A Search for Parent-of-Origin Effects on Honey Bee Gene Expression

Parent-specific gene expression (PSGE) is little known outside of mammals and plants. PSGE occurs when the expression level of a gene depends on whether an allele was inherited from the mother or the father. Kin selection theory predicts that there should be extensive PSGE in social insects because social insect parents can gain inclusive fitness benefits by silencing parental alleles in female offspring. We searched for evidence of PSGE in honey bees using transcriptomes from reciprocal crosses between European and Africanized strains. We found 46 transcripts with significant parent-of-origin effects on gene expression, many of which overexpressed the maternal allele. Interestingly, we also found a large proportion of genes showing a bias toward maternal alleles in only one of the reciprocal crosses. These results indicate that PSGE may occur in social insects. The nonreciprocal effects could be largely driven by hybrid incompatibility between these strains. Future work will help to determine if these are indeed parent-of-origin effects that can modulate inclusive fitness benefits

The Making of a Queen: TOR Pathway Is a Key Player in Diphenic Caste Development

Honey bees (Apis mellifera) provide a principal example of diphenic development. Excess feeding of female larvae results in queens (large reproductives). Moderate diet yields workers (small helpers). The signaling pathway that links provisioning to female developmental fate is not understood, yet we reasoned that it could include TOR (target of rapamycin), a nutrient- and energy-sensing kinase that controls organismal growth.Here, the role of Apis mellifera TOR (amTOR) in caste determination is examined by rapamycin/FK506 pharmacology and RNA interference (RNAi) gene knockdown. We show that in queen-destined larvae, the TOR inhibitor rapamycin induces the development of worker characters that are blocked by the antagonist FK506. Further, queen fate is associated with elevated activity of the Apis mellifera TOR encoding gene, amTOR, and amTOR gene knockdown blocks queen fate and results in individuals with worker morphology.A much-studied insect dimorphism, thereby, can be governed by the TOR pathway. Our results present the first evidence for a role of TOR in diphenic development, and suggest that adoption of this ancestral nutrient-sensing cascade is one evolutionary pathway for morphological caste differentiation in social insects

Behavioral genomics of honeybee foraging and nest defense

The honeybee has been the most important insect species for study of social behavior. The recently released draft genomic sequence for the bee will accelerate honeybee behavioral genetics. Although we lack sufficient tools to manipulate this genome easily, quantitative trait loci (QTLs) that influence natural variation in behavior have been identified and tested for their effects on correlated behavioral traits. We review what is known about the genetics and physiology of two behavioral traits in honeybees, foraging specialization (pollen versus nectar), and defensive behavior, and present evidence that map-based cloning of genes is more feasible in the bee than in other metazoans. We also present bioinformatic analyses of candidate genes within QTL confidence intervals (CIs). The high recombination rate of the bee made it possible to narrow the search to regions containing only 17–61 predicted peptides for each QTL, although CIs covered large genetic distances. Knowledge of correlated behavioral traits, comparative bioinformatics, and expression assays facilitated evaluation of candidate genes. An overrepresentation of genes involved in ovarian development and insulin-like signaling components within pollen foraging QTL regions suggests that an ancestral reproductive gene network was co-opted during the evolution of foraging specialization. The major QTL influencing defensive/aggressive behavior contains orthologs of genes involved in central nervous system activity and neurogenesis. Candidates at the other two defensive-behavior QTLs include modulators of sensory signaling (Am5HT(7) serotonin receptor, AmArr4 arrestin, and GABA-B-R1 receptor). These studies are the first step in linking natural variation in honeybee social behavior to the identification of underlying genes

Relative quantities (RQ) of <i>amTOR</i> mRNA levels in honey bee larvae.

<p>In colonies a–b, queen- (Q) and worker-destined (W) larvae are characterised by a transient two-fold difference in <i>amTOR</i> expression (present in 3^rd instar larvae, absent in spinning 5^th instar larvae). <i>In vitro</i> c–d, RNAi was calibrated to decrease <i>amTOR</i> about two-fold in 3^rd instar knockdowns (RNAi) vs. controls exposed to GFP derived dsRNA (GFP). Suppression was transient, as often observed with RNAi. Thereby natural expression patterns of <i>amTOR</i> were mimicked (a–b vs. c–d). Bars are means±s. e. (asterisk <i>P</i><0.01, three asterisks <i>P</i><1.0 10⁻⁶).</p

Effect of <i>amTOR</i> suppression on caste characters in honey bees.

