Hybridspeciationwithexternalbarriers: Encelia (Asteraceae:Heliantheae),acasestudy CurtisClarkandGerardJ.Allan BiologicalSciences,CaliforniaStatePolytechnicUniversity,Pomona,andRanchoSantaAnaBotanicGarden Abstract Most well-documented cases of homoploid hybrid speciation follow the recombinational model, in which the new species passes through a period of genetic rearrangement and reduced fertility. An alternate model has been presented, in which external barriers to gene flow prevent genetic swamping of the new species by the parent species. The two documented cases of hybrid speciation in Encelia (E. virginensis and E. asperifolia) appear to follow this model. In both cases, the species of hybrid origin are morphologically intermediate to the parents, and E. virginensis is also similar to artificial F1 hybrids. In both cases, the species of hybrid origin share random amplified polymorphic DNA (RAPD) markers with each of the parent species. A single sampled individual of each species of hybrid origin combines the nrDNA internal transcribed spacer (ITS) sequences of its parents. All North American Encelia species are obligate outcrossers with n = 18 chromosomes, and in cultivation they form fertile hybrids apparently in all combinations. In the natural environment, plants of F1 phenotype are locally common, but backcross progeny are rare, being confined primarily to areas of human disturbance, and there is no evidence of introgression. Progeny tests in one case of natural hybridisation suggested that backcross progeny are eliminated after seed dispersal. This suggests that the F1 phenotype represents a “local optimum” on the selective landscape, and that the selection against backcrosses strongly reduces gene flow between the incipient hybrid species and the parents. Espèciation hybride avec barrières externes: Encelia (Asteraceae: Heliantheae), une étude de cas La plupart des cas bien documentés de l’espèciation homoploïde hybride suivent le modèle recombinaisonal, où l’espèce nouvelle passe par une durée de réarrangement génétique et la fertilité reduite. Un modèle alternatif a été présenté, où les barrières externes à flux génétique détournent la submersion génétique de l’espèce nouvelle par l’espèce parentale. Apparemment les deux cas documentés de l’espèciation hybride en Encelia (E. virginensis et E. asperifolia) suivent ce modèle. En tous les deux cas, les espèces d’origine hybride sont intermédiaire morphologiquement aux parents, et E. virginensis resemble aux hybrides artificielles F1. En tous les deux cases, les espèces d’origine hybride partagent indicateurs random amplified polymorphic DNA (RAPD) avec chacun des espèces parentales. Un seul individu échantillonné de chaque espèce d’origine hybride combine les séquences de l’espacement interne transcrit de nrDNA (ITS) des parents. Toutes les espèces de Encelia en Amérique du Nord exigent la pollinisation extérieure et tiennent n = 18 chromosomes. Apparemment les hybrides fertiles forment en culture en toutes combinaisions. En milieu naturel, les plantes de phénotype F1 sont commun de lieu, mais les descendants revers-métis (backcross) sont rare, étant limités surtout aux zones de perturbation anthropogénique, et il n’y a pas d’évidence d’introgression. En un cas d’hybridation naturelle, les épreuves de la descendance suggérèrent que les descendants revers-métis sont éliminés après la dispersion des séminaux. Ce suggére que le phénotype F1 représente un «optimum local» dans le paysage sélectif, et que la sélection contre les descendants revers-métis réduit fortement le flux génétique entre l’espèce hybride nouvelle et ses parents. The purpose of this poster is to propose that two species of Encelia of hybrid origin conform to Grant’s “external barriers” model of hybrid speciation, rather than the better-known “recombinational” model. The species are E. virginensis A. Nels., derived from E. actoni Elmer and E. frutescens (A. Gray) A. Gray, and E. asperifolia (S.F. Blake) Clark & Kyhos, derived from E. californica Nutt. and a glandular form of E. frutescens. Both species satisfy the three criteria for recognition of species of hybrid origin: intermediacy between the parents, similarity to F1 hybrids, and shared apomorphies with both parents. Not all documented species of hybrid origin satisfy all three criteria, and there is no theoretical reason that they should, but the nature of the speciation process outlined here makes it more probable. Enceliavirginensis E.actoni E.virginensis E.frutescens Althoughintermediateinmanyrespects,E.virginensis ismoreeasily confusedwithE.actonithanE.frutescens . Enceliaactoni inhabits Nevada Utah desertmountainsand California E.virginensis thewesternmarginsof thedeserts. Encelia E.actoni Arizona frutescens isfoundin thelowestSonoranand Mojavedeserts. Encelia virginensis occursinthe northeasternMojave Desert.Itissympatric E.frutescens withitsputativeparents Sonora inpartsofitsrange. 