Abstract:
Two forms of A. halimus shrubs: erect habit (A. halimus) and bushy habit shrub (A. schweinfurthii)
are used naturally isolated by a considerable distance from each other and occupy the same area.
To explore the effect of natural isolation on the genetic basis of the two forms, Start Codon Targeted
(SCoT) and the phylogenetic relationships of A. halimus by sequencing ITS1-5.8S-ITS2 regions
of the ribosomal DNA are used. Significant isolation-by-distance relationship was found (r =
0.62, P = 0.001). Soil factors did not influence molecular variations. The natural isolation of A. halimus
habitats restricts gene flow among the populations and the observed high within-population
genetic diversity (74.19%) in this species is best explained by its outcrossing behaviour, longlived
individuals and overlapping generations. The UPGMA analysis of the SCoT results showed
that all the studied populations were divided into two discrete genetic groups with significant separation
of the two forms in Burg El-Arab area (Populations 1 and 2) and insignificant separation
between two forms in El-Hammam area (population 5 and 6). The sequencing of the ITS1-5.8SITS2
rDNA regions also showed the insignificant separation of the two A. halimus forms. We conclude
that gene flow depending on habitat fragmentation was the main factor affecting the population
genetic differentiation. We suggest that the two forms do not merit specific rank in presence
of interference between the two forms and absence of a breeding barrier fail to separate the different
populations when they become sympatric.
