On the fourth Diadema species (Diadema-sp) from Japan

Introgression of the Gamete Recognition Molecule, Bindin, in the Sea Urchin Diadema

Hybridization is important in evolution, because it is a necessary (though not sufficient) step in the introgression of potentially adaptive variation between species. Bindin is a gamete recognition protein in echinoids and asteroids, capable of blocking cross-fertilization between species to varying degrees. Four species of the sea urchin genus Diadema are broadly sympatric in the Indo-Pacific: D. paucispinum, D. savignyi, D. clarki, and D. setosum. Data from three published studies, one of identification of hybrids through allozymes, one of the phylogeography of mitochondrial DNA, and one of the phylogeny of bindin, were combined to assess the degree of bindin introgression between these four species. I analyzed sequences of the ATPase 8 and ATPase 6 mitochondrial genes and of bindin, sampled throughout the species ranges, with an isolation-migration algorithm, IMa3. IMa3 uses a coalescent approach to produce Bayesian estimates of effective population sizes and gene flow between populations. The results showed that bindin alleles coalesce completely within the species bounds of D. clarki and of D. setosum. The sister species D. paucispinum and D. savignyi, however, were estimated as having exchanged a bindin allele at an average of every one to two-and-a-half generations since they speciated from each other. As the allozyme study detected nine hybrids between three of these species in Okinawa (most of them between D. setosum and D. savignyi) in a single sample, hybrids between these species are produced, but bindin does not introgress. Therefore, bindin must not be efficient in blocking heterospecific fertilizations. Complete, or almost complete, reproductive isolation between species of Diadema must result from low hybrid fitness.

Morphological and Molecular Evidence of an Intergeneric Host-Range in Clavisodalis sentifer (Crustacea: Copepoda: Taeniacanthidae) Associated with Diadematid Sea Urchins from the Western Pacific

Sea urchins have a wide variety of symbionts on their body surfaces and inside their bodies. Copepods of the genus Clavisodalis (Taeniacanthidae) collected from the esophagus of sea urchins of the genera Diadema and Echinothrix in southern Japan were identified based on their morphological characteristics, and molecular analysis was conducted to determine whether genetic variation occurs in copepods from different localities and hosts. Morphological observations identified individuals from southern Japan as Clavisodalis sentifer Dojiri and Humes, 1982, making this the first record of this species in the northern hemisphere and the first record of its genus in Japan. Morphological and molecular analysis suggested that the copepod specimens collected from multiple hosts across two genera would be the same species. Considering the typically observed high level of host specificity among taeniacanthid copepods, the utilization of hosts from two genera by C. sentifer is noteworthy.

Phylogeography of Long-spined Sea Urchin Diadema setosum Across the Indo-Malay Archipelago

Widely distributed, broadcast-spawning Diadema sea urchins have been used as model invertebrate species for studying the zoogeography of the tropical Indo-Pacific. So far, the Indo-Malay archipelago, a wide and geographically complex maritime region extending from the eastern Indian Ocean to the western Pacific Ocean, has been under-sampled. This study aims to fill this sampling gap and uncover the phylogeographic structure of the long-spined sea-urchin D. setosum in the central Indo-West pacific region. D. setosum samples (total N = 718) were collected in 13 sites throughout the Indo-Malay archipelago. We sequenced over 1157 bp of COI gene. The Phylogeographic structure was derived from pairwise ФST estimates using multidimensional scaling and hierarchical clustering analysis; biogeographic hypotheses were tested by AMOVA; genetic relationships between haplotypes were summarised in the form of a minimum-spanning network; and pairwise mismatch distributions were compared to the expectations from demographic and spatial expansion models. All samples from the Indo-West Pacific were of the previously uncovered D. setosum-a lineage. Phylogeographic structure was evident: the Andaman Sea population and the northern New Guinea population were genetically distinct. Subtler but significant haplotype-frequency differences distinguished two populations within the Indonesian seas, distributed in a parapatric-like fashion. The phylogeographic partition observed was insufficiently explained by previous biogeographic hypotheses. The haplotype network showed a series of closely related star-shaped haplogroups with a high proportion of singletons. Nucleotide-pairwise mismatch patterns in the two populations from the Indonesian seas were consistent with both demographic and spatial expansion models. While geographic barriers to gene flow were inferred at the western and eastern extremities of the Indo-Malay archipelago, the subtler parapatric pattern observed within the Indonesian seas indicated restriction in gene flow, in a fashion that can hardly be explained by geographic isolation given the dynamic current systems that cross this region. Our results thus raise the hypothesis of subtle reproductive isolation between ecologically incompatible populations. While the coalescence pattern of the Andaman-Sea population suggested demographic stability over evolutionary timescales, that of the two populations from the Indonesian seas indicated recent population expansion, possibly linked to the rapid changes in available D. setosum habitat caused by sea-level oscillations in the late Pleistocene. The phylogeographic patterns observed in this study point to likely allopatric differentiation in the central Indo-West Pacific region. Genetic differences between populations were likely reinforced during interglacials by some form of reproductive isolation.

