Population genetic structure and mycotoxin potential of the wheat crown rot and head blight pathogen Fusarium culmorum in Algeria
Introduction
Fusarium crown rot (FCR; also known as foot and root rot) and Fusarium head blight (FHB) are two of the most economically destructive diseases that affect small grain cereal production worldwide (Goswami and Kistler, 2004). Both diseases are noted for significantly reducing yields and contaminating cereals with trichothecenes and zearalenone mycotoxins, which can render them unsuitable for food or feed (Scherm et al., 2013). Trichothecenes pose a serious threat to food safety and the health of humans and other animals due to diverse toxicological effects (Wu et al., 2014). In addition, trichothecenes are acutely phytotoxic and have been shown to function as virulence factors on sensitive host plants (Proctor et al., 1995, Jansen et al., 2005).
Pathogen surveys indicate that Fusarium culmorum (W.G. Smith) Saccardo is the primary etiological agent of FCR in several countries in the Mediterranean and adjacent regions. This species was reported as the main FCR pathogen in Tunisia (Kammoun et al., 2009, Rebib et al., 2014), Sardinia (Balmas et al., 2015), Iran (Pouzeshimiab et al., 2014), and preliminary surveys suggest that it might be an important FCR pathogen in Algeria (Yekkour et al., 2015, Touati-Hattab et al., 2016). Nested within the B clade of trichothecene toxin-producing fusaria (Sarver et al., 2011), F. culmorum strains are segregating for two of the three known B type trichothecene chemotypes: deoxynivalenol (DON) + 3-acetyl-deoxynivalenol (3ADON chemotype) and nivalenol (NIV chemotype) (Scherm et al., 2013). Differences in the toxicity and bioactivity of the trichothecene toxin types, which appear to have been maintained by balancing selection, are theorized to have significant fitness effects (Ward et al., 2002). A sexual reproductive mode in this important cosmopolitan cereal pathogen is unknown, given that a teleomorph has not been observed. However, it is presumed to be self-sterile or heterothallic because PCR assays for mating type idiomorph have shown that strains possess a MAT1-1 or MAT1-2 idiomorph (Kerényi et al., 2004, Obanor et al., 2010), and because population genetic analyses of this small-grain pathogen have revealed high genetic diversity and low population subdivision consistent with a sexually recombining species (Gargouri et al., 2003, Miedaner et al., 2013, Pouzeshimiab et al., 2014, Rebib et al., 2014).
Due to the importance of Fusarium culmorum induced FCR and FHB, and associated mycotoxin contamination of small-grain cereals to world agriculture, numerous molecular markers have been used to assess its genetic diversity on regional to global scales (Scherm et al., 2013 and references therein). However, for population genetic analyses (Miedaner et al., 2013, Pouzeshimiab et al., 2014, Rebib et al., 2014), microsatellites or simple sequence repeats (SSR) are the genetic marker of choice because: (1) they contain higher allelic variation at each locus due to their high mutation rate, and (2) they can be easily PCR-amplified with fluorescently labelled primers and scored on an automated capillary genetic analyzer as discreet co-dominant, selectively neutral alleles. Recognizing the need for more polymorphic markers for population genetic analyses, Giraud et al., 2002, Suga et al., 2004, Vogelgsang et al., 2009 developed a set of SSR markers that have been used in three separate studies of F. culmorum that found higher genetic diversity within than between populations and low population level subdivision (Miedaner et al., 2013, Pouzeshimiab et al., 2014, Rebib et al., 2014). Because Fusarium culmorum appears to represent the most important FCR and FHB pathogen in Algeria (Touati-Hattab et al., 2016), the present study was conducted to characterize Algerian F. culmorum diversity, toxin potential, and population structure, and compare them with small collections of F. culmorum from Australia, Italy, and the U.S.
Section snippets
Taxon sampling
We conducted FCR and FHB pathogen surveys of 500 wheat fields in 20 semiarid wheat-growing provinces of northern Algeria (Fig. 1) during 2014 and 2015. Pure cultures of the isolates were obtained by disinfecting sections of the symptomatic crown or ear tissue in 2% sodium hypochlorite solution for 5 min, rinsing them in sterile distilled water, and then drying them on sterile filter paper in a biological hood. Strains were purified by growing them on 3% water agar (Bacto, Becton Dickinson and
Molecular identification by TEF1 sequencing
Surveys of diseased durum and common wheat in 500 fields during 2014 and 2015 focused on the genetic diversity of Fusarium culmorum in the 20 main cereal growing provinces of northern Algeria (Fig. 1). Of the 269 strains from Algeria that were identified by sequencing a portion of TEF1, 140 were typed as Fusarium culmorum from 110 different fields, and these included 110 and 30 plants, respectively, symptomatic for FRC and FHB. However, only 106 of these were included in the present study
Discussion
This study represents a detailed analysis of the population biology, mycotoxin potential and genetic diversity of Fusarium culmorum in countries on four different continents. Recent but limited pathogen surveys of these diseases in Algeria, which were focused on evaluating susceptibility of locally grown wheat cultivars to FHB (Touati-Hattab et al., 2016), and the relationship between DON toxin level accumulation in wheat and barley and disease severity (Yekkour et al., 2015), provided
Acknowledgments
The lead author, Imane Laraba, is pleased to thank Abdelazziz Kedad for guidance during the course of this study and Ecole Nationale Supérieure Agronomique, Algiers, Algeria for a scholarship that supported the research visit to NCAUR. Thanks are also due to Gail Doehring, Thomas Usgaard, Christine Hodges, and Nathane Orwig for excellent technical assistance in various aspects of this study and Stephen A. Rehner, ARS-USDA-BARC for a presubmission review. Mention of trade names or commercial
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