Genetic Differentiation of Populations
In our study, the observed genetic differentiation (FST)
between pairs of O. crenata populations by collection location
varied from 0.0076 to 0.1193 (Table 2) and was lower between adjacent
populations and higher between geographically separated ones. Low values
indicate a lower level of genetic differentiation, consistent with gene
flow between populations. In this study, populations with low
FST are geographically close and are located in five
contiguous districts (Algiers, Tipaza, Ain Defla, Blida, Boumerdes).
This region is known for intensive vegetable cultivation where exchange
and/or sharing and trading of agricultural material (crop seeds and
seedlings, manure, machinery, etc.) are common practices. These
practices may have supported seed migrations and resulted in apparent
gene flow among populations. Results from the AMOVA analysis (Table 4)
also support the possibility of high rates of gene flow between
locations since the majority of total variation (63%) was found within
individuals and low average FST values between
populations, suggests very little population differentiation. These
populations are therefore genetically close and likely evolved from the
same source.
Previous studies in O. crenata populations from Morocco (Ennami
et al. 2017a), Spain (Romàn et al., 2001; 2002) and Egypt (Abdalla et
al., 2016) also reported a clear genetic variation at the
intra-population level and only little differentiation among
populations. According to Musselman (1986) and Romàn et al. (2002) these
results are expected considering the predominantly allogamous behavior
of O. crenata and the extremely efficient dispersal of its seeds.
Conversely, high FST values suggest reduced gene flow
between populations leading to population differentiation. In our study,
the highest FST values (0.0858 to 0.1193) were recorded
for the most geographically distant populations, in particular that of
Ain Temouchent (AT), which is the farthest collection location. Genetic
differentiation among populations is expected to increase with increased
geographic distance (Slatkin, 1993). Analysis of the pairwise genetic
differentiation in our study suggests that overall, genetic distances
between populations increased proportionally with geographic distance,
this was also the conclusion of Aouali et al. (2007) regarding O.
crenata populations from the plain of Mitidja in northern Algeria.
However, a Mantel test revealed no support for Isolation By Distance
(IBD) was found across these populations overall. Nonetheless, the Ain
Temouchent population (AT) remained an outlier driving the linear
relationship with geographic distance (Figure 3). Overall, estimates of
FST are consistent with other studies of parasitic weeds
and suggest that host and geography are important factors shaping
genetic differentiation in O. crenata (Román et al., 2002, 2007
b; Stojanov et al. 2019).
Results from prior research support the observation that
inter-population differentiation is likely to be detected between
distant countries rather than within countries (Satovic et al. 2009). In
explaining what could be responsible for this trend, Romàn et al. (2001,
2002) suggested that geographic distance provides a substantial barrier
to gene flow as long as there is no commercial exchange of host seeds
between the regions; whereas within a country migration forces between
populations are continuous and strongly favored by an efficient
dispersal of the parasite seeds via humans, machinery, animals or wind,
as well as on host seeds.