Breeding experiments of hermatypic coralGalaxea fascicularis: partial reproductive isolation between colonies of different nematocyst types and enhancement of fertilization success by presence of parental colonies

Abstract

KEY WORDS: Cnidaria, coral reef, fertilization, mitochondrial DNA, reproductive barrier.The reef-building (hermatypic) corals build the structural foundation of coral reefs in tropical and subtropical zones, and form the trophic basis of coral reef ecosystems. 1 These corals are symbiotic complexes of the cnidarian host, mostly of the order Scleractinia, and dinoflagellate endosymbionts Symbiodinium spp. Most hermatypic corals form colonies, and the bulk of their bodies are made of rigid calcified skeletons. The taxonomy of corals has been traditionally based on the skeletal morphology. In scleractinian corals, however, the skeletal morphology is not always useful in defining the boundary between biological species, and it is often necessary to perform breeding experiments to examine the species status. 2,3 In general, the breeding experiments of hermatypic corals (e.g. Acropora spp.) 2 are straightforward: mixing of eggs with sperm at an appropriate dilution often results in successful fertilization. Galaxea fascicularis is a colonial gonochoric coral, and their populations consist of female colonies producing egg bundles and male colonies that form bundles consisting of sperm and infertile 'pseudoeggs'. Pseudo-eggs contain lipids but lack processed products of vitellogenin, 4,5 and are presumed to carry sperm to the sea surface, where fertilization takes place, by giving buoyancy to male bundles. 4 The populations of G. fascicularis in Okinawa, Japan are likely to consist of two cryptic species. A nematocyst type microbasic p-mastigophore (MpM) in G. fascicularis has been distinguished as two distinct subtypes S and H. 6,7 Type S MpMs have a relatively thick capsule and a shaft approximately one-half of the capsule length, whereas MpMs of type H have a slender capsule with a shaft shorter than one-third of the capsule length. In most G. fascicularis colonies, only one of the two MpM types is found in external tentacles, although a small number of individuals possess a mixture of the S and H types. 7,8 Molecular analyses have shown that the MpM types are highly correlated to different genotypes in a mitochondrial intergenic region 8 and at a nuclear microsatellite locus. 9 Thus, this species in Okinawa is suspected to consist of two reproductive units that occur in sympatry. The present study was initiated to examine the presence of reproductive isolation in G. fascicularis by performing fertilization experiments between individuals of the same or different MpM types.Fragments consisting of 30-50 polyps of G. fascicularis colonies were collected in shallow reefs in Zampa and Teniya of Okinawa Island. All of the subsequent experiments were performed at the Sesoko Station, University of the Ryukyus, Japan. Live corals were kept in tanks under running sea water, and individually transferred to containers with 1-3 L of seawater 2-5 h before the expected time of spawning (19:00-23:00 hours). Two different procedures were used in fertilization tests. In the first procedure ('standard' method), released bundles were ma...

Genetic differentiation and connectivity of morphological types of the broadcast‐spawning coral Galaxea fascicularis in the Nansei Islands, Japan

“…Abe et al. () reported that the seasons are not completely separated because they managed to conduct fertilization experiments between types. Therefore, a reproductive isolation may not be complete between mt‐L and mt‐S, which may have resulted from a recent evolutionary speciation process.…”

“…Although cross‐type hybrids occurred as a result of fertilization experiments between types (Abe et al. ), the incubation time (4–5 h) with higher sperm concentration appears to be longer than those that occur in nature (see Iguchi et al. ; Nozawa et al.…”

“…Therefore, a reproductive isolation may not be complete between mt-L and mt-S, which may have resulted from a recent evolutionary speciation process. Although crosstype hybrids occurred as a result of fertilization experiments between types (Abe et al 2008b), the incubation time (4-5 h) with higher sperm concentration appears to be longer than those that occur in nature (see Iguchi et al 2009;Nozawa et al 2015). Therefore, as Watanabe et al (2005) suggested, actual hybridization between types may be rare, even if spawning seasons partly overlap in some colonies.…”

“…Interbreeding is less likely to occur between the two types than within types, suggesting that spawning seasons do not overlap in some cases (Watanabe et al 2005). Abe et al (2008b) reported that the seasons are not completely separated because they managed to conduct fertilization experiments between types. Therefore, a reproductive isolation may not be complete between mt-L and mt-S, which may have resulted from a recent evolutionary speciation process.…”

Novel Polymorphic Microsatellite Markers Reveal Genetic Differentiation between Two Sympatric Types of Galaxea fascicularis

“…These features are also correlated with the length of the non-coding region between the mitochondrial genes, cytochrome b ( cyt b ) and NADH dehydrogenase subunit 2 ( nad 2 ) (mt-Long: soft colony type and nematocysts with wide capsules and long shafts; mt-Short: hard colony type and nematocysts with thin capsules and short shafts) [ 7 , 8 ]; therefore, these two types represent novel clades or species. Interestingly, gamete fertilization experiments revealed that these two types show partial reproductive isolation [ 9 ]. However, the reproductive barriers that separate these sympatric types and that cause their genetic differentiation are not obvious.…”