Towards an understanding of variation in the Mulga complex (Acacia aneura and relatives)

Abstract

<p><i>Acacia aneura</i> and its close relatives form a highly variable species complex commonly known as Mulga. They are small trees that dominate the vegetation of arid regions, in all occupying around 20% of Australia. This paper discusses and illustrates some of the more important types of variation found in Mulga, especially growth form and phyllode and pod morphology. This variation occurs both between and within populations and often results in a very complex mosaic of mixed Mulga populations. The underlying genetic and biological factors responsible for this variation are explored. While hybridisation is probably one cause of the variation, our use of microsatellite markers has not been able to provide direct evidence of this; however, the sampling done to date has been very small. Genetic developmental mechanisms such as polyploidy, apomixis and neoteny are maintaining this diversity. The Mulga complex contains multiple ploidy levels, including triploids, tetraploids and pentaploids, and polyembryony is a common feature in all these polyploids. Microsatellite data have identified fixed heterozygozity in populations with some genetic differences among morphotypes. Progeny arrays of 24 morphotypes indicated that over 95% of the plants have the same genotype as the mother plant. This accumulating evidence indicates that the Mulga complex is reproducing through facultative apomixis. Additionally the retention of juvenile characteristics (neoteny) is seen in many populations and also increases the variability. Given the importance of Mulga to the ecology, management and sustainable utilisation of arid zone ecosystems, it is important that the classification of the group adequately reflect the biological reality that exists in nature, if indeed this is achievable. The work reported here, and in related molecular and population studies, provides a basis for testing new classifications of Mulga. It also provides new information that can contribute to an improved classification of the group.</p>