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CONSTRAINTS ON UNISEXUAL VERTEBRATESsupports my general <strong>the</strong>sis that strictly asexual reproducti<strong>on</strong>is an evoluti<strong>on</strong>ary dead end. The unisexual lineage that gaverise to this new sexual species was itself drawn from <strong>the</strong> recombinantvariati<strong>on</strong> in a sexual gene pool. When that unisexuallineage gave rise to something new, it did so because cl<strong>on</strong>alreproducti<strong>on</strong> broke down. The new sexual species is a productof recombinati<strong>on</strong>, not cl<strong>on</strong>ing. Although mutati<strong>on</strong>al changeshave occurred in <strong>the</strong> hemicl<strong>on</strong>al genomes that gave rise to thisnew species (Spinella <str<strong>on</strong>g>and</str<strong>on</strong>g> Vrijenhoek, 1982), <strong>the</strong> critical transiti<strong>on</strong>sin this unusual speciati<strong>on</strong> pathway involved recombinati<strong>on</strong>.SUMMARY AND CONCLUSIONSIn general, <strong>the</strong> origin of new unisexual vertebrates depends<strong>on</strong> a delicate balance between meiotic disrupti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> developmentalcoordinati<strong>on</strong> in interspecific hybrids (We<strong>the</strong>ringt<strong>on</strong> etal., 1987; Moritz et al., this volume). Once formed, a new unisexuallineage must ei<strong>the</strong>r invade a sexual populati<strong>on</strong> or becomeestablished in habitats that are <str<strong>on</strong>g>ecological</str<strong>on</strong>g>ly suboptimalfor its sexual ancestors. Some par<strong>the</strong>no<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> lizards arethought adept at col<strong>on</strong>izati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> occupati<strong>on</strong> of marginal habitats,but o<strong>the</strong>rs coexist with <strong>on</strong>e or more of <strong>the</strong>ir sexual relatives.Sperm-dependent unisexual fishes <str<strong>on</strong>g>and</str<strong>on</strong>g> amphibiansmust live with a sexual relative. Fur<strong>the</strong>rmore, <strong>the</strong> c<strong>on</strong>diti<strong>on</strong>snecessary for a stable sperm-dependent relati<strong>on</strong>ship may behighly restrictive (Stenseth et al., 1985). Studies of <str<strong>on</strong>g>ecological</str<strong>on</strong>g>interacti<strong>on</strong>s between unisexual hybrids <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>the</strong>ir sexual ancestorscan offer many new insights <strong>on</strong> factors that promotecoexistence between closely related taxa that are morphologically<str<strong>on</strong>g>and</str<strong>on</strong>g> behaviorally very similar.Discoveries of abundant cl<strong>on</strong>al diversity in many unisexualpopulati<strong>on</strong>s should alert us to <strong>the</strong> danger of erecting simple<str<strong>on</strong>g>ecological</str<strong>on</strong>g> hypo<strong>the</strong>ses that assume <strong>the</strong> existence of <strong>on</strong>ly <strong>on</strong>e"kind" of bisexual form <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>on</strong>e "kind" of unisexual form instable complexes. Evidence for niche partiti<strong>on</strong>ing between coexistingcl<strong>on</strong>es <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>the</strong>ir sexual ancestors is accumulatingfrom careful field studies (Schenck <str<strong>on</strong>g>and</str<strong>on</strong>g> Vrijenhoek, 1986, thisvolume). Most of <strong>the</strong> cl<strong>on</strong>al diversity observed in unisexualvertebrates stems from multiple hybridizati<strong>on</strong> events. Experimentalstudies are showing that new cl<strong>on</strong>es are likely to differsubstantially in many traits that could affect resource use inspatially <str<strong>on</strong>g>and</str<strong>on</strong>g> temporally heterogeneous envir<strong>on</strong>ments(We<strong>the</strong>ringt<strong>on</strong>, 1988). Over <strong>the</strong> l<strong>on</strong>g term, individual cl<strong>on</strong>esprobably are evoluti<strong>on</strong>ary dead ends, but <strong>the</strong> unisexual populati<strong>on</strong>might persist because of recurrent cl<strong>on</strong>al <strong>origins</strong>. Thishypo<strong>the</strong>sis needs to be tested. Applicati<strong>on</strong> of molecular approacheswill help us learn more about <strong>the</strong> relative ages of individualcl<strong>on</strong>es. The limited data currently available suggestthat extant unisexual vertebrates represent relatively recentevoluti<strong>on</strong>ary events (Avise <str<strong>on</strong>g>and</str<strong>on</strong>g> Vrijenhoek, 1987; Goddard etal., this volume; Lowcock, this volume; Moritz et al., this volume).Despite <strong>the</strong> <str<strong>on</strong>g>ecological</str<strong>on</strong>g> success of cl<strong>on</strong>ally diverse populati<strong>on</strong>sof unisexual fishes, amphibians, <str<strong>on</strong>g>and</str<strong>on</strong>g> lizards, we lack evidencethat strictly asexual lineages are