At least three B-opsin losses during hemipteran diversification
Previous studies reported the lack of B-opsin throughout the Heteroptera [
12], in the aphid species
A. pisum and
M. persicae [
19], and in the planthopper
N. lugens [
21]. In isolation, these findings were compatible with a very early loss of the B-opsin gene family in the stem lineage of the Hemiptera. This straightforward scenario, however, was contradicted by the existence of B-opsin orthologs in the green rice leafhopper
N. cincticeps (Auchenorrhyncha: Cicadomorpha) [
21], the hackberry petiole gall psyllid
P. venusta (Sternorrhyncha) (Armisen et al. 2018), and the Asian citrus psyllid
D. citri (Li et al. 2020). The present study documents the existence of B-opsins in further taxa of the Sternorrhyncha (Aleyrodoidea, Psylloidea), Auchenorrhyncha (Cercopoidea, Cicadoidea, Membracoidea), and the Coleorrhyncha (moss bugs). Combined, these findings reveal that the B-opsin gene family had remained conserved in early Hemiptera but was subsequently lost at least three independent times.
Given the current limitation of taxon sampling density, it is possible that future studies will detect additional opsin gene family changes in younger hemipteran taxa. At this point, however, it seems reasonable to assume that the results presented here conclusively capture the number of B-opsin gene losses in the largest hemipteran suborders, i.e. the Heteroptera, Auchenorrhyncha, and the Sternorrhyncha.
Some caution may be warranted concerning the actual number of B-opsin loss events in the Heteroptera. While the most parsimonious explanation for the absence of B-opsins in all Heteroptera is a single loss in the stem lineage of this group, less parsimonious scenarios of multiple early losses cannot be excluded with absolute certainty. In addition to the evolutionary ancientness of these possible events, this is because of the considerable number of independent, and in some cases very likely compensatory, LW-opsin duplications in this clade [
12]. While resolving this ambiguity warrants further study, it is clear that the consequences of B-opsin loss in the Heteroptera differ in major interesting ways from aphids and planthoppers.
Timing B-opsin losses and UV-opsin duplications in planthoppers and aphids
In the aphids, the present study sampled five species from three subfamilies, i.e. the Aphidinae, Chaitophorinae, and Lachninae. Most molecular phylogenetic studies place these groups in distantly related branches of the aphid subfamily tree [
35,
36,
37,
38,
39]. It, therefore, seems reasonable to postulate that the loss of B-opsin and correlated duplication of UV-opsin preceded the last common ancestor of aphids that has been estimated to have diverged about 100 million years ago [
6]. At the same time, it remains possible that more comprehensive species sampling may produce evidence of a later loss of B-opsin in this group. Alternatively, the lack of B-opsins in aphids may even date back to a more ancient gene loss event, preceding the last common ancestor of aphids and scale insects (Coccoidea). Preliminary searches for B-opsin in this group failed to recover B-opsin homologs, despite the availability of high-coverage genome assembly data [
40]. However, sequence information from additional scale insect taxa will need to become available before firm conclusions can be drawn on this issue.
In the planthopper case, the conservation of B-opsin in the Coleorrhyncha and all other clades of Auchenorrhyncha besides planthoppers (Fulgoroidea), i.e. froghoppers (Cercopoidea), cicadas (Cicadoidea), and treehoppers (Membracoidea) makes it straightforward to map the loss of B-opsin to the early evolution of planthoppers [
6]. However, only one of the 13 families of planthoppers was sampled in the present study, i.e. the Delphacidae. Thus, also, in this case, broader species sampling needs to be awaited before this B-opsin loss event can be dated with ultimate confidence.
Combined evidence of B-opsin loss compensation through parallel UV-opsin neofunctionalization in aphids and planthoppers
Gene duplication events can lead to a variety of functionalization outcomes [
41]. This includes the reduction of ancestral pleiotropy via differential inheritance of ancestral functions by the descendant paralogs, leading to subfunctionalization, or the acquisition of novel functions by one paralog, i.e. neofunctionalization. The latter outcome is frequently associated with dramatic protein sequence divergence between the emerging paralogs as one paralog changes more slowly under the continued constraints of ancestral functionality while the second paralog experiences a burst of adaptive protein sequence change while optimizing to a new function. In the evolutionary diversification of opsin genes, the neofunctionalization scenario applies most clearly when a newly emerged paralog shifts its ancestral peak sensitivity. In support of this notion, peak sensitivity-shifted neofunctionalized opsin paralogs have been found to differ more dramatically from the singleton homologs in outgroup species than the paralog that preserves ancestral peak sensitivity [
42]. Based on this criterion, one UV-opsin paralog, labeled UV-der, of each of the aphid and planthopper UV-opsin sister paralog pairs emerged as putatively wavelength sensitivity shifted given its significantly higher overall amount of protein sequence change compared to the sister paralog. The latter, labeled UV-anc by the same logic, represents the candidate for maintaining ancestral UV-sensitivity.
