, 27], but we found lots of Hycleus ORs also in these clades. This may due to the most reports were identified ORs via RNA-seq except T. castaneum (341) [37]. These species only tens of ORs could been found in RNA-seq, such as the leaf beetles P. maculicollis (22) and P. aenescens26) [27], the bark beetles I. typographus (43) and D. ponderosae (49) [20], the long horned beetle Megacyllene caryae (57) [25], and the leaf beetle C. lapponica (42)[49]. This indicted more beetle’s chemosensory family should be annotated comprehensively via whole genome that is better for comprehensive study its evolution in insects.
Moreover, the number of ORs in each blister beetles was obvious differ, ORs identified in H. cichorii 64% more than that in H. phaleratus (191 VS 116). However, the number of others only slightly vary (Table 1). These data hint ORs specific expansion in H. cichorii along both specie differentiation, which occurred around 23 million year ago [56]. ORs is one of the key bridges between animals and its surrounding, and act functions with food, predators and potential mates. These two beetles have largely overlapping sympatric ranges in west south of China. To our field survey experience, in the past years, the H. phaleratus population has declined in the field due to destruction of the living environment by human activity and human catching. In contrast, the H. cichorii population has not declined obviously as the same situation. These ORs maybe acts the keys roles for a stronger adaption ability than H. phaleratus. These ORs needs verified in the future.
For GRs, always divides into three groups by its recognizing materials, CO2, sugar and better receptors. we identified three pairs of CO2 receptors, and nine and eight sugar receptors in two blister beetles (Fig 2). These two groups always more conservative in insects. Notably, a Hycleus-specific expansion clade was found in our phylogenetic tree, and more than 1/2 identified Hycleus-bitter GR were in this clade. That may act important functions in Hycleus genus. On the other hand, there are lack whole GRs identify from genome of other species for comparative analysis. Most published GR family by RNA-seq, only few GRs was been detected due to the GRs expressed very low. such as in P. aenescens (16), P. maculicollis (10) [27], I. typographus (2) and D. ponderosae (6) [20], the leaf beetle C. lapponica (8)[49].
IRs is also membrane proteins, are more closely related to ionotropic glutamate receptors (iGluRs) [38,48].We identified 35 and 42 IRs in H. phaleratus and H. cichorii. IRs could be divided into two groups: ‘antennal IRs’ was always conserved among species, which likely define the olfactory receptor family of insects, and the ‘divergent IRs’ was more divergent, which are expressed in peripheral and internal gustatory neurons, implicating this family in taste and food assessment [38]. All the ten clades of beetle antennal IRs were identified in both blister beetles, and five of these sub-family were conserved with 1:1 ortholog in all the six insects (Fig. 3 and Table 2). However, the IR41a (11 and 15 in blister beetles VS 1–2 in others) and IR75 (10 and 16 in blister beetles VS 3–4 in others) were evidently expensed compare to others (Fig 3 and Table 2). Moreover, there have 5 out 11 copies of IR41a and 7 out of 10 copies of IR75 were detected expression (FPKM ≥1). This data hints that IR41a and IR75 would be specific expansion in the Hycleus genus, that could introduce new characteristics for the Hycleus genus. We first time identified IR60a (HphaIR60a1 and HphaIR60a2) in the beetles based on our phylogenetic tree, it missed in T. castaneum and other beetles. Additionally, HphaIR60a1 has expressed (FPKM: 2.53- 8.81) in antennal tissues. This gene should be verified in the future study.
Candidate chemosensory genes for speciation
Divergent chemosensory genes linked to differences in the sensory tuning of sister species represent a response to divergent selection of chemosensory traits [1]; thus, these genes are candidates for chemical-mediated speciation, which directly or indirectly reflects the ability to adapt to the environment. For instance, a comparison of the gene amplification of chemosensory gene families between the sister beetle species suggested some candidate genes that might have been involved in this ability. In present study, an evident expansion of ORs identified in H. cichorii compare to that in H. phaleratus (191 VS 116). These data hint ORs specific expansion in H. cichorii may promoted both specie differentiation. Which specific ORs and how acts the keys role should be study in the next. On the other hands, we identified 221 pairs of gene between both blister beetles, of which 138 pairs were undergone purifying selection and no gene under positive selection (Additional files 1). This data may hint no positive clue of gene mutation linking to the speciation, the other ways should be attended, such as gene expression change. At least, expression changes between orthologs should be attended in the future study.