P1 ark mbg P2 ARS plb September 28 1998 QC KKK uks 10 30 T1 KKK Annual Reviews AR067 17 Annu Rev Ecol Syst 1998 29 467 501 Downloaded from arjournals annualreviews org by University of Wisconsin Madison on 02 12 07 For personal use only Annu Rev Ecol Syst 1998 29 467 501 c 1998 by Annual Reviews All rights reserved Copyright PATHWAYS MECHANISMS AND RATES OF POLYPLOID FORMATION IN FLOWERING PLANTS Justin Ramsey and Douglas W Schemske Department of Botany University of Washington Seattle Washington 98195 5325 e mail jramsey u washington edu schem u washington edu KEY WORDS polyploidy autopolyploidy allopolyploidy hybridization speciation ABSTRACT Polyploidy is widely acknowledged as a major mechanism of adaptation and speciation in plants The stages in polyploid evolution include frequent fertility bottlenecks and infrequent events such as gametic nonreduction and interspecific hybridization yet little is known about how these and other factors influence overall rates of polyploid formation Here we review the literature regarding polyploid origins and quantify parameter values for each of the steps involved in the principal pathways In contrast to the common claim that triploids are sterile our results indicate that the triploid bridge pathway can contribute significantly to autopolyploid formation regardless of the mating system and to allopolyploid formation in outcrossing taxa We estimate that the total rate of autotetraploid formation is of the same order as the genic mutation rate 10 5 and that a high frequency of interspecific hybridization 0 2 for selfing taxa 2 7 for outcrossing taxa is required for the rate of tetraploid formation via allopolyploidy to equal that by autopolyploidy We conclude that the rate of autopolyploid formation may often be higher than the rate of allopolyploid formation Further progress toward understanding polyploid origins requires studies in natural populations that quantify a the frequency of unreduced gametes b the effectiveness of triploid bridge pathways and c the rates of interspecific hybridization 467 0066 4162 98 1120 0467 08 00 P1 ark mbg P2 ARS plb September 28 1998 468 QC KKK uks 10 30 T1 KKK Annual Reviews AR067 17 RAMSEY SCHEMSKE Annu Rev Ecol Syst 1998 29 467 501 Downloaded from arjournals annualreviews org by University of Wisconsin Madison on 02 12 07 For personal use only INTRODUCTION Polyploidy defined as the possession of three or more complete sets of chromosomes is an important feature of chromosome evolution in many eukaryote taxa Yeasts insects amphibians reptiles and fishes are known to contain polyploid forms 100 and recent evidence of extensive gene duplication suggests that the mammalian genome has a polyploid origin 112 In plants polyploidy represents a major mechanism of adaptation and speciation 24 56 95 104 120 157 159 It is estimated that between 47 and 70 of angiosperm species are polyploid 56 110 Differences in ploidy have been observed among related congeners and even within populations of taxonomic species 24 34 56 100 156 and there is evidence that individual polyploid taxa may have multiple origins 154 These observations suggest that polyploid evolution is an ongoing process and not a rare macroevolutionary event Research in agricultural and natural systems indicates that polyploids often possess novel physiological and life history characteristics not present in the progenitor cytotype 95 104 Some of these new attributes may be adaptive allowing a plant to enter a new ecological niche Because plants of different ploidies are often reproductively isolated by strong post zygotic barriers polyploidy is also one of the major mechanisms by which plants evolve reproductive isolation 34 56 In spite of the prevalence and importance of polyploidy the factors contributing to polyploid evolution are not well understood 165 Two critical stages of polyploid evolution can be identified formation and establishment To understand the process of polyploid formation requires information on the pathways cytological mechanisms and rates of polyploid formation To assess the likelihood that a new polyploid will successfully establish requires information on the viability and fertility of new cytotypes the extent of assortative mating and reproductive isolation within and between different cytotypes and the ecological niche of new polyploids Here we review the literature concerning polyploid formation to answer the following questions a What are the primary pathways and mechanisms of polyploid formation b What are the parameters for each of the steps involved in polyploid formation c What are the numerical values reported for these parameters and d What is the estimated rate of polyploid formation by each pathway One major motivation for this review is to synthesize the diverse literature on polyploid origins and thereby provide a resource for the development of future empirical and theoretical studies of polyploid evolution To this end we have tabulated data from many studies and made this information available on the Annual Reviews web site http www annualreviews org see Supplementary Materials We summarize these data throughout the text and identify the location of each database on the web site P1 ark mbg P2 ARS plb September 28 1998 QC KKK uks 10 30 T1 KKK Annual Reviews AR067 17 Annu Rev Ecol Syst 1998 29 467 501 Downloaded from arjournals annualreviews org by University of Wisconsin Madison on 02 12 07 For personal use only POLYPLOID FORMATION 469 By necessity many of the plants considered in this review are agricultural or horticultural cultivars and their wild relatives as well as taxa widely used in classical genetic studies e g Oenothera and Datura We believe that the studies reviewed here provide insights into the process of polyploid formation in natural populations but caution that further research in natural populations is needed to test our findings Our survey draws from a wide range of plant taxa but because of the limited number of studies we do not interpret our results in a phylogenetic context In this chapter 2n refers to the somatic chromosome number and n to the gametic chromosome number regardless of the degree of polyploidy while x is the most probable base number This gives the following cytological designations diploids 2n 2x triploids 2n 3x tetraploids 2n 4x etc In describing crosses within and between cytotypes the maternal parent is always listed first MECHANISMS OF POLYPLOID FORMATION Several
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