Genetic differentiation and geographical structure
The coefficient of population differentiation with no account the
distances among haplotypes (Gst) and the coefficient of differentiation
based on the pairwise difference between alleles that takes into account
the distances among haplotypes (Nst) were found respectively, equal to
0.196 and 0.374; but the difference was not significant (P
> 0.05). In Citrullus spp.
Mujaju et al. (2011) found Gst = 0.56 and
Nst = 0.49 for sweet watermelon and Gst = 0.71, Nst = 0.81 for cow
watermelon. The fact that the differentiation parameter based on the
pairwise difference between alleles is greater than the one calculated
without permutation (i.e. Nst > Gst) indicates that the
collection is characterized by clear geographic structure
(Grivet, 2002;
Dane et al., 2007;
Guicking et al., 2011). Also the
significant value of the total gene diversity across all four
geographical regions (hT = 0.917, standard error = 0.0320) is indicating
a strong structure in the population (Pons
& Petit, 1996; Sun et al., 2019;
Zhao et al., 2019).
Levi et al. (2017) observed that
accessions of C. colocynthis were sub-divided into five groups in
general agreement with their centres of diversification and
origin. Our findings indicated that regional genetic
differentiation statistics support Levi et
al. (2017)’s conclusions, with sub-samples from different regions
exhibiting genetic differentiation associated with their likely centers
of diversification. Also, haplotypes of C. amarus were mostly
grouped in Southern Africa, which is assumed to be the origin of that
species (Dane & Liu, 2007;
Chomicki & Renner, 2015).
Citrullus chloroplast sequences analysis with TCS 1.21 resulted
in a network where haplotypes widely sampled throughout West Africa were
placed at the root. While coalescence theory predicts that older alleles
will prevail in a population due to a higher number of descending
lineages and associated wider geographic distributions
(Crandall & Templeton, 1993), such an
observation may depend on sample sizes and evolutionary/domestication
histories and also the lack of subs. cordophanus (from northeast
Africa) in the germplasm studied. In this study, H1 is the most
frequently sampled haplotype and has the most connections with other
haplotypes; thus H1 may be considered the most ancient haplotype. This
ancient haplotype was sampled most frequently in West Africa (i.e.
Nigeria and Benin) and was highly shared by accessions of both C.
lanatus and C. mucosospermus . These results support the findings
of Chomicki and Renner (2015) and
Renner et al. (2019) who used eleven gene
regions to infer phylogeny among Citrullus species, and also a
3500-year-old leaf sample from the Egyptian tomb to infer close
relationship between C. lanatus and C. mucosospermus . Our
findings, based upon a large set of egusi melon and watermelon
accessions from four continents, provide further evidence of that close
relationship between these two species. However, they are indeed two
different species, as previous crosses between them (e.g. Charleston
Gray x PI 560006) resulted in high levels of sterility
(Gusmini et al., 2004). The very limited
haplotype diversity among the two species suggests an old split, with
chlorotype fixation (Dane & Liu, 2007)
and ancient types of C . mucosospermus originating from
Western Africa (Renner et al., 2014).
However, to the best of our knowledge, no wild populations have been
confirmed in West Africa. Spontaneous plants may have been found
earlier, but those individuals certainly escaped from cultivation. A
region-wide collecting mission by the first author yielded no wild
population of C. mucosospermus in West Africa
(Achigan-Dako et al., 2015) though, the
presence in West Africa of the ‘neri’ type [Fig. 9f in
Achigan-Dako et al. (2015) and Fig. 1 in
Minsart et al. (2011)], another
cultivated egusi melon that exhibits smaller seeds with yellow soft
coat, should be highlighted as a contributor to the genepool ofCitrullus is the region. While this neri type (C. lanatus )
is morphologically distinct from C. mucosospermus , it has been
rarely studied.
Archaeological evidence indicates north-east Africa as a center of
origin and domestication (Chomicki et al.,
2020). Authors reported wild dessert watermelon in that region
(Paris, 2015) or the genetic affinity
with the C. lanatus var. cordophanus (a sweet
white-fleshed cultivar) (Renner et al.,
2019). However, within the genus Citrullus mucosospermus remains
the closest relative species to C. lanatus . The presence of an
ancient haplotype in West Africa on the one hand and the close
relationship between C. lanatus and subsp. cordophanus of
Darfur in north-eastern Africa as revealed by Renner et al. (2019) on
the second hand, calls for further molecular and archaeological
investigations to generate sufficient knowledge on newly published
results, including those reported here. New molecular investigations
should include more materials from Sudan and neighbouring countries
where wild populations of watermelon have been found
(Paris, 2015). Moreover, our data showed
that one of the Egyptian accessions (PI 525083), indicated to beC. amarus and observed by Levi et
al. (2013) to cluster with dessert watermelon, exhibits a unique
haplotype (H32). That accession is several mutations away from C.
colocynthis and closer to watermelon and egusi melon haplotype.
Previous findings of Levi et al. (2017)
showed that PI 525083 rather clustered with C. lanatus var.
lanatus . In addition, the hypothesis that watermelon is from
north-eastern Africa does not explain how an endemic species such asC. mucosospermus shares the same haplotype with dessert
watermelon, while other accessions from the region (e.g. PI 525083)
shows unique haplotype. If C. lanatus did indeed spread to the
world from west or north-eastern Africa, how and when was it
domesticated in those region as New Kingdom Egyptians were cultivating
sweet red-fleshed watermelon more than 3500 years ago? From which
species was C. mucosospermus domesticated? Through what
mechanisms was C. lanatus spread to Asia, and when? More
germplasm collections from all continents are necessary to fully
understand the phylogeographical relationships among Citrullusspecies. In Africa the focus should be on both west and north-eastern
regions to resolve the domestication history of modern cultivars.