Reticulate Evolution Helps Explain Apparent Homoplasy in Floral Biology and Pollination in Baobabs (Adansonia; Bombacoideae; Malvaceae).

Baobabs (Adansonia) are a cohesive group of tropical trees with a disjunct distribution in Australia, Madagascar, and continental Africa, and diverse flowers associated with two pollination modes. We used custom-targeted sequence capture in conjunction with new and existing phylogenetic comparative methods to explore the evolution of floral traits and pollination systems while allowing for reticulate evolution. Our analyses suggest that relationships in Adansonia are confounded by reticulation, with network inference methods supporting at least one reticulation event. The best supported hypothesis involves introgression between Adansonia rubrostipa and core Longitubae, both of which are hawkmoth pollinated with yellow/red flowers, but there is also some support for introgression between the African lineage and Malagasy Brevitubae, which are both mammal-pollinated with white flowers. New comparative methods for phylogenetic networks were developed that allow maximum-likelihood inference of ancestral states and were applied to study the apparent homoplasy in floral biology and pollination mode seen in Adansonia. This analysis supports a role for introgressive hybridization in morphological evolution even in a clade with highly divergent and geographically widespread species. Our new comparative methods for discrete traits on species networks are implemented in the software PhyloNetworks. [Comparative methods; Hyb-Seq; introgression; network inference; population trees; reticulate evolution; species tree inference; targeted sequence capture.].

The African baobab (Adansonia digitata, Malvaceae): genetic resources in neglected populations of the Nuba Mountains, Sudan.

UNLABELLED: • PREMISE OF THE STUDY: Adansonia digitata L. is one of the most important indigenous fruit trees of mainland Africa. Despite its significance for subsistence and income generation of local communities, little is known about the genetic and morphological variability of East African populations of A. digitata, including those of Sudan. The aim of the current study, therefore, was to analyze genetic and morphological variability of different baobab populations in Kordofan, Sudan and to estimate the effect of human intervention on genetic differentiation and diversity.• METHODS: A total of 306 trees were randomly sampled from seven spatially separated locations in the Nuba Mountains, Sudan, to cover a wide range of differing environmental gradients and management regimes ('homesteads' and 'wild'). Genetic analyses were conducted using nine microsatellite markers. Because of the tetraploid nature of A. digitata, different approaches were applied to estimate patterns of genetic diversity. Investigations were completed by measurements of dendrometric and fruit morphological characters.• KEY RESULTS: Genetic diversity was balanced and did not differ between locations or management regimes, although tendencies of higher diversity in 'homesteads' were observed. A Bayesian cluster approach detected two distinct gene pools in the sample set, mainly caused by one highly diverse population close to a main road. The variability of tree characters and fruit morphometries was high, and significantly different between locations.• CONCLUSIONS: Results indicated a rather positive effect with human intervention. The observed populations provide a promising gene pool and likely comprise ecotypes well-adapted to environmental conditions at the northern distribution range of the species, which should be considered in conservation and management programs.

Radiocarbon dating of a very large African baobab.

In late 2004, Grootboom, probably the largest known African baobab (Adansonia digitata L.), collapsed unexpectedly in northeastern Namibia. Ten wood samples collected from different areas of the trunk were processed and investigated by accelerator mass spectrometry radiocarbon dating. The radiocarbon dates of three samples were greater than 1000 years BP (radiocarbon years before present, i.e., before AD 1950). The corresponding calibrated calendar age of the oldest sample was 1275 +/- 50 years, making Grootboom the oldest known angiosperm tree with reliable dating results. Variations in radiocarbon dates among the wood samples indicated that, morphologically, Grootboom was a quintuple tree, whereas genetically, it was a single individual. Ages of extreme lateral samples revealed that, over the past 500-600 years, Grootbooom had almost ceased growing, providing information about climate changes in central southern Africa. The sudden demise of Grootboom coincided with the spread of the poorly studied baobab disease, which has become epidemic in Namibia.

Patterns of genetic and morphometric diversity in baobab (Adansonia digitata) populations across different climatic zones of Benin (West Africa).

BACKGROUND AND AIMS: Baobab (Adansonia digitata) is a multi-purpose tree used daily by rural African communities. The present study aimed at investigating the level of morphometric and genetic variation and spatial genetic structure within and between threatened baobab populations from the three climatic zones of Benin. METHODS: A total of 137 individuals from six populations were analysed using morphometric data as well as molecular marker data generated using the AFLP technique. KEY RESULTS: Five primer pairs resulted in a total of 217 scored bands with 78.34 % of them being polymorphic. A two-level AMOVA of 137 individuals from six baobab populations revealed 82.37 % of the total variation within populations and 17.63 % among populations (P < 0.001). Analysis of population structure with allele-frequency based F-statistics revealed a global F(ST) of 0.127 +/- 0.072 (P < 0.001). The mean gene diversity within populations (H(S)) and the average gene diversity between populations (D(ST)) were estimated at 0.309 +/- 0.000 and 0.045 +/- 0.072, respectively. Baobabs in the Sudanian and Sudan-Guinean zones of Benin were short and produced the highest yields of pulp, seeds and kernels, in contrast to the ones in the Guinean zone, which were tall and produced only a small number of fruits with a low pulp, seed and kernel productivity. A statistically significant correlation with the observed patterns of genetic diversity was observed for three morphological characteristics: height of the trees, number of branches and thickness of the capsules. CONCLUSIONS: The results indicate some degree of physical isolation of the populations collected in the different climatic zones and suggest a substantial amount of genetic structuring between the analysed populations of baobab. Sampling options of the natural populations are suggested for in or ex situ conservation.

Baobab trees (Adansonia) in Madagascar use stored water to flush new leaves but not to support stomatal opening before the rainy season.

Baobab trees (Adansonia, Bombacaceae) are widely thought to store water in their stems for use when water availability is low. We tested this hypothesis by assessing the role of stored water during the dry season in three baobab species in Madagascar. In the dry season, leaves are present only during and after leaf flush. We quantified the relative contributions of stem and soil water during this period through measures of stem water content, sap flow and stomatal conductance. Rates of sap flow at the base of the trunk were near zero, indicating that leaf flushing was almost entirely dependent on stem water. Stem water content declined by up to 12% during this period, yet stomatal conductance and branch sap flow rates remained very low. Stem water reserves were used to support new leaf growth and cuticular transpiration, but not to support stomatal opening before the rainy season. Stomatal opening coincided with the onset of sap flow at the base of the trunk and occurred only after significant rainfall.