genetic and ecological constraints on the origins ... - Nativefishlab.net
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VRIJENHOEK<str<strong>on</strong>g>and</str<strong>on</strong>g> abundance of individual diploid <str<strong>on</strong>g>and</str<strong>on</strong>g> triploid unisexualcl<strong>on</strong>es are far more complex (Schenck <str<strong>on</strong>g>and</str<strong>on</strong>g> Vrijenhoek, 1986,this volume).Heterotic generalists.- Because of <strong>the</strong> str<strong>on</strong>g coupling betweenhybridizati<strong>on</strong> <str<strong>on</strong>g>and</str<strong>on</strong>g> unisexuality, some investigators believeunisexual organisms might be heterotic (Schultz, 1971,1977, 1982; Cole, 1975; White, 1978). The high levels of heterozygosityfound in unisexual organisms (reviewed by Moore,1984; Vrijenhoek, 1989) might allow <strong>the</strong>m to exploit a broaderrange of envir<strong>on</strong>mental c<strong>on</strong>diti<strong>on</strong>s than <strong>the</strong>ir parental species.However, Moore (1984) cauti<strong>on</strong>ed that a distincti<strong>on</strong> must bemade between hybrid vigor in somatic characteristics <str<strong>on</strong>g>and</str<strong>on</strong>g> trueheterosis in characters affecting Darwinian fitness. Based <strong>on</strong>his review of <strong>the</strong> populati<strong>on</strong> dynamics <str<strong>on</strong>g>and</str<strong>on</strong>g> biogeography ofunisexual Poeciliopsis, Moore (1984) found no substantive evidencefor heterotic fitness.Never<strong>the</strong>less, evidence for hybrid vigor in <strong>the</strong> form of broadenvir<strong>on</strong>mental tolerances <str<strong>on</strong>g>and</str<strong>on</strong>g> rapid growth has been reportedfor some unisexual strains (Schultz, 1982). When c<strong>on</strong>sidered in<strong>the</strong> c<strong>on</strong>text of envir<strong>on</strong>mental tolerances al<strong>on</strong>e, <strong>the</strong> heterosishypo<strong>the</strong>sis becomes a special form of <strong>the</strong> "general-purposegenotype" hypo<strong>the</strong>sis (Baker, 1965). Lynch (1984) clarified thishypo<strong>the</strong>sis by stating that general-purpose genotypes have <strong>the</strong>lowest variance in fitness in a temporally variable envir<strong>on</strong>ment(i.e., exhibit <strong>the</strong> highest geometrical mean fitness). Thehybrido<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> frog, Rana esculenta, appears to have broaderopti<strong>on</strong>s for overwintering survival than ei<strong>the</strong>r of its parentalspecies gunner <str<strong>on</strong>g>and</str<strong>on</strong>g> Nopp, 1979). Also, <strong>the</strong> hybrido<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> fishP. m<strong>on</strong>acha-lucida has broader <strong>the</strong>rmal tolerances than ei<strong>the</strong>rof its parental species, but this result could not be extended too<strong>the</strong>r unisexual forms (Bulger <str<strong>on</strong>g>and</str<strong>on</strong>g> Schultz, 1979). The triploidgyno<str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> form, P. m<strong>on</strong>acha-2 lucida, is more heterozygousthan <strong>the</strong> hybridogen but not more tolerant to <strong>the</strong>rmal extremes.Also, P. m<strong>on</strong>acha-occidentalis has high heterozygosity,but its <strong>the</strong>rmal tolerance is intermediate to its parentalspecies (Bulger <str<strong>on</strong>g>and</str<strong>on</strong>g> Schultz, 1982). Broad envir<strong>on</strong>mental tolerancesare not a generalizable c<strong>on</strong>sequence of hybridity inunisexual hybrids.We<strong>the</strong>ringt<strong>on</strong> et al. (1987) used syn<strong>the</strong>tic