Origins of PolyploidyAn observation…Slide 3Slide 4Slide 5Slide 6Key literaturePowerPoint PresentationSlide 9Chromosome doubling mechanismSlide 11Detection of 2n gametesUnreduced gametes- 2 main meiotic pathways (Bretagnolle and Thompson 1995)Slide 14Slide 15Slide 16Ramsey & Schemske pathways to polyploidizationTriploid BlockTriploidsSlide 20Triploid bridge- production of tetraploidsSlide 22The genetic basis of unreduced gametesPrimula kewensisOrigins of PolyploidyBotany 940Rachel Schmidt Jabaily2/6/07 (Happy Birthday, Dad)An observation…•A cross between good Midwestern stock…xAn observation…•Produced one “wild type”An observation…•And, at high elevation, an aberrant, highly vigorous specimen showing an increased tolerance to cold and a five-fold increase in love of plantsAn observation…•But upon outcrossing,An observation…•a lone, runty, sterile offspring was produced…Key literature•Bretagnolle F and JD Thompson. Tansley Review No. 78: Gametes with the somatic chromosome number; mechanisms of their formation and role in the evolution of autopolyploid plants. 1995. New Phytol. 129: 1-22.•Carputo D, L Frusciante, SJ Peloquin. 2n gametes and endosperm balance number in the origin and evolution of polyploids in tuber-bearing Solanums. 2003. Genetics 163: 287-294.•Harlan JR and JMJ deWet. On O. Winge and a prayer: the origins of polyploidy. 1975. The Botanical Review 41(4): 360-390.•Ramsey J and DW Schemske. Pathways, mechanisms and rates of polyploid formation in flowering plants. 1998. Annu. Rev. Ecol. Syst. 29: 467-501.“I hope I’m not polyploid!!!”•Kihara & Ono (1926)Autopolyploidy- polyploidy within a speciesAllopolyploidy- hybridization and polyploidy between specieswww.micro.utexas.edu/courses/levin/bio304•Which is more likely? Which is more important?–Allopolyploidy more likely than autopolyploidy (but both are still unlikely)–Autopolyploidy may be as important as allopolyploidy•Many intermediary forms•What to call hybridization between populations (ecotypes?)•Stebbins (1970) suggests we abandon the terms•Strict cytogenetic definition-multivalents mean autopolyploids, only bivalents mean allopolyploidsChromosome doubling mechanism•Early researchers assumed mechanism of polyploidy was hybridization followed by chromosome doubling-somatic cell•Winge, (1917)- chromosomes, if alike “enough” in the zygote pair, then split longitudinally to produce 4x polyploid.–Evidence: Oenothera, Nicotiana•Karpechenko (1927) was unsatisfied with this explanation, showed meiotic failures in his own Raphanus x Brassica system•Bukasov (1939) suspected unreduced gametes were involved with polyploidy in Solanum•Clausen, Keck, Hiesey (1945) summarized research on many amphiploids and determined they were produced by unreduced gametes.•Key paper- Harlan and deWet 1975- unreduced gametes are THE primary mechanism of polyploidization–Stebbins had downgraded the importance of this mechanism, thought it was rare and flushed out by natural selection•Somatic chromosome doubling in sporophyte, embryo, zygote–rare, but it happens (wounds, colchicine) Primula kewensis•Polyspermy in orchidsDetection of 2n gametes•Flow cytometry (pollen)•Pollen volume (not good for wind pollinated species!)•Simple crossing experimentsUnreduced gametes- 2 main meiotic pathways (Bretagnolle and Thompson 1995)•First Division Restitution: failure at anaphase I- possession of two non-sister chromatids•Second Division Restitution: sister chromatids fail to separate in meiosis II- possession of sister chromatids=–Direct viewing of meiosis to determine which occurs–Generally occurs in meiosis of micro/mega sporogenesis, but some in somatic cells of ovules before meiosisPioneering work done at Univ. Wisconsin!From Bretagnolle & Thampson, 1998From Bretagnolle & Thampson, 1998Main types of meiotic problems- Spindle failure in MI or MII (3,4,5,11)Abnormal synapse of chromosomes during prophase of MI (1,2,8,9)Omission of first or second division (10)•Polyploidy can increase heterozygosity and fitness (e.g. Solanum- Carputo et al 2003)•Polyploidy reduces the chances of establishment of recessive mutations, buffers intermediate genotypes, reduces the effect of genetic segregation.•FDR= 2n gamete with crossing over has 80% of parental heterzygosity (increase)•SRD= 40% (less than parental heterzygosity)Ramsey & Schemske pathways to polyploidization•Triploid bridge-autotetraploid•One step-autotetraploid•Triploid bridge- allotetraploid•One step- allotetraploid•Higher ploidy one step•Autopp hybridize, create allopp•Allopp created by hybridization of different cytotypesTriploid Block•2n gametes may cause failure of seed production because of failure of endosperm (Carputo et al 2003). •Ratio of female/male genomes is key to endosperm success–2f:1m is most common “endosperm balance number”–2x-4x= 2:2–4x-2x= 4:1–This mechanism may make certain polyploid combinations impossible (e.g. Solanum)Triploids•2x-x most common (but a lack of studies on the mechanism of unreduced chromosomes in eggs!)•Triploid most likely will be sterile-triploid block/meiotic instabilities- BUT some are fertile (and more than was previously thought!)–Ramsey and Schemske 1998- 23% allopolyploids, 39% autopolyploids•Hexaploids produced from 3x-3x or 3x self•Triploids not generally perpetuated sexually: most descendents diploids or aneuploidsFrom Ramsey & Schemske 1998Triploid bridge- production of tetraploids•2x-2x = 4x pairing relatively unlikely•4x-2x produce 3x, backcross to a 2x parent to produce 4x•6.3% of 3x progeny are tetraploid (Ramsey and Schemsky 1998) •Fertile hybrids produce sterile tetraploids; sterile hybrids produce fertile tetraploidsFrom Ramsey & Schemske 1998The genetic basis of unreduced gametes•Strong genetic basis, (can be rapidly selected for in crop experiments ) but frequency varies widely across groups, within species•Some key genes have been identified that contribute to a high frequency of unreduced gametes (e.g. elongate in eggs)•Multiple genes are involved with complex, pleiotropic effects•Environmental impact - massive increase in unreduced gametes seen in cold treated plants•Hybrids 50 times more likely to produce unreduced gametesPrimula kewensis•A single hybrid plant, presumably between P. floribunda and P. verticillata appeared amoung P. floribunda seedlings at Kew in
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