population has been founded by only a few individuals, and the geographic isolation may have increased
the distinction.
A real surprise is the second cluster from the dendrogram, O. aegyptiaca/O. oxyloba. These species
are the most closely related (GD 0.13). This is similar to the findings of Zeid et al. [31], who calculated
a genetic similarity of 68% between the populations of the same species in the same habitat. The results
may be due to the fact that we examined representatives of the second and third generations of the para-
sitic plants produced by autogamous reproduction, which by decreasing the amount of heterogeneity may
have contributed to the lesser difference. Yet another possibility is that because of the relatively close ge-
ographic habitats and easy seed dispersal some hybridizations between the species have occurred. That
hypothesis can be proved with further molecular genetic studies with the inbred lines that have been ob-
tained. However, the great similarity of O. oxylobatoO. aegyptiacaat the DNA level is a good reason to
consider this species an important “new player” in the game. This is also supported by the knowledge that
the mode of evolution of parasitic plants is by passing free-living stage, wild host, and crop host stages
[1,10,84].
IV. SOME ASPECTS OF THE EVOLUTION OF PARASITIC PLANTS
Within angiosperms, haustorial parasitism has evolved independently at least eight or nine times
[3,85–87].
Atsatt [10] considers it an answer to competition for limited resources. In nutrient-poor and arid habi-
tats parasites have competitive advantage over autotrophic plants because the host plant derives nutrients
not otherwise available.
Searcy [88] offers a molecular evolutionary hypothesisrelated to parasitic plants. He proposes that
parasitic plants would undergo distinct phases of evolution, each of which is composed of a multitude of
individual evolutionary events. During the firstphase, a free-living organism is involved in an initial par-
asitic or symbiotic relationship with the host. There would be a requirement for new or modification of
the existing genetic information because of the evolution of a functional haustorium and the crucial struc-
tural and physiological innovations in the parasites. At the secondphase, after the parasitic relationship
is established, natural selection would be relaxed on functions required of a free-living organism and the
genes for these functions would be free to accumulate random mutations. They would eventually be lost
from the genome, which would lead to reduction of its total size and locking the plant irreversibly into an
obligate parasitic relationship. A thirdphase of evolution would be possible as the obligate parasite
PARASITIC FLOWERING PLANTS OF GENUS OROBANCHE 793
TABLE 1 Genetic Distances Between Five OrobancheGenotypes, Calculated from the Presence and
Absence of 968 Amplification Products of six RAPD Primers
O. ramosaSpain (S) O. aegyptiaca O. oxyloba O. ramosaBulgaria (BG)O. aegyptiaca 0.717
O. oxyloba 0.753 0.133
O. ramosa,BG 0.246 0.687 0.746
O. ramosa, S 0.321 0.707 0.730 0.150
Figure 3 A dendrogram of five Orobanchegenotypes based on average linkage cluster analysis using RAPD
markers. Genetic distances between the species are indicated on the right of the dendrogram.