High genetic variation and low gene flow among populations of Viscum verrucosum in semi-arid savanna, southwest Zimbabwe
Introduction
Mistletoes, through high transpiration rates, high litter nutrient concentrations, and extended periods of litterfall, act as ecosystem engineers and keystone species [1] altering host performance, soil moisture content, litterfall rates, and nutrient availability [2], [3], [4], which can ultimately influence local plant biomass, species composition and spatial distribution (e.g. grasses, [5], [6]. These changes to the understorey properties, together with additional mistletoe resources such as nutrient-rich foliage, flowers, fruits and nectar in infected host canopies, also alter the structure and function of animal assemblages (e.g. insects, [7], [8]; birds [9], [10]). However, despite the significant role played by parasitic plants in ecosystem structure and function, little is known about their genetic diversity.
In semi-arid savanna southwest Zimbabwe, woodlands are under increasing disturbance due to human activities such as deforestation and agricultural expansion. This has resulted in highly fragmented woodlands, with the remaining undisturbed woodlands occurring on private lands and conservation areas. In addition to reducing the size of woodland patches, fragmentation is expected to increase the geographical distance between woodland patches [11]. These changes are likely to affect gene flow between populations of both host trees and associated mistletoes, and consequently the genetic structure and diversity of mistletoes may be altered. Additionally, together with woodland fragmentation, the patchy distribution of mistletoes [12], [13] may further enhance isolation and increase genetic differentiation between populations [14]. However, studies on the genetic structure and diversity of mistletoe populations, and contributing factors are lacking in subtropical savanna.
Viscum is a genus of mistletoes represented by at least 17 species in southern Africa, including Viscum verrucosum [15]. Viscum verrucosum Harv. (Viscaceae) forms a leafless aerial shrub 0.5–1 m tall in host trees [16]. Flowering occurs in the dry season, mainly September–October, but fruits found throughout most of the year. Viscum verrucosum is found in mixed woodlands and bushlands at altitude ranging between 500 and 1 550 m. Host range in V. verrucosum is narrow, preferring high nutrient hosts mainly Vachellia species, but also on Ficus, Combretum and Syzygium species [17]. In addition to providing nutritive and highly digestible dry-season browse for livestock in savanna rangelands [18], [19], V. verrucosum also contributes significantly to carbon and nutrient cycling in semi-arid savanna [20], [21], [22].
In this study, we analyzed the patterns of random amplified polymorphic DNA (RAPD) variation of Viscum verrucosum populations in semi-arid savanna, south-western Zimbabwe. Our main study objectives were to: (i) determine genetic variability among and within V. verrucosum populations and (ii) examine the relative role of geographical distance in determining the patterns of genetic variation among populations.
Section snippets
Study species and field sampling
Fresh material was collected from adult Viscum verrucosum Harv. (Viscaceae) plants growing on Vachellia karroo Hayne. in three locations (with geographical coordinates, elevation and area): Bulawayo (20°08ʹS, 28°36ʹE; elevation 1341 m; 160 ha), Matopo Research Station (20°23′S, 28°28′E; elevation 1340 m; 500 ha), and Dambari Wildlife Trust (20°25′S, 28°77′E; elevation 1340 m; 21 ha), all in semi-arid savanna, southwest Zimbabwe (Fig. 1). Viscum verrucosum does not have leaves [16], and
Genetic diversity
Levels of genetic diversity were high at the mean population level (Table 2a). The percentage of polymorphic loci (P) averaged 77.19% with a range from 63.16% (Matopo), 78.95% (Bulawayo) to 89.47% (Dambari) (Table 2a). The mean number of alleles per (A) locus was 1.58 ± 0.11, while the effective number of alleles (Ae) was 1.53 ± 0.05. The mean Nei's genetic diversity (h) for individual populations was 0.30 ± 0.03 on average, ranging between 0.22 and 0.35 (Table 2a). Bulawayo and Matopo had high
Discussion
In the present study, RAPD markers used to assess the genetic diversity within populations of V. verrucosum showed that the species has high levels of genetic diversity (P = 77.19%, A = 1.58, h = 0.30, Hʹ = 0.44). The genetic diversity detected here is high relative to other mistletoes such as V. album L [14], [31]. Because the small seed size facilitates fast handling by birds, smaller fruited Viscum species are dispersed by a wider variety of birds [32], including mousebirds with relatively
Conclusions
In conclusion, our findings revealed high levels of genetic diversity in V. verrucosum populations, also with low gene flow (less than one migrant per generation) and high genetic differentiation between populations. As a result of the high genetic differentiation, the majority of the variation was retained within than among populations. Finally, the data fit isolation-by-distance model as genetic distance and geographic distance were correlated suggesting that gene flow between populations was
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgements
Special thanks to the anonymous reviewers for their comments on earlier versions of this manuscript that allowed us to increase the quality and clarity of our work. We are also grateful to Dambari Wildlife Trust and Matopo Research Station for giving us permission to collect plant samples in their properties. The Department of Applied Biology and Biochemistry is acknowledged for providing laboratory facilities and their staff for assisting with laboratory work. Map of the study area was
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