Abstract
Knowledge of rangewide variation in DNA content and ploidy level may be valuable in understanding the evolutionary history of a species. Recent studies of Acacia senegal report diploids and occasional tetraploids in the Sudano-Sahelian region of sub-Saharan Africa, but nothing is known about the overall extent of DNA ploidy variation within the species. In this study, we determine the DNA content and ploidy level of A. senegal across its native range, and explore whether the variation is related to its evolutionary and colonization history. We used propidium iodide flow cytometry (FCM) to estimate DNA content (2C value) and infer ploidy in 157 individuals from 54 populations on various tissues, using seeds, fresh leaves, dried leaves and twigs and herbarium specimens. The mean 2C DNA (pg ± s.d.) contents detected were 1.47 ± 0.09, 2.12 ± 0.02, 2.89 ± 0.12, and a single individual with 4.51 pg, corresponding to a polyploid series of diploid, triploid, tetraploid and hexaploid individuals. Diploids were confirmed by chromosome counts (2n = 2x = 26). Most populations (90.7 %) were of single ploidy level, while mixed ploidy populations (9.3 %) comprising mostly diploids (2x+3x, 2x+4x and 2x+6x) were restricted to the Sudano-Sahelian and Indian subcontinent regions, its northern range. The species is predominantly diploid, and no mixed ploidy populations were detected in east and southern Africa, its southern range. The geographic pattern of ploidy variation in conjunction with existing phylogeographic and phylogenetic data of the species suggests that polyploids have occurred multiple times in its evolutionary and recent colonization history, including contemporary ecological timescales. The successful use of external tissues of dried twigs in FCM is new, and presents the opportunity to study numerous other dryland woody species.
Highlights
Polyploidy, known as whole-genome duplication, is widely recognized as a major driver of evolution, diversification and speciation in angiosperms (Soltis et al 2009; Wood et al 2009; Robertson et al 2010)
Analysis of variance results showed that the 2C DNA value of diploid individuals was dependent on tissue type (F3,121 1⁄4 40.8, P, 0.0001); seed and fresh leaves were not significantly different according to Tukey’s post test (Fig. 3C), and were lower than dried leaves and twigs
Our main objective to survey and assess the rangewide distribution patterns of DNA contents and ploidy levels of A. senegal was dependent on the utility and reliability of the flow cytometry (FCM) methodological innovations we developed in order to circumvent the difficulties associated with collecting fresh leaf samples in the species’ native range
Summary
Polyploidy, known as whole-genome duplication, is widely recognized as a major driver of evolution, diversification and speciation in angiosperms (Soltis et al 2009; Wood et al 2009; Robertson et al 2010). Cytological, fossil and genomic studies suggest that polyploidy is ancient. Odee et al — Rangewide ploidy variation and evolution in Acacia senegal accompanied by ploidy increase (Otto and Whitton 2000; Wood et al 2009), this number is likely to grow as more species are studied. Novel genetic combinations arise that may confer polyploids with different reproductive, morphological, ecological, physiological or cytological characteristics from their progenitors, allowing them to exploit new environments (Kolarˇ et al 2009; Manzaneda et al 2012; Hao et al 2013)
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