Nuclear microsatellite variation in Malagasy baobabs (Adansonia, Bombacoideae, Malvaceae) reveals past hybridization and introgression

The rise of baobab trees in Madagascar

The baobab trees (genus Adansonia) have attracted tremendous attention because of their striking shape and distinctive relationships with fauna1. These spectacular trees have also influenced human culture, inspiring innumerable arts, folklore and traditions. Here we sequenced genomes of all eight extant baobab species and argue that Madagascar should be considered the centre of origin for the extant lineages, a key issue in their evolutionary history2,3. Integrated genomic and ecological analyses revealed the reticulate evolution of baobabs, which eventually led to the species diversity seen today. Past population dynamics of Malagasy baobabs may have been influenced by both interspecific competition and the geological history of the island, especially changes in local sea levels. We propose that further attention should be paid to the conservation status of Malagasy baobabs, especially of Adansonia suarezensis and Adansonia grandidieri, and that intensive monitoring of populations of Adansonia za is required, given its propensity for negatively impacting the critically endangered Adansonia perrieri.

Molecular and morphological evidence of hybridization between two dimorphic sympatric species of Fuchsia (Onagraceae)

Hybridization is commonly reported in angiosperms, generally based on morphology, and in few cases confirmed by molecular markers. Fuchsia has a long tradition of ornamental cultivars with different hybrids produced by artificial crosses, so natural hybridization between sympatric Fuchsia species could be common. Natural hybridization between F. microphylla and F. thymifolia was tested using six newly developed microsatellites for F. microphylla in addition to other molecular markers with codominant and maternal inheritance. Geometric morphometrics of leaves and floral structures were also used to identify putative hybrids. Hybrids showed a different degree of genetic admixture between both parental species. Chloroplast DNA (cpDNA) sequences indicated that hybridization occurs in both directions, in fact, some of the hybrids showed new haplotypes for cpDNA and ITS (internal transcriber spacer of nuclear ribosomal RNA genes) sequences. The morphology of hybrid individuals varied between the two parental species, but they could be better identified by their leaves and floral tubes. Our study is the first to confirm the hybridization in natural populations of Fuchsia species and suggests that hybridization has probably occurred repeatedly throughout the entire distribution of the species. Phylogeographic analysis of both species will be essential to understanding the impact of hybridization throughout their complete distribution.

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.]

Hybridization and differential introgression associated with environmental shifts in a mistletoe species complex

Host specialization after host shifting is traditionally viewed as the pathway to speciation in parasitic plants. However, geographical and environmental changes can also influence parasite speciation, through hybridization processes. Here we investigated the impact of past climatic fluctuations, environment, and host shifts on the genetic structure and patterns of hybridization and gene flow between Psittacanthus calyculatus and P. schiedeanus, a Mesoamerican species complex. Using microsatellites (408 individuals), we document moderate genetic diversity but high genetic differentiation between widespread parental clusters, calyculatus in dry pine-oak forests and schiedeanus in cloud forests. Bayesian analyses identified a third cluster, with admixture between parental clusters in areas of xeric and tropical dry forests and high levels of migration rates following secondary contact. Coincidently host associations in these areas differ from those in areas of parental species, suggesting that past hybridization played a role in environmental and host shifts. Overall, the observed genetic and geographic patterns suggest that these Psittacanthus populations could have entered a distinct evolutionary pathway. The results provide evidence for highlights on the importance of the Pleistocene climate changes, habitat differences, and potential host shifts in the evolutionary history of Neotropical mistletoes.