Genetic Variation in Native Populations of the Laurel Wilt Pathogen,Raffaelea lauricola, in Taiwan and Japan and the Introduced Population in the United States

Abstract

Laurel wilt is a vascular wilt disease caused by Raffaelea lauricola, a mycangial symbiont of an ambrosia beetle, Xyleborus glabratus. The fungus and vector are native to Asia but were apparently introduced to the Savannah, GA, area 15 or more years ago. Laurel wilt has caused widespread mortality on redbay (Persea borbonia) and other members of the Lauraceae in the southeastern United States, and the pathogen and vector have spread as far as Texas. Although believed to be a single introduction, there has been no extensive study on genetic variation of R. lauricola populations that would suggest a genetic bottleneck in the United States. Ten isolates of R. lauricola from Japan, 55 from Taiwan, and 125 from the United States were analyzed with microsatellite and 28S rDNA markers, and with primers developed for two mating-type genes. The new primers identified isolates as either MAT1 or MAT2 mating types in roughly equal proportions in Taiwan and Japan, where there was also high genetic diversity within populations based on all the markers, suggesting that these populations may have cryptic sex. Aside from a local population near Savannah and a single isolate in Alabama that had unique microsatellite alleles, the U.S. population was genetically uniform and included only the MAT2 mating type, supporting the single introduction hypothesis. This study suggests the importance of preventing a second introduction of R. lauricola to the United States, which could introduce the opposite mating type and allow for genetic recombination.

First report of a sexual state in an ambrosia fungus:Ambrosiella cleistominutasp. nov. associated with the ambrosia beetleAnisandrus maiche

“…Cryptic sex was hypothesized for Raffaelea lauricicola T.C. Harr., Fraedrich, & Aghayeva, the mycangial symbiont of Xyleborus glabratus Eichhoff., but ascomata were not identified (Wuest et al 2017). …”

Genetic Analyses of the Laurel Wilt Pathogen, Raffaelea lauricola, in Asia Provide Clues on the Source of the Clone that is Responsible for the Current USA Epidemic

“…Primers have been previously developed that amplify portions of the MAT1-1 genes (MAT1-1-3) [17] and MAT1-2 genes (MAT1-2-1) [20].…”

“…Recently, this hypothesis was investigated with genetic markers in R. lauricola. Wuest et al [17] used two simple sequence repeat (SSR), a single nucleotide polymorphism (SNP) and mating type markers with isolates from the USA, Japan and Taiwan, whereas Hughes et al [5] used amplified fragment length polymorphism (AFLP) and SSR markers to examine isolates recovered early in the USA epidemic. Although the studies used a limited number of markers or isolates, Hughes et al [5] and Wuest et al [17] suggested that a single introduction of the pathogen had occurred in the USA.…”

“…The present study had several objectives. To robustly test the single introduction hypothesis, we developed a multiplexed panel of SSR markers based on those developed by Dreaden et al [18] and used by others [5,17,19]. Isolates of R. lauricola from the USA and the only three countries in Asia from which isolates of the pathogen were available (Myanmar, Japan and Taiwan) were then assayed with SSR and mating type data (Table 1).…”

Host density dependence and environmental factors affecting laurel wilt invasion

“…Although R. lauricola has been detected in X. glabratus in Asia (Dreaden et al 2019;Harrington et al 2011), laurel wilt disease has not been found in the US prior to the appearance of the X. glabratus (Ploetz et al 2013). R. lauricola populations are quite diverse in East Asia, but the pathogen and its vector seem to be clonal in the US (Cognato et al 2019;Dreaden et al 2019;Hughes et al 2017;Wuest et al 2017). This suggests that both were introduced in a single event, possibly being carried to the port of entry in untreated packing material Ploetz et al 2017).…”