The effects of genotype and/or environment on the phenotypic expressions of mandibular gland signals in honeybees (Apis mellifera)

Noach-Pienaar, Lee-Ann (2011-03)

Thesis (PhD (Botany and Zoology))--Stellenbosch University, 2011.

Thesis

ENGLISH ABSTRACT: Insect societies utilize advanced chemical communication systems to organize many aspects of their social life, which among others, include reproduction, thus maintaining colony homeostasis. The queen pheromone complex (QMP), dominated by (E)-9-keto-2-decenoic acid (9ODA) is of integral importance in regulating worker reproductive development. Unique characteristics, associated with reproductive dominance, enabled the successful establishment of Apis mellifera capensis workers as social parasites (or pseudoqueens) in colonies of the neighbouring A. m. scutellata. This suggested that producing a queenlike pheromonal bouquet is one of the proximate factors in their success. In this study we attempted to address the pheromone communication dilemma by investigating whether the phenotypic expression of mandibular gland signals in honeybee workers are under genetic and/or environmental influence. It was hypothesized that the mandibular gland profiles of queens and workers may be closely correlated to specific genotypes in the colony. However, different ageing and rearing environments (social context) can ultimately influence gene expression with respect to mandibular gland signals, highlighting the fact that environmental influences are not necessarily non-significant. In our experiments, both environmental/social conditions and genotypes of our test individuals were manipulated. The capensis workers used in our experiment from their native range (Western Cape area are refered to as native workers, while capensis parasitic workers, from the clonal parasitic lineage, were obtained from the Gauteng area. A. m. scutellata workers were obtained from their native range, north of the hybrid zone. Both native and parasitic workers showed the potential to become reproductively active, but the rapid pheromonal development of parasitic workers placed them at a reproductive advantage. Parasitic workers started producing low levels of 9HDA, the precursor to the queen substance 9ODA, between 12-24 hrs, while native workers only did so after 24 hrs. Despite this, rapid signal development did not culminate in the parasitic clones always pheromonally out-competing native workers. Withinin groups of native workers and a single clonal parasitic worker, the mandibular gland profiles of most workers were dominated by 9ODA and 9HDA (> 80% of extracts) with only 43% of the single parasitic workers producing higher amounts of 9ODA than native workers. Mandibular gland pheromone profiles converged in groups of workers sharing a greater proportion of genes, providing support for a link to genotypic affects. Workers that were 75 – 99% related diverged significantly from groups with lower levels of relatedness was largely due to the presences of 9ODA (Spearman’s rank correlation r = 0.66, p < 0.0001). Despite the tendency for signal to convergence in groups of closer relatedness a considerable amount of signal variability was also observed under varying social conditions. Workers originating from a single capensis queen but aged under queenright and queenless conditions had very distinct mandibular gland profiles (Wilks’ lambda λ = 0.118, χ2 = 331.002, p < 0.0001). This variability was thus a result of the social environment that the workers were exposed to. The physiological traits, namely mandibular gland pheromone production, linked to reproductive potential in honeybee workers seem to be determined by a combination of environmental and genetic factors. Queen mandibular gland pheromone biosynthesis is genetically predisposed in certain workers however the final oxidation step to 9ODA is strongly influenced by the social environment. The signal plasticity observed in this study is adaptive and assists workers to realize their reproductive potential.

