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      Molecular phylogeography of the troglobiotic millipede Tetracion Hoffman, 1956 (Diplopoda, Callipodida, Abacionidae)

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      International Journal of Myriapodology

      Pensoft Publishers

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          Abstract

          More than 85 species of cave-obligate (troglobiotic) millipede have been described from North America. Understanding the patterns and processes that determine their distribution in this region is an area of recent research. Here, we present the first molecular phylogeographic study of troglobiotic millipedes. Millipedes of the genus Tetracion Hoffman, 1956 (Callipodida: Abacionidae) inhabit caves on the Cumberland Plateau in Tennessee and Alabama, a global hotspot for cave biodiversity. Three species have been described: T. jonesi Hoffman, 1956, T. antraeum Hoffman, 1956, and T. tennesseensis Causey, 1959. To examine genetic divergence within and between species of Tetracion we sequenced part of the mitochondrial cytochrome oxidase 1 gene from 53 individuals from eleven caves across the range of T. tennesseensis and in the northern part of the range of T. jonesi. We found: (1) little variation within species (six haplotypes in T. tennesseensis and four haplotypes in T. jonesi, with a maximum of 1.4% intraspecific divergence between haplotypes), (2) that gene flow between caves is limited (7 of 10 haplotypes were restricted to a single cave, and FST > 0.80 and P < 0.05 for fifteen of eighteen comparisons between caves), and (3) significant genetic divergence between species (8.8% between T. tennesseensis and T. jonesi). Our results are consistent with previous morphology-based species definitions showing T. tennesseensis and T. jonesi belonging to distinct taxa. Our research contributes to the growing body of phylogeographic information about cave species on the Cumberland Plateau, and provides a point of comparison for future studies of troglobionts and millipedes.

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          Revisiting the insect mitochondrial molecular clock: the mid-Aegean trench calibration.

          Phylogenetic trees in insects are frequently dated by applying a "standard" mitochondrial DNA (mtDNA) clock estimated at 2.3% My(-1), but despite its wide use reliable calibration points have been lacking. Here, we used a well-established biogeographic barrier, the mid-Aegean trench separating the western and eastern Aegean archipelago, to estimate substitution rates in tenebrionid beetles. Cytochrome oxidase I (cox1) for six codistributed genera across 28 islands (444 individuals) on both sides of the mid-Aegean trench revealed 60 independently coalescing entities delimited with a mixed Yule-coalescent model. One representative per entity was used for phylogenetic analysis of mitochondrial (cox1, 16S rRNA) and nuclear (Mp20, 28S rRNA) genes. Six nodes marked geographically congruent east-west splits whose separation was largely contemporaneous and likely to reflect the formation of the mid-Aegean trench at 9-12 Mya. Based on these "known" dates, a divergence rate of 3.54% My(-1) for the cox1 gene (2.69% when combined with the 16S rRNA gene) was obtained under the preferred partitioning scheme and substitution model selected using Bayes factors. An extensive survey suggests that discrepancies in mtDNA substitution rates in the entomological literature can be attributed to the use of different substitution models, the use of different mitochondrial gene regions, mixing of intraspecific with interspecific data, and not accounting for variance in coalescent times or postseparation gene flow. Different treatments of these factors in the literature confound estimates of mtDNA substitution rates in opposing directions and obscure lineage-specific differences in rates when comparing data from various sources.
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            Rapid morphological radiation and convergence among races of the butterfly Heliconius erato inferred from patterns of mitochondrial DNA evolution.

             A V Brower (1994)
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              Recent divergence with gene flow in Tennessee cave salamanders (Plethodontidae: Gyrinophilus) inferred from gene genealogies.

              Cave organisms occupy a special place in evolutionary biology because convergent morphologies of many species demonstrate repeatability in evolution even as they obscure phylogenetic relationships. The origin of specialized cave-dwelling species also raises the issue of the relative importance of isolation vs. natural selection in speciation. Two alternative hypotheses describe the origin of subterranean species. The 'climate-relict' model proposes allopatric speciation after populations of cold-adapted species become stranded in caves due to climate change. The 'adaptive-shift' model proposes parapatric speciation driven by divergent selection between subterranean and surface habitats. Our study of the Tennessee cave salamander complex shows that the three nominal forms (Gyrinophilus palleucus palleucus, G. p. necturoides, and G. gulolineatus) arose recently and are genealogically nested within the epigean (surface-dwelling) species, G. porphyriticus. Short branch lengths and discordant gene trees were consistent with a complex history involving gene flow between diverging forms. Results of coalescent-based analysis of the distribution of haplotypes among groups reject the allopatric speciation model and support continuous or recurrent genetic exchange during divergence. These results strongly favour the hypothesis that Tennessee cave salamanders originated from spring salamanders via divergence with gene flow.
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                Author and article information

                Journal
                International Journal of Myriapodology
                IJM
                Pensoft Publishers
                1875-2543
                1875-2535
                October 11 2011
                October 11 2011
                : 5
                : 35-48
                Article
                10.3897/ijm.5.1891
                © 2011
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