4.0 30 E. frutescens Encelia frutescens 3.5 25 3.0 E. virginensis 20 2.5 15 Achene Width (mm) E. actoni Encelia virginensis Mean Annual Temperature (C) Encelia actoni 2.0 10 6.0 6.5 7.0 7.5 8.0 10 15 20 25 30 Achene Length (mm) Mean Annual Range of Temperature (C) The achenes of E. virgin- Encelia virginensis inhabits ensis are intermediate in regions with an inter- length and width to those mediate mean temper- of the putative parents. ature and a slightly more extreme range of temper- ature. 20 E. frutescens A principal co-ordinate analysis based on eight 10 characters of the leaves and heads shows E. 0 virginensis to be generally E. actoni intermediate to its putative -10 parents. Second Coordinate Axis E. virginensis -20 -20 -10 0 10 20 30 40 First Coordinate Axis Characters Used pedicel width length of petiole number of rays width of leaf length of ray height of head length of leaf width of head E.virginensis has chimericITS IntheITSregionofthe nrDNA,E.actoniand E. frutescens differby13 E.frutescens bases.Thesingle examined E.virginensis hasnouniquebases, E.virginensis andsharesthebaseof oneparentortheother atallsites.Thechart E.actoni depictsascenariofor thechimericDNA, 0 200 400 600 assumingtheminimum Length(bases) crossing-over. Shared Apomorphies E. actoni E. virginensis E. frutescens morphological broad, multicellular- broad, multicellular- based hairs based hairs RAPD UBC 301 900bp UBC 301 900bp UBC 301 600bp UBC 301 600bp UBC 302 1200bp UBC 302 1200bp (E. actoni has no clear morphological autapomorphies) Enceliaasperifolia E.californica E.asperifolia E.frutescens glandularform EnceliaasperifoliaissimilarinappearancetoE.californica ;itwas originallynamedasavarietyofthatspecies. Enceliacalifornica livesin coastalsagescrub.The California Arizona glandularformof E. frutescens inhabitsthe drydesertintherain shadowofthenorthern E. californica mountainsofBaja California. Encelia asperifolia isfoundinthe E.asperifolia rockycoastaldesertsof Baja BajaCalifornia.Itis narrowlysympatricto E. California californica ,butallopatric toE.frutescens . E.asperifolia has IntheITSregionofthe chimericITS nrDNA,E.californica and E.frutescens differby21 bases.Thesingleexam- E.frutescens inedE.asperifolia differs fromeitherparentat threeofthosesites,but E.asperifolia sharesthebaseofone parentortheotheratthe rest.Thechartdepictsa E.californica scenarioforthechimeric DNA,assumingmini- 0 200 400 600 mumcrossing-over.The Length(bases) threeuniquesitesin E. asperifolia areinthered region. Shared Apomorphies E. californica E. asperifolia E. frutescens morphological UV-reflective ray UV-reflective ray corollas corollas brown disk corollas brown disk corollas moniliform hairs moniliform hairs broad multicellular- broad multicellular- based hairs based hairs no benzopyrans or no benzopyrans or benzofurans benzofurans yellow stigmas yellow stigmas Shared Apomorphies E. californica E. asperifolia E. frutescens RAPD UBC 218 800bp UBC 218 800bp UBC 218 1600bp UBC 218 1600bp UBC 382 1400bp UBC 382 1400bp UBC 409 500bp UBC 409 500bp UBC 478 1400bp UBC 478 1400bp Operon B8 750bp Operon B8 750bp UBC 149 700bp UBC 149 700bp UBC 375 1000bp UBC 375 1000bp The Encelia species described here clearly do not fit the model of recombinational speciation: Recombinational Model Encelia Chromosome differences No apparent chromosome between parent species differences between parents Hybrids of reduced fertility, Hybrids of full fertility, often often uncomon common Internal reproductive barriers Fertile backcrosses are between hybrids species and possible, but eliminated in parent species the wild after seed dispersal However, they do agree with Grant’s model of speciation with external barriers. Although Grant provided more examples of hybrid speciation with external barriers than of recombinational speciation, until recently most of the cases confirmed by molecular evidence have been of the latter. Wolfe and Elisens re-examined Grant’s example from Penstemon, demonstrating that, although the F1 hybrids had reduced fertility, external barriers, including floral differences that affect pollination, were important in maintaining the new species. In Encelia, hybrids have full fertility, so in a sense this represents an extreme case of this form of speciation. In addition, there are no floral isolating mechanisms in the genus. Kyhos et al. showed that backcross progeny in Encelia are routinely produced but almost never survive. This phenomenon provides the necessary external barrier, although its basis is still unknown..