Interspecies relationships in Acropora (Cnidaria: Scleractinia)
Date Issued
2007
Date
2007
Author(s)
Wei, Nu-Wei Vivian
DOI
en-US
Abstract
Acropora species often coexist sympatrically and their mass-spawning behavior provides the opportunities for interspecific hybridization. The reticulate patterns of Acropora have been widely documented from 1997 to 2006 and have been used as a generalisation of reticulate history for scleractinian evolution. The possible reproductive isolations including temporal isolation of spawning time, the post-mating isolations, and the cytogenetic types were studied. In addition, the phylogenetic analyses of ribosomal internal transcribed spacer (ITS) and 5.8S gene region were conducted to deduce the reticulate evolutionary history.
Four sympatric Acropora corals, including A. muricata, A. valida, A. hyacinthus, and A. humilis, were studied in the Chinwan Inner Bay, Penghu Islands from 2002 to 2005. The spawning dates of Acropora are highly variable among years and months, ranging from 7 days before full moon to 10 days after full moon. Major spawning peaks can be splitted on April or May. Within the spawning days, field monitoring indicated an overlapping in spawning time among these four species in the Chinwan Inner Bay, suggesting synchronously spawning occurred. On the other side, the four Acropora species shared some overlapping in spawning days and spawning time in Chinwan Inner Bay. However, the probabilities of synchronization showed that synchrony mainly occurred within species and between two species. Cross experiments showed very low or none interspecific hybridization among species, indicating gametic incompatibilities and prezyotic isolation existed. Cytogenetic typing showed the chromosome numbers were 28 in developed coral larvae for all the four Acropora species.
For the molecular phylogenetic analysis, I examined the molecular evolution of the internal transcribed spacers (ITS) and 5.8S rDNA sequences from 78 species, representing 28 genera, and 12 families of scleractinian corals. Genetic distances (measured by p-distances) and frequency distribution analyses revealed that both extremely high intra- and interspecific heterogeneity of the ITS-5.8S rDNA were specific to the Acropora. The 5.8S rDNA phylogeny clearly showed a significantly long branch length leading to the cluster of Acropora. The molecular-clock hypothesis tested by likelihood ratio test indicated a highly significant difference in global evolutionary rate for scleractinian 5.8S rDNA. The relative rate tests showed that Isopora, Caribbean Acropora, and Indo-Pacific Acropora evolved at constant tempos in their rDNA, indicating that highly divergent rDNA persisted in Acropora before splitting into these three lineages. In contrast, rate constancy was rejected in most of comparisons between Acropora/ Isopora and other coral genera, suggesting that the rates of evolution of 5.8S differed between Acropora/ Isopora and other lineages, and the evolutionary rate of Acropora/ Isopora was accelerated after divergence from the common ancestor of scleractinian corals.
In summary, I have applied the BSC to examine the species boundary of symatric Acropora corals which spawned relatively synchronous in their reproductive seasons. Strong prezygotic isolation existed among Acropora suggested that a gamete recognition system could have operated the incomparability of sperm and egg interaction during mass spawning events the corals in Chinwan Inner Bay, Penghu. In addition, revisiting the molecular phylogenetic analysis of ITS clearly showed the high heterogeneity of rDNA is a unique feature of Acropora. Thus using this unique case to conclude the reticulate evolution as a general pattern of scleractinain corals should be cautious. The unusual heterogeneous rDNA characteristics might be applied to the whole nuclear genome of Acropora, and thus deductions of species boundary based on molecular markers are still far from completed. Under this circumstance, ecological and reproductive examinations based on the BSC criterion continue to be an effective approach to delineate the species boundary in corals. Although debates of reticulate evolution in scleractinian corals remain unsettled, future applications of ecological, behavioral, reproductive, and molecular genetics are still to be the coordinated approaches to reveal the evolutionary mechanism of coral speciation.
Subjects
軸孔珊瑚
同步集體產卵
網狀演化
生物種概念
祖先多型性
種的範圍界定
染色體組型
Acropora
mass spawning
Biological Species Concept
reticulate evolution
ancestral polymorphism
species boundary
Karyotype
Type
thesis