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%).