Genetic Structure in Tunisian Apricot, Prunus armeniaca L., Populations Propagated by Grafting: A Signature of Bottleneck Effects and Ancient Propagation by Seedlings

Abstract

In order to give insights into the origin and historical selection process of Tunisian apricot propagated by grafting, 31 cultivars from three areas presenting contrasting ecological conditions -Kairouan, Testour and Ras Jbel were compared to cultivars from Europe, North America, North Africa, Turkey, Iran and China, using 234 AFLP markers. The phenetic analysis allowed to distinguish 5 clusters, the four previously defined groups: -'diversification', 'geographically adaptable', 'continental European' and 'Mediterranean' -groups and the Tunisian one. The partitioning of genetic diversity within and between cultivar groups assessed according to the Bayesian approach and assuming Hardy-Weinberg equilibrium, showed a loss of 21.81-38.49% of genetic diversity in Tunisian apricot compared to Mediterranean and diversification groups, respectively. Genetic variation occurred within Tunisian subgroups rather than among (F ST = 0.060) evidencing a narrow genetic pool. Mediterranean and Tunisian groups were the least differentiated. Comparing them, 24 AFLP fragments discriminated the Mediterranean group from the Tunisian group but most of them where also shared by the other groups. Strongly differentiated gene pool and low genetic diversity are probably the result of bottleneck effects linked to the occurrence of propagation by seedlings rather than by grafting during the introduction periods in the North and the Centre of Tunisia. This study points at the propagation by seedlings as an important factor which should be taken into account to understand the evolution of apricot in South Mediterranean areas.

Genetic relationships between local North African apricot (Prunus armeniaca L.) germplasm and recently introduced varieties

“…As expected, we noted a highly significant proportion of admixed accessions derived from seed propagated populations (68.3%) since gene flow can only occur when farmers propagate their apricots through seedlings. However, we observed a similar proportion of admixed accessions among graft (24%) and seed propagated apricots (21.7%), thus supporting the assumption that graft and seed propagated apricots share a common gene pool, as previously suggested in Tunisia (Khadari et al, 2006;Bourguiba et al, 2010) and the Maghreb region (Bourguiba et al, 2012a).…”

Impact of Mapped SSR Markers on the Genetic Diversity of Apricot (Prunus armeniaca L.) in Tunisia

“…Thus, the Tunisian traditional apricot cultivars showed a relatively high level of polymorphism as compared with the Mediterranean materials. Moreover, our results confirmed the presence of a decreasing gradient of genetic diversity from East to West the Mediterranean area, which may be explained by bottleneck effects related to historical introduction and apricot propagation in Tunisia by numerous seedling events (Hagen et al 2002;Khadari et al 2006).…”

“…These results suggested that the genetic background of cultivars from the northern, central, and southern areas of apricot culture in Tunisia was close. The same results were revealed also by Khadari et al (2006), which showed that the Tunisian apricot cultivars represented a distinct group with a narrow genetic basis.…”

Loss of genetic diversity as a signature of apricot domestication and diffusion into the Mediterranean Basin

“…Such a loss of genetic diversity was significant when comparing clusters 1 (‘Iran-Caucasian’) and 3 (‘North Mediterranean Basin’) and clusters 1 and 4 (‘South Mediterranean Basin’). Nevertheless, it was not significant when cluster 3 was compared to cluster 4, despite the different modes of apricot propagation: vegetative in North Mediterranean vs. sexual reproduction in South Mediterranean, especially in the oasis agroecosystems in the Maghreb area [33,34]. …”

“…In North Africa ( Algeria, Morocco, and Tunisia), apricot germplasm contained accessions propagated by grafting, but also by seeds specifically located in oasian regions. A fine-scale genetic diversity study conducted using AFLP markers at the within-population level, focusing on Tunisian grafted apricot cultivars, supported the assumption of few introduced genotypes that have been firstly propagated by seeds [33]. Analysing a larger set of Tunisian apricots, including both vegetatively propagated cultivars and seed propagated accessions, Bourguiba et al [34] identified two main gene pools according to their propagation mode and confirmed the assumption that these two gene pools shared the same origin.…”