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.

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Pathways, mechanisms, and rates of polyploid formation in flowering plants

Publisher Summary Dioecism has arisen independently in different plant families and plant genera. In the majority of cases, the evolution of dioecism has apparently taken place on the species level; in some cases, on the subgeneric level (rumex) or generic level (humulus). Few families (salicaceae) comprise only dioecious species. Dioecious plants offer, in many cases, better tools for studying the genetics of sex determination than dioecious animals. First, the fact that dioecious plants have arisen independently gives an opportunity to study the different ways in which dioecism may become established. Second, in plants the step from dioecism to bisexuality is often a short one and in most cases, dioecism is not clear-cut. In many dioecious species (e.g. mercurialis), bisexual types are found in nature often with a rather high frequency. Such bisexual individuals of normally dioecious plant species are almost always fertile and can be genetically studied, whereas similar bisexual animals are sterile intersexes. In plants it is possible to follow both the evolution of dioecism from hermaphroditism (or monoecism) and the reverse process.

The mechanism of sex determination in dioecious flowering plants.

CONTENTS INTRODUCTION ... ..... ... .. 54 Cross-Specific Comparisons 55 LOWER EUCARYOTES .... 57 Criteria for Mutant Isolation 57 Fertility 57 Effects on specific meiotic events 58 Mitotic disturbances 58 Yeasts 59 Fertility mutants ........ ... ... 60 Mutants with altered meiotic exchange .. ...... 62 Defective mating-type control of sporulation 63

The genetic control of meiosis.

Publisher Summary In this chapter, the review of available data clearly points to the conclusion that the causes underlying the erection of barriers of reproductive isolation, therefore of the origin of species, differ considerably from one group of organisms to another. Furthermore, these differences are related to other differences among groups of organisms, such as size and structure of populations, rates of reproduction, and methods of adaptation. In the future, therefore, the study of the origin of species should not be a search for any more general causes, which might be considered as responsible for speciation in all groups of organisms, but a series of special and comparative investigations, by means of which the interrelationships mentioned above, as well as others as yet unknown, can be further explored and more firmly established.

The inviability, weakness, and sterility of interspecific hybrids.

Natural populations of plants show intricate patterns of variation. European botanists of the eighteenth and nineteenth centuries used this variation to classify different 'kinds' into a hierarchy of family, genus, and species. Although useful, these classifications were based on a belief in the fixity of species and the static patterns of variation. Darwin's theory of evolution changed this view; populations and species varied in time and space and were part of a continuing process of evolution. The development of molecular techniques has transformed our understanding of microevolution and the evolutionary history of the flowering plants. This new edition reviews recent progress in its historical context, showing how hypotheses and models developed in the past have been critically tested. The authors consider the remarkable insights that molecular biology has given us into the processes of evolution in populations of cultivated, wild and weedy species, the threats of extinction faced by many endangered species and the wider evolutionary history of the flowering plants as revealed by cladistic methods.

Plant variation and evolution

Meiotic aberrations and sterility in alopecurus myosuroides huds

Cytological studies of diploid and triploid Populus tremula and of crosses between them.

Interspecific hybridization within the genus Betula.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1601-5223.1945.tb02761.x

The triploid progeny of the cross diploid x tetraploid Populus tremula.

Vilmorin23 is an internationally used differential host variety for studies on the interactions between wheat stripe rust and wheat. It contains the stripe rust resistance gene YrV23 and is potentially an important source of stripe rust resistance worldwide. SSR analysis was performed on the wheat NIL Taichuang 29*6/YrV23 carrying the resistant gene YrV23 against stripe rust, Vilmorin 23 and its recurrent parent Taichung 29. Fifty pairs of SSR primers on wheat chromosome 2B were screened and a reproducible polymorphic DNA fragment amplified by Xwmc356 was found. Genetic linkage was tested on 150 segregating F2 plants. It showed that the microsatellite marker Xwmc356 was linked to the resistance gene YrV23 with a genetic distance of 9.4 cM.

Helge Johnsson

Papers

Meiotic aberrations and sterility in alopecurus myosuroides huds

Cytological studies of diploid and triploid Populus tremula and of crosses between them.

Interspecific hybridization within the genus Betula.

The triploid progeny of the cross diploid x tetraploid Populus tremula.

Cytological studies of triploid progenies of populus tremula