Origins of Polyploidy Botany 940 Rachel Schmidt Jabaily 2 6 07 Happy Birthday Dad An observation A cross between good Midwestern stock x An 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 plants An observation But upon outcrossing An observation a lone runty sterile offspring was produced Key literature I hope I m not polyploid 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 Kihara Ono 1926 Autopolyploidypolyploidy within a species Allopolyploidyhybridization and polyploidy between species www micro utexas edu cours Many intermediary forms What to call hybridization between populations ecotypes Stebbins 1970 suggests we abandon the terms Strict cytogenetic definitionmultivalents mean autopolyploids only bivalents mean allopolyploids 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 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 Chromosome doubling mechanism 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 orchids Detection of 2n gametes Flow cytometry pollen Pollen volume not good for wind pollinated species Simple crossing experiments Unreduced 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 meiosis Pioneering work done at Univ Wisconsin From Bretagnolle Main types of meiotic problemsSpindle 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 From Bretagnolle 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 bridgeautotetraploid One step autotetraploid Triploid bridgeallotetraploid One step allotetraploid Higher ploidy one step Autopp hybridize create allopp Allopp created by hybridization of different cytotypes Triploid 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 2x x most common but a lack of Triploids studies on the mechanism of unreduced chromosomes in eggs Triploid most likely will be steriletriploid block meiotic instabilitiesBUT 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 aneuploids From Ramsey Schemske Triploid 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 tetraploids From Ramsey Schemske 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 gametes The genetic basis of unreduced gametes Primula kewensis A single hybrid plant presumably between P floribunda and P verticillata appeared amoung P floribunda seedlings at Kew in 1899 It was sterile with 9 pairs of chromosomes like its parents In 1905 a single pin eyed flower appeared and was fertilized by pollen of a thrum eyed flower both on sterile stock The seed produced fertile amphiploid plants 2n 36 Since the stock remained sterile and diploid the most likely interpretation is a 2n 2n mating In 1926 tetraploid branches appeared on diploid stock From Harlan deWet 1975
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