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<br />.L~.TERS TO NATURE
<br />
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<br />debated)-S. Allhough recent discussions on mantle convection
<br />and the driving forces for plate motions have favoured plate
<br />velocities that are similar to mantle convective velocities5, further
<br />documentation of the DM M II component beneath several con-
<br />'2. tinents of distinct tectonic and accretionary history would indi-
<br />l~q -J cate that asthenosphere-lithosphere ~I~p repres~nts a significant
<br />~ proportion of absolute plate velocllles, partIcularly beneath
<br />\)o.,J\~ ~ relatively slowly moving continents4. 0
<br />
<br />~~
<br />
<br />Receiveo 14 October 1992: acceplecl 2 Match 1993.
<br />
<br />1. -eS_IIl. C. J.. Kempton. PO.. Rogers." W. E"am. R M. & van Calsleren. P W. Ear/tl pI_e
<br />Sci. Lere ... 256-268 119901
<br />2. M:00n0u1l\ W F. EMltI pI_e Sci. Lert. 101. 1-18119901
<br />3. Rlcharason. R. M.. Solomon. S. C & Sleep. N H. Re. Geopnys. ~PnYS.17. 981-1019119791.
<br />4. GortlOn. R. G. & .lJrdy. O. M. 1. ,eopfJyS. Res 91.12389-12406 119861
<br />5. Oa.ies. G. F. & _as. M. A 1. GeoI. 100, 151-206 119921.
<br />6. Kemplon.P.O..F.ncn.JG.._es_lh.C.J &o.merO<l.O.S. J teoPhrs Res 12. 13713-13735
<br />CI9911.
<br />7. Patchell P. J & Tatsumolo. M. Geot>hys. Res Lerr 7, 1077-1080 119801
<br />8. Palchell P. J Urhos 15. 47-51 C19631.
<br />9. Pllchell P. J. Geodtim CDSmOChim. Acta 47. 81.91 (19831.
<br />10. SaIlers. V. J M. & Hart. S. R. ENth plaNt Set Lerr 104. 364-380 119911.
<br />11. SaIlers, V. l M. & twt. S. R. ~ture 342. 420-422 119891.
<br />12. Menzies. M. A.. L-. W. P. & Hawkes_lh. C. J NaIUle 303, 205-209 (19831.
<br />13. Perry. F. V..IlaIG-idp. W. S. & 0eP1OIo. O. J J ,eor>hys Res !l2, 9193-9213 (l98n
<br />14. Johnson. C. M. & Thompson. R. A. J geophys. Res ... 13593-13608 119911.
<br />15. 1lar0Yich. K. M. & ~ C. M. Scieru CSUIlmillecll.
<br />16. Harl S. A_ Gerta:t\. O. C. & While. W. M. GeoctIim. cosmoctltnt Acta SO. 1551-1557 (19861.
<br />17. Patchell P. J.. White. W. M.. Felcmam. H.. Kielincz~. S. & Hotrnam. A. W. ENthp/Mtee Set Lele
<br />n. 365-378 (1~1
<br />
<br />V D t ') &2 t ~11 i
<br />
<br />lit,:! /
<br />
<br />
<br />""II
<br />
<br />,
<br />18. Chauvel. C.. Ho'mann. W. A. & Vidal. P. Ear/tl pI_t Sei. Lerr. 110. 99-119 (19921
<br />19. Takahashi. E. & Kushiro. I. Am MinetM. ... 859-879 (19831.
<br />20. Olsen. K. H.. Balchl&e. W. S. & '*'-. J. F. r<<tonophysICs 143. 119-139119871.
<br />21. Simo. Y. A.. Dallel. P. H.. Keller. G. A. Morean. P. & Harder. S H. 1. 'eor>hys Res !II. 6143-6156
<br />(19861.
<br />22. Oav;es. P. M.. Parker. E. C.. Evans. J. A.lyer. 11. M. & Olsen K. H. New Mo."", '<<JI. Soc Guidr/>ooI.
<br />.. 29-38119841.
<br />23. Iyer. H. M. & Hitchcocl<. T. Geol. Sac. Am. Atom 144, 179-209119891.
<br />24. SliIIe. P..l.Wuh. O..M. & TIlSumo\o. M. Geodrirn. cosmoctlun. Acta 50. 2303-2319119861.
