Molecular evidence for natural hybridization in the mangrove fern genus Acrostichum

Natural hybridization is prevalent in ferns, and plays an important role in fern evolution and speciation. In the Indo West-Pacific region, the mangrove fern genus Acrostichum consists of two largely sympatric species, A. aureum and A. speciosum. Although there has been no report of interspecific hybridization before, we found some individuals morphologically intermediate between them in Guangdong and Hainan, China, for the first time, which were suspected to be hybrids. In this study, we aimed to test the hypothesis of natural hybridization between A. aureum and A. speciosum in Guangdong and Hainan using three low-copy nuclear genes. A chloroplast intergenic spacer was used to infer the hybridization direction once the hybrid status was confirmed. In addition, we examined spore shapes and germination for these taxa.

Both A. aureum and A. speciosum showed a low level of polymorphism at all three nuclear genes; however, they were well separated at these loci. At both locations, each individual of the putative hybrid showed additivity in chromatograms at all sites where the two species showed fixed differences. Haplotype analysis at all three nuclear genes indicated that each individual of the putative hybrid possessed two haplotypes, matching with those of A. aureum and A. speciosum, respectively. Sequencing of the chloroplast trnV-trnM regions showed that A. aureum differed from A. speciosum by eleven nucleotide substitutions and three indels (insertions/deletions), and all sampled individuals of the putative hybrid had the identical sequences with A. speciosum. Compared with A. aureum and A. speciosum, the putative hybrid had much reduced spore germination rate.

Sequence data of the three nuclear genes provide compelling evidence for natural hybridization between A. aureum and A. speciosum, and all the hybrid individuals are likely F1s. The hybridization is unidirectional and A. speciosum is the maternal parent of the hybrid based on the assumption of maternal inheritance of chloroplast DNA. Human disturbance on mangrove habitats may facilitate the establishment of hybrids of Acrostichum.