Comparison of genotypic diversity in native and introduced populations of Sphaeropsis sapinea isolated from Pinus radiata

Abstract

Sphaeropsis sapinea is an endophyte and latent pathogen of pines, assumedly introduced to the Southern Hemisphere along with its host. There are at least three recognised forms of S. sapinea that differ from each other morphologically and can also be separated based on molecular characteristics. Pinus radiata is a native to California but has been used extensively for afforestation in the Southern Hemisphere. For this study, populations of S. sapinea were collected from exotic P. radiata plantations in South Africa, South and Western Australia, Tasmania and New Zealand and from native P. radiata in California. The genotypic diversity of the populations was assessed and compared using vegetative compatibility tests. SSR markers were used to determine the morphotype of isolates from each vegetative compatibility group. All Californian isolates of S. sapinea were found to be of the ‘B’ morphotype, while all introduced isolates in the Southern Hemisphere were of the ‘A’ morphotype. The genotypic diversities of S. sapinea populations ranged from extremely low in Australia to very high in South Africa with New Zealand having an intermediate genotypic diversity. S. sapinea is an asexual fungus and, therefore, different genotypes in an exotic population represent separate introductions. Our results suggest there have been very few introductions into Australia and multiple introductions into South Africa. In addition, it appears that S. sapinea isolates from P. radiata in the Southern Hemisphere did not originate from native P. radiata, but rather the widely planted exotic P. radiata has acquired this fungal endophyte from other Pinus within the exotic environment.