Neotropical plant radiations provide a perfect system for examining population divergence, sympatric coexistence and potential gene flow among close relatives. However, little is known about natural hybridization and gene flow between species or about levels of gene flow among geographically disparate populations within species.

Dr. Yann Surget-Groba of Xishuangbanna Tropical Botanical Garden (XTBG) and his collaborator investigated patterns of intraspecific and interspecific gene flow among populations of two recently derived species of Neotropical spiral gingers, Costus pulverulentus and Costus scaber (Costaceae). The two species co-occur in the tropical rainforest and share pollinators, but are reproductively isolated by a series of prezygotic barriers, some of which show evidence of reinforcement at sympatric sites.

By genotyping a set of rapidly evolving microsatellites, they sought to answer several questions about gene flow both within and between those species. First, they asked how isolated populations within a species were from each other across a range of geographical distances using summary genetic distances and multivariate analysis. Then, they asked whether there was evidence for hybridization and introgression between species. Moreover, they asked whether there was admixture or depressed genetic distance between sympatric populations compared with allopatric populations and whether phenotypically intermediate individuals from sympatric sites were evidence for hybridization and introgression in this putatively reinforced system.

 The researchers found high levels of genetic isolation among populations within each species and low but detectable levels of introgression between species at sympatric sites.  Putative hybrids identified by morphology were consistent with F1 or more advanced hybrids. Their results highlighted the effectiveness of prezygotic isolating mechanisms at maintaining species boundaries in young radiations and provided empirical data on levels of gene flow consistent with reinforcement.

The researchers suggested that future work will focus on genomic data and will evaluate genealogies of loci linked and unlinked to quantitative trait locus underlying different forms of reproductive isolation.

   The study entitled “Restricted gene flow within and between rapidly diverging Neotropical plant species” has been published online in Molecular Ecology.