AFRIKAANSE OPSOMMING: Insek gemeenskappe gebruik gevorderde chemiese kommunikasie sisteme om verskeie aspekte van sosiale lewe, onder andere reproduksie, te organiseer en sodoende word korf homeostasis handhaaf. Die feromoon kompleks van die koninginby, wat hoofsaaklik uit (E)-9-keto-2-decenoic acid (9ODA) bestaan speel ʼn belangrike rol in die regulering van reproduksie in heuningby werkers. Die suksesvolle vestiging van Apis mellifera capensis werkers as sosiale parasiete (pseudo koninginne) in die korwe van die naburige A. m. scutellata, is bewerkstellig deur hul unieke kenmerke, wat met reproduktiewe oorheersing verband hou. Dit suggereer dat die produksie van ʼn tipiese koningin feromoon sein een van verskeie beduidende faktore is in capensis werkers se sukses. In hierdie studie het ons die dilemma van feromoon kommunikasie probeer aanspreek deur te ondersoek of die fenotipiese uitdrukking van seine van die mandibulêre kliere deur genetiese en/of omgewings faktore beïnvloed word. Die hipotese was dat die mandibulêre klier profiele van koninginne en werkers korreleer met spesifieke genotipes in die korf. Die verskillende omgewings waarin werkers groot gemaak word en verouder (sosiale konteks), kan uiteindelik die uitdrukking van gene, raakende mandibulêre kliere, beïnvloed. Dit beklemtoon die feit dat omgewings faktore nie noodwendig onbeduidend is nie. Beide omgewings/sosiale toestande and genotipes van toets individue is in ons eksperimente gemanipuleer. Die capensis werkers afkomstig uit hul natuurlike habitat (Weskaap area) wat in ons eksperimente gebruik is word na verwys as inboorling werkers, terwyl parasitiese capensis werkers, van klonies parasitiese afkoms, vanuit die Gauteng area verkry is. A. m. scutellata werkers was vanuit hul natuurlike habitat, noord van die, hybried sone, verkry. Beide inboorling en parasitise werkers het die potensiaal getoon om reproduktief aktief te word, maar versnelde feromoon ontwikkeling van parasite werkers het hulle ʼn reproduktiewe voordeel gegee. Parasiet werkers het reeds lae hoeveelhede 9HDA, die voorganger van 9ODA, begin produseer tussen 12 – 24 uur, terwyl inboorling werkers produksie eers na 24 uur begin het. Ten spyte van die versnelde ontwikkeling in parasiet werkers het dit nie gelei daartoe dat kloniese parasiete altyd feromonies die oorhand oor inboorling werkers gekry het nie. In groepe bestaande uit inboorling werkers en ʼn enkele parasite werker, was die mandibulêre klier profiele altyd deur 9ODA en 9HDA (> 80% van ekstrakte) gedomineer. Slegs 43% van parasite werkers het groter hoeveelhede 9ODA as inboorling werkers geproduseer. In groepe werkers, wat ʼn groter proporsie gene in gemeen gehad het, het mandibulêre klier profiele konvergeer. Dit ondersteun die bestaan van ʼn verband met genotipiese invloed. Werkers van 75 – 99% verwantskap het beduidend verskil van groepe met laer verwantskapsvlakke, hoofsaaklik as gevolg van die teenwoordigheid van 9ODA (Spearman’s rank korrelasie r = 0.66, p < 0.0001). Ten spyte van die konvergerende neiging van profiele, van meer verwante groepe, was aansienlike veranderlikheid onder verskillende sosiale toestande waargeneem. Werkers, afkomstig vanaf ʼn enkele capensis koninginby, maar òf in die teenwoordigheid òf afwesigheid van ʼn koningin verouder is, het baie kenmerkende mandibulêre klier profiele getoon (Wilks’ lambda λ = 0.118, χ2 = 331.002, p < 0.0001). Die veranderlikheid was dus ʼn gevolg van die sosiale omgewing waaraan die werkers blootgestel was. Dit blyk asof die fisiologiese kenmerke wat met reproduksie potensiaal in heuningbye verband hou, naamlik mandibulêre klier feromoon produksie, deur ʼn kombinasie van genetiese – en omgewings faktore beïnvloed word. Sekere werkers is meer geneig tot die biosintese van koningin mandibulêre klier feromoon as gevolg van hul genetika, terwyl die finale oksidasie na 9ODA onder sterk omgewings invloed is. Die plastisiteit in mandibulêre seine waargeneem in hierdie studie, is aanpasbaar en help werkers om hul reproduksie potensiaal te bereik.

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