J Appl Genetics (2012) 53:237–247 exhibit a cryptic sexual cycle (Turgeon 1998). Similar results were reported by Arie et al. (2000) for F. oxysporum isolates and by Kerényi et al. (2004) for several Fusarium species. The amplification of the MAT idiomorph sequence revealed that 30% of the examined F. oxysporum isolates possess the MAT1-1 idiomorph and as many as 70% possess the MAT1- 2 allele. Similar results for mating types distribution in the fungal population were described for F. oxysporum originated from common bean (Karimian et al. 2010). The maximum effective reproductive strategy occurs when the mating type idiomorphs are present in a 1:1 ratio (Britz et al. 1998). In many Fusarium species, the ratio of MAT idiomorphs was significantly different from the theoretical 1:1 ratio expected in an idealised population, i.e. in a population of F. verticillioides, the mating ratio was 2:1 for isolates originated from maize in Italy, Brazil and the Philippines (Cumagun 2007; de Oliveira Rocha et al. 2011; Venturini et al. 2011). The same mating ratio was observed for an F. subglutinans population in South Africa (Britz et al. 1998). Among F. culmorum isolates originated from wheat and rye, the mating ratio was estimated as 2:3 (Irzykowska and Kosiada 2011). The achieved results may suggest that MAT1-2 is a predominant mating type in the F. oxysporum population in Poland, although a larger population of fungus originated from different geographic locations should be analysed before reaching a final conclusion. The predominance of one mating type can cause a limitation or lack of the possibility of sexual reproduction inside the population (Venturini et al. 2011). RT-PCR analysis proved that the F. oxysporum MAT genes are expressed (Yun et al. 2000). Lack of sexual reproduction may also be a result of the functional disorder of the yet-unidentified genes that are involved in successful sexual reproduction (Arie et al. 2000). Alternatively, asexual Fusarium spp. may require environmental conditions for mating that are uncommon when disease epidemics occur (Kerényi et al. 2004). To summarise, the molecular discrimination of MAT1-1 and MAT1-2 in F. oxysporum pop- ulations make possible the recognition of potentially compat- ible isolates that could be used in future experiments revealing whether the species is truly asexual or not. The genetic variability of F. oxysporum isolates was esti- mated previously by different molecular methods. Genetic fingerprinting by RAPD-PCR is an effective method for deter- mining an inter- and intra-species genetic variation without prior knowledge of the genome sequence. RAPD has been used successfully to analyse the genetic variation of several Fusarium species, including F. oxysporum (Assigbetse et al. 1994; Clark et al. 1998; Werner and Irzykowska 2007; Karimian et al. 2010), F. proliferatum (von Bargen et al. 2009), F. culmorum (Miedaner et al. 2004; Irzykowska and Baturo 2008), F. graminearum (Ouellet and Seifert 1993; Carter et al. 2002), F. moniliforme, F. solani and F. avenaceum (Khalil et al. 2003). In the present study, a significant genetic variation among F. oxysporum isolates was revealed and 245 shown in the form of a dendrogram. Clustering analysis resolved several fingerprint groups, six of which contained one isolate. This is consistent with the results of another study on the genetic diversity of F. oxysporum, where from 6 to 10 fingerprint groups were found (Cramer et al. 2003; Zanotti et al. 2006; Karimian et al. 2010). The DNA sequences of the most unlike isolates (O-02 and O-12) differ in analysed genome parts by almost 70%. Isolate O-02 originated from green spears of asparagus ‘Gijnlim’, whereas isolate O-12 originated from white spears of asparagus ‘Eposs’, thus, it is possible that the revealed variation between them resulted from race existence. However, we have found five pairs of isolates which were identical in the examined genome parts. All of these isolates originated from white spears of asparagus ‘Eposs’. Such high genetic similarity can also be the result of the relatively low number of markers used in the study. It is well known that a high level of intra-species genetic diversity is typical for species undergoing sexual cycle with the meiotic recombination (McDonald 1997; Kerényi et al. 2004). The ways available for genetic change in F. oxysporum are still largely unknown, but some possibilities exist beyond simple sexual or clonal reproduction (Kistler 1997). Daboussi and Langin (1994) reported that active fungal transposable elements may comprise up to 5% of the F. oxysporum genome and may have an impact on gene structure and function. Other molecular studies suggest a genetic duplication in the rDNA regions (O’Donnell and Cigelnik 1997). Moreover, in this study, nine molecular markers associated with MON or FB1 formation levels were found (Table 4). The obtained PCR products will be used in future for the prelim- inary discrimination of toxigenic isolates. RAPD marker OPC-15b (∼1,150 bp), connected with MON biosynthesis, will be cloned and sequenced to converse them to the specific and more universal SCAR marker. Amplified regions of the F. oxysporum genome may be involved in the study of toxin biosynthesis pathways. A genetic background of toxin bio- synthesis is complex and, probably, some genes still remain unknown. Toxicity as the majority of significant physiological traits (i.e. toxin or protein biosynthesis) is inherited quantita- tively, so continuous variation arises from the segregation of alleles at many interacting loci, whose effects depend on the environment (Edwards et al. 1987; Irzykowska et al. 2001; Bocianowski and Krajewski 2009). Proctor et al. (2008) reported that the FUM cluster is located in different genome parts in F. oxysporum, F. proliferatum and F. verticillioides. Perhaps the genetic bases of FB1 formation by F. oxysporum strains have not yet been fully elucidated. In conclusion, we postulate that F. oxysporum present in asparagus spears produces not only MON but also highly toxic FB1, which can be a significant concern for consumers. The mechanism of genetic changes occurring in F. oxysporum populations and the connection between the toxin profile and the genetic variability of fungus demand further attention. More