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Figure 4. Dendrogram of 16 wild Brassica populations from the distance matrix described in Table 1 and the UPGMA algorithm.

a population but does not describe the organization of presented an excess of heterozygotes.This could be
the variation in the population. For example two explained by assuming the presence of subpopulations
populations of B. incana are very different for H or competition phenomena in a population as well as
values (0.23 pop14, 0.09 pop15) but their allelic extensive gene exchanges (Brown 1979) and also
composition is very similar; on the contrary B. incana hypothesizing the self-incompatibility systems, often
(pop14) B. villosa subsp. bivoniana (pop2), B. rupes- found in many Brassica populations (Watanabe and
trus subsp. hispida (pop13) have similar values (0.23, Hinata 1999). The presence of rare alleles could also
0.24 and 0.21 respectively) but a very different allelic have an influence on the number of heterozygotes,
composition. because the probability to find them as heterozygotes
Several factors have been associed with differences is higher than to find them as homozygotes. For two
in values of polymorphism (P), allelic number (A), populations, pop4 (B. villosa subsp. drepanensis) and
gene diversity (H) such as life form, regional dis- pop14 (B. incana), an excess of homozygous indi-
tribution, geographical range, breeding system (Ham- viduals was found. For these populations, the gene
rick et al. 1979; Hamrick 1990). According with these exchange seems restricted either by geographical
parameters, high values are generally associed with isolation or by biological process.
mixed system of reproduction between autogamy and Gst values could be related with breeding system
allogamy; low values are found in autogamous and in and seed dispersal mechanism (Loveless and Hamrick
endemic plants. In our case, we must keep in mind 1984). In Brassica populations investigated, mean
that three species (B. macrocarpa, B. rupestris, B. values of Gst were comprised between 0.30–0.37.
villosa) are endemic, some populations (as pop5) are Therefore a breeding system mixed between au-
constitued by very few individuals and other popula- togamy and allogamy (probably prevailing because of
tions as B. rupestris subsp. hispida (pop12) and B. self-incompatibility) and a seed dispersal-mechanism
macrocarpa (pop16) that are larger in size, are iso- for gravity or provoked by animals could be hypoth-
lated from the geographic point of view. esized.
Considering the fixation index, a great number of The same structure of the diversity was observed
populations, in particular large sized populations, for each species with the intra-populational diversity