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CONSTRAINTS ON UNISEXUAL VERTEBRATESual cl<strong>on</strong>es of an asexual "species" tend to exploit differentpatches in a spatially heterogeneous envir<strong>on</strong>ment, <strong>the</strong>n <strong>the</strong>l<strong>on</strong>gevity of each cl<strong>on</strong>e would be a functi<strong>on</strong> of <strong>the</strong> predictability<str<strong>on</strong>g>and</str<strong>on</strong>g> l<strong>on</strong>gevity of its specialized subniche. However, in anenvir<strong>on</strong>ment that changes unpredictably between generati<strong>on</strong>s,cl<strong>on</strong>al specialists will rapidly disappear. Such temporalunpredictability should favor <strong>the</strong> cl<strong>on</strong>e with a general-purposegenotype (Lynch, 1984). The maintenance of cl<strong>on</strong>al diversity intemporally unpredictable envir<strong>on</strong>ments requires c<strong>on</strong>stant replacementby new cl<strong>on</strong>es as old cl<strong>on</strong>es are lost. Over <strong>the</strong> l<strong>on</strong>gterm,sexual species are more likely to persist in temporallyunpredictable envir<strong>on</strong>ments, because in each generati<strong>on</strong> <strong>the</strong>yregenerate a broad array of genotypes that exploit ephemeralresources as <strong>the</strong>y appear <str<strong>on</strong>g>and</str<strong>on</strong>g> disappear (Bell, 1982).Destabilizing hybridizati<strong>on</strong>.- The geographical distributi<strong>on</strong>of some par<strong>the</strong>nogens may be limited by <strong>the</strong>ir inability toblock <str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> interference by <strong>the</strong>ir sexual relatives (Lynch,1984). Occasi<strong>on</strong>al matings of par<strong>the</strong>no<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> lizards with <strong>on</strong>eof <strong>the</strong>ir sexual ancestors can lead to higher levels of polyploidy<str<strong>on</strong>g>and</str<strong>on</strong>g> sterility. The disjunct distributi<strong>on</strong>s of many par<strong>the</strong>no<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g>forms <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>the</strong>ir sexual ancestors may be due to <strong>the</strong> inabilityof par<strong>the</strong>nogens to protect <strong>the</strong>ir mode of reproducti<strong>on</strong>from disruptive matings. This interesting c<strong>on</strong>straint <strong>on</strong> <strong>the</strong>geographical distributi<strong>on</strong> of par<strong>the</strong>nogens warrants fur<strong>the</strong>rinvestigati<strong>on</strong>. It might also affect <strong>the</strong> distributi<strong>on</strong> of some hybridogens(Lynch, 1984).Muller's ratchet.- Muller (1964) proposed that asexual reproducti<strong>on</strong>is like a ratchet mechanism; <strong>the</strong> fitness of an asexuallineage cannot increase from generati<strong>on</strong> to generati<strong>on</strong>, but instead,it can <strong>on</strong>ly decline because of an accumulati<strong>on</strong> of deleteriousmutati<strong>on</strong>s. The ratchet will advance if <strong>the</strong> genome-widemutati<strong>on</strong> rate is high, if selecti<strong>on</strong> against new mutati<strong>on</strong>s isweak, <str<strong>on</strong>g>and</str<strong>on</strong>g> if <strong>the</strong> effective size of <strong>the</strong> asexual populati<strong>on</strong> issmall (Maynard Smith, 1978). For vertebrates, <strong>the</strong> genomicmutati<strong>on</strong> rate is probably high enough that most eggs willcarry at least <strong>on</strong>e slightly deleterious mutati<strong>on</strong> (Vrijenhoek,1984b). Small populati<strong>on</strong>s are affected because offspring carryingno new mutati<strong>on</strong>s will have a good chance of not occurringin <strong>the</strong> next generati<strong>on</strong>. Whe<strong>the</strong>r <strong>the</strong> ratchet mechanismreally limits unisexual vertebrates is unknown, but if unisexualhybrids have elevated mutati<strong>on</strong> rates due to hybrid dysgenesis(Kidwell et al., 1977), <strong>the</strong> ratchet mechanism mightc<strong>on</strong>tribute to a significant reducti<strong>on</strong> in mean fitness over a fewhundred generati<strong>on</strong>s [see Pamilo et al. (1987) for computersimulati<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> an analysis of this problem]. Evidence for anaccumulati<strong>on</strong> of potentially deleterious mutati<strong>on</strong>s has beenfound in hybrido<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> Poeciliopsis <str<strong>on</strong>g>and</str<strong>on</strong>g> Rana (Leslie <str<strong>on</strong>g>and</str<strong>on</strong>g> Vrijenhoek,1978, 1980; Spinella <str<strong>on</strong>g>and</str<strong>on</strong>g> Vrijenhoek, 1982; Hotz,1983; Graf <str<strong>on</strong>g>and</str<strong>on</strong>g> Polls, this volume). Never<strong>the</strong>less, in regi<strong>on</strong>swhere endemic cl<strong>on</strong>al <strong>origins</strong> are possible, <strong>the</strong> ratchet mechanismprobably does not limit unisexual populati<strong>on</strong>s, becausenew cl<strong>on</strong>es might arise often enough to replace deterioratingcl<strong>on</strong>es.FROM NO-HOPERS TO HOPEFUL MONSTERSThe c<strong>on</strong>clusi<strong>on</strong> that asexuality per se is an evoluti<strong>on</strong>ary deadend for higher organisms is well supported by <strong>the</strong> overall taxo-nomic rarity of asexual "species" of vertebrates <str<strong>on</strong>g>and</str<strong>on</strong>g> insects(Maynard Smith, 1978; White, 