PhD Candidate, Hawai‘i Institute of Marine Biology
Using genetics to trace the Pacific origins of endemic Hawaiian coral reef fishes
Since the Hawaiian Islands are volcanic, every marine species that occurs there had to originate somewhere else. There are two major hypotheses as to where they come from: either from the direction of Japan via an ocean current called the Kuroshio Current, or from the south, using the Line Islands and Johnston Atoll as oceanic "stepping stones". The majority of marine species have a larval form during which they drift in ocean currents, which is their primary means of dispersing over long distances. While it is not possible to directly trace the pathway taken by marine larvae, we can use genetics as a proxy, allowing us to go back in time to trace the journeys taken by species across the oceans. Genetic studies so far have provided evidence for both hypotheses. The endemic Hawaiian limpet ‘opihi (genus Cellana) arrived from Japan approximately 3-7 million years ago (Bird et al., 2011). Orange-band surgeonfish came from the Central Pacific (southern route) about 0.5 million years ago (Gaither et al., 2015), and endemic Hawaiian butterflyfishes show evidence of both colonization routes (Craig et al., 2010). Overall, Hodge and colleagues (2014) provide evidence for two major waves of reef fish colonization: one at 0-3 million years and one at 8-12 million years ago. In this study, we synthesize available data on Hawaiian reef fish colonization routes, and we introduce a new dataset based on endemic Hawaiian blennies compared to widespread sister species to further elucidate the ages and origins of Hawaiian reef fauna. One emerging conclusion is that a dispersal corridor between the Line Islands / Johnston Atoll and the Papahānaumokuākea Marine National Monument (in the Northwest Hawaiian Islands) may constitute an important entry point for colonization into Hawaii, which provides important insight into factors that promote and maintain endemic reef biodiversity.
Abstract: The Hawaiian Archipelago has become a natural laboratory for understanding genetic connectivity in marine organisms as a result of the large number of population genetics studies that have been conducted across this island chain for a wide taxonomic range of organisms. However, population genetic studies have been conducted for only two species occurring in the mesophotic or submesophotic zones (30+m) in this archipelago. To gain a greater understanding of genetic connectivity in these deepwater habitats, we investigated the genetic structure of two submesophotic fish species (occurring ∼200-360 m) in this archipelago. We surveyed 16 locations across the archipelago for submesophotic snappers Etelis coruscans (N = 787) and E. "marshi" (formerly E. carbunculus; N = 770) with 436-490 bp of mtDNA cytochrome b and 10-11 microsatellite loci. Phylogeographic analyses reveal no geographic structuring of mtDNA lineages and recent coalescence times that are typical of shallow reef fauna. Population genetic analyses reveal no overall structure across most of the archipelago, a pattern also typical of dispersive shallow fishes. However some sites in the mid-archipelago (Raita Bank to French Frigate Shoals) had significant population differentiation. This pattern of no structure between ends of the Hawaiian range, and significant structure in the middle, was previously observed in a submesophotic snapper (Pristipomoides filamentosus) and a submesophotic grouper (Hyporthodus quernus). Three of these four species also have elevated genetic diversity in the mid-archipelago. Biophysical larval dispersal models from previous studies indicate that this elevated diversity may result from larval supplement from Johnston Atoll, ∼800 km southwest of Hawaii. In this case the boundaries of stocks for fishery management cannot be defined simply in terms of geography, and fishery management in Hawaii may need to incorporate external larval supply into management plans.