<p>In comparison to control (GFP), <i>amTOR</i> RNAi (RNAi): a, reduced larval growth (exemplified by 4-day-old larvae in sections of a microtiter plate, larval volume is mean±s. e. arbitrary units a.u.); delayed development, b–c, where b is a snapshot of the full phenotypic variance in each group (individuals were 19 days old) – controls emerged with queen morphology, or had advanced pigmentation (indicates more rapid development) and large size. RNAi bees were lightly pigmented and small in size; the latter effect is shown also in d, the wet-weight at adult emergence and in e, the final adult size. Adult size did not overlap between treatment groups (e vs. f, where f shows the two control bees with lowest wet-weight: 155 and 161 mg; weight-range after RNAi was 108–136 mg). <i>amTOR</i> RNAi also reduced ovary size g, to ovariole numbers characteristic of workers (GFP range was 12–180, RNAi range was 2–7 ovarioles per ovary). In sum, the divergence between control and <i>amTOR</i> RNAi is characteristic of queen vs. worker development <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000509#pone.0000509-Winston1" target="_blank">[5]</a>. Bars are means±s. e. (three asterisks <i>P</i><1.0 10⁻³). Scale bars a: 10 mm; b, e, f: 5 mm.</p

Effect of rapamycin/FK506 pharmacology on caste characters in honey bees (Vehicle = 2% ethanol in insect saline; FK+R = sequential treatment of FK506, rapamycin; R = rapamycin; W = worker as comparison).

<p>In queen-destined individuals, rapamycin: a, prolonged development and b, reduced wet-weight (size) at adult emergence; c, ovary size was not affected by pharmacology but rapamycin d, induced the worker-specific trait corbicula (arrow in top panel shows location on hind leg); pictures of hind leg basitarsus covered with short hairs in Vehicle and FK+R, and by long corbicular hairs (e.g. arrows) in R and W (scale bar 0.25 mm). Corbicula is associated with low basitarsus length/width ratio <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000509#pone.0000509-Dedej1" target="_blank">[9]</a>, and in accord R individuals showed a decrease in this Basitarsal Index (BI) mean±s. e. noted in d (ANOVA: <i>F</i>_3,21 = 21.48, <i>P</i><0.00001, Fisher LSD: <i>P</i><0.00001; vs. between Vehicle and FK+R controls and between worker-destined groups <i>P</i>>0.50). e–g, rapamycin/FK506 pharmacology did not affect worker-destined bees. Bars are means±s. e. (asterisk <i>P</i><0.02, two asterisks <i>P</i><0.005, three asterisks <i>P</i><1.0 10⁻⁶).</p

Control experiment comparing <i>amTOR</i> suppression (<i>amTOR</i> RNAi) with <i>vitellogenin</i> suppression (<i>vg</i> RNAi).

<p>In 3^rd instar larvae, <i>vitellogenin</i> mRNA a, was reduced by <i>amTOR</i> RNAi. Yet when b, <i>vitellogenin</i> was similarly suppressed by <i>vg</i> RNAi, <i>amTOR</i> c, remained unaffected. Thus, <i>vg</i> RNAi is an unconfounded control for <i>amTOR</i> RNAi. In comparison to <i>vg</i> RNAi, <i>amTOR</i> RNAi: d, reduced larval growth (exemplified by 5-day-old larvae, larval volume is mean±s. e., arbitrary units); and delayed development (days until adult emergence: <i>vg</i> RNAi, 20.14±0.14; <i>amTOR</i> RNAi, 24.00±0.58) – also shown in e, a snapshot of phenotypic variance (20-day-olds), which demonstrates that <i>vg</i> RNAi bees had emerged with queen characters or had advanced pupal pigmentation; while <i>amTOR</i> RNAi bees were pupae, lightly pigmented and small. f, adult size of the last <i>vg</i> RNAi bees to emerge (21^st day) <i>vs.</i> TOR RNAi bees (adult wet-weight mg: <i>vg</i> RNAi, 197.14±7.94; <i>amTOR</i> RNAi 119.67±12.03). In comparison to <i>vg</i> RNAi, <i>amTOR</i> RNAi also reduced ovary size g, (range <i>vg</i> RNAi: 62–210; <i>amTOR</i> RNAi: 4–26 ovarioles/ovary, ANOVA: <i>F</i>_1,8 = 118.92, <i>P</i><0.00001). These data accurately replicate our comparison of GFP controls and <i>amTOR</i> RNAi (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000509#pone-0000509-g003" target="_blank">Fig. 3</a>). Bars are means±s. e. (asterisk <i>P</i><0.05). Scale bars d: 10 mm; e, f: 5 mm.</p

A Search for Parent-of-Origin Effects on Honey Bee Gene Expression

Parent-specific gene expression (PSGE) is little known outside of mammals and plants. PSGE occurs when the expression level of a gene depends on whether an allele was inherited from the mother or the father. Kin selection theory predicts that there should be extensive PSGE in social insects because social insect parents can gain inclusive fitness benefits by silencing parental alleles in female offspring. We searched for evidence of PSGE in honey bees using transcriptomes from reciprocal crosses between European and Africanized strains. We found 46 transcripts with significant parent-of-origin effects on gene expression, many of which overexpressed the maternal allele. Interestingly, we also found a large proportion of genes showing a bias toward maternal alleles in only one of the reciprocal crosses. These results indicate that PSGE may occur in social insects. The nonreciprocal effects could be largely driven by hybrid incompatibility between these strains. Future work will help to determine if these are indeed parent-of-origin effects that can modulate inclusive fitness benefits