Kyoto University and University of the Ryukyus confirm through gene analysis that the sea hibiscus scattered its seeds into ocean currents to achieve a global distribution in 'only four million years'

The sea hibiscus is widely distributed in the tropics and subtropics and achieved distribution across the globe by dispersing its seeds on ocean currents. But when did it appear on the earth and how long did it take to spread around the world? A research group led by Associate Professor Koji Takayama and master's student Yuri Yamazaki (now with Hitachi, Ltd.) of the Graduate School of Science, Kyoto University, and Professor Tadashi Kajita of the Tropical Biosphere Research Center, University of the Ryukyus, has unraveled how the sea hibiscus expanded its distribution using genome analysis of samples of the species and closely related species collected to date from around the world. The group's research was published in Molecular Ecology.

In their analysis the group used a next-generation sequencer to detect the entire chloroplast DNA sequence (approximately 162,000 base pairs) and single nucleotide polymorphisms (891 loci) in the nuclear DNA. Phylogenetic analysis at the genome level revealed that the ancestors of the sea hibiscus diverged from an inland species in South-East Asia about four million years ago and spread to tropical and subtropical regions all over the world. Furthermore, several domestic species that grow in the area around the distribution of the sea hibiscus were shown to have arisen around one million years ago. Population structure analysis has also shown that there is no clear genetic differentiation in the Indian Ocean-Pacific populations of the sea hibiscus, and that gene flow between populations is maintained to a large extent even after their global distribution.

On the other hand, when the researchers focused on the phylogenetic relationships between the sea hibiscus and related species, they found that the phylogenetic trees of chloroplast DNA and nuclear DNA did not always match. Whereas nuclear DNA phylogenetic trees were divided into separate lineages by species or by region, chloroplast DNA phylogenetic trees were partially nested between different species. This discrepancy between chloroplast and nuclear DNA phylogenetic trees has been observed in many plants, and gene penetration due to hybridization and incomplete gene sorting have been thought to be the main reasons for this.

In their research, the group tested the possibility of interspecific hybridization using data on single nucleotide polymorphisms in nuclear DNA, and traces of hybridization were detected between the closely related American species, H. pernambucensis, and the West African population of the sea hibiscus. This result is also consistent with the possibility of chloroplast capture occurring in H. pernambucensis shown in previous studies. At present, sea hibiscus does not grow naturally in the New World, but it is thought that long-distance seed dispersal, such as crossing the Pacific Ocean, caused genetic infiltration from sea hibiscus to H. pernambucensis.

On the other hand, the researchers found no evidence of interspecific hybridization between other closely related species and the sea hibiscus, suggesting that not only gene penetration due to hybridization but also incomplete phylogenetic sorting of chloroplast DNA may be a factor in the discrepancy between the two phylogenetic trees.

"The seed dispersal mode of ocean current dispersal has a very low potential for establishment and seems to be a very wasteful dispersal mode," said Takayama. "However, given the current distribution range of pan-tropical marine plants, it is a very good method in terms of expanding distribution. I admire the sea hibiscus for abandoning its inland environment and spreading to pan-tropical and subtropical coasts in just four million years."

Journal Information
Publication: Molecular Ecology
Title: Spatiotemporal process of long-distance seed dispersal in a pantropically distributed sea hibiscus group
DOI: 10.1111/mec.16836