In the long term, it will be desirable to obtain definitive evidence of compensatory blue-shifted UV-opsin gene family paralogs in the planthoppers and aphids by in vitro spectral sensitivity analyses [
31,
43]. This will be particularly attractive for exploring the two new candidate tuning sites 47 and 183, which are characterized by parallel amino acid substitutions in the aphid and planthopper UV-der paralogs. At this point, further support for the parallel compensatory UV-opsin neofunctionalization scenarios can be drawn from the findings of behavioral wavelength discrimination and electroretinogram (ERG) studies.
In the Sternorrhyncha, critical data are available from studies in the greenhouse whitefly
Trialeurodes vaporariorum [
44], the pea aphid
A. pisum [
45], and the green peach aphid
M. persicae [
32]. Following up on earlier behavioral and ERG studies that produced evidence of UV- and LW peak sensitivities in the greenhouse whitefly
T. vaporariorum compound eyes [
46], Stukenberg & Poehling (2019) demonstrated high-resolution behavioral wavelength discrimination assays that
T. vaporariorum is trichromatic with peak sensitivities in the UV (340–370nm), blue (480–490nm), and green (510–520nm) [
44]. Moreover, their data led to the model that blue-sensitivity boosts the accuracy of settling onto green food plant targets. A key observation leading to this conclusion was the observation of an activity-inhibiting effect of exposure to blue light [
44].
While sequence data are not available for
T. vaporariorum at this time, its trichromacy aligns well with the ancestral possession of the B-opsin in the Aleyrodidae as documented by the silverleaf whitefly
B. tabaci (
Table 1). The available ERG data for the B-opsin lacking
A. pisum are considered ambiguous regarding the possibility of distinct spectral sensitivities in the blue and UV ranges [
45]. ERG studies on the green peach aphid
M. persicae, however, have shown that this species is characterized by a trichromatic profile of peak sensitivities, which match those of
T. vaporariorum despite the lack of B-opsin [
32]. It thus seems safe to assume that the detected blue sensitivity in aphids is supplied by the UV-der opsin paralogs as previously proposed [
19].
In the Auchenorhyncha, behavioral and ERG data have been generated for the B-opsin possessing green rice leafhopper
N. cincticeps [
47] and the B-opsin-lacking brown planthopper
N. lugens [
21]. Phototaxis assays revealed highest attraction of
N. cincticeps to green, blue, and yellow wavelengths [
47]. ERG data revealed pronounced sensitivity peaks in the UV- and LW-ranges but the best modeling fit was obtained with a trichromatic composite of wavelength sensitivities centered on UV- (354nm), blue (449nm), and LW (527nm) absorption maxima [
47]. The same primary UV- and LW-sensitivity peaks were reported for
N. lugens as well as specific and pronounced attraction to blue light (470nm) besides UV (365nm, 385nm), and green (525nm) light [
21]. Mirroring the situation in the Sternorrhyncha, these data are compatible with preserved trichromacy in
N. lugens through compensatory neofunctionalization of the planthopper UV-der opsin paralog.
Another key requirement for color vision is the differential expression of opsin paralogs with different peak sensitivities in defined photoreceptor subtypes [
26]. To date, retinal opsin gene expression data have been reported for
N. lugens [
21],
N. cincticeps [
47], and
A. pisum [
19]. Of these,
N. cincticeps represents the ancestral state, possessing singleton homologs of the UV-, B-, and LW-opsin subfamilies [
47]. In this species, the singleton LW-opsin homolog is expressed in a defined set of seven photoreceptors per ommatidium. The photoreceptor subtype not expression LW-opsin is characterized by the seemingly stochastic but mutually exclusive expression of the UV- or B-opsin homolog [
47]. Taken together, the retinal mosaic defined by the differential expression of LW-, B- and UV-opsin is consistent with the behavioral and physiological evidence of trichromacy in
N. cincticeps [
47].
In the B-opsin lacking
N. lugens and
A. pisum, strong expression of the singleton LW-opsin was detected uniformly throughout the retina, suggesting expression in most, if not all, photoreceptors. The UV-opsin paralogs, in contrast, were detected in more selective patterns. In
N. lugens, the candidate blue-shifted UV-der paralog (BPH_UVop1: XP_039285232) was detected in the ventral periphery of the compound eye while the ancestral UV-anc paralog (BPH_UVop2: XP_039285228) was detected in a subset of photoreceptors that were more evenly distributed across the retina [
21]. In
A. pisum, both the UV-der opsin (Ap-SWO2: XP_001951588) and the UV-anc opsin paralog (Ap-SWO3: XP_001951613) were detected in similar punctuated low density of photoreceptors across the retina [
19], highly reminiscent of the retinal mosaic of
N. cincticeps [
47]. While the cellular resolution of the expression studies in
N. lugens and
A. pisum is too limited for conclusive inferences, the broader LW-opsin and the selective UV-opsin expression patterns in these species are consistent with the existence of LW-, B-, and UV-specific photoreceptors as in the ancestrally organized
N. cincticeps [
47].