Population structure and host differentiation
Population differentiation by geographic distance was more evident across crop hosts and was lowest between tomato and chickpea in adjacent populations (TT& TH, FST = 0.0162) and highest between carrot and pea in distant populations (AT& BM, FST = 0.1193). These data are consistent with the AMOVA results (Table 4) supporting a role for an interaction between host and location partitioning genetic variance among populations.
Clustering analyses supported the existence of 4 clusters based on genetic differentiation by host/population. Cluster 1 contained predominantly populations from “Tipaza-Hadjout” (90 m altitude / 635 mm annual rainfall / 18.5°C annual average temperature) harvested on chickpea (TH) and tomato (TT) samples. Cluster 2 is made exclusively of carrot samples all from the population (AT) of “Ain Temouchent” (235 m alt. / 485 mm ann. rainfall / 17.4 °C ann. average temp.). Cluster 4 grouped pea samples, all from the population (BM) from “Blida-Mouzaia” (120 m alt. / 684 mm ann. rainfall / 18.1°C ann. average temp.). Cluster 3 and cluster 4 shared populations harvested on faba bean (AA, AD, AS, BE, TB, TC), almost all from the coastline (Except of AD: 1 to 41m alt. / 619 to 739 mm ann. rainfall / 17.7 to 18.7°C ann. average temp.).
This number of clusters might suggest that the sampled O. crenatapopulations could derive from four genetically different gene pools. However, overall, populations maintain close genetic relatedness, as shown by the low differentiation among them, which might support the idea of host specialization and adaptation to local ecological conditions from a single Algerian introduction rather than different introduction events. The AT population is a striking example. The most geographically separated populations AT and AA (432 km straight line distance), expected to have the highest FST value, appeared less genetically differentiated with an FST of 0.0858. This outcome may fit the hypothesis that both populations could have evolved from the same origin and that AT was further locally adapted. This also implies the possibility of spread of O. crenata seeds from northeast to southwest. This direction seems most plausible since the oldest infestations were reported in the central part of the Algiers coastline (Sahel of Algiers), which is consistent with the study of Aouali et al. (2007) that suggested the center of dissemination of O. crenata might be the region of ‘Mitidja’.
In addition, from data in Table 2, populations AT and BM appeared to share fewer genes with all other populations as they present in general the highest pairwise FST values. It is worth noting that the geographical situation of the population in the Blideen Atlas (BM) contrasts with that of the majority of the populations located on the coastal plains. This situation could have eventually allowed isolation of this population. Hence, it is very likely that host-based genetic differentiation is due to a host specialization process driven by either selection pressure for agronomic traits, or a co-accommodation between host and parasite accentuated by specific local ecological and geographical factors. However, more populations from more locations are needed to better understand how and when host genetic differentiation occurs.
Several authors have investigated host differentiation among parasitic weeds, with sometimes contrasting results. Aouali (2005) could not find evidence of host-differentiation among eight O. crenatapopulations parasitizing faba bean, chickpea, pea and carrot in the Mitidja plain in Algeria using RAPD and RFLP markers. Similarly, Ennami et al. (2017 b) did not detect host-specialization among O. crenata accessions from 3 regions in Morocco collected from faba bean and lentil hosts using RAPD markers. Conversely, evidence of host-differentiation has been found in other parasitic weeds, such asO. foetida (Román et al. 2007b; Vaz Patto et al., 2008),Striga hermonthica (Unachukwu et al. 2017) and Phelipanche ramosa (Stojanov et al. 2019). These differences may be due to specific methods / sampling design or may arise from the type of genetic marker selected, as single locus co-dominant markers are more efficacious for population biology insights (Sunnucks 2000).
The present study provides relevant information that may benefit future breeding programs and management practices aimed at bolstering resistance against this parasitic weed. Other aspects that are worth further investigation may include cross-infestation experiments to ascertain host preferences and specialization. Also, it would be interesting to study genetic interactions between wild and weedy forms of O. crenata . In Algeria, the host range of O. crenataincludes both cultivated and wild plant species belonging to at least eight families. Host crops include : Carthamus tinctorius, Cicer arietinum, Daucus carota, Lactuca sativa, Lathyrus sativus, L. ochrus, Lens esculenta, Lupinus sp., Pisum sativum, Solanum lycopersicum, Vicia faba and V. sativa . Wild hosts include: Dipsacussp., Geranium sp., Lathyrus odoratus, Medicago hispida, Pichris echioides, Plantago lanceolata and Trapaeolum majus . While not yet investigated, host-relationship studies between wild and weedy O. crenata populations in Algeria would provide useful insights since wild vegetation may act as a reservoir of genetic diversity for overcoming genetic resistance mechanisms in host crops (Pineda-Martos et al. 2014).