Genetic diversity in the biological control process : Acacia nilotica as a test case

Abstract

Prickly acacia, Acacia nilotica (L.) Delile (Leguminosae: Mimosoideae), is a weed of national significance in Australia, covering approximately 6.5 million hectares of grazing land. Biological control of this and other weed species has not been achieved to date, and it has been suggested that various genetic based methodologies may assist with current classical biological control efforts. To support the biological control of A. nilotica in Australia, the country of origin of the Australian weed populations was determined using molecular methods, and a novel rearing method for biological control agents was assessed for its capacity to maintain genetic diversity in mass reared insect populations. Acacia nilotica is distributed from southern Africa through the Arabian peninsular to the Indo-Pakistan subcontinent. Patterns of DNA sequence variation (in two DNA fragments) in all nine described subspecies of A. nilotica were used to determine the genetic relatedness of subspecies, to verify if the Australian populations were A. nilotica subsp. indica (Benth.) Brenan, and to establish if any other subspecies were present in Australia. This study confirmed the Australian populations were A. nilotica subsp. indica, and the origin of the Australian weed source was most likely India and Pakistan. At least two genotypes of A. nilotica were identified to occur in Australia but it is unknown whether current or future biological control agents will behave differently for each genotype. This research demonstrated the importance of genotyping weed species that may be easily misidentified, particularly when they are targeted for biocontrol and/or listed as host specificity test species. Standard insect rearing practices may cause a reduction in genetic diversity and fecundity through inbreeding depression, hence lessening the successes of biological control programs due to reduced agent quality. This study used an approved host specific biological control agent of A. nilotica, Chiasmia assimilis (Warren, 1899) (Lepidoptera: Geometridae) imported from South Africa, to investigate an isofemale rearing methodology that aims at maintaining the genetic diversity of organisms that are being mass reared for release. The standard group rearing approach used in many current biocontrol programs interbreeds field collected adults in either a single or a few subpopulations, and migration or mating between subpopulations is not prevented. In the isofemale method of rearing, 30 plus lines (“populations”) are initialised using a randomly selected male and female from field collected adults. Subsequent offspring from each line are kept genetically isolated and become parents of the next generation. After five to eight generations, isofemale lines are subsequently hybridised over several generations to restore allele frequencies and fecundity, immediately prior to field release. Measurements of the number of neonates produced (fecundity) were determined for both group reared lines of insects and isofemale reared lines over multiple generations. Experimental results showed a significant reduction in the fecundity of the isofemale line treatments. When isofemale lines were then hybridised to form replicate hybridised populations, the level of fecundity returned to levels that were statistically similar to the group reared lines. Further evidence from the group rearing of C. assimilis for 32 generations, demonstrated that standard group rearing methods may lead to a significant loss in fecundity.