Genetic diversity and sequence variation
Within the genus Citrullus genetic diversity analyses have been
investigated since the second middle of the 20thcentury (Hashizume et al. 1996) revealing
various trends. Previous knowledge revealed lower genetic diversity inCitrullus for breeding purpose
(Levi et al., 2001;
Levi et al., 2004). Recent studies shed
light on obvious genetic diversity within the genus. For instance, a
study using High Frequency Oligonucleotide Target Active Genes
(HFO-TAGs) revealed high genetic diversity among Citrullus spp.
and highlighted the potential importance of PI accessions as sources of
valuable traits like disease resistance
(Levi et al., 2013).
Our findings revealed low cpDNA variability among C. lanatus andC. mucosospermus . This was also observed by
Dane and Lang (2004) and
Dane et al. (2004) who revealed low
nucleotide variability based on a low number of parsimony-informative
sites within each of the studied species. Most haplotypes were found
within non-cultivated (C. colocynthis ) rather than cultivated
(C. lanatus and C. mucosospermus ) species. Taxa were
highly separated from one another with divergence based mainly on indels
and transition events (Dane et al.,
2004). However, there was sufficient resolution of the trn T-L
and ndh F-rpl 32 non-coding regions to reveal intraspecific
variability.
Chloroplast sequence analysis revealed that the ndh F-rpl 32
region exhibits comparatively higher variability within the two
cultivated species than the trn T-L region.
Dane and Lang (2004) analyzed four
chloroplast regions (nhd F, ycf 6-psb M,ycf 9-trn G, and atp A-trn R) and found no
variability within cultivated accessions, grouped either by
morphological traits or geographical origin. In this study, we used a
large number of C. lanatus accessions from a wide geographical
range and observed low haplotype diversity within that species, as also
revealed by Guo et al. (2013). While many
factors can influence sequence diversity, selection is a major
contributor via the imposition of bottlenecks that can substantially
reduce diversity (Dane & Lang, 2004;
Levi et al., 2013). The lack of haplotype
divergence within C. lanatus and C. mucosospermus is
likely the result of selection or other bottlenecks in the domestication
histories of watermelon and egusi melon. Certainly, selection for sweet
red-fleshed cultivars with high lycopene content or selection of seed
type as source of protein/oil for consumption might contribute to
current genetic structure in those cultivated species
(Achigan-Dako et al., 2015;
Renner et al., 2019).
C. colocynthis exhibited a relatively high number of
parsimony-informative characters. Dane et
al. (2004) revealed that haplotypes detected within C.
colocynthis were associated with geographical origin and that was also
confirmed by Levi et al. (2017). The
haplotype diversity within C. colocynthis suggests cryptic
evolution and calls for a comprehensive morphological comparison of
Asian and African colocynths. Such an investigation is exemplified by
the recent studies on Cucumis melo that revealed modern melon
cultivars go back to two lineages and was domesticated at least twice:
in Asia and in Africa (Endl et al.,
2018).
Citrullus haplotype evolution
Thirty-eight haplotypes were detected among the cultivated and wildCitrullus accessions used in this study.
Dane et al. (2004), found seven
haplotypes within the genus, using 55 accessions of C. lanatus,15 accessions of C. colocynthis , and a total of seven cpDNA
regions (HinfI, RsaI, TaqI, AluI, HaeIII, MboI, and BglII). With two
cpDNA regions and 135 accessions carefully selected to represent a wide
geographical region, we detected an even higher haplotype diversity
among Citrullus spp. This situation can be expected to continue
to evolve as more watermelon accessions from Sudan or northeast Africa
are sequenced, particularly, the Sudanese sweet white-fleshed melon.
Unfortunately, sampling of C. lanatus from the Darfur region of
Sudan has been scarce (Renner et al.,
2019).
On average, we observed 9.5 haplotypes per species, varying from 5 to
16. In comparison with other species,
Guicking et al. (2011) found 9.8
haplotypes per species in Macaranga and
Jakob and Blattner (2006) found 2.83
haplotypes per species in Hordeum . In Citrullus spp.,
nucleotide substitutions appear to have evolved at different rates, an
observation supported by the Fisher’s test for homogeneity of nucleotide
substitution. Fu’s test Fs also rejected the null hypothesis of
neutrality of evolution of nucleotide substitution, further supporting
the hypothesis that the polymorphism pattern observed is non-random.
Population expansions tend to produce significantly negative values of
D, while population bottlenecks tend to produce significantly positive
values of D. In our case the departure from neutrality might indicate
that there is a high demographic expansion and a pattern of isolation by
distance would be occurred between the continents
(Jiang et al., 2016).