Data from cultivation experiments of potato onion at NordGen and the different Nordic programs for biodiversity in cultivated plants were assembled and compared. Many accessions were included in repeated test cultivations, and thus the reliability of the scored characters could be compared. Although there was obvious morphological variation among the material (Fig. 1), characters seemed to be strongly influenced by environment and set onion properties besides genotype. This was especially the case for quantitative traits, such as scores of the size and number of onions produced from each set onion, with large variation among experiments and a strong influence from the size of the set onion. The large variation observed, combined with effects of differences of years and growing locations, limits the possibility to draw conclusions of any quantitative data, and these are therefore not presented. Bulb shape is more of a qualitative character, but was partly differently scored among experiments, where the same accession could produce flattened, round, rhomboid, and high top bulbs. A predominant shape could, however, be assigned to most accessions (ESM 1).
The only character with consistent scoring among experiments was the bulb skin color. This could be classified as either predominantly yellow-brown or red (ESM 1). In all cases where the bulb skin color was red, the flesh color was violet-white, whereas accessions with yellow-brown skin had green-white flesh. Of the 86 accessions scored, nine had red skin and the remaining accessions had yellow-brown skin. The red-skinned accessions were found in all Fennoscandian countries except Denmark.
We performed molecular genotyping using 12 microsatellite markers. The markers, originally developed for bulb onion, worked well with multiplying onions and yielded relatively consistent results with one or two deviating alleles when the same accessions were subjected to repeated genotyping.
Genetic diversity was calculated for accessions from each country as well as the group of modern varieties (Table 1). The highest diversity was found among the modern varieties, whereas accessions from Georgia were the most homogenous. For the four Fennoscandian countries, only small differences in within-country diversity were found. Genetic differentiation was calculated between groups of accessions from each country (Table 3). Pairwise comparisons between Denmark, Norway, and Sweden resulted in low and non-significant FST values, indicating low differentiation between accessions in these countries. In contrast, Finland, compared to the Scandinavian countries, showed higher and significant FST values. The accessions from Georgia were highly differentiated from all other groups. The modern variety group showed high FST values in comparison to all other groups except the accessions from Finland, suggesting a closer genetic relationship between the Finnish accessions and the modern varieties included in the study.
To visualize differentiation of accessions within and between countries and investigate links to morphology and nomenclature, we performed principal component analysis (PCA). This analysis again showed the clear differentiation of the homogenous group of accessions from Georgia from the Fennoscandian material. The modern varieties were well separated from each other and in many cases located away from the Fennoscandian onions, however with notable exceptions (Fig. 2a). Looking only at accessions from the Fennoscandian countries, a large group of genetically similar accessions was identified (Fig. 2b, group I). This group included several accessions from all four countries, in particular Denmark. Two minor groups with genetically similar accessions were also found (Fig. 2b, groups II and III) as well as several accessions that were more genetically unique. We further investigated whether the morphologically stable skin color trait was mirrored in the genetic analyses (Fig. 2c). Although the major cluster only contained yellow-brown skinned accessions, no clear clustering of red and yellow-brown accessions, respectively, was found. Similarly, genetic clustering was not associated with nomenclature, and the same cluster could contain both accessions called “shallot” and those called “potato onion.” On the contrary, the different clusters contained accessions with both names (Fig. 2d).
To geographically visualize distribution of genetic variability, we performed discriminant analysis of principal components (DAPC). In the model, we used ten different genetic groups as BIC values formed a plateau around this number (data not shown). The proportion of assignment to each group for each accession was plotted on a map at the location of the origin of the accession (Fig. 3). The most common group (light green) was frequently found in Denmark, especially Jutland, but also in Sweden and along the Norwegian coast. In addition, three of the Finnish accessions primarily belonged to this group. Another group (purple) seems particularly common in eastern Denmark and southernmost Sweden, but single accessions belonging to this group were also found in Norway, northern Sweden, and Finland. The most common Finnish group (brown) was also represented, with one accession in Denmark and one in Norway. The yellow group was only found in a few accessions from southernmost Sweden. Accessions including the dark blue and purple group were found in Norway and Finland, but not in Denmark or southern Sweden. Other minor groups had no obvious geographical distribution. In summary, although some geographical distribution patterns can be seen, many genetic groups are spread over all of Fennoscandia.
Since multiplying onions are mainly vegetatively propagated, a high degree of clonality among accessions was expected. We identified eight genotypes that were shared by two or more accessions. The most common genotype was found in 13 accessions, mainly from Denmark. The two second most common genotypes were each found in five accessions. The first of these two genotypes was represented by accessions from Denmark, Norway, and Sweden, and the second genotype by accessions from Denmark and Sweden and the modern variety ‘Success.’ All the three most common genotypes (comprising 23 accessions) were closely related to each other, differing only in single locus. They belong completely or mainly to the light green genetic group in Fig. 3 and are included in group I in Fig. 2b. The remaining shared genotypes were represented by two or three accessions. Noteworthy are the accessions NGB8315 and NGB17967 from Finland that have identical genotypes to the modern variety ‘Santé.’
A total number of 24 survey responses were received (48%). The majority of responses were from Denmark and Sweden. The replies are summarized in Table 4, but not all respondents replied to all questions. All respondents from Denmark and Norway responded they knew only “shallot” and therefore could not distinguish between the types “potato onion” and “shallot” (Table 4). Two of the Swedish respondents replied similarly (respondents SWE85 and SWE86) and another two that shallots and potato onion were the same (respondents SWE96 and SWE84). One respondent claimed that potato onion stores better than shallots (respondent SWE87), and another claimed that potato onion was more circular in shape and tasted better than shallots (respondent SWE89).
Regarding cultivation and phenology, most respondents used small- or medium-sized bulbs for cultivation (Table 4). Cultivation was done by planting in rows (at various distances) in spring. Harvesting was done in late summer when leaves were dry. No clear patterns were detected between donor taxon name and the size of bulbs used for cultivation or other cultivation practices. More than half of the respondents had never observed flowering plants among the onions (Table 4). Of the respondents that confirmed flowering, most claimed that this was occurring only at low frequency. One of the respondents (SWE82) reported that 1–2% of the onions planted flowered. Another respondent claimed that flowering occurred after early planting (respondent SWE89). No clear relationship was detected between donor taxon name and flowering. None of the respondents had ever obtained or used true seed for propagation.
The keeping ability varied from over the winter until more than 2 years (Table 4). Dry and cool storage conditions were mentioned as important by most respondents. A proper outdoor curing period prior to storing was mentioned by a few respondents as important to avoid decay (SWE82, SWE106). Storing on nets or in thin layers in boxes was reported (respondents NGB16552 and 16549). Others mention storing in the boiler room or cellar (SWE58, SWE89, SWE96).
For many of the respondents, the histories of the plants were highlighted, in some cases with a long story dating back at least 50 years. Often, the onions were given from one generation to the next within a family. When details were present, a mother to daughter line could often be detected (in Scandinavian “mormor”). Only in one case was a male mentioned (respondent NGB16555). The onions were used in a lot of different dishes and no clear pattern or country differences were detected.