Possible Natural Hybridization of Two Morphologically Distinct Species of Acropora (Cnidaria, Scleractinia) in the Pacific: Fertilization and Larval Survival Rates

Naoko Isomura,Hironobu Fukami,Kenji Iwao,Sofia Consuegra

doi:10.1371/journal.pone.0056701

Abstract

Natural hybridization of corals in the Indo-Pacific has been considered rather rare. However, field studies have observed many corals with intermediate interspecific or unusual morphologies. Given that the existence of F1 hybrids with intermediate interspecific morphologies has been proven in the Caribbean, hybrids may also inhabit the Indo-Pacific and occur more frequently than expected. In this study, we focused on two morphologically different species, Acropora florida and A. intermedia, and performed crossing experiments at Akajima Island, Japan. Results showed that these species could hybridize in both directions via eggs and sperm, but that fertilization rates significantly differed according to which species provided eggs. These results are similar to those reported from the Caribbean. Although all embryos developed normally to the planular larval stage, the developmental processes of some hybrid embryos were delayed by approximately 1 h compared with conspecific embryos, suggesting that fertilization occurred 1 h later in interspecific crosses than in intraspecific crosses. More successful hybridization could occur under conditions with low numbers of conspecific colonies. Additionally, a comparison of survival rates between hybrid and intraspecific larvae revealed that intra- and interspecific larvae produced from eggs of A. florida survived for significantly longer than those produced from eggs of A. intermedia. Considering these data, under specific conditions, hybrids can be expected to be produced and survive in nature in the Pacific. Furthermore, we identified one colony with intermediate morphology between A. florida and A. intermedia in the field. This colony was fertilized only by eggs of A. florida, with high fertilization rates, suggesting that this colony would be a hybrid of these two species and might be backcrossed.

Highlights

Hybridization occurs in many animals and plants
In 2007, bundle setting, during which egg-sperm bundles were set on the polyp mouths, started in A. florida and A. intermedia from 20:10 until 20:30, with sunset occurring at 19:21
In 2012, bundle setting started in A. florida, A. intermedia, and ‘‘int-flo’’ from 20:15 until 20:45, with sunset occurring at 19:23

Summary

Introduction

Hybridization occurs in many animals and plants. In plants, reticulate evolution, in which hybridization and speciation are repeated in evolutionary time, has been reported [1,2]. Many species of coral spawn synchronously on the same night and time in early summer (i.e., mass spawning) [6,7,8]. In the Indo-Pacific, such mass spawning of the most abundant corals, genus Acropora, might cause hybridization because large quantities of their eggs and sperm mix in mass spawning events. Molecular phylogenetic analyses have revealed that hybridization occurs only among genetically closely related species [11], suggesting the presence of the repeated hybridization of Acropora, i.e., reticulate evolution. Study of Acropora chromosomes supported a hypothesis of reticulate evolution [14]

Methods

Results

Conclusion