Population differentiation
Population differentiation was estimated using Weir and Cockerham’s (1984) FST (Table 2). FST estimates indicate a degree of separation among populations where 0 indicates that populations are sharing genes to 1 indicating complete isolation. The overall population FST was estimated at 0.0522 indicating some degree of structure among populations, supported by an exact test using the G-statistic (population P = 0.002, population nested within host P = 0.01). Pairwise FST values ranged from a low of 0.0162 (adjacent populations TH, TT: Figure 1) to a high of 0.1193 between geographically separated populations (AT, BM: Figure 1). Interestingly, the most geographically separated populations, AT and AA, appeared less genetically differentiated with an FSTof 0.0858. Comparing pairwise FST estimates across crop hosts also indicated genetic differentiation (Table 3). Here the lowest, 0.0162 between tomato (TT) and chickpea (TH), and the highest, 0.1193 between carrot (AT) and pea (BM), are confounded by short geographic distance. In contrast, FST between O. crenatagrowing on pea (BM) and chickpea (TH) was the second highest at 0.1135. Within faba bean host samples (AA, AD, AS, BE, TB, TC), the pairwise FST values ranged from 0.0216 to 0.0354 indicating that there was less genetic differentiation within faba bean hosts than across all samples.
Nei’s genetic distance (Nei 1972) was calculated and used to construct a neighbor joining tree (Figure 2) which grouped samples by host plant with strong bootstrap support (99-100%). Intriguingly, while carrot (AT) is a geographic outlier, tomato (TT) and chickpea (TH), which were collected in adjacent fields, were split with 100% bootstrap support. Faba bean host samples were split into 4 groups with moderate bootstrap support (95%). Within this grouping support for differentiation by location varied (74.3-100%).