Poeciliopsism<strong>on</strong>acha-lucida strains to test whe<strong>the</strong>r <strong>the</strong> hybrids were sp<strong>on</strong>taneouslyheterotic for characters related to fitness. In <strong>the</strong> relativelyc<strong>on</strong>stant envir<strong>on</strong>ment of our laboratory, <strong>the</strong> syn<strong>the</strong>tichybridogens exhibit, <strong>on</strong> average, significantly lower survivorship,lower fertility, <str<strong>on</strong>g>and</str<strong>on</strong>g> higher frequencies of developmentalabnormalities than <strong>the</strong> sexual strains from which <strong>the</strong>y werederived. Although some syn<strong>the</strong>tic strains are comparable to<strong>the</strong> parental strains for <strong>the</strong>se characteristics, n<strong>on</strong>e are better.In c<strong>on</strong>trast, two natural unisexual strains have superior survival.Since all <strong>the</strong> unisexuals, syn<strong>the</strong>tic <str<strong>on</strong>g>and</str<strong>on</strong>g> natural, have essentially<strong>the</strong> same level of heterozygosity, <strong>on</strong>e cannot attribute<strong>the</strong> qualities found <strong>on</strong>ly in <strong>the</strong> natural strains to heterosis. Instead,<strong>the</strong> variance am<strong>on</strong>g strains is attributable to <strong>the</strong> differentcombining abilities of <strong>the</strong> haploid m<strong>on</strong>acha genomes thatwere 'frozen" in <strong>the</strong>se strains. Some m<strong>on</strong>acha genomes makegood combinati<strong>on</strong>s with lucida, <str<strong>on</strong>g>and</str<strong>on</strong>g> some do not. The fitness of<strong>the</strong>se unisexual hybrids is a property of <strong>the</strong> general combiningabilities of <strong>the</strong>ir hemicl<strong>on</strong>al m<strong>on</strong>acha genomes, not sp<strong>on</strong>taneousheterosis.Great cauti<strong>on</strong> must be exercised in making inferences about<strong>the</strong> sp<strong>on</strong>taneous fitness <str<strong>on</strong>g>and</str<strong>on</strong>g> niche breadth of all unisexual hybridsfrom studies of a few naturally occurring forms. Extantunisexuals are <strong>the</strong> good combinati<strong>on</strong>s, <strong>the</strong> genotypes that survivedin nature because <strong>the</strong>y can compete with or displace <strong>the</strong>bisexual ancestors from some porti<strong>on</strong> of <strong>the</strong> available niche(see below). We d<strong>on</strong>'t often get to see nature's failures. Laboratorysyn<strong>the</strong>sis of more unisexual hybrid forms is needed to establish<strong>the</strong> generality of this result.Frozen niche-variati<strong>on</strong>.- Vrijenhoek (1979, 1984a) proposedthat <strong>the</strong> l<strong>on</strong>g term success of unisexual populati<strong>on</strong>s depends<strong>on</strong> <strong>the</strong> opportunity for recruitment of new cl<strong>on</strong>es from <str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g>allyvariable sexual ancestors. Ge<strong>net</strong>ic variati<strong>on</strong> for lifehistory<str<strong>on</strong>g>and</str<strong>on</strong>g> niche-related characters is frozen <str<strong>on</strong>g>and</str<strong>on</strong>g> faithfullyreplicated in new cl<strong>on</strong>es. The c<strong>on</strong>sequence of intercl<strong>on</strong>al selecti<strong>on</strong>is a structured assemblage of cl<strong>on</strong>es that can subdivide<str<strong>on</strong>g>and</str<strong>on</strong>g> efficiently exploit food <str<strong>on</strong>g>and</str<strong>on</strong>g> spatial resources. Extensivecl<strong>on</strong>al diversity has been found in many unisexual vertebratepopulati<strong>on</strong>s (reviews