Slow evolution under purifying selection in the gamete recognition protein bindin of the sea urchin Diadema

Bindin is a sperm protein that mediates attachment and membrane fusion of gametes. The mode of bindin evolution varies across sea urchin genera studied to date. In three genera it evolves under positive selection, in four under mostly purifying selection, and in one, results have been mixed. We studied bindin evolution in the pantropical sea urchin Diadema, which split from other studied genera 250 million years ago. We found that Diadema bindin is structurally similar to that of other genera, but much longer (418 amino acids). In seven species of Diadema, bindin evolves under purifying selection, more slowly than in any other sea urchin genus. Only bindin of the recently rediscovered D. clarki shows evidence of positive selection. As D. clarki is sympatric with D. setosum and D. savignyi, positive selection could arise from avoidance of maladaptive hybridization. However, D. setosum and D. savignyi overlap in the Indo-West Pacific, yet their bindins show no evidence of positive selection, possibly because the two species spawn at different times. Bindin in the East Pacific D. mexicanum, the West Atlantic D. antillarum, the East Atlantic D. africanum, and the Indo-Pacific D. paucispinum also evolves slowly under purifying selection.

DNA barcoding and morphological analyses revealed validity of Diadema clarki Ikeda, 1939 (Echinodermata, Echinoidea, Diadematidae)

A long-spined sea urchin Diadema-sp reported from Japanese waters was genetically distinct from all known Diadema species, but it remained undescribed. Extensive field surveys in Japan with molecular identification performed in the present study determined five phenotypes (I to V) in Diadema-sp according to the presence and/or shape of a white streak and blue iridophore lines in the naked space of the interambulacral area. All phenotypes were distinct from Diademasetosum (Leske, 1778) and Diademasavignyi (Audouin, 1829), of which a major type (I) corresponded to Diademaclarki Ikeda, 1939 that was questioned and synonymized with Diademasetosum by Mortensen (1940). The holotype of Diademaclarki has not been found, but three unlabeled dried tests of Diadema were found among Ikeda’s original collection held in the Kitakyushu Museum of Natural History and Human History, Fukuoka, Japan. A short mtDNA COI fragment (ca. 350bp) was amplified from one of the tests, and the nucleotide sequence determined (275bp) was nearly identical with that of Diadema-sp. Arrangements of the primary tubercles on the coronal plates in Diadema-sp and the museum specimen also conformed with Diademaclarki, indicating that Diadema-sp is identical to Diademaclarki and a valid species. Narrow latitudinal distribution (31°N to 35°N) of Diademaclarki in Japan was observed, where it co-existed with abundant Diademasetosum and rare Diademasavignyi. No Diademaclarki was found in the southern islands in Japan, such as Satsunan Islands to Ryukyu Islands and Ogasawara Island, where Diademasetosum and Diademasavignyi were commonly observed.