Abstract
Ochnaceae is a pantropical family with multiple transoceanic disjunctions at deep and shallow levels. Earlier attempts to unravel the processes that led to such biogeographic patterns suffered from insufficient phylogenetic resolution and unclear delimitation of some of the genera. In the present study, we estimated divergence time and ancestral ranges based on a phylogenomic framework with a well-resolved phylogenetic backbone to tackle issues of the timing and direction of dispersal that may explain the modern global distribution of Ochnaceae. The nuclear data provided the more robust framework for divergence time estimation compared to the plastome-scale data, although differences in the inferred clade ages were mostly small. While Ochnaceae most likely originated in West Gondwana during the Late Cretaceous, all crown-group disjunctions are inferred as dispersal-based, most of them as transoceanic long-distance dispersal (LDD) during the Cenozoic. All LDDs occurred in an eastward direction except for the SE Asian clade of Sauvagesieae, which was founded by trans-Pacific dispersal from South America. The most species-rich clade by far, Ochninae, originated from either a widespread neotropical-African ancestor or a solely neotropical ancestor which then dispersed to Africa. The ancestors of this clade then diversified in Africa, followed by subsequent dispersal to the Malagasy region and tropical Asia on multiple instances in three genera during the Miocene-Pliocene. In particular, Ochna might have used the South Arabian land corridor to reach South Asia. Thus, the pantropical distribution of Ochnaceae is the result of LDD either transoceanic or via land bridges/corridors, whereas vicariance might have played a role only along the stem of the family.
Highlights
Dispersal is amongst the most important biogeographical mechanisms shaping the distribution of plant species
The plastid dataset with gap-based trimming generally inferred slightly younger age estimates for the selected 14 clades compared to the un-trimmed plastid dataset
The infinite-sites plots (Supplementary Figure 4) reveal a stronger correlation between the posterior mean ages and the width of their 95% highest posterior density (HPD) for the nuclear dataset (R2 = 0.85) than for the plastid datasets (R2 = 0.76 for the untrimmed and R2 = 0.74 for the trimmed), indicating that adding more DNA sequence data is less likely to increase the precision of posterior age estimates in the nuclear relative to the plastid datasets, all three would still benefit from more sequence data
Summary
Dispersal is amongst the most important biogeographical mechanisms shaping the distribution of plant species. If vicariance fails to explain transoceanic disjunctions, such distributions can be attributed either to dispersal via land connections between continents or direct dispersal across oceans The latter may have been facilitated by island chains between continents (stepping-stone dispersal; e.g., Morley, 2003; Harbaugh and Baldwin, 2007; Kainulainen et al, 2017; Ahlstrand et al, 2019) and can be achieved by transport of diaspores in water (e.g., Gallaher et al, 2015), by wind (Muñoz et al, 2004) or animals (e.g., Nogales et al, 2012; Viana et al, 2016) or by rafting of seeds or plants on floating islands (Renner, 2004; van Duzer, 2004). More data are needed to reveal potential patterns across time strata and regions
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