Molecular confirmation of natural hybridization between Lumnitzera racemosa and Lumnitzera littorea

The hypothesis of natural hybridization between Lumnitzera racemosa and Lumnitzera littorea, two mangrove species distributed in the Indo-West Pacific region, was proposed in 1970s based on morphological traits; however, no molecular evidence has been reported to support it. In this study, we sequenced two low-copy nuclear genes and one chloroplast intergenic spacer (trnS-trnG) in the two Lumnitzera species and their putative hybrid to test this hypothesis. Our results revealed that there were 9 and 27 nucleotide substitutions at the two nuclear loci, respectively, between one haplotype of L. racemosa and L. littorea, and that the putative hybrid showed additivity in chromatograms at these sites. Sequencing the chloroplast intergenic region trnS-trnG showed that the two Lumnitzera species differed by seven fixed nucleotide substitutions and four fixed insertions/deletions in this region, while the putative hybrid had identical sequences to L. racemosa. Molecular data clearly demonstrated that there indeed existed natural hybridization between L. racemosa and L. littorea and that L. racemosa was the maternal parent in this hybridization event. The uncommon direction of hybridization and F1 nature of hybrids in this case, and in mangroves in general, is discussed.

► Molecular means can confirm hybrid status of taxa in question.
► Lumnitzera × rosea is a F1 hybrid between Lumnitzera racemosa and Lumnitzera littorea.
► Hybridization is unidirectional and L. racemosa was the maternal parent.
► Reinforcement hypothesis can account for the uncommon direction of hybridization.