Genetic diversity in Asteraceae endemic to oceanic islands: Baker’s Law and polyploidy.

CRAWFORD D.; LOWREY T.; ANDERSON G.J.; BERNARDELLO G.; STUESSY T.

Systematics, Evolution, and Biogeography of Compositae.

International Assotiation of Plant Taxonomists

Lugar: Vienna; Año: 2009; p. 139 - 151

When considering the two factors ploidy and breedingsystem of ancestral colonizers together from the fi ve archipelagos,two things are most evident. First, the CanaryIslands are exceptional because of the frequency of diploidcolonizers compared to the other island groups ( Table9.2). The hypothesis advanced to explain this is the muchcloser proximity of the Canaries to a continental sourcearea than the other island systems; thus, multiple early introductionsmay have reduced the selective advantage ofthe greater genetic diversity aff orded the single polyploidpropagules dispersed to more remote archipelagos.The other notable observation is the greater frequencyof SC ancestors for Hawaii relative to the other island systems( Table 9.2). It is tempting to suggest that the higherfrequency of SC colonizers in Hawaii is due to the greaterdistance to source areas; however, the most spectacularradiation in Hawaii, the silversword alliance, originatedfrom SI or PSF ancestors ( Table 9.2). An equally largeradiation in Hawaii, but perhaps not as spectacular interms of ecological and morphological diversity as the silverswords,is Bidens where the thirty species originatedfrom a SC ancestor (Table 9.2). Bidens in Hawaii is a highpolyploid ( Table 9.2), so a single ancestor colonist couldhave carried extensive diversity to the island. Although148 Crawford, Lowrey, Anderson, Bernardello, Santos-Guerra and StuessyLiterature citedAdams, K.L., Cronn, R., Percifi eld, R. & Wendel, J.F.2003. Genes duplicated by polyploidy show unequal contributionsto the transcriptome and organ-specifi c silencing.Proceedings of the National Academy of Sciences of the United Statesof America 100: 4649–4654.Adams, K.L. & Wendel, J.F. 2005. Polyploidy and genome evolutionin plants. Current Opinion in Plant Biology 8: 135–141.Anderson, G.J., Bernardello, G., Stuessy, T.F. & Crawford,D.J. 2001. Breeding system and pollination of selected plantsendemic to Juan Fernández Islands. American Journal of Botany88: 220–233.Andrus, N., Trusty, J., Santos-Guerra, A., Jansen, R.K.& Francisco-Ortega, J. 2004. Using molecular phylogeniesto test phytogeographical links between East/South Africa-Southern Arabia and Macaronesian islands —a review, andthe case of Vierea and Pulicaria section Vieraeopis (Asteraceae).Taxon 53: 333–346.Archibald, J.K, Crawford, D.J., Santos-Guerra, A. &Mort, M.E. 2006. The utility of automated analysis of intersimplesequence repeat (ISSR) loci for resolving relationshipsin the Canary Island species of Tolpis (Asteraceae). AmericanJournal of Botany 93: 1154–1162.Bain, J.F. & Golden, J.L. 2000. A phylogeny of Packera (Senecioneae:Asteraceae) based on internal transcribed spacer regionsequence data and a broad sampling of outgroups.Molecular Phylogenetics and Evolution 16: 331–338.Baker, H.G. 1955. Self-compatibility and establishment after“long-distance” dispersal. Evolution 9: 347–348.Baker, H.G. 1967. Support for Baker’s Law—as a rule. 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Interspecifi c hybrid ancestry of a plantadaptive radiation: allopolyploidy of the Hawaiian silverswordalliance (Asteraceae) inferred from fl oral homeotic gene duplications.Molecular Phylogenetics and Evolution 16: 1105–1113.Barrier, M., Robichaux, R.H. & Purugganan, M.D. 2001.Accelerated regulatory gene evolution in an adaptive radiation.Proceedings of the National Academy of Sciences of the UnitedStates of America 98: 10208–10213.Beck, J.B., Al-Shehbaz, A. & Schaal, B.A. 2006. Leavenworthia(Brassicaceae) revisited: testing classic systematic andmating system hypotheses. Systematic Botany 31: 151–159.species of Bidens are SC, there has been evolution of sexualexpression in the lineage, including factors that promoteoutcrossing, with the net result that species exhibita mixed mating system (Ritland and Ganders 1985; Sunand Ganders 1986, 1988).One of the lineages that seems to “break the rules”for Hawaii, and indeed for the other four archipelagos, isTetramolopium. The colonizing ancestor of this ecologicallyand morphologically diverse lineage (Lowrey 1986, 1995;Lowery et al. 2005) was both SC and diploid (Table 9.2).The progenitor of Hawaiian Tetramolopium is from NewGuinea and possible dispersal mechanisms are birds andwind; the pappus and glandular trichomes on the fruitscould facilitate adherence to the feathers and feet of birds(Lowrey 1995; Lowrey et al. 2005). Though highly speculative,it is suggested that there may have been multiplefruits in a single dispersal event, and Tetramolopium evolvedfrom more than a single colonizer. Tetramolopium is an illustrationof the limitation of attempting to interpret thegenetic diversity of colonizers with only two variables ofploidy and breeding system. While the dispersal agent ofAsteraceae is normally the individual fruit, this does notmean that single dispersal events involve only one fruit,as was admitted by Baker (1967). However, an importantpoint with regard to Tetramolopium, and other lineagesoriginating from SC colonizers, is that if a single dispersalevent included more than one fruit from the same selfi ngpopulation, then the multiple fruits may not collectivelycontain signifi cantly more genetic diversity than a singlefruit. In contrast to multiple propagules from selfi ng sourcepopulations, dispersal of multiple fruits in a single eventfrom an outcrossing source population would enhance geneticdiversity in a founding population relative to a singlepropagule. This may explain why the colonizing ancestorsof so many island lineages were SI (or more likely PSF)regardless of whether there was one or multiple propagulesin a single dispersal event. Despite the limitations of consideringonly the two variables of ploidy and breedingsystem, observations for the fi ve archipelagos show thatboth polyploidy and SI accompanied by PSF are commonattributes of successful lineages. It seems reasonable to hypothesizethat these two factors, both alone or in concert,provide colonizers with the requisite diversity necessaryfor radiation and speciation in an archipelago. There areplausible hypotheses to explain the two notable exceptionsto the above generalizations, the paucity of polyploidy inthe Canary Islands and SC colonizers in Hawaii.