Genetic structure of aMycosphaerella crypticapopulation

Abstract

We studied the genetic structure of Mycosphaerella cryptica following natural infection of Eucalyptus globulus in a genetic trial. Results from this study indicated significant genetic variation within, and between, E. globulus families (three cloned and control-crossed F 2 and four open-pollinated families). Single-ascospore isolates were sampled from 72 E. globulus trees with contrasting levels of resistance within the trial, 21 E. nitens trees in an adjacent plantation, and five distant Eucalyptus plantation trees (two E. globulus 100-2400 km; two E. nitens 50-2200 km; one E. grandis × E. tereticornis hybrid 1500 km from the trial). Deoxyribonucleic acid from these isolates was scored for the presence/absence of 75 random amplified polymorphic DNA (RAPD) loci. Only 18 RAPD genotypes, which appeared to recombine rarely, were present among the 98 isolates, which indicates that M. cryptica is not strictly heterothallic. Cluster analysis using genetic distance revealed that M. cryptica genotypes from the field trial grouped into two clusters that matched differences in isolate culture morphology, indicating that the fungal population comprised two distinctive biotypes. The two biotypes differed markedly in their host interactions. Biotype 2 was found only on E. globulus, whereas biotype 1 infected both E. globulus and E. nitens eucalypt species. Within E. globulus, biotype 2 was almost exclusively collected on resistant trees whereas biotype 1 was found on both resistant and susceptible trees. The hypothesis that there could be specialisation of M. cryptica biotypes occurring at two levels, among host species and within host species, is discussed.

Few Mendelian Genes Underlie the Quantitative Response of a Forest Tree, Eucalyptus globulus, to a Natural Fungal Epidemic

“…Quantitative genetic studies have reported genetic variation in the susceptibility of E. globulus to MLD, both within and between provenances (Carnegie et al 1994;Dungey et al 1997;Milgate et al 2005a,b) as well as within segregating families (Milgate et al 2005a). There is some evidence for a positive correlation between resistance to the disease and rainfall, consistent with more intense natural selection for disease resistance in high-rainfall areas (Carnegie et al 1994;Dungey et al 1997).…”

“…The three controlled-cross families were planted in a field trial at Woolnorth in northwest Tasmania (Figure 1) in May 1998, as part of an experiment also containing open-pollinated families collected from four of the grandparents (KI2, KI5, T7, T5; see Milgate et al 2005a for a full description of the trial design). The trial featured an incomplete block design, consisting of two replicates, each with 20 incomplete blocks of 30 or 36 trees.…”

“…In terms of genetic variability within the pathogen, in general, a broad range of host adaptation as evident in M. cryptica would suggest a lack of specialization in response to host species or resistance (Milgroom and Peever 2003). However, Milgate et al (2005a) provided evidence for a close association between host and pathogen, finding specialization in the occurrence of two different biotypes of M. cryptica between resistant and susceptible E. globulus individuals. Specifically, RAPD analysis showed that 18 M. cryptica genotypes clustered into two biotypes, one of which was more frequent on resistant trees, whereas the other was more frequent on susceptible trees (Milgate et al 2005a).…”

A latitudinal cline in disease resistance of a host tree

“…Studies by Freeman et al (2008) and Milgate et al (2005b) suggest that a gene-for-gene relationship may exist between pathogen and host in this pathosystem. Freeman et al (2008) identified two major QTL (quantitative trait loci) for resistance to T. cryptica, one of which was validated in an additional E. globulus family and the second across two additional families, and Milgate et al (2005b) identified a T. cryptica biotype only associated with resistant host genotypes. This association is further complicated by the potential for multiple mechanisms of resistance, tolerance and/ or avoidance, which are at play in the relationship between the host and pathogen (Rausher, 2001).…”

Mycosphaerella and Teratosphaeria diseases of Eucalyptus; easily confused and with serious consequences

“…Eucalyptus spp. are generally more susceptible to infection by Mycosphaerella and Teratosphaeria species during their juvenile leaf phase and the leaf infections caused by these fungi reduce the photosynthetic capacity, leading to premature defoliation and stunting of growth (Park and Keane 1982b;Lundquist and Purnell 1987;Carnegie and Ades 2003;Milgate et al 2005b;Pinkard and Mohammed 2006).…”