BIOL2609_L19_2007

BIOL2609_L19_2007 - Molecular Ecology ECOL 2007 Lecture 19:...

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Unformatted text preview: Molecular Ecology ECOL 2007 Lecture 19: Phylogeography: Gene flow among highly mobile populations Phylogeography of billfishes Billfish are successful predators in the epipelagic region Taxonomy of billfishes Family Istiophoridae Genus Makaira • M.nigricans (blue marlin) • M.indica (black marlin) Genus Istiophorus • I.platypterus (sailfish) Genus Tetrapturus • T.albidus (white marlin) • T.audax (striped marlin) • T.pfluegeri (longbill spearfish) • T.angusirostris (shortbill spearfish) • T.belone (Mediterranean spearfish) • T.georgei (roundscale spearfish) Family Xiphidae Genus Xiphias • X.gladius (swordfish) We will concentrate on the family Istiophoridae • Billfish are an important commercial and recreational fisheries resource – Catch of the genus Makaira alone was >40x103t in 1990 (FAO, UN) • They are fished commercially by long line, and are a considerable part of by-catch in tuna fishing operations • All billfish are categorized as: fully-exploited or over-exploited by ICCAT*, and so their continued fishing is not ecologically sound or sustainable – * = International Commission for the Conservation of Atlantic Tunas Non-molecular views on intraspecific population structure • Little was known of intraspecific population structure for any of the billfish • Assumed that due to mobility and continuous distribution – there would be little population subdivision between the oceans • Some tag-recapture data seemed to support this but based upon very few recoveries (0.5-2%) and unequal efforts in recovery at different locations • More recent molecular analysis has revealed considerable intraspecific genetic differentiation with location • Genetic variation vs location = phylogeography Striped marlin • Distribution: Pacific & Indian Ocean • Traditional studies (Squire & Suzuki 1990): single Pacific stock • However, RFLP analysis of mtDNA (Graves & McDowell 1994) suggests the existence of >1 Pacific population • ~40 fish per site • mtDNA haplotypes differed by 1-2 restriction site changes (nucleotide sequence divergence measured at 0.16-1.11%) • Phylogeographic analysis: genetic variability vs location • Revealed shallow structuring, consistent with evolutionarily recent restrictions to gene flow Striped marlin mtDNA haplotypes location Haplotype Mexico Ecuador Hawaii Australia A B C D E F G Unique Totals 12 6 8 0 0 0 6 4 36 20 6 9 2 1 1 0 1 40 14 7 4 10 2 0 0 6 43 12 3 1 8 7 11 0 5 47 Phylogeographic map of Pacific striped marlin Based upon 166 marlin samples, RFLP using 11 restriction endonucleases, circle size proportional to abundance of haplotype (smallest circle = 2 individuals) White marlin • Very similar to striped marlin • Atlantic distribution only • RFLP analysis of mtDNA from 3 sites (~15 fish per site) revealed no significant heterogeneity among white marlin (Graves & McDowell 1995) – unlike the striped marlin considered in the previous example • HOWEVER – white marlin territory overlaps with that of the striped marlin around South Africa – Two haplotypes are common to both species, acounting for 87% and 25% respectively – The other haplotypes were also similar, differing by only 1-2 restriction sites • This suggests that if striped and white marlin are valid species, then: they separated recently or, introgression has occurred since speciation Phylogeographic analysis of striped and white marlin Haplotypes included are 166 striped marlin & 52 white marlin, generated with 10 retsriction endonucleases Blue marlin The relationship of Atlantic and Indo-Pacific blue marlin has always been considered problematic • Single circum-tropical population Rivas 1975) • Atlantic & Indo-Pacific population (Nakamura 1985) – based on slight differences in lateral line morphology • Blue marlin exhibit considerable genetic variation (Graves & McDowell 1995) • RFLP analysis of mtDNA, using 11 restriction enzymes • Significant difference in haplotype distribution between Atlantic and Indo-Pacific • Also significant within-ocean population structure – E.g. one haplotype apparently confined to Atlantic fish was found in 11/74 Jamaican fish, but only 1/74 of nonJamaican Atlantic fish • This level of population structure is strange given the mobility of the fish – E.g. Tag/recapture suggests 10% W.Atlantic fish are recaptured in E.Atlantic • Why? – Spawning site fidelity? Phylogeographic analysis: • Reveals two groups of haplotypes that differ by several restriction site differences – Nucleotide divergence measured as 1.35% • One group of haplotypes was ubiquitous – Found in 55% Atlantic fish and all Indo-Pacific fish • Ten haplotypes were present in both Atlantic and Indo-Pacific fish • The remaining haplotypes (accounting for 45% of individuals caught) were found only in Atlantic fish • These two genetically divergent groups suggest historical isolation Phylogeographic analysis of Atlantic and Indo-Pacific blue marlin Included haplotypes from 138 Atlantic and 68 Pacific fish Sailfish As for blue marlin, the taxonomy of sailfish has been problematic: • Morrow & Harbo (1969) – single circumtropical species • Nakamura (1985) – two species based on morphology • RFLP of mtDNA from 56 Atlantic & 33 IndoPacific fish • Higher haplotype diversity in Atlantic fish • Accounted for by presence of two distinct groups of haplotypes, only one of which appeared in Indo-Pacific fish – These two groups were separated by 3 restriction site differences, and had nucleotide divergence of 0.65% • In contrast to blue marlin, the ubiquitous sailfish haplotypes were not found in high number among Atlantic fish • Only two haplotypes were shared between oceans, accounting for 76% of Indo-Pacific samples and 9% of Atlantic samples Phylogeographic analysis: • Similar pattern to blue marlin • Atlantic fish – two distinct haplotypes, only one of which is present in Indo-Pacific fish • How did this occur? – Extinction of intermediate haplotypes? - but unlikely to occur in both species – Long period of isolation between conspecific populations, coupled with recent unidirectional exchange? – possible as cool waters may have prevented inter-oceanic movement in the past Phylogeographic analysis of sailfish Haplotypes included are from 56 Atlantic and 33 Indo-Pacific fish ...
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