in Parker, 1979; Moore, 1984; but seeCuellar, 1977b).Support for <strong>the</strong> frozen niche-variati<strong>on</strong> model has been foundin field <str<strong>on</strong>g>and</str<strong>on</strong>g> laboratory studies of unisexual Poeciliopsis. Ourprimary observati<strong>on</strong>s have been <strong>the</strong> following: (1) Relative to<strong>the</strong>ir sexual progenitors, unisexual populati<strong>on</strong>s composed ofmultiple cl<strong>on</strong>es are numerically more successful than m<strong>on</strong>ocl<strong>on</strong>alpopulati<strong>on</strong>s (Vrijenhoek, 1979, 1984b). (2) Multicl<strong>on</strong>alpopulati<strong>on</strong>s occupy a broader range of microhabitats thanm<strong>on</strong>ocl<strong>on</strong>al populati<strong>on</strong>s (Vrijenhoek, 1984a). (3) Differencesexist am<strong>on</strong>g cl<strong>on</strong>es in <strong>the</strong> use of food <str<strong>on</strong>g>and</str<strong>on</strong>g> spatial resources (Vrijenhoek,1978; Schenck <str<strong>on</strong>g>and</str<strong>on</strong>g> Vrijenhoek, 1986, this volume).Additi<strong>on</strong>al examples of <str<strong>on</strong>g>ecological</str<strong>on</strong>g> differentiati<strong>on</strong> am<strong>on</strong>g coexistinganimal cl<strong>on</strong>es have recently been reviewed (Bell, 1982;Lynch, 1984; Vrijenhoek, 1984a, 1989; Case <str<strong>on</strong>g>and</str<strong>on</strong>g> 'Paper, 1986;see also, Schultz <str<strong>on</strong>g>and</str<strong>on</strong>g> Fielding, this volume).The frozen niche-variati<strong>on</strong> model is based <strong>on</strong> <strong>the</strong> hypo<strong>the</strong>sisthat <strong>the</strong> sexual ancestors of cl<strong>on</strong>al lineages c<strong>on</strong>tain <str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g>variati<strong>on</strong> for traits affecting niche-breadth. My colleagues <str<strong>on</strong>g>and</str<strong>on</strong>g>I are using <strong>the</strong> syn<strong>the</strong>tic P. m<strong>on</strong>acha-lucida hybridogens totest this hypo<strong>the</strong>sis. The syn<strong>the</strong>tic hemicl<strong>on</strong>es have frozen significantdifferences in size at birth <str<strong>on</strong>g>and</str<strong>on</strong>g> juvenile growth rate(We<strong>the</strong>ringt<strong>on</strong>, 1988). They exhibit different growth rate reacti<strong>on</strong>norms under high <str<strong>on</strong>g>and</str<strong>on</strong>g> low quality diets (We<strong>the</strong>ringt<strong>on</strong>,1988). Early reproductive investment differs markedly am<strong>on</strong>g<strong>the</strong> strains (Weeks, pers. comm.). We are also examining <strong>the</strong>syn<strong>the</strong>tic hemicl<strong>on</strong>es for a variety of characteristics includingtrophic morphology, <str<strong>on</strong>g>and</str<strong>on</strong>g> foraging <str<strong>on</strong>g>and</str<strong>on</strong>g> predatory behavior.IT'S HARD TO PERSIST AS AUNISEXUAL VERTEBRATEThe disappearing niche.- The idea that asexual species areextincti<strong>on</strong>-pr<strong>on</strong>e has had a l<strong>on</strong>g history (reviewed by Bell,1982). The absence of <str<strong>on</strong>g>ge<strong>net</strong>ic</str<strong>on</strong>g> recombinati<strong>on</strong> in an asexual lineageis believed to prevent adjustments to a c<strong>on</strong>stantly changingphysical <str<strong>on</strong>g>and</str<strong>on</strong>g> biotic envir<strong>on</strong>ment. The c<strong>on</strong>cept of a changingenvir<strong>on</strong>ment must be c<strong>on</strong>sidered in two parts: spatialheterogeneity <str<strong>on</strong>g>and</str<strong>on</strong>g> temporal variati<strong>on</strong> (Bell, 1982). If individ-27