Implications of the changing phylogenetic relationships of Acacia s.l. on the biological control of Vachellia nilotica ssp. indica in Australia

Abstract

Plant relationships have implications for many fields including weed biological control. The use of DNA sequencing and new tree building algorithms since the late 1980s and early 1990s have revolutionised plant classification and has resulted in many changes to previously accepted taxonomic relationships. It is critical that biological control researchers stay abreast of changes to plant phylogenies. One of the largest plant genera, Acacia, has undergone great change over the past 20 years and these changes have ramifications for weed biological control projects in a number of countries. Vachellia nilotica (prickly acacia) is a major weed in Australia, originating from the Indian subcontinent and Asia, and it has been a target for biological control since 1980. Once a member of Acacia, a large (>1,000 spp.) and iconic group in Australia, prickly acacia is now part of the genus Vachellia. Current knowledge suggests that Vachellia is more closely related to mimosoid genera than it is to Acacia s.s. There has also been a recent reclassification of legume subfamilies with subfamily Mimosoideae now part of subfamily Caesalpinioideae, and four new subfamilies. In this paper we review the changes that have occurred to this group since the prickly acacia biological control project began and discuss the implications for the project. A new host test list for quarantine testing is proposed. Developed following the modernisation of the centrifugal-phylogenetic method, it is shorter than past lists, containing 46 species, although still lengthy because of the expectations of regulatory bodies, which are slower to accept advances in scientific knowledge. The list includes five Vachellia species, six “Mimoseae” species and 26 Acacia species. The number species from legume subfamilies other than the new Caesalpinioideae is greatly reduced.