AFLP analyses of California and Mediterranean populations of Seiridium cardinale provide insights on its origin, biology and spread pathways

Abstract

Seiridium cardinale is regarded as the most important agent responsible for the disease of cupressaceous hosts referred to as Cypress canker. The fungus was first described in California and is currently reported in all continents. A recent study based on seven SSR loci has suggested that California populations may represent the source of the epidemic in the Mediterranean. In this study, 185 AFLP markers were used on an expanded sample size of 125 isolates to determine whether the Mediterranean population may indeed be derived from the California one and to compare the reproductive modes of populations in the two regions of the world. Additionally, AMOVA, NJ, STRUC-TURE, principal component analysis and b-tubulin sequence analyses were employed to infer the presence of genetic structure within and between populations. The distribution of pairwise AFLP similarity coefficients suggests Mediterranean populations are reproducing only clonally, while California populations are reproducing both clonally and sexually. AMOVA indicates Mediterranean and California populations are currently genetically isolated, but NJ and STRUCTURE analyses both suggest ancestral Mediterranean genotypes belong to the California population. No alleles were private to either population, and the presence of identical or quasi-identical genotypes at large distances supports the notion that movement of infected cypress plants is responsible for the global spread of the disease. Surprisingly, STRUCTURE identified a second cluster of Mediterranean genotypes distinct from the basal mixed California-Mediterranean cluster. This second cluster either may have originated from the first one under the selection pressure imposed on the pathogen in the new Mediterranean environments or may be the result of a further introduction from California or elsewhere. Cumulatively, the evidence presented here suggests that S. cardinale may be native or long naturalized in California and that two genetically distinct groups are present in the Mediterranean, with obvious implications for further studies on this disease.

Epidemiological History of Cypress Canker Disease in Source and Invasion Sites

“…The higher genetic diversity of the Cal population is also supported by sequencing of the β-tubulin region, which revealed the presence of two distinct haplotypes within the S. cardinale population in Cal, whereas only one of the two haplotypes was present among isolates from the Med. According to amplified fragment length polymorphism (AFLP) data, the two β-tubulin haplotype groups of the Cal population clustered separately in a PCA analysis [34]. This suggests that the two haplotypes are not simply two different alleles at one locus, but may be associated with two distinct groups of isolates potentially characterized by genotypic and phenotypic differences.…”

“…Network analysis (MSN) based on SSR multilocus genotypes and Neighbour-Joining analysis of AFLP both suggest that the Cal population of the fungus represented a source for the Med invasion [34]. In fact, isolates from California and the Mediterranean formed two distinct clusters of genotypes.…”

“…The Med 1 population would include the founder Mediterranean population (directly derived from California), while the Med 2 population would derive from Med 1 and may be the result of a genetic process which led to a distinct population that then expanded in the Mediterranean [34].…”

“…After this first outbreak, cypresses have not been used for extensive plantings in California, but they were still planted for hedges or as shade trees or, in some cases, for protective purposes, e.g., C. macrocarpa along the coastal area. Today, both S. cardinale and S. unicorne are known to be present in California [33][34][35][36][37][38][39], threatening ornamental plantations of Cupressus sp.-mainly C. sempervirens, C. macrocarpa, ×Cup. leylandii, and Juniperus spp.…”

“…A total of 2385 trees of C. sempervirens (790), C. macrocarpa (611) (Figure 2), and ×Cup. leylandii (984) were Today, both S. cardinale and S. unicorne are known to be present in California [33][34][35][36][37][38][39], threatening ornamental plantations of Cupressus sp.-mainly C. sempervirens, C. macrocarpa, ×Cup. leylandii, and Juniperus spp.…”