Abstract
Biodiversity of mangrove ecosystems is difficult to assess, at least partly due to lack of genetic verification of morphology-based documentation of species. Natural hybridization, on the one hand, plays an important role in evolution as a source of novel gene combinations and a mechanism of speciation. However, on the other hand, recurrent introgression allows gene flow between species and could reverse the process of genetic differentiation among populations required for speciation. To understand the dynamic evolutionary consequences of hybridization, this study examines genomic structure of hybrids and parental species at the population level. In the Indo-West Pacific, Bruguiera is one of the dominant mangrove genera and species ranges overlap extensively with one another. Morphological intermediates between sympatric Bruguiera gymnorrhiza and Bruguiera sexangula have been reported as a variety of B. sexangula or a new hybrid species, B. × rhynchopetala. However, the direction of hybridization and extent of introgression are unclear. A large number of species-specific inter-simple sequence repeat (ISSR) markers were found in B. gymnorrhiza and B. sexangula, and the additive ISSR profiling of B. × rhynchopetala ascertained its hybrid status and identified its parental origin. The varying degree of scatterness among hybrid individuals in Principal Coordinate Analysis and results from NewHybrids analysis indicate that B. × rhynchopetala comprises different generations of introgressants in addition to F 1s. High genetic relatedness between B. × rhynchopetala and B. gymnorrhiza based on nuclear and chloroplast sequences suggests preferential hybrid backcrosses to B. gymnorrhiza. We conclude that B. × rhynchopetala has not evolved into an incipient hybrid species, and its persistence can be explained by recurrent hybridization and introgression. Genomic data provide insights into the hybridization dynamics of mangrove plants. Such information can assist in biodiversity assessment by helping detect novel taxa and/or define species boundaries.
Highlights
Mangrove forests consist of an important group of woody plants occupying coastal zone habitats
Parental origin and genetic relatedness A total of 284 inter-simple sequence repeat (ISSR) marker loci were recorded which represent all fragments amplified with the 12 ISSR primers for 112 individuals belonging to five different taxa (B. gymnorrhiza, B. sexangula, B. cylindrica, B. parviflora and the hybrid B. 6 rhynchopetala) from four geographically isolated populations
Over 90% of the bands detected in B. gymnorrhiza were present in B. 6 rhynchopetala, except for the Johnstone River population where only one hybrid individual was detected based on morphological criteria
Summary
Mangrove forests consist of an important group of woody plants occupying coastal zone habitats. Global distributions of mangroves are mainly influenced by temperature [1], restricting them to warm tropical and subtropical latitudes in the Indo West Pacific (IWP) and Atlantic East Pacific (AEP) regions These plants and associated organisms and habitats constitute one of the world’s most productive ecosystems [2], extant mangrove taxa worldwide remain incompletely described and poorly identified, which limits our understanding of mangrove biodiversity and evolutionary relationships among the major constituents. To understand the dynamic evolutionary consequences of hybridization, more genetic studies are needed to compare the hybrids and their parental species at the population and genomic level Such studies are important for accurately classifying and managing biodiversity
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