1978). Although morphologically<str<strong>on</strong>g>and</str<strong>on</strong>g> <str<strong>on</strong>g>ecological</str<strong>on</strong>g>ly diverse cl<strong>on</strong>al lineages have arisen polyphyleticallyfrom extant sexual relatives, J am unaware of substantiveevidence that any strictly asexual lineage of higherplants or animals has generated morphologically or <str<strong>on</strong>g>ecological</str<strong>on</strong>g>lydiscrete descendent forms that also reproduce asexually[bdelloid rotifers <str<strong>on</strong>g>and</str<strong>on</strong>g> traminine aphids might be excepti<strong>on</strong>s,but careful <str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> studies are needed (Maynard Smith,1986)]. Lynch <str<strong>on</strong>g>and</str<strong>on</strong>g> Gabriel (1983) found that phenotypic diversificati<strong>on</strong>could occur at similar rates in sexual <str<strong>on</strong>g>and</str<strong>on</strong>g> asexualpopulati<strong>on</strong>s, but <strong>the</strong> asexual mutati<strong>on</strong> rate had to be at leasttwice that of <strong>the</strong> sexual lineage. Elevated mutati<strong>on</strong> ratesmight occur in asexual lineages, but never<strong>the</strong>less <strong>the</strong> assumpti<strong>on</strong>that most mutati<strong>on</strong>s might c<strong>on</strong>tribute to adaptive variati<strong>on</strong>is not realistic. Because most new mutati<strong>on</strong>s are mildlydeleterious (Simm<strong>on</strong>s <str<strong>on</strong>g>and</str<strong>on</strong>g> Crow, 1977), <strong>the</strong> ratchet mechanism<str<strong>on</strong>g>and</str<strong>on</strong>g> associated <str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> "hitchhiking" are more likely to causedeteriorati<strong>on</strong> of cl<strong>on</strong>es (Felsenstein, 1974).If asexuality c<strong>on</strong>tributes anything to <strong>the</strong> evoluti<strong>on</strong> of biologicaldiversity, it might serve as a temporary vehicle for <strong>the</strong> replicati<strong>on</strong>of unusual gene combinati<strong>on</strong>s. In his seminal papers<strong>on</strong> unisexuality, hybridizati<strong>on</strong>, <str<strong>on</strong>g>and</str<strong>on</strong>g> polyploidy in <strong>the</strong> vertebrates,Schultz (1969, 1980) suggested that asexual reproducti<strong>on</strong>might be a stepping-st<strong>on</strong>e in <strong>the</strong> evoluti<strong>on</strong> of gene duplicati<strong>on</strong><str<strong>on</strong>g>and</str<strong>on</strong>g> higher ploidy levels. For example, additi<strong>on</strong> of a fourthgenome (A) to a cl<strong>on</strong>ally reproducing allotriploid form (ABB)may balance meiosis <str<strong>on</strong>g>and</str<strong>on</strong>g> <strong>the</strong>reby produce a new sexually reproducingallotetraploid (AABB) species (see also Astaurov,1969; Vasilev et al., this volume). Perhaps <strong>the</strong> tax<strong>on</strong>omicallydiverse, sexually reproducing tetraploid fishes in <strong>the</strong> familiesSalm<strong>on</strong>idae <str<strong>on</strong>g>and</str<strong>on</strong>g> Catastomidae arose in this way; but, unfortunately,<strong>the</strong> evoluti<strong>on</strong>ary histories of <strong>the</strong>se fishes are obscuredby <str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> changes that have accumulated in <strong>the</strong> 25 to 100 milli<strong>on</strong>years since <strong>the</strong>y arose (see Allendorf <str<strong>on</strong>g>and</str<strong>on</strong>g> Thorgaard, 1984;Ferris, 1984). Unisexuality appears to be involved in <strong>the</strong> morerecent <strong>origins</strong> of some polyploid cyprinid fishes (Collares-Pereira, 1985; this volume).A sexual species of unisexual origin.- Given <strong>the</strong> pessimistic<strong>the</strong>me expressed in <strong>the</strong> introducti<strong>on</strong>, it is perhaps fitting that Ishould c<strong>on</strong>clude this paper by reporting <strong>the</strong> discovery of a newsexual species whose immediate ancestor was a unisexual vertebrate(a formal descripti<strong>on</strong> of this new species is in preparati<strong>on</strong>;herein I summarize <strong>the</strong> salient points). A morphologicallydistinct, diploid species of Poeciliopsis inhabits a marshyarea in <strong>the</strong> headwaters of <strong>the</strong> Rio Magdelena (S<strong>on</strong>ora, Mexico).Allozyme studies indicate that it arose from a hybrido<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g>form, P. m<strong>on</strong>acha-occidentalis hemicl<strong>on</strong>e Ia (Angus, 1980),with which it still occurs. Apparently, <strong>the</strong> hybrido<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g>mechanism broke down, <str<strong>on</strong>g>and</str<strong>on</strong>g> recombinati<strong>on</strong> occurred between<strong>the</strong> m<strong>on</strong>acha <str<strong>on</strong>g>and</str<strong>on</strong>g> occidentalis genomes. This resulted in a new,self-perpetuating, mosaic genotype that reproduces sexually,has normal Mendelian inheritance, <str<strong>on</strong>g>and</str<strong>on</strong>g> has a 1:1 primary sexratio.Crossing experiments revealed that <strong>the</strong> new species isreproductively isolated from both its hybrido<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> ancestor<str<strong>on</strong>g>and</str<strong>on</strong>g> P. occidentalis. I plan to name this new species in h<strong>on</strong>or ofR. Jack Schultz, a foresighted pi<strong>on</strong>eer in this field of research.Although an apparent c<strong>on</strong>tradicti<strong>on</strong>, this discovery clearly28