<br />25. s<<ne. P..lNuh. o. M. & TalsumolO. M. Nature 304. 25-29 (19831.. @
<br />26. Otlonello. G.. unsl W. G. & Joron. J. l. 1. Pelrol. 25. 343-372 (19841.
<br />27. Fujimaki. H.. Talsumolo. M. & -. K. J 'eophys. Res n. 662-672 (19841.. ~"2 e:.
<br />28. Irvin&. A. J. & Frey. F. A. Go!oaIirn. cosmor::him Acta q, 1201-1221119841 J
<br />29. Aamilone. E- Bollazzi. P. & Oteolini.L. ~tlle 3S4,. 518-520 (19911.
<br />30. While. W. M. & Palchell P. J. E,th pI_e Sci. Lere 17. 167 -185119841
<br />31. WOO<I1ead. J O. Chem. Geol. 7S, 1-24 119891.
<br />32. Kempton. P. 0.. Duncan. M. A. & Blanchard. O. P. Geot Soc. Am. Spec. Pap 21S. 347-370
<br />(1987l.
<br />33. Dungan. M. A. el M. 1. 'eor>hys Resll1. 5999-6208 119861
<br />34. Phelps. O. W.. GuSl O. A. & WOOllen. J. L. ConlriO. Miner. Pelrol. .... 182-190 119831.
<br />35. Slor....... l C. Jr GeoI. Sac. Am Bull. U. 2443-2448 119721.
<br />36. Johnson. C. M. & Lipman. P. W. ConrriO. Mi_. Perroll00. 107-128 <19881
<br />37. Johnson. C. M..Lipman, P. W. & Cz_. G. K. Conlr"'. Miner. Petrol. 104. 99.124 <19901
<br />38. Thompson. A. A.. Johnson. C. M. & Mehnert. 11. 11. 1. 'eop/Iys. Res. !II. 13577-13592119911.
<br />39. Palchell P. J. & T.lsumoro. M. Nature 2a, 571-574 (19801.
<br />40. TIlsumoro. M.. Lnruh. O. M. & Palchell P. J. Proc. 61h Sy,"". AntarctiC Moleorlles 237 -249 (Nil
<br />1nSl. Polar Res.. Tokyo. 19811.
<br />41. Beard. 8. L.. lhesis,tnv. Wisconsin-Maclison (19921.
<br />42. Beard. 8. l. & Johnson. C. M. ENth p/arJee Sei. Lete (SUbmillecll
<br />
<br />ACKNOWlEDGEMENTS. We - S.llaIG-id&e. J. Davidson. M. Duncan. K. FUla. S. Gollson. P. Kempton.
<br />. S. Moorbal/\ F. Perry. J. Ratle. J. Stormer. A A. Thompson ana R N. Thompson 'or prOVidinC samples
<br />and -.. unpublished c:hemlcaI .., Nd iSOlope data. We _ K. Bar....." 'or imprOYinC HI
<br />~ieaI proc:eo..res and 'or . review. and J. Patchell 'or a rev.ew. This rese.eII..as ~ by
<br />the NSF.
<br />
<br />Evolutionary significance of
<br />introgressive hybridization
<br />in cyprinid fishes
<br />
<br />~.
<br />..'\ ~ Thoma E. Dowlng & Bruce D. DeMarais
<br />
<br />\, \\~ Department of Zoology, Arizona ~tate UniverSity. Tempe.
<br />tf Arizona 85287.1501. USA
<br />fJ ,,'I .
<br />~'/1 \ BOTANISTS have 10nK recognized intr?fressive hybridization as
<br />) 'v "'\.... Important i. the evolution of plants' . Hybridization among
<br />f. r animals is also common), yet few zoologists have considered it
<br />)(~ evolutionarily important-. We report here that in the genus Gila,
<br />, .. a morphologically diverse group of minnows from western North
<br />~(" America 7, we haye discovered a pervasive Inftuence or hybridization
<br />througbout their evolutionary histories. CeDe exchaage among_
<br />distiactive forms bas contributed to existing divenity, supporting
<br />the hypothesis that introgressive hybridization can play a sig-
<br />nificant role i. eyolution or vertebrates.