clado<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g>, that <strong>the</strong>y cansplit into diversified descendent cl<strong>on</strong>al lineages. Although <strong>the</strong>apparent example of speciati<strong>on</strong> reported here does involve aunisexual ancestor, <strong>the</strong> product of this event was a new sexualspecies, not a new cl<strong>on</strong>e. Fur<strong>the</strong>rmore, <strong>the</strong> origin of this morphologically<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>ecological</str<strong>on</strong>g>ly distinct species involved <str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g>recombinati<strong>on</strong>, <strong>the</strong> essence of sexuality. Clado<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> speciati<strong>on</strong><str<strong>on</strong>g>and</str<strong>on</strong>g> sexual recombinati<strong>on</strong> may be closely intertwined(Stanley, 1975). The phylo<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> rarity of asexual "species"would not appear, <strong>the</strong>refore, to require any special explanati<strong>on</strong>s.A low rate of origin of novel unisexual forms might simplybe balanced by a high rate of extincti<strong>on</strong>, keeping <strong>the</strong> overallfrequency of unisexual "species" low. lb draw an analogywith events at <strong>the</strong> genic level, we could imagine all unisexual"species" as mutants <str<strong>on</strong>g>and</str<strong>on</strong>g> all <strong>the</strong> bisexual species as wild-typealleles at a hypo<strong>the</strong>tical locus c<strong>on</strong>trolling meiosis. As such, unisexualityis not even a valid polymorphism. The tax<strong>on</strong>omicrarity of unisexual "species" could be determined by a higherlevel process comparable to mutati<strong>on</strong>-selecti<strong>on</strong> balance at <strong>the</strong>genic level.ACKNOWLEDGEMENTSI wish to thank my colleagues, J. We<strong>the</strong>ringt<strong>on</strong>, R. Schenck,K. Kotora, S. Weeks, J. Quattro, D. Baldwin, L. Vukovich, <str<strong>on</strong>g>and</str<strong>on</strong>g>C. Sadowski for <strong>the</strong>ir collaborati<strong>on</strong> <strong>on</strong> <strong>the</strong> Poeciliopsis project.Research was supported by Grant BSR86-00661 from <strong>the</strong> Nati<strong>on</strong>alScience Foundati<strong>on</strong>.LITERATURE CITEDALLENDORF, F. W, AND G. H. THORGAARD. 1984. Thtraploidy<str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>the</strong> evoluti<strong>on</strong> of salm<strong>on</strong>id fishes, pp. 1-53. In: Evoluti<strong>on</strong>aryGe<strong>net</strong>ics of Fishes. B. J. Turner (ed.). Plenum Press, New York.ANGUS, R. A. 1980. Geographical dispersal <str<strong>on</strong>g>and</str<strong>on</strong>g> cl<strong>on</strong>al diversity inunisexual fish populati<strong>on</strong>s. Amer. Natur. 115:531-550.ASTAUROV, B. L. 1969. Experimental polyploidy in animals. Ann.Rev. Ge<strong>net</strong>. 3:99-126.AVISE, J. C., AND R. C. VRIJENHOEK. 1987. Mode of inheritance<str<strong>on</strong>g>and</str<strong>on</strong>g> variati<strong>on</strong> of mitoch<strong>on</strong>drial DNA in hybrido<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> fishes of<strong>the</strong> genus Poeciliopsis. Mol. Biol. Evol. 4:514-525.AVISE, J. C., J. C. PATTON, AND C. F. AQUADRO. 1980. Evoluti<strong>on</strong>ary<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g>s of birds. I. Relati<strong>on</strong>ships am<strong>on</strong>g North Americanthrushes <str<strong>on</strong>g>and</str<strong>on</strong>g> allies. The Auk 97:135-147.BAKER, H. G. 1965. Characteristics <str<strong>on</strong>g>and</str<strong>on</strong>g> modes of origin of weeds, pp.147-172. In: Ge<strong>net</strong>ics of Col<strong>on</strong>izing Species. H. G. Baker <str<strong>on</strong>g>and</str<strong>on</strong>g> G.L. Stebbins (eds.). Academic Press, New York.BELL, G. 1982. The Masterpiece of Nature: The Evoluti<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> Ge<strong>net</strong>icsof Sexuality. Univ. Calif. Press, Berkeley.BERGER, L. 1971. Viability, sex <str<strong>on</strong>g>and</str<strong>on</strong>g> morphology of F2 generati<strong>on</strong>within forms of Rana esculenta-complex. Zoo!. Pol<strong>on</strong>iae 21:373-393.BULGER, A. J., AND R. J. SCHULTZ. 1979. Heterosis <str<strong>on</strong>g>and</str<strong>on</strong>g> intercl<strong>on</strong>alvariati<strong>on</strong> in <strong>the</strong>rmal tolerance in unisexual fish. Evoluti<strong>on</strong>33:848-859.BULGER, A. J., AND R. J. SCHULTZ. 1982. Origins of <strong>the</strong>rmal adaptati<strong>on</strong>in nor<strong>the</strong>rn vs. sou<strong>the</strong>rn populati<strong>on</strong>s of a unisexual hybridfish. Evoluti<strong>on</strong> 36:1041-1050.CASE, M. L., AND T. J. TAPER. 1986. On <strong>the</strong> coexistence <str<strong>on</strong>g>and</str<strong>on</strong>g> coevoluti<strong>on</strong>of asexual <str<strong>on</strong>g>and</str<strong>on</strong>g> sexual competitors. Evoluti<strong>on</strong> 40:366-387.29