Paternity, floral morphology and pollen viability analyses of African Baobab (Adansonia digitata L.) populations in South Africa

Abstract

The African baobab (Adansonia digitata L.) is a valuable flagship, cultural and socio-economic species for communities across Africa. Not only is the baobab an important medicinal plant, but products such as seed oils and fruit pulp are sold to supplement rural community income. Baobab trees produce hermaphroditic flowers, but fruit numbers vary between individuals, with some trees producing many fruit (50 –200) every season, termed producers, and other trees producing little to no fruit every season (< 5), poor producers. This difference is not due to environmental conditions such as rainfall, soil conditions or land use type as tree types occur in the same environment, sometimes only meters apart. The aim of this study was to evaluate how floral reproductive traits and the movement of pollinators between A. digitata producer and poor producer trees may affect the paternity of seed sets from producer trees. I measured seven floral traits of producers and poor producers to assess floral differences using one-way ANOVA between the tree types across three populations in the Vhembe region of South Africa. Floral traits included flower diameter, stamen ball diameter, filament length, anther length, stigma surface diameter, style length from stamen ball emergence and peduncle length. Producers had longer styles emerging from the stamen balls and longer peduncles, whereas poor producers had larger stamen balls, longer filaments, and larger anthers. There were no differences in flower diameter and stigma surface diameter between producer and poor producer trees. Similarly, canopy volume, width, and stem diameter revealed no difference between tree types. Additionally, I evaluated differences in pollen viability between producer and poor producer trees using Alexander’s stain. Although both tree types possess viable pollen, poor producers had nearly 50% higher percentage viability compared to producers. There appears to be some level of functional dioecy across baobab individuals as we found that trees either allocate resources predominantly to either male or female floral structures. Assessing the movement of alleles as a measure of pollen movement between trees may aid in our understanding of how this functional dioecy is influencing allele movement within the population. Eleven microsatellite loci were amplified to assess the paternity of 117 offspring from 13 ‘maternal’ fruit-producing trees across three A. digitata subpopulations in Vhembe, South Africa. v The six nearest potentially paternal trees to each maternal tree were genotyped to test whether they were pollen donors. Parentage analyses conducted using POLYGENE revealed that offspring of maternal trees had alleles from multiple genotyped pollen donor trees outside of their six nearest trees with selected paternal trees contributing pollen to multiple maternal trees. Analyses suggested that pollen dispersed an average of 300 m within subpopulations, but also repeatedly dispersed distances over 40 km between subpopulations. The duration of pollen viability was determined through pollen tube germination trials and aids in our understanding of how long pollen remains viable for in the environment. Pollen grains that were 24, 48 and 72 hours old were deposited on receptive stigmas and allowed to germinate overnight. Scanning electron microscopy was used to confirm pollen tube growth for the three ages of pollen; it indicated that the pollen remains viable for at least 72 hours as it successfully germinated on stigmatic surfaces across these three treatments. This indicates that pollen present in the environment, especially on the surface of pollinators (e.g., moths, bats, or beetles) has the potential to pollinate flowers when up to 72 hours old, although pollen that is older than 72 hours may still be viable, which requires further investigation. The degree of functional dioecy in individual baobab trees, proportion of viable pollen, and extensive potential range of pollen dispersal, all contribute to our knowledge of A. digitata fruit production. Understanding baobab fruiting may aid in future conservation and regeneration strategies and would improve agroforestry security of the species.