<br />Cyprinid fishes of the genus Gila have attracted considerable
<br />attention because of their unique morphological adaptations to
<br />dive~e riverine habitats8. Three morphologically distinctive
<br />species were broadly sympatric throughout the Colorado River
<br />b~sin. Gila cypha once occupied canyon-bound reaches of the
<br />middle and upper basin. whereas G. elegans fonnerly occupied
<br />~ost of t~e ba~in's largest rivers. Both possess slender, stream-
<br />lined bodies With leathery skins, embedded scales. and falcate
<br />fins, .characteristics well suited for surviving in abrasive, high.
<br />gradient, flood-~rone rivers. A third species, G. robusla, exhibits
<br />a. more generalized morphology and lives in large and small
<br />nvers throughout the basin.
<br />Morphologica~ an~lysesB-IO indicated that G. cypha, elegans
<br />and robu~la m~lDtalD their distinctiveness in sympatry, with
<br />pben~K:aJly mtcrmediate individuals occasionally notedll,12.
<br />In a~dltlon, geographically isolated stocks such as G. seminuda
<br />(until recently a subspecies of G. robusta) were found to be
<br />morphol.ogically intennediate to these three species, prompting
<br />specu.'atl?n of hybrid originS. Molecular and morphological
<br />e~~n of .G: s~minuda supponed its hypothesized hybrid
<br />onglD, as It exhibited a mosaic of allozymic and mitochondrial
<br />DNA characteristics otherwise unique to G. elegans or robusta u.
<br />
<br />444
<br />
<br />To funher study the potential significance of introgression
<br />among Colorado River Gila, we examined allozymic and
<br />mtDNA. variation within and among taxa. Morphologically
<br />identified Go cypha, elegans and robusla from now-allopatric
<br />populations exhibited unique allozymic and mtDNA charac-
<br />teristics (Table I), identifying them as distinct phylogenetic
<br />lineages. Despite this evolutionary independence. local
<br />. intro&res~ion among all three taxa has clearly occurred in the
<br />"past: In tile upper Colorado River basin where G. robusla and
<br />cypha now coexist, morphologically identified G. rob uSIa
<br />exhibited allozyme characters of both species and mtDNA typi.
<br />cal of G. cypha (Table I). Isolates (such as G. seminuda, from
<br />the Virgin River; G. robusla jordani from the pluvial White
<br />River, a now-isolated tributary to the Virgin River; and G. -
<br />robusla from the upper Little Colorado River) also possessed
<br />derived allozyme alleles of all three species (Table I) and G.
<br />cypha or elegaRS mtDNA (Table I). Neither G. cypha nor elegans
<br />have been recorded from the Virgin, pluvial White, or upper
<br />Little Colorado rivers, with the last two systems isolated by
<br />barriers at least since mid.Holocenel4 and late PleistoceneU,
<br />respectively.
<br />Hybridization was funher examined by contrasting phy-
<br />logenies inferred from allozymes and mtDNA 16. Gila atraria,
<br />native to the adjacent Bonneville basin", was included because
<br />of similarity of its mtDNA to that of Colorado basin Gila
<br />(T.E.D., unpublished data). Gila bieolor, a distant relative'8. was
<br />included as an outgroup. Phenetic and cladistic analyses pro-
<br />duced similar topologies within character sets, but results were
<br />discordant between sets (Fig. J). With allozyme data (Fig. I a, b),
<br />Colorado basin Gila were monophyletic. Where hybridization
<br />was suspected and all tissues available. populations were
<br />phenetically intennediate to G. robusla and cypha. The distantly
<br />related G. alraria and bicolor comprised independent
<br />clusters/clades, neither similar to Colorado River forms.
<br />Topologies based on mtDNA characters (Fig. Ie, d) were in
<br />sharp contrast. Although populations of G. robusta from the
<br />lower Colorado basin were distinct, robusta from the Lillie
<br />Colorado and Green rivers and G. robustajordani haJ mtDNA
<br />similar to that of cypha, and G. seminuda exhibited mtDNA
<br />almost identical to G. elegans. Most notably, Colorado basin
<br />Gila were not monophyletic, as G. cypha mtDNA was more
<br />closely related to that of G. atraria (especially from the Sevier
<br />River) than it was to either G. elegans or robusta mtDNAs.
<br />
<br />NATURE. VOL 362 . 1 APRIL 1993
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