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.