Polyploid complex

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 i5 ), 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.

Abstract: Polyploidy has played a major role in the evolution of many eukaryotes. Recent studies have dramatically reshaped views of polyploid evolution, demonstrating that most polyploid species examined, both plant and animal, have formed recurrently from different populations of their progenitors. Populations of independent origin can subsequently come into contact and hybridize, generating new genotypes. Because of the frequency of polyploidy in plants, many recognized species are probably polyphyletic. Extensive and rapid genome restructuring can occur after polyploidization. Such changes can be mediated by transposons. Polyploidization could represent a period of transilience, during which genomic changes occur, potentially producing new gene complexes and facilitating rapid evolution.

Abstract: Chloroplast DNA sequences were obtained from 331 Asplenium ceterach plants representing 143 populations from throughout the range of the complex in Europe, plus outlying sites in North Africa and the near East. We identified nine distinct haplotypes from a 900 bp fragment of trnL-trnF gene. Tetraploid populations were encountered throughout Europe and further afield, whereas diploid populations were scarcer and predominated in the Pannonian-Balkan region. Hexaploids were encountered only in southern Mediterranean populations. Four haplotypes were found among diploid populations of the Pannonian-Balkans indicating that this region formed a northern Pleistocene refugium. A separate polyploid complex centred on Greece, comprises diploid, tetraploid and hexaploid populations with two endemic haplotypes and suggests long-term persistence of populations in the southern Mediterranean. Three chloroplast DNA (cpDNA) haplotypes were common among tetraploids in Spain and Italy, with diversity reducing northwards suggesting expansion from the south after the Pleistocene. Our cpDNA and ploidy data indicate at least six independent origins of polyploids.

Abstract: Summary The perennial soybeans (Glycine subgenus Glycine), are the sister group of the annual cultivated soybean (G. max). Among the approximately 20 species are diploids and polyploids, the former confined to Australia and neighboring islands and the latter more widespread. Although most subgenus Glycine species reproduce predominantly by selfing in cleistogamous flowers, phylogenetic evidence exists for reticulate evolution throughout the history of the subgenus. The entire genus is a paleopolyploid, and could possibly be allopolyploid, though there is as yet no evidence for a hybrid origin. Incongruence among the major nuclear genome groups in nuclear and chloroplast gene trees can be explained by several ancient introgressions. Within the B-genome group there is substantial incongruence between chloroplast and nuclear single copy gene trees that is explained better by introgressive hybridization than by stochastic sorting of ancestral lineages. Several allopolyploids originated by hybridization among a subset of genome groups to form a single large interconnected polyploid complex. A number of allopolyploid combinations have arisen recurrently, some bidirectionally. Some recurrent polyploids show evidence of lineage recombination, indicating that their populations comprise a single biological species. Neopolyploidy has involved hybridization among a subset of subgenus Glycine genome groups, and appears to have occurred recently, whereas hybridization at the diploid level has occurred throughout the history of the group.