GEOGRAPHIC VARIATION IN THE MILKFISH CHANOS CHANOS. I. BIOCHEMICAL EVIDENCE

Implication of single year seasonal sampling to genetic diversity fluctuation that coordinates with oceanographic dynamics in torpedo scads near Taiwan

Many fisheries management and conservation plans are based on the genetic structure of organisms in pelagic ecosystems; however, these structures tend to vary over time, particularly in cyclic ocean currents. We performed genetic analyses on the populations of the pelagic fish, Megalaspis cordyla (Osteichthyes: Carangidae) in the area surrounding Taiwan during 2000–2001. Genotyping was performed on M. cordyla collected seasonally around Taiwan as well as specimens collected from Singapore (Malacca strait) and Indonesia (Banda Sea). Gonadosomatic indices (GSI) revealed that M. cordyla does not spawn near Taiwan. Data related to the mitochondrial control region revealed that the samples from Singapore and Indonesia represented two distinct genetic cohorts. Genotyping revealed that during the summer (June–August 2000), the Indonesian variant was dominant in eastern Taiwan (presumably following the Kuroshio Current) and in the Penghu region (following the Kuroshio Branch Current). During the same period, the Singapore genotype was dominant along the western coast of Taiwan (presumably following the South China Sea Current); however, the number dropped during the winter (December–February 2001) under the effects of the China Coast Current. Divergence time estimates indicate that the two genetic cohorts split during the last glacial maximum. Despite the fact that these results are based on sampling from a single year, they demonstrate the importance of seasonal sampling in unravelling the genetic diversity in pelagic ecosystems.

Genetic diversity and patterns of demographic expansion in natural populations of milkfish, Chanos chanos (Forsskål, 1775), in the Philippines

The milkfish, Chanos chanos (Forsskål, 1775) is an important aquaculture species produced in the tropical Pacific region. Milkfish aquaculture is heavily reliant on seedstock; thus, there is a need to assess the genetic diversity of wild populations to identify areas that could be sources of high quality seedstock and broodstock. In this study, 631 milkfish specimens from 11 Philippine and, for comparison, two Indonesian wild populations were analysed for their genetic diversities using mitochondrial control region and cytochrome b. High genetic diversity indices were found for Sarangani, Claveria, and Camarines Sur populations in the Philippines. Philippine samples had either comparable or higher genetic diversity values than Indonesian samples from Medan and Sulawesi. Based on neutrality tests and mismatch distribution analysis, Philippine samples exhibited population expansion. Such pattern was not observed among Indonesian specimens. Genetic structure among populations was weak based on analysis of molecular variance (AMOVA). Correlation between genetic distance and geographic distance was not significant. Differentiation among marine biogeographic regions was small based on hierarchical AMOVA. High gene flow due to natural dispersal and influx of haplotypes brought about by importation may have contributed to the patterns observed, but diversification events within the Philippines were still evident.

POPULATION GENETIC CONSEQUENCES OF DEVELOPMENTAL EVOLUTION IN SEA URCHINS (GENUS HELIOCIDARIS)

Within the sea urchin genus Heliocidaris, changes in early embryonic and larval development have resulted in dramatic differences in the length of time larvae spend in the plankton before settling. The larvae of one species, H. tuberculata, spend several weeks feeding in the plankton before settling and metamorphosing into juveniles. The other species, H. erythrogramma, has modified this extended planktonic larval stage and develops into a juvenile within 3-4 days after fertilization. We used restriction site polymorphisms in mitochondrial DNA to examine the population genetic consequences of these developmental changes. Ten restriction enzymes were used to assay the mitochondrial genome of 29 individuals from 2 localities for H. tuberculata and 62 individuals from 5 localities for H. erythrogramma. Within H. tuberculata, 11 mitochondrial genotypes were identified. A G_ST analysis showed high levels of genetic exchange between populations separated by 1,000 kilometers of open ocean. In contrast, in H. erythrogramma, 13 mitochondrial genotypes differing by up to 2.33% were geographically partitioned over spatial scales ranging from 800 to 3,400 kilometers. Between distant localities, there was complete mitochondrial lineage sorting and large sequence divergence between resulting clades. Over much smaller spatial scales (< 1,000 km), genetic differentiation was due to the differential sorting of very similar genotypes. This pattern of mitochondrial variation suggests that these population differences have arisen recently and may reflect the historical interplay between the restricted dispersal capabilities of H. erythrogramma and the climatic and geological changes associated with Pleistocene Ice Ages.

High connectivity in the deepwater snapper Pristipomoides filamentosus (Lutjanidae) across the Indo-Pacific with isolation of the Hawaiian archipelago

In the tropical Indo-Pacific, most phylogeographic studies have focused on the shallow-water taxa that inhabit reefs to approximately 30 m depth. Little is known about the large predatory fishes, primarily snappers (subfamily Etelinae) and groupers (subfamily Epinephelinae) that occur at 100-400 m. These long-lived, slow-growing species support fisheries across the Indo-Pacific, yet no comprehensive genetic surveys within this group have been conducted. Here we contribute the first range-wide survey of a deepwater Indo-Pacific snapper, Pristipomoides filamentosus, with special focus on Hawai'i. We applied mtDNA cytochrome b and 11 microsatellite loci to 26 samples (N=1,222) collected across 17,000 km from Hawai'i to the western Indian Ocean. Results indicate that P. filamentosus is a highly dispersive species with low but significant population structure (mtDNA Φ(ST)=0.029, microsatellite F(ST)=0.029) due entirely to the isolation of Hawai'i. No population structure was detected across 14,000 km of the Indo-Pacific from Tonga in the Central Pacific to the Seychelles in the western Indian Ocean, a pattern rarely observed in reef species. Despite a long pelagic phase (60-180 days), interisland dispersal as adults, and extensive gene flow across the Indo-Pacific, P. filamentosus is unable to maintain population connectivity with Hawai'i. Coalescent analyses indicate that P. filamentosus may have colonized Hawai'i 26 K-52 K y ago against prevailing currents, with dispersal away from Hawai'i dominating migration estimates. P. filamentosus harbors low genetic diversity in Hawai'i, a common pattern in marine fishes, and our data indicate a single archipelago-wide stock. However, like the Hawaiian Grouper, Hyporthodus quernus, this snapper had several significant pairwise comparisons (F(ST)) clustered around the middle of the archipelago (St. Rogatien, Brooks Banks, Gardner) indicating that this region may be isolated or (more likely) receives input from Johnston Atoll to the south.