Pub.: 12 Apr '14, Pinned: 19 Jul '17
Abstract: Speciation remains a central enigma in biology, and nowhere is this more apparent than in shallow tropical seas where biodiversity rivals that of tropical rainforests. Obvious barriers to gene flow are few and most marine species have a highly dispersive larval stage, which should greatly decrease opportunities for speciation via geographic isolation. The disparity in the level of geographic isolation for terrestrial and marine species is exemplified in Hawai'i where opportunities for allopatric speciation abound in the terrestrial realm. In contrast, marine colonizers of Hawai'i are believed to produce only a single endemic species or population, due to the lack of isolating barriers. To test the assertion that marine species do not diversify within Hawai'i, we examine the evolutionary origin of three endemic limpets (Cellana exarata, C. sandwicensis and C. talcosa) that are vertically segregated across a steep ecocline on rocky shores. Analyses of three mtDNA loci (12S, 16S, COI; 1565bp) and two nDNA loci (ATPSβ, H3; 709bp) in 26 Indo-Pacific Cellana species (N=414) indicates that Hawai'i was colonized once ∼3.4-7.2Ma from the vicinity of Japan. Trait mapping demonstrates that high-shore residence is the ancestral character state, such that mid- and low-shore species are the product of subsequent diversification. The Hawaiian Cellana are the first broadcast-spawners demonstrated to have speciated within any archipelago. The habitat stratification, extensive sympatry, and evolutionary history of these limpets collectively indicate a strong ecological component to speciation and support the growing body of evidence for non-allopatric speciation in the ocean.
Pub.: 13 Apr '11, Pinned: 19 Jul '17
Abstract: The drivers of speciation remain among the most controversial topics in evolutionary biology. Initially, Darwin emphasized natural selection as a primary mechanism of speciation, but the architects of the modern synthesis largely abandoned that view in favour of divergence by geographic isolation. The balance between selection and isolation is still at the forefront of the evolutionary debate, especially for the world's tropical oceans where biodiversity is high, but isolating barriers are few. Here, we identify the drivers of speciation in Pacific reef fishes of the genus Acanthurus by comparative genome scans of two peripheral populations that split from a large Central-West Pacific lineage at roughly the same time. Mitochondrial sequences indicate that populations in the Hawaiian Archipelago and the Marquesas Islands became isolated approximately 0.5 Ma. The Hawaiian lineage is morphologically indistinguishable from the widespread Pacific form, but the Marquesan form is recognized as a distinct species that occupies an unusual tropical ecosystem characterized by upwelling, turbidity, temperature fluctuations, algal blooms and little coral cover. An analysis of 3737 SNPs reveals a strong signal of selection at the Marquesas, with 59 loci under disruptive selection including an opsin Rh2 locus. While both the Hawaiian and Marquesan populations indicate signals of drift, the former shows a weak signal of selection that is comparable with populations in the Central-West Pacific. This contrast between closely related lineages reveals one population diverging due primarily to geographic isolation and genetic drift, and the other achieving taxonomic species status under the influence of selection.
Pub.: 11 Mar '15, Pinned: 19 Jul '17
Abstract: On evolutionary time scales, sea level oscillations lead to recurrent spatio-temporal variation in species distribution and population connectivity. In this situation, applying classical concepts of biogeography is challenging yet necessary to understand the mechanisms underlying biodiversity in highly diverse marine ecosystems such as coral reefs. We aimed at studying the outcomes of such complex biogeographical dynamics on reproductive isolation by sampling populations across a wide spatial range of a species-rich fish genus: the sergeants (Pomacentridae: Abudefduf). We generated a mutli-locus data set that included ten species from 32 Indo-West Pacific localities. We observed a pattern of mito-nuclear discordance in two common and widely distributed: A. sexfasciatus and A. vaigiensis. The results showed three regional sub-lineages (Indian Ocean, Coral Triangle region, western Pacific) in A. sexfasciatus (0.6-1.5% divergence at cytb). The other species, A. vaigiensis is polyphyletic and consists of three distinct genetic lineages (A, B, and C) whose geographic ranges overlap (9% divergence at cytb). Although A. vaigiensis A and A. sexfasciatus were found to be distinct based on nuclear information, A. vaigiensis A was found to be nested within A. sexfasciatus in the mitochondrial gene tree. A. sexfasciatus from the Coral Triangle region and A. vaigiensis A were not differentiated from each other at the mitochondrial locus. We then used coalescent-based simulation to characterize a spatially widespread but weak gene flow between the two species. These fishes may be considered as candidates of choice to investigate the complexity of the discrepancies between phenotypic and genetic evolution among sibling species. This article is protected by copyright. All rights reserved.
Pub.: 06 Feb '17, Pinned: 19 Jul '17