Molecular Plant Breeding 2015, Vol.6, No.18, 1
-
8
4
candidate loci, like Tajima’s
D
(1989)
,
Fu and Li’s
tests (1993) and Fu’s
Fs
(1997). Tajima’s
D
‐
statistic
was computed for each locus and reflects the
difference between π and θ
W
. Fu and Li’s and Fu’s
Fs
neutrality tests were computed without outgroup.
For
TIP
and
PIP2
genes, allele frequency distribution
is indicated by the value of Tajima's
D
which was
positive, not significantly and different from 0. As
showed by Audigeos et al. (2010) positive Tajima's
D
value indicates a deficit of low frequency alleles
relative to neutral expectations in a randomly mating
population of constant size and could be the
consequence of population bottlenecks, population
subdivision or balancing selection as would be
expected in breeding populations. Fu and Li’s tests
and Fu’s
Fs
neutrality tests were positive for the two
genes suggesting a deficiency of recent mutations and
a lower number of haplotypes than expected. Fu’s
F
test was negative for
PIP2
gene (-0.129) indicating
that the departure from neutrality is only apparent.
Interpretation of the neutrality tests is difficult since
many recombination events could occur in natural
populations. Fu (1996) note that recombination events
reduce the variances of nucleotide genetic diversity
estimators and increase the number of alleles in a
sample. Accordingly, further research on the genetic
diversity of Tunisian olive by SNP analysis related to
drought stress is needed to investigate other Tunisian
hybrids and rootstocks. The attainment of better levels
of discrimination of olive cultivars related to their
environment requires the testing of more candidate
genes and more primers via classic sequencing
methods as well as using NGS platform.
2.3 Genetic relationships
In order to analyze the genetic relationships among
studied cultivars for the two genes, dendrograms were
constructed based on the UPGMA cluster analysis of
SNP data. The clustering patterns obtained are given
in Figures 1–2.
The analysis of the SNP data for
TIP
gene revealed a
clear separation of cultivars into 3 groups by cutting
the dendrogram at a genetic similarity value of 0.17,
while the dendrogram in
PIP2
gene separate cultivars
in two clear separate groups, with greatest separation
of cultivars. Our results failed to differentiate cultivars
into separate clusters according to their susceptibility
to drought (resistant vs. susceptible). No further
discrimination was, however, visible among the
different varieties to their geographical origins (south,
north or center of Tunisia) and environmental ranges
for the two genes. However, an overlap between
groups is reported. Thus, we note the clustering of
some cultivars from the north in group composed by
southern varieties was showed in the two data.
Moreover, it could be found that cultivars from
different regions and different susceptibility to water
scarcity such as ‘Zarrazi Zarzis’, ‘Fokhari’, ‘Chamlali
Sfax’, ‘Chamlali-Jerba’, ‘Zalmati’ and ‘Toffehi’ which
are “resistant” and ‘Chétoui’ and ‘Neb-Jamal’ are
“sensitive” were clustered together. These results
show a high genetic variation and high degree of
polymorphism in Tunisian olive cultivars. Comparable
results were found by Ercisli et al. (2011), who shown
that Turkish olive cultivars exhibit a high genetic
diversity by using SSR molecular markers techniques.
Meanwhile, some studied cultivars originated from the
southern of Tunisia considered as “medium resistant”
cultivars were clustered with cultivars from the north.
Probably, amplified
TIP
and
PIP2
loci were not
closely associated with major loci controlling drought
resistance in olive. These results were in agreement
with catalase-based markers applied in apple genetic
analysis and PCR-based strategy failed to discriminate
resistant and susceptible apple accessions to
scab
and
powdery mildew
responses (Gulsen et al. 2010).
The results presented here were the first report on
molecular diversity in aquaporines genes of
commercially relevant species of one of the most
diverse in Mediterranean countries. AQPs-based
markers may be applied in Tunisian olive germplasm
genetic diversity analysis. The fragments of
TIP
and
PIP2
genes analyzed here have revealed moderate
large variability with patterns that vary along the gene.
Our results demonstrated that genetic variation at
studied aquaporin genes may represent a response to
variable environmental conditions. Knowledge about
genetic relationships between Tunisian olive cultivars
could be helpful in olive improvement strategies and
could facilitate association mapping in olive. An
extended characterization of these loci and other loci
involved in drought stress could provide more
information on the link among genetic diversity,
environmental conditions and adaptive traits related to
water stress.
