Format: ORAL

Sen Li1, Miles Tracy1, Ziwen He1, Chung-I Wu1, Suhua Shi1

1 State Key Laboratory of Biocontrol, Guangdong Provincial Key Laboratory of Plant Resources, School of Life Sciences, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Sun Yat-sen University, Guangzhou 510275, China

Reinforcement, the process by which premating isolation is strengthened by selection against the investment in maladaptive hybrids, is a key prediction of the evolutionary theory. Exceptions may therefore shed light on this controversial topic. Two mangrove species (Bruguiera gymnorhiza and B. sexangula) on the Indo-Malayan coasts come into secondary contact on the Hainan island, China, but rarely elsewhere. Strikingly, hybrids account for 30% of the individuals in sympatry and exceed the number from the parental species, B. gymnorhiza. Furthermore, while the two species in allopatry have different flowering times, they both flower year-round in Hainan. Counter-reinforcement thus appears to be at work in Bruguiera. To analyze the unusual hybridization, we sequenced and de novo assembled the high-quality genomes of the two species, followed by the re-sequencing of 63 individuals from allopatric and sympatric populations. The genomic analysis identifies both F1s and multiple generations of backcrosses. The hybridization pattern suggests weak postmating isolation and, likely, ecological speciation. Based on these patterns, we develop a theoretical model whereby the allele that does not exercise mate choice can be advantageous. In this model, the mating allele of the weaker species may do better by mating with the stronger and more common species. The case of Bruguiera thus shows that the evolution of mating preference as well as reinforcement is a function of genic fitness, rather than the fitness of the species itself.