Genetic diversity of root-infesting woolly apple aphid Eriosoma lanigerum (Hausmann) (Hemiptera: Aphididae) populations in the Western Cape

Timm, Alicia (Alicia Eva) (2003-03)

Thesis (MScAgric)--University of Stellenbosch, 2003.

Thesis

ENGLISH ABSTRACT: Characterizing the genetic structure of a pest population can provide an understanding of the factors influencing its evolution and assist in its ultimate control. The aim of the present study was to characterize the genetic structure of woolly apple aphid Eriosoma lanigerum (Hausmann) populations in the Western Cape Province in South Africa. Since this economically important apple pest has not previously been characterized at molecular level, it was necessary to evaluate methods for determining the genetic structure of E. lanigerum populations. Two different molecular techniques were evaluated viz. random amplification of polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP). This study represents the first application of the latter technique to members of the Aphididae. Aphids were sampled from four regions in the Western Cape in South Africa viz. Elgin, Ceres, Vyeboom and Villiersdorp. A spatially nested sampling design was used to establish the distribution of the genetic variance of aphids. A total of 192 individuals from 13 farms were analysed. Ten RAPD primers were chosen for analysis from an initial assay of 25 after fragment reproducibility had been confirmed. For AFLP analysis three different rare-cutting restriction enzymes were evaluated for AFLP analysis, viz. EcoRI, SseI and MluI. The latter yielded the best results in combination with the frequent-cutting enzyme MseI. Twenty-five AFLP selective primer pairs were evaluated, out of which five were chosen for analysis of the total population. Two hundred and fifty AFLP fragments and 47 RAPD fragments were scored for analysis. Both analyses indicated that a low level of genetic variation was apparent in E. lanigerum populations and that no differentiation resulted from geographic isolation. From RAPD analyses it was deduced that all variation could be attributed to differences between individuals. AFLP analysis indicated that, whereas genetic differences in E. lanigerum populations between orchards were negligible, a significant portion of genetic variation could be attributed to differences between farms and individuals within farms. Therefore, AFLP analysis allowed for finer discrimination of the genetic structure of E. lanigerum populations than RAPD analysis and is recommended for studies of other aphid species. The fact that most of the genetic variation present in E. lanigerum populations could be found on small spatial scales indicated that sampling individuals over a wide geographic area was an ineffective way of detecting the genetic diversity present in E. lanigerum populations. The low level of variation in populations is most likely due to the exclusive occurrence of parthenogenetic reproduction, founder effects (including distribution of infested plant material from a limited source) and selective factors such as the use of resistant rootstocks or pesticides. Furthermore, the low level of variation found indicated that the possibility of controlling E. lanigerum in the Western Cape using host plant resistance is favourable. Thus, plant breeders developing resistance to E. lanigerum can expect plant entries to be exposed to most of the genetic diversity present in Western Cape populations, regardless of location.

AFRIKAANSE OPSOMMING: Die bepaling van die genetiese struktuur van 'n landboukundige plaagpopulasie kan lei tot begrip van die faktore wat die populasie beïnvloed en kan uiteindelike beheer vergemaklik. Die doel van die huidige studie was om die genetiese struktuur van die appelbloedluis Eriosoma lanigerum (Hausmann) in die Wes-Kaap Provinsie van Suid-Afrika te bepaal. Aangesien hierdie belangrike appelplaag nie van tevore op molekulêre vlak bestudeer is nie, was dit nodig om metodes vir die bepaling van die genetiese struktuur van E. lanigerum populasies te evalueer. Twee molekulêre tegnieke is geëvalueer, nl. lukraak geamplifiseerde polimorfiese ONS (RAPD) en geamplifiseerde fragment-lengte polimorfismes (AFLP). Hierdie studie is die eerste om laasgenoemde tegniek te gebruik om lede van die Aphididae te bestudeer. Plantluise is verkry van vier verskillende gebiede in die Wes-Kaap Provinsie van Suid-Afrika nl. Elgin, Ceres, Vyeboom en Villiersdorp. 'n Hierargiese sisteem is gebruik om die verspreiding van die genetiese variasie van plantluise te bepaal. In totaal is 192 individue van 13 plase geanaliseer. Tien RAPD inleiers is gekies uit 'n analise van 25 verskillende inleiers nadat fragment reproduseerbaarheid bevestig is. Drie verskillende restriksie ensieme is geëvalueer vir AFLP analise nl. EcoRI, SseI en Mlul. Die beste resultate is verkry toe MluI saam met MseI gebruik is. Vyf-en-twintig AFLP selektiewe inleier pare is geëvalueer waarvan vyf gekies is vir analise van die totale populasie. Twee-honderd-en-vyftig AFLP fragmente en 47 RAPD fragmente is gedokumenteer vir analise. Beide RAPD en AFLP analises het getoon dat daar 'n lae vlak van genetiese variasie in E. lanigerum populasies is en dat geen differensiasie as gevolg van geografiese isolasie ontstaan het nie. Uit RAPD analise is daar afgelei dat al die variasie toegeskryf kon word aan verskille tussen individue. AFLP het aangetoon dat alhoewel verskille in E. lanigerum populasies tussen boorde laag was, kon 'n hoë persentasie van die variasie toegeskryf word aan verskille tussen plase en individue binne plase. AFLP analise het meer insig in die genetiese struktuur van E. lanigerum populasies verskaf, en word dus aanbeveel vir studies van ander plantluise. Omdat meeste van die genetiese variasie oor klein geografiese afstande verkry word, is steekproefueming oor groot gebiede 'n ondoeltreffende manier om die genetiese variasie binne 'n monster te meet. Die lae vlak van genetiese variasie is waarskynlik te wyte aan partenogenetiese vermeerdering, stigter gevolge (insluitend verspreiding van geïnfesteerde plantmateriaal vanaf 'n beperkte bron), sowel as selektiewe faktore soos die gebruik van bestande onderstokke en insekdoders. Verder dui die lae vlak van variasie aan dat die moontlikheid vir beheer deur gasheerplantbestandheid goed is in die Wes-Kaap. Planttelers kan verseker wees dat hulle plante blootgestel sal wees aan meeste van die genetiese variasie in die Wes-Kaap appelbloedluis populasies ongeag hulle ligging.

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